Rm... 2.3.2: .It. 1\.n.-..+.u a T M'LFII'Jnl-‘n' lC lgan State $00“ L University This is to certify that the thesis entitled FUNDAMENTAL POWER COUPLER DEVELOPMENT FOR LOW-BETA SUPERCONDUCTING CAVITIES presented by Jon Joseph Wlodarczak has been accepted towards fulfillment of the requirements for the Master of degree in Electrical and Computer Science Engineering @K C..‘__/ Major Professor’s Signature 0] A123 7.90? Date MSU is an affirmative-action, equal-opportunity employer .—.—n—o-u-.-o-o-u-u-cuo-a-u-u-n—c—a—a-nu-o-.—- PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 5/08 KIProj/Acc8-PresICIRC/DaieDue indd FUNDAMENTAL POWER COUPLER DEVELOPMENT FOR LOW-BETA SUPERCONDUCTING CAVITIES By Jon Joseph Wlodarczak A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Electrical and Computer Engineering 2008 ABSTRACT FUNDAMENTAL POWER COUPLER DEVELOPMENT FOR LOW-BETA SUPERCONDUCTING CAVITIES By Jon Joseph Wlodarczak The construction of a re—accelerator for secondary ion beams is currently underway at the National Superconducting Cyclotron Laboratory (N SCL). This re-accelerating linear accelerator (linac) will use superconducting quarter-wave resonators Operating at 80.5 MHz with fl = 0.041 and fl = 0.085, where U is the velocity of a particle relative to the speed of light. Input coupling will be done using a coaxial probe-type radio frequency (RF) fundamental power coupler (FPC) for both quarter-wave resonator (QWR) cavities. Custom-designed power couplers are expensive, complicated, and can have long lead times. The FPCS presented in this thesis will be operated with fixed coupling, using continuous wave (CW) input power operating at a maximum of 1 kW. The key component of the coupler is a commercially-available RF feedthrough to reduce the cost and production time. This feedthrough allows the cavity to maintain a clean vacuum with pressure less than 1 x 10‘8 torr. Another concern is the heat leak to the cryomodule’s cold mass at 4.5 K. The design goal for each coupler is to produce less than 1 W of static heat load to the cold mass. Four prototype F PCs were fabricated. Two of the FPCS were assembled back- to—back, pumped out, and conditioned with both traveling wave and standing wave power using a 1 kW RF amplifier. Experience with the prototype FPCS provided the basis for the design of production FPCS for the re—accelerator. For Fred iii ACKNOWLEDGMENT This thesis would have never been possible without the help of many people. First I would like to thank my advisors Dr. Leo Kempel and Dr. Walter Hartung for providing me the opportunity to work on this project. Their collective guidance was immeasurable. Thanks are also in order to the professors and students in the electromagnetics group at MSU. They provided me with the knowledge to make it through. I would also like to thank everyone at the National Superconducting Cyclotron Laboratory for having me as part of their team. I would especially like to thank Jim Wagner and Kurt Kranz for getting my foot in the door as an undergraduate. Working with the SRF group has given me the support and equipment which enabled me to work on this project. Without the technical knowledge of John Popielarski, I would not have been able to accomplish as much. I would also like to thank David Norton for all of his work with the data acquisition during conditioning, Laura Popielarski for her clean room assistance, Patrick Glennon for his mechanical design work, Matt Johnson for his work with ANSYS, Steve Bricker, John Bierwagen, and everybody else that provided assistance. Most of all I would like to thank my wife for supporting me throughout this entire process, both financially and emotionally. Without her, I would never have gotten to where I am today. This is for Danielle. iv TABLE OF CONTENTS List of Tables vii List of Figures viii Key to Abbreviations xi Key to Symbols xii Introduction 1 Superconducting Cavities and Linear Accelerators 6 2.1 Superconducting Cavities ......................... 6 2.2 Coaxial Quarter-Wave Resonators .................... 9 2.3 Linacs and Re-accelerators ........................ 11 RF Power Coupling and Couplers 15 3.1 Fundamental Power Couplers, Higher Order Mode Couplers and Pick- Up Couplers ................................ 15 3.2 Coupling Methods and Locations .................... 17 3.3 RF Window ................................ 19 3.4 Material Considerations ......................... 22 3.4.1 Calculating Heat Load and RF Losses ............. 23 3.5 Conductor Sizing ............................. 27 3.6 Finding the External Quality Factor .................. 29 3.7 Potential Faults and Failures ....................... 31 Design Approach and Methodology 34 4.1 FPC Placement .............................. 34 4.2 Minimizing Heat Load and Losses .................... 35 4.3 Determining Conductor Dimensions ................... 36 4.4 Ceramic Window Simulations ...................... 37 4.5 Setting QM ................................ 44 4.6 Diagnostics ................................ 51 Coupler Cleaning, Conditioning and Testing 54 5.1 Coupler Preparation and Cleaning .................... 54 5.2 Bake-out .................................. 57 5.3 Coupler Conditioning ........................... 61 5.4 High Power Testing ............................ 71 5.5 Issues ................................... 76 6 Conclusions A Mechanical Drawings of the Coupler and Test Assemblies B Clean Room Assembly Procedure C Conditioning Procedure Bibliography vi 82 85 91 94 97 2.1 3.1 4.1 4.2 LIST OF TABLES Low beta QWR cavity parameters. ................... 14 Comparison of coaxial and waveguide couplers .............. 17 Power coupler heat load analysis. .................... 36 Mechanical mode analysis for inner conductor deformation with various length stainless steel liners ......................... 44 vii 1.1 2.1 3.1 3.2 3.3 3.4 3.5 4.1 4.2 4.3 4.4 4.5 4.6 LIST OF FIGURES A conceptual view of the N SCL gas stopper and re—accelerator. . . . . Superconducting QWRs to be used for the re-accelerator. The niobium inner and outer conductors are shown in gray, and the titanium helium vessels are shown in green. ........................ Analyst model of E = 0.041 QVVR showing the surface magnetic field distribution ................................. Analyst model of 5 = 0.041 QWR showing the surface electric field distribution ................................. Thermal conductivity of OF HC copper and select stainless steels. Electrical resistivity of OFHC copper and select stainless steels. Geometry of partially filled transmission line. ............. A 3D model of the coupler window assembly. The blue areas are vac- uum space, and the orange ones are high purity alumina. ....... Analyst simulation of the electric field in the coupler window assembly. Analyst simulation of magnetic field in the coupler window assembly. Simulated reflection coefficients of selected coupler window assemblies using Analyst ................................ Simulated transmission coefficients of selected coupler window assem- blies using Analyst ............................. ANSYS simulation for the first harmonic deformation of the inner con- ductor with a 11” stainless steel liner ................... viii 13 20 21 24 25 28 39 40 41 42 43 45 4.7 4.8 4.9 4.10 4.11 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 ANSYS simulation for the second harmonic deformation of the inner conductor with a 11” stainless steel liner ................. ANSYS simulation for the third harmonic deformation of the inner conductor with a 11” stainless steel liner ................. ANSYS simulation for the deflection of the inner conductor with a 11” stainless steel liner, from 1 N applied laterally to the tip. ....... RF power vacuum feedthroughs from Insulator Seal. The white portion is the cylindrical alumina window ..................... Qm versus coupler penetration length for the 80.5 MHz, ,6 = 0.041 cavity. The length is measured from the inside of the bottom flange. . Assembly of the power coupler test fixture inside a class 10, 000 clean room. ................................... Outer conductor of a power coupler wrapped in heat tape prior to bake-out. The ceramic window is visible on the left ........... Power coupler conditioning fixture wrapped in foil for baking. Temperature and pressure measurements taken during the bake-out. The vacuum pressure on plotted along the right vertical axis ...... Residual gases in the conditioning assembly prior to baking out. . . . Increased gases in the fixture 2 hours after baking ............ Remaining gases after pumping on fixture for 24 hours after baking. . The measured and simulated transmission coefficients through the two— coupler assembly. ............................. The measured and simulated transmission coefficients through the two- coupler assembly. ............................. Schematic diagram of the power coupler conditioning system ...... The sliding short assembly as installed for conditioning in standing wave mode. The large diameter pipe in the foreground is the input side, and the small diameter one near the upper-right corner was used to adjust the resonant frequency. .................... ix 46 47 48 49 50 56 58 59 60 62 63 64 66 67 68 5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19 Al A2 A3 A4 A5 C.1 Standing wave voltage sweep pattern. The vertical black lines are the window locations; the dark blue line is the initial sweep position. . . . 72 Standing wave current sweep pattern. The vertical black lines are the window locations; the dark blue line is the initial sweep position. . . . 73 Increased RCA reading resulting from conditioning a “dirty” area of the power coupler assembly. Note the increasing trend indicating gas levels are still climbing ........................... 74 Remaining residual gases after conditioning. .............. 75 Final RCA reading showing the removal of nearly all the residual gases from the system. ............................. 77 Failed inner conductor solder joint due to excessive heating. The piece of solder at the bottom of the picture was found inside the conditioning fixture during disassembly ......................... 79 Solder deposited on the interior surface of the copper conditioning sleeve, near the location of the failed solder joint. ........... 80 Improved inner conductor tip assembly with silver brazing and smoothed surfaces. .................................. 81 Power coupler components and assembly diagram. 1) Vacuum side outer conductor, 2) Air side outer conductor, 3) Vacuum feedthrough, 5) Spacing ring, 10) Spark detector, 11) Current probe, 14) Vacuum gauge, 19) Rigid line coax to N-type adapter. ............. 86 Important dimensions of an assembled input power coupler. ..... 87 Power coupler attachment to the bottom flange of a cavity. The center port is for the tuning actuator, and the third port is for the pick—up coupler. NOT TO SCALE ......................... 88 Modified 6-way cross use in the conditioning assembly. 1) Copper inner sleeve, 2) Connecting barrel, 3) Modified 6-way cross, 4) Copper gasket. 89 Assembled power coupler conditioning fixture. NOT TO SCALE. . . 90 Measurement points for system calibration ................ 94 KEY TO ABBREVIATIONS N SCL: National Superconducting Cyclotron Laboratory MSU: Michigan State University ReA3: Re-Accelerator, 3 MeV LINAC: Linear Accelerator RIBS; Rare Isotope Beams RIA: Rare Isotope Accelerator F RIB: Facility for Rare Isotope Beams QWR: Quarter Wave Resonator RF: Radio Frequency FPC: Fundamental Power Coupler SRF: Superconducting Radio Frequency CW: Continuous Wave VNA: Vector Network Analyzer RRR: Residual Resistance Ratio RFQ: Radio Frequency Quadrupole HOM: Higher Order Mode OFHC: Oxygen-Free High Conductivity IR: Infrared CF: ConFlat RGA: Residual Gas Analyzer xi KEY TO SYMBOLS (1: Diameter of inner conductor [In] A, A”, AC”, Arc, ADC: Cross-sectional area [m2] b: Diameter of outer conductor [m] Bpk: Peak magnetic field [T] 8.1,: Magnetic flux density, azimuthal [T] BW: Bandwidth [Hz] c: Speed of light [2.998 x 108 m/s] c: Diameter of dielectric liner [m] (1: Width of rectangular waveguide [111] Em: Accelerating (electric) field [V/m] Epic: Peak electric field [V/ m] Er: Electric field, radial [V/m] f0: Resonant frequency [Hz] G: Geometry Factor [9] H: Magnetic field intensity [A / m] [0: Peak current [A] MT)”, k(T)Cu,: Temperature dependent thermal conductivity [W/(mK)] L: Gap length [m] me: Minimum coupler length [inches] Pbeam: Beam power [W] PC: Power dissipated in a cavity [W] Pm: Transient power emitted from a cavity [W] wad: Forward power [dBm] Pg: Power supplied by RF generator [W] Pp: Power dissipated through pick-up coupler [W] P,: Reflected power [W] xii Paw: Standing wave equivalent power [dBm] Pt: Transmitted power [W] (hand: Conducted heat transfer [W] qmd: Radiated heat transfer [W] qu: RF conduction heat transfer [W] qtotal: Total heat transfer [W] Qext: External quality factor Q L: Loaded quality factor Q0: Intrinsic quality factor Qsm: Static heat leak [W] Qtot: Total heat leak [W] RL: Load resistance [Q] R3: Surface resistance [9] t”: Wall thickness of stainless steel [inches] to”: Copper plating thickness [# of skin depths] U: Stored energy in a cavity [J] Vacc: Accelerating voltage [V] VP: Peak voltage [V] Z9: Impedance of partially-filled transmission line [D] Z0: Characteristic impedance of a transmission line [D] ,8: Relative velocity 5m: Coupling factor flextm‘ Pick-up coupling factor flex”: Input coupling factor I‘L: Reflection coeffecient of coupler transmission line I‘m: Reflection coeffecient of coupler window 6: Skin depth [m] xiii so: Permittivity of free space [1/c2,u0 = 8.8541878 x 10‘12 F/m] 81, 52: Relative permittivity of a dielectric e: Emissivity 770: Intrinsic impedance of free-space [Q] A: Wavelength [m] AC: Cut—off wavelength [m] _ a0: Permeability of free space [47r x 10‘7 N/Az] p: Electrical resistivity [Rm] 0: Electrical conductivity [(Qm)1] 033: Stefan-Boltzmann constant [5.6703 x 10‘8 W / m2K4] w: Angular frequency [radians/sec] xiv Chapter 1 Introduction A particle accelerator is a device which is used to accelerate charged particles to high velocities using electric fields. These high speed particles are then collided with a target to create new nuclei. This is why particle accelerators are sometimes called “atom smashers.” These new isotopes do not normally exist on Earth, and last for only minute fractions of a second. Measuring the properties of these rare isotopes helps researchers begin to understand how an element is formed, or what holds a nucleus together. These answers can begin to unravel the origin of elements, or even why stars shine. Particle accelerators also have applications in archeology, national security, and medicine. The National Superconducting Cyclotron Laboratory (N SCL) is a world leader in the research of rare isotopes. Located on the campus of Michigan State University (MSU), N SCL operates two coupled superconducting cyclotrons. Beams are injected into the K500 cyclotron then sent to the K1200 cyclotron for additional acceleration [1]. Together, these two cyclotrons can accelerate particles up to 200 MeV per nu- cleon. NSCL has also built the first superconducting cyclotron for medicine, the K100 Neutron Therapy Cyclotron at Harper-Grace Hospital in Detroit [2]. NSCL will be the first facility in the world to demonstrate the ability to create, stop and re—accelerate exotic beams. A conceptual diagram of the system, currently titled ReA3, is shown in Figure 1.1 [3]. The construction of ReA3 to re-accelerate these secondary ion beams is currently underway. Ion "‘ ., . . . {I . K500 sources ' ; cyclotron K1 200 cyclotron A1900 fragment separator ~100 MeV/nucl . i Gas stopping .L Char 9 hrs ing < 12 MeV/nucl . A < 3 MeV/nucl .1 ~ keV Eta-acceleration Flo-acceleration Figure 1.1: A conceptual view of the NSCL gas stopper and re—accelerator. Linear accelerators (linacs) are able to accelerate particles to much higher energies than circular accelerators. This is due to synchrotron radiation which is created as the particles accelerate around the center. The radiation energy lost by the particle beam must be restored by the accelerator. This limits the amount of energy that can be transferred to the beam. Using the cyclotron as an injector for ReA3, rare isotope beams (RIBS) can be accelerated at a lower cost than a dedicated facility, such as the Rare Isotope Accel— erator (RIA) [4], or the Facility for Rare Isotope Beams (FRIB). The cavity designs of ReA3 are the same as those already developed and tested for RIA, requiring less development time and cost. ReA3 will utilize three cryomodules, containing a total of fifteen 80.5 MHz quarter- wave resonator (QWR) cavities. The modules will also contain eight superconducting solenoid magnets to be used for focusing [5]. A cryomodule is a complex component of the linac that contains liquid helium and operates at temperatures as low as 2 K. The module contains the accelerator cavities, focusing elements, as well as well as the tuners and shielding necessary for particle acceleration [6]. The re—acceleration of the beams will rely heavily on low velocity QWR cavities operating at 80.5 MHz. The QWR cavities that will be used are 6 = v/c = 0.041 [7] and fl = 0.085 [8], each requiring less than 1 kW of operating power from the radio frequency (RF) generation system. In addition to the internal elements, the cryomodule also supports the necessary auxiliary components necessary for operation. This includes vacuum pumps, tuning actuators, gas analyzers and fundamental power couplers (FPCS). The FPC is very important to the operation of a superconducting cryomodule; it is used to transport the RF power from the generation system into the cryomodule, and to the cavity. There are many approaches used to couple power into cavities [9, 10]. Power cou- plers can be quite complicated, and requires the cooperation of several engineering disciplines. This makes the design of the coupler one of the key areas of research in superconducting RF (SRF) development of accelerating cavities. A prototype cryomodule has been constructed at NSCL to measure the perfor- mance of the QWR cavities and FPCS initially designed for RIA [11]. This module uti- lizes a coaxial coupler originally designed by Moblo [12]. Using information gathered during the testing of the module, a new prototype power coupler has been designed. The existing design has a small diameter inner conductor and thin ceramic window. These flaws make it susceptible to breakage during assembly. The new F PC design was optimized for continuous wave (CW) operation up to 1 kW of input power at 80.5 MHz. The F PC has fixed coupling utilizing a semi—custom vacuum feedthrough from industry. The addition of a larger center conductor, thicker alumina, and less obtru- sive air-side geometry will increase durability. This feedthrough allowed the design to remain affordable, and yet be robust enough to handle the power requirements of the NSCL re—accelerator project. Additionally, coupler diagnostic ports have been added to monitor the condition of the window, a feature that was not included previously. Most commercially built input couplers are designed for much higher power applica- tions, and are quite expensive because of this. Typically the cost of manufacturing a coupler is very high, in the tens of thousands of dollars [13]. This is close to the cost of the cavities themselves. Additionally, the range and type of superconducting accelerators is so varied, there is no one coupler that fits all applications [14]. This thesis will discuss the continuing development of an aflordable FPC for NSCL. A brief introduction to superconducting cavities and linear accelerator con- cepts will be provided in Chapter 2. Additionally, Chapter 2 will present the param- eters and specifications for the cavities that the couplers will be used to excite. A more detailed analysis of coupler design is presented in Chapter 3. This includes background on the various methods used to get power in and out of a cavity, and coupler placement. In addition, common problem areas will be addressed. Chapter 4 discusses the design considerations that were used in the development of the couplers. This includes both electrical and mechanical analysis. The diagnostic methods used to minimize the most common design issues will also be described. In Chapter 5, an overview of the processes used during the conditioning of the prototype assemblies will be presented, including the procedures and the equipment used. Also provided in Chapter 5 are the results of conditioning. This chapter will discuss the outcome of the conditioning in addition to addressing issues that arose during conditioning, such as problems that developed in the design, assembly, and testing of the prototypes. Chapter 6 is the conclusion to this thesis. A summary of the findings and results will be presented. Recommendations for adjusting the conditioning procedures as well as considerations for future work on the FPC will be provided. Chapter 2 Superconducting Cavities and Linear Accelerators The re-accelerator project at NSCL relies heavily on superconducting cavities. These SRF cavities provide greater accelerating capabilities than their normal conducting counterparts. Trade-offs include increased initial costs in materials and facilities, with gains in accelerating gradient and reduced RF generation costs. An introduction to SRF cavities is given in Section 2.1. Section 2.2 provides further detail on the QWR cavities that will be used in the re-accelerator. Finally, Section 2.3 explains the application of the cavities for the acceleration of exotic beams, and future acceleration projects at MSU. 2.1 Superconducting Cavities The primary function of a resonant cavity is to impart energy to a beam of charged particles using a time-varying electric field. The Til/[010 mode is commonly chosen for operation of pillbox cavities operating in 7r mode, because it is the lowest order mode that has an electric field along the beam axis, which is what provides acceleration. Coaxial resonators use a TEM mode to create the accelerating field. Magnetic fields do not add energy to the beam, however they are useful for focusing. Focusing is done elsewhere in the linac using superconducting solenoids. Superconductivity is desirable because it allows large accelerating fields to be generated without a large amount of power loss in the cavity walls. Much of the losses occur due to surface resistance. Normal conducting copper cavities have surface resistance on the order of 10‘3 52, depending on the frequency and temperature. In comparison the typical surface resistance of superconducting niobium cavities range from 10 to 100 n9 [15]. These losses within the cavity can be described by the quality factor Q0, or the intrinsic Q of the cavity. The quality factor is a figure of merit for resonant cavities; it is a measure of the number of RF cycles that a cavity will go through before it dissipates the energy stored within. The quality factor is expressed as Energy Stored in Cavity _ wU Q wPower Dzsszpated zn Cavzty PC ( ) The quality factor can also be written as G 0 = _ 2.2 Q R. < > where R, is the surface resistance and G is the geometry factor: or o H 2 dv G = # fv l l (2.3) f3 [HI2 ds It is easily seen from Equation (2.2) that the quality factor of a superconducting cavity is approximately five orders of magnitude higher than that of copper because of the reduced surface resistance. The geometry factor depends on the shape of the cavity, not its size or material, so it is convenient to use when comparing different cavity configurations. The intrinsic Q of the cavity is useful for finding the losses in a cavity, but this is not the only mechanism for losses to occur. Additional energy can be lost through the input and pick-up couplers. Another useful term is the external quality factor, or Qext. The Qext quantifies how much energy leaks out of a port in cavity and is lost. Assuming a port terminated with an external load, we define Energy Stored in Cavity wU = 2.4 wPower Dissipated in the Erternal Load Pm ( ) Qext : We can now use the above definitions to describe the total loss in the cavity, or average power dissipated. This is the loaded Q and is what can be measured using a vector network analyzer (VNA). The loaded quality factor is Q _ wEnergy Stored in Cavity __ Lil-f— L _ Total Power Dissipated _ PT (2.5) where PT 2 PC + Part is the total power dissipated. For a one-port system, the relationship between the Q factors can be expressed as 1 1 1 22: — a + Q... (26) Once the cavity is designed and built, Qext is the only quality factor than can be readily adjusted. The external Q depends on the coupling between the power generator and the cavity. This coupling factor is a measure of the coupling strength to the cavity, and is referred to as flat and defined as 1 Pert : Q0 PC Qext 186m E (2.7) Pm is the power emitted from the cavity to the input coupler, under a transient 1 The ‘ext’ subscript will be used to differentiate the coupling factor from the normalized beam velocity ([3), which is unrelated. condition. A more detailed explanation is available in [15]. If flex; < 1 the coupler and cavity are considered undercoupled. When Bat > 1 the combination is considered overcoupled. When 3m 2 1 the cavity and coupler are considered critically coupled, or matched. When this happens, Qext = Q0 and QL = Q0 / 2. Substituting Equation (2.7) into Equation (2.6) gives Q0 : QL(1 + fiext) (2'8) In the case of two couplers, one to input the power (flaky), and a second used as a pick up (flaw), Equation (2.8) becomes Q0 : QL(1 + [Betty + fiextp) (2.9) The coupling factor of the second port can be expressed as Pp (Be-rip : Fe (2.10) where PD is the power lost to the pick-up coupler. There are many more important formulas pertaining to cavities, the above provide the information needed to design a power coupler. A more in-depth analysis on cavity design and performance can be found in the text by Padamsee [15]. 2.2 Coaxial Quarter-Wave Resonators ReA3 will rely on a total of sixteen QWR cavities of ,8 = 0.041 and fl = 0.085 operating at 80.5 MHz. These cavities consist of a single inner conductor, or loading element, that is /\/4 long, centered in a cylindrical outer conductor. The conductor walls are formed from niobium sheets 2 mm thick, with RRR Z 250 [8]. This residual resistance ratio (RRR) is a measure of the purity of a metal. The RRR can be formally defined as Resistivity at 300 K RRR 2 Residual Resistivity at Low Temperature (Normal Conducting) (2.11) The larger the RRR, the more pure the metal is considered, up to the theoretical limit of 35,000 [15, 16]. It has already been shown that ,8 is the velocity of a particle relative to the speed of light. The product 5). is the distance a particle of velocity 5 travels in a single RF period. In order to provided a positive field in each gap, cavities are designed with an inter-gap spacing of ,BA/ 2. Because 6 is small, a lower frequency allows for a larger gap, which is more useful for acceleration [15]. However, this low frequency requires rather large cavity dimensions, which is why QWRs are chosen over other cavity types. Cavities with B < 0.2 typically have operating frequencies between 50 and 200 MHz. The accelerating field (Em) of a cavity is given by V Em, : -—p 2.12 L ( > where V,, is the peak voltage and L is the distance over which the beam experiences electric field during acceleration. Care must be taken when defining this distance, as the definition may vary between institutions. The superconducting cavities that will be used for ReA3 are shown in Figure 2.1, and the cavity parameters are provided in Table 2.1 [17, 18]. A quarter—wave cavity is a modification of a simple coaxial resonator. A coaxial resonator is formed by placing conducting end walls onto a simple coaxial transmission line. The enclosed length allows standing-wave TEM modes to exist, which have only transverse fields. In his text, Wangler [19] provides the following three formulas for 10 coaxial resonators: 010 - Ba 2 # cos(p7rz/l)ej“" (2.13) 7rr and E, = —2j Egg-sinwvrz/Dem (2.14) 60 27rr where p = 1,2,3, . . ., w = p7rz/l, l = pA/ 2 and z is the position along the cavity, from 0 to l. The quality factor of the coaxial cavity can be expressed as b Q0 2 if“ [53 [r (l + $811.19)] (2'15) 8 0 a b a where a and b are the inner and outer conductor diameters respectively. The lowest order mode is p = 1 which corresponds to a half—wave resonator. A modification of the half-wave coaxial resonator is the quarter-wave. This is made by shorting one end of the center conductor, and leaving a gap at the opposite end, creating a capacitance. Beam holes are placed near the end gap, where the electric field is the greatest; these fields will be discussed further in Section 3.2. 2.3 Linacs and Re-accelerators One would assume that the term linear accelerator would refer to any device that accelerates particles in a straight line. In fact, linac refers to an accelerator that accelerates a charged particle in a linear path using electromagnetic fields [19]. Other paths of resonant RF accelerators are spiral for a cyclotron, or circular for a syn- chrotron. One benefit of straight-line trajectory is that a linac provides beams of high energy and quality without the power losses caused by synchrotron radiation, which is present in circular accelerators. An advantage that cyclic accelerators offer is continuous acceleration of a particle as it circles. Another advantage is that circular accelerators take up much less space than a linac of comparable power. 11 ReA3 will consist of a normal conducting radio frequency quadrupole (RF Q) con— nected in series with three superconducting cryomodules containing a total of fifteen QWR cavities [18]. The particles being accelerated will first pass through the RFQ and then into two modules containing 3 = 0.041 cavities, then through one module containing 3 = 0.085 cavities. The beam velocity range that the cavity can efficiently accelerate is the cavity’s velocity acceptance, and the range of the re—aceelerator will be from v = 0.05c to v = 0.15c; providing energies up to 3.0 MeV. A fourth module near the end of the accelerator will contain a single 6 = 0.041 QWR cavity and will be used as a beam rebuncher. The size of a linac can range from a few meters to several kilometers in length, and the costs can range from a few million to a billion dollars. Both the length and cost depend on the desired final energy. The information provided here just scratches the surface on linacs and their applications. A much more detailed history and explanation can be found in the text by Wangler [19]. 12 [30,,=o.041 pom=o.oss Figure 2.1: Superconducting QWRs to be used for the re—accelerator. The niobium inner and outer conductors are shown in gray, and the titanium helium vessels are shown in green. Table 2.1: Low beta QWR cavity parameters. [Cavity Type [[ x\/ 4 /\/ 4 Optimum fl (flopt) 0.041 0.085 Resonant frequency (MHz) 80.5 80.5 Number of cavities 8 8 E10,, (MV/m) 16.5 20 13,. (mT) 28.2 49.5 V... (MV) 0.46 1.18 L (m) 0.095 0.21 Em:C (MV/m) 4.84 5.62 Operating Temperature (K) 4.5 4.5 0(a) 15.7 19 U (J) 0.99 6.69 Q0 5 x 108 5 x 108 QL 1.4 x 106 3.2 x 106 Pg (W) 236 700 Pb..." (W) 118 350 14 Chapter 3 RF Power Coupling and Couplers There are many systems necessary to successfully operate a superconducting cavity. Perhaps none are more critical than the fundamental power coupler. Without the FPC supplying RF power to the cavity, there would be no electric field to accelerate the beam. If a coupler fails, it can disable the cavity or even the entire accelerator. Section 3.1 provides a brief introduction to the three most common types of power couplers. F PC location and style considerations are discussed in Section 3.2. The function and design analysis of the RF window are given in Section 3.3. Material constraints and analysis of heat load are given in Section 3.4. Conductor sizing and matching are presented in Section 3.5. Section 3.6 explains how the desired coupling strength (Qm) is determined. Lastly, Section 3.7 introduces various faults and the consequences of a coupler failure. 3.1 Fundamental Power Couplers, Higher Order Mode Couplers and Pick-Up Couplers RF couplers serve a single purpose for superconducting cavities and linacs, and that is transporting power. There are three basic types of couplers used for this objective, 15 fundamental power couplers, higher order mode (HOM) power couplers, and pick-up couplers. These three types of couplers serve very different functions. The role of the FPC is to transport RF power to the cavity, while the pick-up and HOM couplers remove power from the cavity. Pick-up couplers are also used to extract power from the cavity in order to measure the transmitted power. These are typically very weakly coupled in order to prevent large amounts of power from being removed. Pick-ups use a simple geometry based on commercially available N-type feedthroughs. FPCS are designed to operate at the resonant frequency of the cavity. This fre- quency can vary depending on several factors, such as the temperature, pressure, or tuning of the cavity, however this range is rather small. The amount of power that is input to a cavity can be up to several megawatts, transmitted either CW or pulsed. More information on current activities involving pulsed and CW input power couplers can be found in the presentation by Garvey [9]. When there is no beam in the cav- ity, nearly all of the input power is reflected back through the coupler, as the cavity losses are quite small. This reflected power creates a standing wave with twice the voltage and current. Thus an input coupler must be capable of handling four times the forward power. Section 2.1 explained how time-varying fields impart energy to the beam. The converse is true as well, the charge moving through the cavity can excite higher order modes. HOMs do not provide useful acceleration and can be detrimental to the beam stability. Additionally, these modes also increase the cryogenic load due to the added power dissipated in the walls of the cavity. The higher the beam current is, the more excitation of HOMs occur. A method of determining these modes is given in the thesis by Popielarski [20]. HOM couplers are used to extract power from the cavity only at frequencies higher than the fundamental. The design of the HOM coupler must ensure that it does not couple to the accelerating mode, as this will produce a 16 degradation in the cavity performance and additional RF power dissipation for the HOM load. There are two types of HOM couplers [14]. The first carry the power away from the cavity to be dissipated in an external load. The second type of coupler uses RF absorbers embedded in the coupler walls to dissipate the energy within the module. The QWRs being used have only three modes total, and are designed for low beam current, so extracting the HOM power is not necessary. The focus of this thesis is the design and analysis of a F PC, but the processes and methods are similar for designing HOM couplers as well. 3.2 Coupling Methods and Locations Many coupler geometries are possible, but the two most common are the waveguide and coaxial. Both methods of coupling have benefits and drawbacks. There is not a clear advantage that makes one style the best for all applications. Table 3.1 provides a quick comparison of these two types of couplers [21]. Table 3.1: Comparison of coaxial and waveguide couplers. [ Pros Cons Waveguide oSimple design oLarge oHigh power oLarge heat load oSimple cooling oDifficult variability oHigh pumping speed Coaxial oCompact oComplicated design oSmall heat leak oLower power oCoupling variability oComplicated cooling oAdjustable impedance oSlow pumping speed Waveguide couplers are less complicated, and many styles and frequencies are readily available from industry. Waveguide couplers are capable of handling large amounts of power. The simple geometry of a rectangular waveguide makes it easy to cool and the open structure allows high vacuum pumping speed. One major drawback 17 of the waveguide coupler is its size, especially at low frequencies. The dominant mode of a rectangular waveguide is the TE01 mode [22]. The cutoff wavelength of a waveguide is A. = 2d (3.1) where d is the width of the waveguide. Thus the width of the waveguide must be larger than one-half of the wavelength corresponding to the desired operating frequency of the cavity. A 80.5 MHz waveguide would be 1.86 meters wide, but only 50 millimeters wide at 3 GHz. Therefore waveguides are not typically used for frequencies less than 500 MHz. Although waveguide couplers provide several mechanical benefits, coaxial couplers have one distinct advantage. That advantage is the ability to change the impedance of the coupler to provide different matching conditions. A coaxial coupler can also be more compact. Additionally, the coaxial coupler is basically a transmission line, so multiple frequencies are able to propagate. This allows a single design to be used for different frequencies if necessary. The condensed geometry of this type of coupler generally allows for a smaller heat load, at the expense of slower vacuum pumping speed. Another asset of the coaxial design is the ability to make the coupling adjustable. It is possible to make an adjustable waveguide coupler, however, it is much more complicated. Just like there are two common types of input couplers, there are also two methods of coaxial coupling to a cavity. The first is coupling to the magnetic field using a current 100p, and the second is using a voltage probe to couple to the electric field. Using Equation (2.13) and Figure 3.1, we can see that the peak magnetic field is near the top of the cavity and is azimuthal. Therefore a loop antenna would be ideally placed near the top of the cavity, where the magnetic field is the strongest. The coupling strength can then be adjusted by changing the size and orientation of the loop. Figure 3.2 and Equation (2.14) show the peak electric fields are located at the 18 tip of the inner conductor, near the bottom of the cavity. The electric field is radial, and decreases along the length of the cavity. Ideally, the probe would be located along the beam axis where the electric fields are strongest, but the bottom of the cavity is close to the peak fields and allows for a simpler mechanical design. The coupling strength can be adjusted by the voltage probe’s length and diameter. Most accelerators use fixed coupling, since the beam current is fixed and loading is well defined [13]. If there is a large amount of beam loading, and the range of the beam current is too great to be compensated by the RF controls alone, a variable coupler can be used. This variability requires more complex feedthroughs and actuators for control. In the case of voltage probe type couplers, a linear actuator can be used to increase the amount of penetration. Current loops can use a linear or rotary actuator to change the magnetic flux through the loop. The easiest method to move either of these would be to put the actuator inside the cryomodule, and run the controls out of the module. However, it is difficult to find reliable actuators that operate at cryogenic temperatures. Externally mounted methods are more reliable, but require actuator feedthroughs that can transmit motion and still maintain vacuum. Additionally, it is sometimes diflicult to achieve strong coupling without disturbing the cavity fields. 3.3 RF Window The main function of the RF window is to isolate the cavity vacuum from the atmo- sphere while transmitting RF power through the coupler. There are several aspects that require consideration when designing a window. One of the first issues that needs to be addressed is the window location. If the window is located inside the module it is considered a cold window. This method allows the cavity to be completely sealed while still inside the clean room. However this approach is more complicated as it requires vacuum on both sides of the window. Additionally, because this window is close to the cavity itself, and is more prone to damage from electrons and x—rays 19 Vol. ROIH) 2,539+CO4 'I.896+(D4 l.26e+(D4 Figure 3.1: Analyst model of 5 = 0.041 QWR showing the surface magnetic field distribution. 20 Vol. RelE) l .746+(I)7 l.3le+007 1069-0131 Figure 3.2: Analyst model of )6 = 0.041 QWR showing the surface electric field distribution. 21 emitted from the cavity [15]. The electrons can charge up the window and create arcing. As clean room procedures have improved, sealing the cavity early on is not as important as it once was. The warm window has become more popular, as it does not have the drawbacks of the cold window. This method is preferred for high power applications because any multipacting or RF heating that occurs at the window is far away from the cavity. Some facilities opt to implement double windows to provide extra protection. The warm window design used in our application has the benefit of sealing the cavity while it is still inside the clean room. The cryomodule construction is such that each of the completed coupler assemblies can be inserted through the cryomodule walls. Another important design consideration is the geometry of the window. This is often decided by the type of coupler used. Waveguide couplers typically rely on planar windows mounted in the waveguide. These windows can be rectangular, or can consist of one or more circular disks brazed to a vacuum tight flange. In the case of coaxial couplers, the window requires a hole in the center for the inner conductor to pass through. This window is usually a cylinder, disk, or cone. Conical windows can be used for impedance matching, but it is not clear if it provides any benefit against multipacting. 3.4 Material Considerations Common materials used in power couplers include stainless steel, oxygen-free high- conductivity (OFHC) copper, and ceramics. The ceramic that is most common is high purity alumina (> 95% AL203)1. Although it has low thermal conductivity (30 W/mK), it is often chosen because of its availability and purity. The thermal conduc- tivity of alumina does increase as the temperature decreases, making it advantageous 1 ISI refers to (> 95%) as high purity, but other sources consider it medium purity, and (> 99%) as high purity [13] 22 for cold windows. Knife-edge flanges are commonly made from stainless steel, and are sealed together with copper gaskets. Selecting the materials for the rest of the coupler is slightly more complex. One issue that the coupler materials present to the cryogenic system is a thermal load. It takes approximately 1 kW of electric power to remove 1 W of heat from liquid at 2 K due to Carnot losses and inefficiencies. Therefore it is important to minimize both the RF heating and the thermal leakage to the cold mass at 2 to 4.5 K. The thermal conductivity of stainless steel is much lower than that of copper, as shown in Figure 3.3 [23]. AISI 317 stainless steel appears to be the best choice of material for the outer conductor to minimize heat transfer. However, another source of heating is from the ohmic losses inside the outer conductor. The electrical resistivity of stainless steel is much greater than that of copper, as presented in Figure 3.4. OFHC copper would be the preferred outer conductor material to minimize the ohmic losses, but it would transfer too much ambient heat into the module. A compromise must be made between thermal conduction and ohmic losses. The solution is to plate the interior surface of the stainless steel outer conductor with a layer of copper. The majority of the conductor is stainless steel with poor thermal conductivity, while the copper lining is sufficient to reduce the ohmic losses. The thermal conduction along the copper inner conductor is not an issue because it is not in direct contact with the helium system, so there is no path for the heat to conduct. 3.4.1 Calculating Heat Load and RF Losses The total static heat leak from the RF input coupler can be estimated from the sum of the calculations for both the inner and outer conductor. The total heat leak includes the case when the RF is turned on and includes the RF power conducted along the coupler [24]: qtotal : (Icond + qrad + qrf (3.2) 23 ()Iw /M) 1: Jeddoo onso os=uuu 10 Auniionpuoo inweui O O O O O O O C) O O O l0 0 ID 0 O m N N r- Y— ID . If 8 I I I ’ :2 I I I I / o co // 8 / r e x 5 / 82 x’ 3 / 5 / 1- / / / // 8 / / I I I \\ a \ x x \ I x \ \\ \ weroo __ \\ 2 55.55903 \\ @EEN‘ZZ’E“ ‘\ <<<3<<08 \‘ . . . . 0 (MW IM) )1 Sl°°ls ssolums 10016310 Minitonpuoo Inuuout Figure 3.3: Thermal conductivity of OFHC copper and select stainless steels. 24 ow on 00 0: 222883 cm 0». ow 5&8 0:51 I one _m_m¢4 UZHZZDQ um Q¢Q mEo mw>nm Q¢DJ ya zIa om 31H mags zzu om .\ /, monasao nazaagommam musaaoo nazaakummHn WHDQ 1/ \V mmahamuazfizxg amen: , mm>aa mmaaammazma \. ta muao moao moan: .o¢> .o¢> mu>aa ea mam ///mWM; mmkqmmzmo nconw Figure 5.10: Schematic diagram of the power coupler conditioning system. 68 Figure 5.11: The sliding short assembly as installed for conditioning in standing wave mode. The large diameter pipe in the foreground is the input side, and the small diameter one near the upper—right corner was used to adjust the resonant frequency. 69 the shorts were located at current maxima and voltage minima. This allowed either coupler window to be subjected to both voltage and current maxima by moving both shorts by equal amounts. During conditioning, the position of the peaks were swept across the entire length of both couplers. The shorts were moved 24 times in 3” increments in order to cover the entire length of the coupler assembly. The patterns of the standing wave voltage and currents used to condition are shown in Figures 5.12 and 5.13 respectively. The conditioning was relatively simple, starting with the smaller diameter short completely inserted. The large diameter short was then positioned so that the as- sembly resonated at 80.5 MHz. Power was then applied to the structure, and slowly ramped up until the standing-wave power reached 70 dBm, while the pressure inside the fixture was closely monitored. It is recommended to not allow the pressure inside the assembly to increase above 10“7 torr in order to prevent damage to the windows [13, 15, 37]. During conditioning, some short positions did not produce any increase in pressure and were conditioned for approximately 15 minutes. Other short positions, such as near the windows produced large pressure increases, and required heavy conditioning. As the pressure began to approach the safe limit, the input power was set to slowly pulse. When the vacuum began to recover, the pulse length was slowly increased until the input power was again at CW. A more detailed description of the conditioning procedure is provided in Appendix C. The problematic short positions were the result of materials embedded in the windows and multipacting. The multipacting also caused an increase in the outer conductor temperatures. Conditioning the problem areas created a substantial in- crease in the gases measured by the RCA. Compared to Figure 5.7, which was prior to starting the conditioning, Figure 5.14 shows the increased levels caused by the conditioning of a vacuum window. As the vacuum recovered and the contaminants 7O were removed, the residual gases were depleted, as shown in Figure 5.15. After conditioning in standing wave mode, the assembly was tested with standing- wave power equivalent to 10 kW, much greater than what is necessary for normal operation. Some multipacting was observed near the ceramic windows, but these barriers were easily conditioned away by pulsing the input power as necessary. 5.4 High Power Testing After completing the conditioning of the couplers, a long-term durability test of the couplers was performed. In order to do this, the sliding shorts were removed and the N-type adapters were reinstalled. One end of the coupler assembly was connected to the amplifier through a directional coupler. The other end was connected to a 1 kW, oil-filled matched load. The input power was steadily increased until 500 W of forward power was applied to the system and measured. At this point the directional coupler was removed because it was rated for only 500 W. Power was reapplied to the system until the 1 kW limit of the amplifier was reached. The system was left to run at full power for seven days uninterrupted, while the pressure and temperature were continuously monitored. After seven days, the power was turned off and a temperature sensitive label mounted to a stainless steel rod was inserted into an inner conductor. Full power was then reapplied for three hours to measure the temperature of the inner conductor near the tip. The label was removed and the tip temperature was noted as reaching 49° C. This process was then repeated with a new label, and the test was repeated for 24 hours. The internal temperature peaked at 54° C this time. The temperature variation was not large, so that the conductive cooling of the inner conductor should be sufficient for operation of the re-accelerator. One final RCA reading was taken after the endurance test. Figure 5.16 shows that the majority of the residual gases were removed from the system and the vac- uum pressure improved to 5.6 x 10"9 torr. The testing of the coupler assembly was 71 225 59.3 ....................... 00000000000000000 .I-n-4-t-¢-i-c-u-o-1 . 3...oo.o%.o%.o%oo%o 000......ooooooooooooooooooooo.. 0000000000000 .0» Wonononowowonowonououowouooo.o. V... 0.1.. I---0-I-I-I.I.D-I-I'i-b- ............. ennuidwv Figure 5.12: Standing wave voltage sweep pattern. The vertical black lines are the window locations; the dark blue line is the initial sweep position. 72 7 120 k 80 Length (Inch) 20 1 0 1 \\ . emuldtuv Figure 5.13: Standing wave current sweep pattern. The vertical black lines are the window locations; the dark blue line is the initial sweep position. Figure 5.14: Increased RCA reading resulting from conditioning a “dirty” area of the power coupler assembly. Note the increasing trend indicating gas levels are still climbing. 74 Figure 5.15: Remaining residual gases after conditioning. 75 complete. The entire assembly was again wiped down with acetone and methanol, and transferred into the class 10, 000 clean room. Once inside, the system was slowly backfilled with clean nitrogen gas. Currently the system is being stored until further testing on the re—conditioning of the structure can be done. 5.5 Issues Although the conditioning of the power couplers was successful, issues arose along the way. The first obstacles were encountered during the assembly. When inserting the copper sleeve into the six-way cross, it was found to only fit along one axis. The manufacturing welds along the inside flanges were thicker in some areas, preventing the sleeve from fitting correctly. This required some components to be removed and assembled to other flanges in order to complete the assembly. The problem was corrected by simply installing the copper sleeve earlier in the process. Another problem which occurred during assembly was with the diagnostic ports on the outer conductors. The tubing on these ports was 0.25” diameter stainless tubing with a 0.03” wall thickness. When tightening the fasteners on the flanges of these ports, the torque applied caused the tubes to twist. This put excess stress on the welds and could result in a vacuum leak. For the production couplers, these pieces will be made of 0.375” diameter tubing with thicker 0.049” walls. Not only will this provide greater strength during assembly, but the increased radius will also allow a larger diameter antenna to be used for the current probe, providing more reliability in multipacting detection. A more dramatic and costly issue arose during the first attempt at conditioning. In Section 4.2, calculations found that the static heat load for unplated stainless steel outer conductors was very low. The first conditioning that was performed used unplated outer conductors, as well as soft soldered tips on the inner conductors. After conditioning along the first 21” of the couplers, the voltage peak was located near 76 Figure 5.16: Final RCA reading showing the removal of nearly all the residual gases from the system. 77 the center of the assembly by the connecting barrel of the inner conductors. As the power was increased, the vacuum steadily got worse. The forward power was then pulsed for approximately two hours at various rates, and then switched back to CW. The standing wave power applied was 50 dBm for nearly 10 minutes. At this point, the pressure spiked dramatically upwards. Then the vacuum gauges and RCA tripped off, and the turbomolecular pump began to make noise. The vacuum had been compromised, and the test fixture required disassembly in order to locate the breach. It was found that the solder connection on one of the feedthroughs had failed. The faulty tip is shown in Figure 5.17. Somehow during conditioning, the solder reached its melting point and began sputtering material onto the inside of the copper sleeve (Figure 5.18) because of the vacuum inside. Pieces of solder also fell into the turbomolecular pump, shearing off several rotor blades. The exact cause of the failure was not determined, but steps were taken to prevent future occurrences. First, by using copper plated outer conductors the temperature peaks during conditioning were reduced from 60° C for the unplated case to less than 50° C for the plated case. Another issue may have been the solder itself. The first material used only had a melting point of 250° C, and the bake-out was performed at 200° C. It is possible that the connection was weakened during the bake. The final failure may have resulted from RF heating caused by conditioning with unplated outer conductors. The solder was replaced with silver braze material with a much higher melting point, 635° C. Additionally, the seam of the inner conductor joints were buffed smooth to reduce the possibility of “hot spots” on the tip. The smoothed seams can be seen in Figure 5.19. These two corrections allowed the second round of conditioning to proceed with any issues. 78 Figure 5.17: Failed inner conductor solder joint due to excessive heating. The piece of solder at the bottom of the picture was found inside the conditioning fixture during disassembly. 79 Figure 5.18: Solder deposited on the interior surface of the copper conditioning sleeve, near the location of the failed solder joint. 80 Figure 5.19: Improved inner conductor tip assembly with silver brazing and smoothed surfaces. 81 Chapter 6 Conclusions The goal of this thesis was to develop a fundamental power coupler for low-beta superconducting cavities, with immediate application to the re—accelerator project at NSCL. A fundamental power coupler was designed, fabricated, conditioned and tested. The coupler was able to meet the design goals. The simulations accurately predicted the transmission and reflection coefficients of the final model. This will allow further Optimization of the coupler components without the cost of machining additional test pieces. Four prototype power couplers were fabricated and two of those couplers were successfully conditioned. The equip« ment and methods necessary to begin processing production FPCS is in place. One solder joint failure occurred in the first round of conditioning and provided additional information that was used to improve the coupler design. It is believed that using the silver braze will be sufficient to prevent future solder joint failures. The solder joint failure showed that the coupler window was not the weakest aspect of the design. Typically, the window is considered fragile and prone to breakage. The ceramic in this case was able to withstand full standing-wave power and remained intact through— out a catastrophic failure. The performance of unplated outer conductors was also investigated, and they did not meet expectations, so copper plating will be used for production FPCS. 82 There is still much work to be done before the 20 couplers for ReA3 are completed. Production is currently underway for the next set of FPCS, using information gained during the design and testing of the prototype F PCs. The time needed to condition the couplers depends on many factors, such as multipacting and the cleanliness of the components. Many areas of the coupler were conditioned in as little as 15 min— utes, while other areas required as much as eight hours of pulsed and CW power to condition. It is believed that a more thorough ultrasonic cleaning of the windows will reduce conditioning time. Several problems were encountered with the prototype FPCS, prompting a number of improvements in the design and procedures, as detailed in Section 5.5. Due to time constraints, many aspects of the coupler design have not been in- vestigated. Outstanding issues include further investigation of the performance of the current configuration, further optimization of the design, and refinement of the assembly and conditioning procedures. The first priority will be to pump down and retest the current assembly after it has been sitting idle in the clean room. This will provide an idea of how much reconditioning will be necessary on the production FPCS after they are installed onto the cavities. All of the couplers will require some in-situ conditioning prior to operation of the linac. If the amount of reconditioning necessary is small after sitting idle for several months, then conditioning of the pro- duction couplers can be done at will. Otherwise, the conditioning will need to be done just prior to attachment to the cavity. Design improvements include the re- moval of the canted spring on the air-side outer conductor. The spring is included to ensure a good RF contact of the air side outer conductor to the feedthrough flange. However, TRIUMF has successfully removed an RF finger contact from their coupler design without any degradation in performance [38]. If this spring is removed from our assembly, the geometry inside of the outer conductor can be modified, providing a better impedance match. Improving the match at the window should be more ben- 83 eficial than the spring. The addition of IR diagnostics to monitor the temperature of the window should also be looked into. Finally, the conditioning can be automated. LabVIEW is already used to monitor power, pressure and temperature. Interlocking the amplifier control to the system will allow an extra layer of protection in order to prevent a vacuum breach or sputtering at the windows. None of these improvements are costly to implement, they only require additional time. The information gained while performing the research presented in this thesis has already been useful in developing a power coupler that will be implemented in the near future. Additional opportunities remain for further research, and the findings can be applied to power coupler designs for other cavities and other frequencies. 84 Appendix A Mechanical Drawings of the Coupler and Test Assemblies 85 Figure A.1: Power coupler components and assembly diagram. 1) Vacuum side outer conductor, 2) Air side outer conductor, 3) Vacuum feedthrough, 5) Spacing ring, 10) Spark detector, 11) Current probe, 14) Vacuum gauge, 19) Rigid line coax to N—type adapter. 86 <.< ZOE—Own $30 . “i mum R lash-Ill. .n ..H.............. \\\\\\\\\\\\\. -. fin -- _ as Figure A.2: Important dimensions of an assembled input power coupler. 87 ‘ t .33» mtthtittm. . 9. 29.53 S . WNW 4w§3¢<§§§h - - - . Mafiuunlnx\\\\\\.\\\\\\\\. .1 _ l .1... “is , E I‘VRMKANMM‘Mthiittlttl . “V 35988.“ [$23 “—3.an port is for the tuning actuator, and the third port is for the pick-up coupler. NOT 88 Figure A.3: Power coupler attachment to the bottom flange of a cavity. The center TO SCALE. Figure A.4: Modified 6-way cross use in the conditioning assembly. 1) Copper inner sleeve, 2) Connecting barrel, 3) Modified 6—way cross, 4) Copper gasket. 89 m>00¢0 «(52% 2. 03.252. mm Op 025% 4.00 0m._.Zoomo «5:25. 2. in; ma 9 02?.» =00 9:26 3. ma 0 / Eczema: «0a 230:”. on: £2 08 - .363— Figure A.5: Assembled power coupler conditioning fixture. NOT TO SCALE. 90 Appendix B Clean Room Assembly Procedure 990$?” 10. 11. 12. . Wipe down all parts with acetone, then methanol, and place in the clean room. Ultrasonically clean power coupler components in Micro-90 solution for 20 min- utes, then rinse in ultra pure water for 40 minutes. a) Ultrasonically clean the feedthroughs separately for 15 minutes, and rinse for 30 minutes. Allow all components to dry in the clean room for at least 24 hours. Assemble the RCA quadrupole to a standard 2 3/ 4” CF nipple. Attach (2)-Zero length adapters to the mini elbows, then connect the cold cath- ode gauges to the zero length adapters and set aside. Attach the 2 3/ 4” zero length window to the “front” of the 6—way cross. Attach the RCA quadrupole nipple to the “bottom” of the 6-way cross. Attach leak valve assembly to cross, opposite of the window. Attach right-angle valve to turbo pump. Attach turbo pump to cross, opposite the RCA quadrupole. NOTE: Always mount turbo pump with opening facing down, and with an inlet screen! Attach one copper-plated outer conductor to the 6—way cross using a double length spacer, one custom copper gasket, one standard gasket, and 3” long bolts. Re—insert the copper sleeve with the canted springs pushed over the ends. 91 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. Attach the second outer conductor, across from the first outer conductor, using 3” bolts. Attach a single ISI vacuum feedthrough to one outer conductor. Place the copper shorting sleeve on the tip of the second power coupler feed- through. Insert the second coupler, attempting to line up the end of the shorting sleeve with the tip of the first coupler. Using a multi-meter, check the continuity of the inner conductors to determine whether the copper sleeve is joining them. Using the multi-meter, make sure that neither of the inner conductors are shorted to the outer conductor. If either test fails, remove the second coupler and repeat steps 16 through 18. Attach windows and current probe to diagnostic ports on the outer conductor. NOTE: Be careful when tightening bolts to avoid torquing small tubes. Attach elbows on cold cathode to the remaining diagnostic ports, being careful to orient correctly so the plug does not interfere with the conditioning assembly. Attach cooling fan and controller to the turbo pump. Go through and re—tighten all bolts and fasteners. Attach RCA control unit to the analyzer mounting flange. Move assembly to test cart and roll over to the vacuum pump—out station. Connect all power and controller cords. Connect vacuum line from scroll pump to right angle valve on turbo. Turn on scroll pump and pump down the vacuum line before completely opening the right angle valve. Pump the conditioning assembly to g 100 millitorr. Turn on turbo pump and cold cathode(s). Connect RCA ethernet cable to second port on clean room PC, allowing initial communication with RCA for leak checking. Open e—Vision software and turn on RCA filament. Leak check with helium. 92 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. Continue to pump-down the coupler conditioning assembly until the pressure is < 1 x 10‘6 torr. Close the right-angle valve completely, then unplug the turbo pump, and shut off scroll pump. Disconnect all diagnostic cables. Wait for turbo pump to completely ramp down. Transfer the assembly from the clean room to conditioning area. Mount assembly to support frame in the power coupler testing area. Reconnect the scroll pump and turn on. Pump vacuum line to g 100 millitorr, then turn on turbo pump. Slowly open the right-angle valve completely. Reconnect cold cathode and RCA cables. 93 Appendix C Conditioning Procedure Fitpt‘nr‘k F ‘ ;‘,k‘;3\\\\ ‘. 7.! f ' 3“ [ \{i/ ] Di,[$ L [Vt/l [ / ,_. L 1 ,7, [.IK\7\1\I,‘;K\‘I/] Figure C.1: Measurement points for system calibration. 1. Complete bake—out procedure. 2. Set up conditioning system in traveling wave mode with N—type adapters. a) Connect VNA to coupler assembly and measure S“ and 321. b) Measure and record the traveling wave transmitted power (Paw) through 1 5/8” EIA directional coupler. 3. Calibrate system (see Figure CI). The goal is to be able to calculate the forward power (at Point B), reverse power (at Point B), and transmitted power (at Point C) from measured values from the power meter sensors at Points D, E, and F. a) Set up VNA with test cables on ports 1 and 2. b) Disconnect the amplifier and connect VN A port 1 cable to the directional coupler at Point A. Disconnect DUT from upstream directional coupler and connect VNA port 2 cable to the directional coupler at Point B. Perform a THRU calibration to establish a reference plane at B. c) Disconnect the forward power sensor and connect VNA port 2 cable at Point D. Attach 50 ohm termination at Point B. Record 321. This is mel. (1) Connect the VNA port 1 and port 2 cables together and repeat the THRU calibration. 94 10. 11. e) Connect the VNA port 1 cable to the directional coupler at Point B. Disconnect the reverse power sensor and connect VNA port 2 cable at Point E. Attach 50 ohm termination at Point A. Record 321. This is PM“. f) Disconnect DUT from downstream directional coupler and connect VNA port 1 cable to the directional coupler at Point C. Disconnect the transmitted power sensor and connect VNA port 2 cable at Point F. Record 8'21. This is P t,cal- Make sure the power meter sensors are connected to Points D, E, and F again. Reconnect the amplifier and DUT. Make sure that the output is terminated in a matched load. a) Slowly increase the forward power until Pf z 60 dBm, while monitoring power levels and pressure. NOTE: Make sure power levels input to power meter sensors do not exceed 20 dBm. b) Check for proper transmission or excessive reflection powers, then reduce input power. Connect sliding shorts for standing wave mode. If any cables were added or removed, repeat the process from step 3. Connect VNA to the input (large diameter) short. a) Adjust both shorts so the voltage peak is at the 1 5/8” EIA directional coupler, and resonating at 80.5 MHz. Record Pam, and the power at the pick-up coupler of the input short, Ppml. b) Determine equivalent power in standing wave using, Paw = Pt,” + PM”, using PM,” measured in step 2. . Fully insert tuning (small diameter) short completely. With input short connected to VNA, adjust the input short position to resonate at 80.5 MHz, full wave. a) Record 311 and .921. Connect RF amplifier to input short, and power meter (Pt) to pick-up 100p. Slowly ramp power, while monitoring all diagnostics. a) Starting with the signal generator (Pf) at z —30 dBm, slowly increase power until Paw = Pt + [Ppmzl = 73 dBm. b) Reduce input power so Paw = 70 dBm, watching for pressure spikes. c) Adjust frequency of signal generator slightly in order to minimize Pr. 95 12. 13. 14. 15. 16. 17. 18. 19. Maintain input power and monitor pressure, which may be increasing. a) If not substantial increase is observed over 15 minutes of conditioning, continue to next step. b) If pressure increases over 5 x 10‘6 torr, begin to pulse input power. i) If using the DB Broadcast amplifier, longer pulses at low frequency work best. e) As pressure begins to recover, slowly increase pulse duration until power is again CW, and condition an additional 15 minutes. Turn off RF power. Move tuning short out 3” and repeat steps 9 through 13. a) Repeat process until entire length of both couplers have been conditioned, approximately 24 steps. Remove sliding shorts and reassemble coupler in traveling wave mode, using a 1 kW matched load. a) Measure S11 and 321. b) Slowly increase power until Pf = 60 dBm, the limit of the RF amplifier. NOTE: Be cautious of the power limit of directional couplers and other devices. Continue high—power testing uninterrupted for the desired length of time. a) Repeat durability test in opposite direction if desired. The conditioning is now complete. a) Close right angle valve. b) Turn off both vacuum pumps. i) Disconnect scroll pump. c) Wipe entire assembly with acetone and then methanol. (1) Move assembly back into clean room. Connect nitrogen line to gas fitting on coupler assembly. a) Adjust metering valve to slowly bleed nitrogen into fixture through the gas filter. b) Open the all-metal valve completely. c) Allow fixture to come to equilibrium. (1) Close all—metal valve completely. e) Remove nitrogen line. The couplers can now be stored as-is, or disassembled for attachment to a cavity. 96 [ll [2] [3] [4] [5] [6] [7] BIBLIOGRAPHY F. Marti, P. Miller, D. Poe, M. Steiner, J. Stetson, X.Y. Wu, “Commissioning of the Coupled Cyclotron System at NSCL,” Proceeding of the 16th Interna- tional Conference on Cyclotrons and their Applications, pp. 64, East Lansing, Michigan, 2001. H. Blosser, J. DeKamp, J. Griffin, D. Johnson, F. Marti, B. Milton, J. Vincent, G. Blosser, E. Jemison, R. Maughan, W. Powers, J. Purcell, W. Young, “Com- pact Superconducting Cyclotrons for Neutron Therapy,” IEEE Transaction on Nuclear Science, Vol. NS—32, No. 5, pp. 3287-3291, October 1985. 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