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LOW BETA SUPERCONDUCTING RF CAVITY POWER COUPLER
DEVELOPMENT FOR THE RARE ISOTOPE ACCELERATOR

presented by
Adam Daniel Moblo

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Low BETA SUPERCONDUCTING RF CAVITY POWER COUPLER
DEVELOPMENT FOR THE RARE ISOTOPE ACCELERATOR
By

Adam Daniel Moblo

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Electrical and Computer Engineering

2004

ABSTRACT
Low BETA SUPERCONDUCTING RF CAVITY POWER COUPLER
DEVELOPMENT FOR THE RARE ISOTOPE ACCELERATOR
By

Adam Daniel Moblo

The larger more powerful accelerator facilities of tomorrow are a good
match for high gradient superconducting radio frequency (rf) accelerating
cavities. Coupling rf power into a superconducting rf accelerating cavity is done
through a fundamental power coupler (FPC) which consists either of a
waveguide or coaxial transmission line and a window to isolate the cavity

vacuum. A low cost FPC design solution has been sought for the ,6 = v/c = 0.085
80.5 MHz quarter-wave, and the ,6 = 0.285 322 MHz half-wave cavities being
prototyped for the Rare Isotope Accelerator (RIA). The ,6 of a cavity is the

velocity at which a particle receives its optimum energy gain from the cavity and
is expressed relative to the speed of light. The use of a commercially available
vacuum rf window is the basis for the low cost design. Thermal and electrical
conductivities of coupler materials are analyzed and the ohmic losses are
accounted. An experimental approach is used to determine the coupler
penetration lengths needed to satisfy the beam power and cavity bandwidth
requirements. Two prototype FPC’s and a standing wave test setup have been
constructed to verify assembly procedures, power transmission, power

capabilities and conditioning methods.

ACKNOWLEDGEMENTS

These accomplishments could not have made without the help and
support of my family, friends, and colleagues. Foremost I would like to thank my
father and mother, Daniel and Suzanne, for the wonderful home they provided,
their never-ending loving support. Also my brother, sisters and their families for
always expecting more out of their younger brother. I deeply appreciate the love
and support from the Ackerson family has also provided me with.

Everyone in the NSCL community has provided a friendly and enjoyable
atmosphere for learning and working. The teamwork and support of the SRF
accelerator group was the backbone of my research. I would like to specially
thank Roger Zink for getting my foot in the door at the NSCL and Dr. Terry
Grimm for the graduate educational opportunity he provided me with. I could
have never done without the friendships and assistance of John Popielarski, Dr.
Walter H. Hartung, Dr. John Vincent, Matt Johnson, Chris Compton, James
Colthorp, Steve Bricker and John Bierwagen.

The department of Electrical and Computer Engineering has provided me
with the tools and knowledge needed to become a top contributing engineer in
the field of electrical engineering. I thank my advisor Prof. Leo Kempel for his
dedication to my education. I also thank all the professors in the department for
the wisdom they passed on to me. Finally, I thank the National Science
Foundation and the US. Department of Energy for funding my graduate

education and research.

TABLE OF CONTENTS

LIST OF TABLES ................................................................................................. v

LIST OF FIGURES ............................................................................................... vi

INTRODUCTION .................................................................................................. 1

CHAPTER 1

THE RIA AND SUPERCONDUCTING RF CAVITIES .......................................... 2
1.1 The Rare Isotope Accelerator Superconducting LINAC ........................... 2
1.2 The Low Beta Cavities ............................................................................. 3
1.3 Cavity Parameters and the Lumped Element Model ................................ 4

CHAPTER 2

COUPLING POWER INTO THE CAVITY ........................................................... 10
2.1 Beam Loading and Power Requirements ............................................... 10
2.2 Coupler Style and Location .................................................................... 12
2.3 Measuring External Quality Factors ........................................................ 15

CHAPTER 3

MECHANICAL CONSRAINTS ............................................................................ 20
3.1 Material Constraints ................................................................................ 20
3.2 Thermal Analysis .................................................................................... 24
3.3 Plating Measurements ............................................................................ 27
3.4 Vacuum Window, Conductor Size and Multipacting ............................... 29

CHAPTER 4

PROTOTYPE ASSEMBLEY, TESTING AND CONDITIONING ......................... 33
4.1 Prototype Assembly and Cleaning .......................................................... 33
4.2 Bake-out Procedure and Results ............................................................ 34
4.3 Matching Parameters ............................................................................. 37
4.4 Power Testing and Conditioning ............................................................. 39
4.5 Test Results ........................................................................................... 46

CONCLUSION .................................................................................................... 49

APPENDICES .................................................................................................... 50

REFERENCES ................................................................................................... 52

LIST OF TABLES

Table 1: Low beta cavity parameters ................................................................. 4

Figure 1:
Figure 2:
Figure 3:
Figure 4:

Figure 5:

Figure 6:

Figure 7:

Figure 8:

Figure 9:

Figure 10:

Figure 11:
Figure 12:
Figure 13:
Figure 14:
Figure 15:
Figure 16:

Figure 17:

LIST OF FIGURES

(Images in this thesis are presented in color.)

RIA superconducting LINAC cavities ................................................. 3
RLC model of a superconducting RF cavity ...................................... 6
Half-wave coaxial cavity electrical and magnetic fields ................... 13
Quarter-wave coaxial cavity electrical and magnetic fields .............. 14

Qext vs antenna length for the 80.5 MHz cavity where the length is
measured into the cavity from the stainless steel mini CF flange of
the bottom plate of the cavity ........................................................... 18

Qext vs antenna length for the 322 MHz cavity where the length is
measured into the cavity from the surface of the NbTi CF flange of

the cavity .......................................................................................... 19
Thermal conductivity of select materials ........................................... 22
Electrical resistivity of select materials ............................................ 23

Heat disipated in the helium system for select copper plating
thicknesses due to the power coupler for the 322 MHz half-wave
cavity ............................................................................................... 26

Heat disipated in the helium system for select copper plating
thicknesses due to the power coupler for the 80.5 MHz quarter-wave

cavity ............................................................................................... 26
Resistance per unit length ............................................................... 29
Two-coupler test setup conductor joints .......................................... 34

Temperature profile during the two-coupler test setup bake-out ..... 36
Vacuum pressure of the two-coupler test setup during bake-out ..... 36
The reflection coefficent through the two-coupler test setup ........... 38
The transmission coefficent through the two-coupler test setup ...... 39

Two-coupler test setup for high power testing and conditioning ...... 41

vi

Figure 18:

Figure 19:

Figure 20:

Figure 21:

Figure 22:

Figure 23:

Figure 24:

Figure 25:

LIST OF FIGURES CONTINUED

Electric field simulation for the coupler at 322 MHz with 1 kW

applied to the 1-5/8” EIA input of the coupler and the cavity port
shorted to produce the equivalent maximum standing wave fields

the coupler will endure during Operation using Ansoft’s HFSS
software package. ............................................................................ 42

The first four resonant frequencies of the test setup of
length 1.02 (m) ................................................................................. 44

The voltage standing wave pattern for the resonant modes of the
test setup ......................................................................................... 44

Voltage standing wave pattern for coupler conditioning spaning
330-395 MHz ................................................................................... 46

High power standing wave voltage and current pattern
approximation for tests conducted at 321 MHz ................................ 48

High power standing wave voltage and current pattern

approximation for conducted at 161 MHz ........................................ 48
322 MHz F PC cross section ............................................................ 50
80.5 MHz FPC cross section ........................................................... 51

vii

INTRODUCTION

The quest for high gradient accelerating cavities has sparked the demand
for affordable superconducting rf structures. A superconducting accelerator
requires a more expensive initial investment per cavity in comparison to the
alternative “normal conducting” copper structures but saves on the required
quantity of cavities and the reduced rf generator power needed to power the
cavity. In the Rare Isotope Accelerator (RIA) design, the reduced radio
frequency (rf) power needed for the low beta cavities has opened up the
opportunity for the use of commercially available hardware as an alternative to
expensive over-powered fundamental power couplers (FPCs) currently in use.

The ,6 = v/ c = 0.085 80.5 MHz quarter-wave, and the ,6 = 0.285 322 MHz

half-wave cavities only require a generator power of 344 W and 1012 W,
respectively [1]. Current commercial markets provide rf vacuum feedthroughs
capable of more than 30 kW at lower frequencies. The FPC designs for the low
beta cavities are adaptations of these feedthroughs. The methods used to
compare electrical and mechanical tradeoffs of the design are presented. A
thermal analysis was conducted to produce a low loss coupler. The construction
of two prototype couplers provided a test bed for assembly procedures,

impedance matching, power capabilities and conditioning procedures.

CHAPTER 1

THE RIA AND SUPERCONDUCTING RF CAVITIES

Numerous superconducting rf accelerating cavities are required for the
Rare Isotope Accelerator (RIA) linear accelerator (LINAC) design. In all, six
different cavity types will be used. The FPC designs in this paper are for specific
quarter-wave and the half-wave cavities. Several cavity parameters are needed

for the FPC design and can be represented in a RLC Circuit model.

1.1 The Rare Isotope Accelerator Superconducting LINAC
The RIA is a proposed linear accelerator under prototype development. The
RIA is a heavy ion accelerator, which will be able to accelerate uranium ions up
to 400 MeV/u and lighter ions to higher energies with a final beam power of 400
kW.
The RIA superconducting driver LINAC consists of 496 superconducting

cavities consisting of 332 low beta cavities (,6 <04) and 164 high beta cavities
(,6 >04). The low beta cavities are divided into three types: ,6 = v/ c = 0.041 80.5
MHz quarter-wave [2], ,6 = 0.085 80.5 MHz quarter-wave [3], and the ,6 = 0.285
322 MHz half-wave [4]. The power coupler development of the latter two cavities
is discussed in detail throughout this paper. The high beta section consists of 6-
cell elliptical structures operating at 805 MHz with betas of 0.49, 0.63, and 0.83

[5-6]. The different types of cavities are shown in Figure 1. The velocity at which

a particle receives its optimum energy gain from a cavity is referred to as the

cavity ,6 expressed relative to the speed of light. So a particle traveling through

the 322 MHz half-wave cavity section would receive a maximum energy gain at a
velocity of 0.285c. Several of each type of cavity will be arranged into a
cryomodule. The cryomodules will then be aligned in ascending order according
to the beta of the cavities they contain. A particle being accelerated by the
LINAC would first pass through the quarter-wave cavities and into the half-waves

and finally through the 6-Cell elliptical cavities.

___.
—v-——-—'

 

 
  
  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

gig—:1 __ _ l3
i . , 4 ' ' ' " i =1: '- = 0.49
. , , pt
I I ML: "'"8’ _, 805 Mill.
~ : 8”" “H’ ' ‘é‘, ,4 , f MSU/JLAB
I I MSU . n .: -. . 1,
l
I [3,," = 0.03
805 MHz.
SNS
I7 U jfl
so 5 \Illz [ll—Iggy :11: 2.85 4 Burn = 0.83
”gm” SU " 305 MHz
M SNS

Figure 1: RIA superconducting LINAC cavities.

1.2 The Low Beta Cavities

The two low beta cavities of focus are the ,6 = 0.085 80.5 MHz quarter-
wave, and the ,6 = 0.285 322 MHz half-wave. These low beta cavities are formed

from RRR>150 niobium where the RRR factor is a measure of purity. The larger

the RRR, the more pure the material. Both are coaxial cavities with an outer and

inner conductor diameter of 24 cm and 10, cm respectively. The cavities are

indented in the beam axis region to enhance the accelerating voltage and for the

quarter-wave the indentation angles are unequal to steer the beam. The quarter-

wave cavities will be used for acceleration of particles in the velocity range of

v = 0.05c to v = 0.15c. The half-wave cavities will cover the v = 0.15c to v = 0.4c

range for uranium ions and up to v = 0.6c for protons. A list of operating

parameters referenced throughout this paper is shown in Table 1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cavity Type xi/ 4 1/2
,6 = We 0.085 0.285
Frequency (MHz) 80.5 322
Temperature (K) (operating temperature) 4.5 2.0
Vacc (MV) (accelerating voltage) 1.18 1.58
Epea k (MV/m) (peak surface electric field) 20 25
Bpea k (mT) (peak surface magnetic field) 46.5 68.6
U (J) (stored energy) 6.69 6.19
90 (“n'oaded Q) 5x108 5x109
9,, (Ioaded Q) 6.3x106 8.3x106
beam (W) (beam power) 172 506
P8 (W) (generator power) 344 1012
allowed (Hz) (control bandwidth) 12 36

 

Table 1:

Low beta cavity parameters.

1.3 Cavity Parameters and the Lumped Element Model

The introduction of superconductivity introduces some parameters that

may be otherwise neglected or may play a less significant role in “normal

 

conducting” resonators. There are several terms that have relevant importance
in Characterizing superconducting cavities. A simple cavity model can be used to
help explain these figures of merit. A more detailed derivation can be found in [7]
or [8].

Superconducting RF cavities can obtain extremely large fields

(Eacc > SOMV/m and Epe >100MV/m) within the cavity with relatively little

ak

input power, which explains their attractiveness as an accelerator, due to their

large quality factors (108 to 1010 ). In the optimal case the maximum fields would
be along the axis at which the particles pass through the cavity. This is not

always the case so we define the accelerating field Eacc and the corresponding
voltage Vacc as the field that the particle undergoes as it passes through the

cavity, where their relationship is

V
E zfi 1.1
acc d '

A superconducting RF cavity can be modeled as a resonant circuit using a
simple parallel RLC circuit driven by a current source with generator impedance
as shown in Figure 2. The RLC Circuit can be represented as a combination of

quality factors at its resonant frequency a).

 

 

I

 

 

Figure 2: RLC model of a superconducting RF cavity.

The losses in the cavity are characterized by the unloaded quality factor

Q0 and is defined as:

 

(0 Energy Stored m Cavity _ (OH = 2 797% C,

Power Disipated in Cavity - PC 1-2

Q0:

Q0 is therefore a measure of the ratio of the energy stored to the power

dissipated in the cavity walls in roughly one rf cycle. The time average energy
stored in the cavity corresponds to the equivalent circuit model time average
energy stored in the electric field of the capacitor, which is equal to the time
average energy stored in the magnetic field of the inductor. So the energy stored

in the cavity can be expressed as

U=1CV H2=%L12, 1.3

and the power dissipated in the cavity is simply the losses across the resistor

R .
c

V 2
P 2199— 1.4

C 2R
C
with Vacc being the peak voltage of the oscillator. In terms of field quantities the
time averaged stored energy is
_ 1 2 _ 1 2 .
U — 3,110 LIHI dv — EEO LIEI dv 1.5
and the dissipated power in the cavity becomes
P :11: “HIzds, 1.6
C 2 S s
where R5 is the surface resistance of the cavity walls, so the Q of the cavity is

_ capo LIHIZdv
O _

 

2 . 1.7
RS IS|H| ds

Next we can define the amount of energy leaked out Of the cavity and lost
in the generator in terms of a quality factor. We call this the external quality

factor Qext , which is defined as:

Q = (0 Energy Stored m Cavrty _ (0U : 2

ext Power Disipated in the Generator - P

 

’VRgC 1.8

We can express all the losses in the circuit, both the cavity losses and the
generator losses as a quality factor also. This is called the loaded quality factor

Q L and is defined as:

Energv Stored in Cavity (0U _ 2

 

 

: a) = C
QL Total Power Disipated (Cavity + Generator) PT f’r/RT 1'9
Where RT is the parallel combination of the generator and cavity resistances.
RcRg
R = ——
T (R + R ) 1.10
C g
The relationship between the Q’s based upon their definitions is given by
1 1 + 1
_ = —‘ 1.11
QL Q0 Qext

Next a coupling factor 7g can be defined between the generator and the

 

cavity.
Q P
yg= 0 =P—g. 1.12
Qext C

Utilizing 1.12 into equation 1.11 we see that the relationship between Q0 and

Q L becomes

Q0=QL(l+yg). 1.13

This equation becomes important later on in the method used to measure Qext.

For the case of two couplers, an input coupler and a pickup coupler, equation

1.13 becomes
Q0=QLI1+7g+ypI 1.14
Where 7g is the coupling factor between the generator and the cavity, and 7p is

the coupling factor between the cavity and the pickup. Coupling factor is an

appropriate name for the y ’5 since they can also be written as

7 =P—g’ 1.15
g PC
and
7 =P—p. 1.16
P P

C

Although there are several other important parameters that are considered
in the design of a superconducting cavity, the terms defined above are sufficient

for designing the cavity power coupler.

CHAPTER 2

COUPLING POWER INTO THE CAVITY

An accelerating cavity is not complete without a FPC properly designed for
the beam it’s accelerating. The coupling must be set to maintain control of
amplitude and phase Of the beam over the cavity detuning range. The power the
FPC must be able to handle can be determined from the detuning range of the
cavity and the beam power. The location of the FPC on the cavity must be taken
into consideration and can determine its effectiveness and adjustability. From

the coupling factor we can determine the penetration length of the FPC.

2.1 Beam Loading and Power Requirements

The overall goal of the LINAC is to produce a beam with specified
characteristics such as current and energy. The beam Characteristics are what
drive the cavity design and therefore the coupler design too. Once we determine
the power delivered to the beam the coupler penetration length and power
requirements can be determined.

The beam current is denoted as 1b and the power of the beam in the

cavity is defined as

Pb = IbVacc cos¢, 2.1

where ¢ is the phase with respect to the beam of the rf voltage of the cavity. In

this accelerator, the phase will be -30 degrees to maintain a synchronous particle

10

bunch and stable longitudinal motion. Now that the power transferred to the
beam is known a quality factor for the beam can be defined as was done with the
cavity. The quality factor Of the beam is defined as

E nergv Stored in Cavity (0U

= a) = .
Qb Power Disipated in the Beam Pb 2'2

 

Ideally the generator power would equal the beam power and any cavity
detuning would be within the system bandwidth. For the case of RIA, cavity
detuning will be greater than the system bandwidth so additional generator power
is required to maintain amplitude and phase of the beam for the shift in the

resonant frequency of the cavity. The required generator power Pg for a given

beam loading, coupler strength Qext ; Q L and maximum detuning i (y is given

by [1]:

2 2
i:l$ 1+.gl; + 1Q_L ’ 23
Pb 4QL

where the half beam bandwidth is
Ab = —- 2.4
Assuming Pg = ZPb, a system of equations can written, to determine Q L

and therefore Qext , from equation 2.3 and by setting its derivative with respect to

Q = Q L / Qb to zero. Doing so the following is obtained

11

P 2 —.5
0:211— _8 =Q_1+ _‘>/_ , 2.5
Q Pb Ab

By solving the system we find Qext ; Q = ‘33Qb’ so the desired Qext is 6.3x106

L

and 8.3x106 for the quarter-wave and half-wave cavities, respectively.

For Pg = 2Pb the required power handling of the F PCs will be at least

4Pg since there is a possibility of full reflection of power from the cavity. In the

case of full reflection a standing wave with twice the voltage and current of the

forward wave will be formed, which equates to four times the forward power.

2.2 Coupler Style and Location

The focus of this paper is on the FPC development for the MSU-designed
low beta cavities for the RIA. The low beta portion of the LINAC consists of half-
wave and quarter-wave superconducting coaxial resonant cavities. The half-
wave type is also referred to as 3 spoke cavity.

For the 6 = 0.285 322 MHz half-wave, the fields in the cavity will be
defined as follows. The z-axis is in the direction of the inner conductor axis with
z = O at the beam axis and the ends of the cavity are located at z = i1 shown in
Figure 3. The peak electric fields occur in the z = 0 plane, are radially polarized

and sinusoidally goes to zero at z = i]. The peak magnetic fields occur around

the z = i1 plane, are ¢ polarized and go to zero at the z = 0 plane.

12

Z=-| Z=+|

 

coupler
location

peak E

Figure 3: Half-wave coaxial cavity electrical and magnetic fields.

There are two practical coupler styles to choose from: a coaxial coupler
with a loop to inductively couple to the magnetic field of the cavity or a coaxial
coupler with a monopole antenna that capacitively couples to the electric field of
the cavity. A waveguide coupler operating at 322 MHz or 80.5 MHz is impractical
because of its required size. For the loop coupler the ideal location would be on
the end plates of the cavity where the magnetic fields are the strongest and there
are small electric fields reducing the risk of arcing. The coupling strength can
then be adjusted by the size and orientation of the loop. For the capacitive
coupler the ideal location would be near the z = 0 plane where the electric fields
are the strongest and there are small magnetic fields reducing the currents on

the tip of the coupler. The coupling strength can be adjusted by the length and

diameter of the monopole antenna. Capacitive coupling in the z = 0 plane was
Chosen because of its mechanical simplicity for the half-wave cavities.

For the 6 = 0.085 80.5 MHz quarter-wave we will define the fields in the

cavity as follows. The z-axis is in the direction of the inner conductor axis with
z = 0 at the beam axis and the shorted end of the cavity located at z = —l and the
open termination at z = +d shown in Figure 4. The peak electric fields occur at
the open end of the cavity near the z = 0 plane. At 2 = 0 the electric field radially
polarized and sinusoidally goes to zero at z = —I. For 2 > 0 the electric field is

normal to the nose of the inner conductor as well as the outer conductor in this
region. The peak magnetic fields occur around the z = —1 plane, are ¢ polarized

and go to zero at the z = 0 plane.

 

 

 

 

 

 

 

 

coupler
location

peak E

Figure 4: Quarter-wave coaxial cavity electrical and magnetic fields.

14

The practical coupler styles for the quarter-wave cavities are the same as
for the half-wave explained above. The ideal location for the loop coupler is
again on the short plate in the z = —1 plane. An appropriate location for the
monopole capacitive coupler is in the z = 0 plane but the open end plate is a
suitable choice also. The open end plate was Chosen for the coupler location

because of the mechanical simplicity of the design.

2.3 Measuring External Quality Factors

The external quality factor Qext plays an important role in coupling power

to the beam in the cavity since it is a determining factor in the system bandwidth.

An experimental approach is used to measure the Qext as a function of the

coupler penetration length. A simple two-coupler method is used to measure the

Qext independent of the cavity losses. This means that it doesn’t matter if the

measurements are done when the cavity is superconducting or at room
temperature.

The external quality factor Qext can be calculated from the three
measured parameters: the loaded quality factor Q L’ the reflection coefficient
511’ and the transmission coefficient 521. All three parameters (Q L’ S1 1, 521)
can be measured with a vector network analyzer (VNA). The two-coupler

method consists of a matched input coupler ( yg z 1) and a pickup coupler for

15

which we are interested in calculating the Qext. First, the definition 0f Qext p

that is the Qext of the pickup must be defined:

Energv Stored in Cavity (0U

 

Q = w . . . . = 2 6
ext, P Power Dzszpated m the Pickup coupler Pp °

where Pp is the power of the pickup coupler which is equal to the transmitted

power measured with the VNA in this setup. The Q can also be expressed

ext,p
intermsonO
P P
coU
gm, =_2.[_]=_c~.g,
,P P P P 0
P C P
where
Q0=Il+7g+yPIQD 2'8

7g is the coupling factor between the generator and the cavity and 7p is the

coupling factor between the cavity and the pickup. For the input coupler

Q0 _liISIII

Qext,g TTISl 1l

 

2’ 2.9

Where the sign is chosen so that 7g is greater than one when over-coupled and

less than one when under-coupled. QO may be approximated as

Q0z(l+yg)QLz2QL. 2.10

16

This approximation of Q0 is most accurate when the input coupler is matched

. . 6 7 . .
( 7g z 1), Since the range of Qemp IS 1x10 to 1x10 , then 7p << 7g IS satisfied.

The power dissipated in the cavity PC can be approximated as the forward

power less the reflected power of the matched input coupler,

PC=Pf-Pr. 2.11

Where the reflected power is (Pr z 0 for the matched input coupler) given by
2 2 12
Pr _ IS] I] P f. .
The power of the pickup is
2 2 13
Pp = I521I P f. .

Substituting equations 2.11, 2.12, and 2.14 into equation 2.7 results in the

 

calculated value of Qext from the measured parameters of Q L ,511 and 521.
l S 2
ext,p 2 g L 2 ' '
I521I I521I

By means of the two-coupler method described above, the Qext of the

pickup coupler in the measurement is plotted as a function of antenna length in
Figure 5 and Figure 6. From these plots we can determine the length of the

antenna to achieve the proper coupling of the FPCs.

17

10
10 . l I I r I

 

10:“ 1

Qext

10:-

llLi

10: _:

7171‘

 

 

I I I l

40 50 60 7O 80 90 1 00
Probe Length (m)

 

10

Figure 5: Qext vs coupler probe length for the 80.5 MHz cavity where the length is measured
into the cavity from the stainless steel mini CF flange of the bottom plate of the
cavity.

18

13

 

 

 

 

10 E T T I I I
1012? 1
, I
I
10":
I .
10ml .
32' E
(D I
0 I
109 - -
108 Er :
107 r 1
106 I I I I I
—10 0 10 20 30 40 50

Probe length (mm)

Figure 6: Qext vs coupler probe length for the 322 MHz cavity where the length is measured
into the cavity from the surface of the NbTi CF flange of the cavity.

19

CHAPTER 3

MECHANICAL CONSRAINTS

Coupling power into a superconducting RF cavity poses several
mechanical design issues. First of all the materials must be able to withstand
cryogenic temperatures, withstand large pressure differentials, create an ultra-
high vacuum seal with the mating cavity, and be unsusceptible to outgassing
harmful contaminants into the rest of the cavity. Second, the thermal load to the
cryogenics system should be minimized to maximize the cryomodule efficiency (it
takes about 1 kW Of electrical power to remove 1 watt of heat from 2 K liquid
helium due to Carnot and mechanical efficiencies). Lastly there are constraints
on the physical size of the coupler to create the correct impedance as well as
reduce the effects of multipacting. A balance must be made between the

mechanical constraints and the electrical performance.

3.1 Material Constraints

The materials of Choice for ultra-high vacuum systems are 304 stainless
steels, OFHC (oxygen free high conductivity) copper, aluminum and ceramics [9].
Since ceramics are not a good electrical conductor, it is a good Choice as an
electrical insulator of vacuum window in our case. Aluminum is subject to
outgassing due to its inherent oxide layer present on its surface. Stainless steel
and OFHC copper would be a good choice of materials for the power coupler.

The mating flange on the cavity is a stainless steel conflat flange that can only be

20

sealed with another mating conflat with a copper vacuum gasket between them.
The airside end of the coupler must be welded to a stainless steel bellows to
allow for the shrinkage of the cavity when it cools down to less than 5 K. For
these reasons investigation of the use of 304 stainless steel as a potential
material for the outer conductor is necessary.

Thermal conduction properties must be investigated to minimize the
conduction of heat into the helium cooling system. A simple solution to minimize
the heat flow is to make the walls of the outer conductor thin, which reduces the
cross sectional area for the heat to flow through. This concept is easily seen

from the following heat equation known as Fourier’s Law [10]

dT
qcond :k(T)AE’ 3.1
where qwn d is heat conduction in watts, k(T) is the thermal conductivity of the

material which is dependent on temperature T and A is the cross sectional area

at which the heat passes through. If we reduce A then qcon d is reduced. For

stainless steel k(T) decreases as T decreases, this is ideal and shown in Figure

7 along with the thermal conductivity of OFHC copper as well [11]. It can be
seen that the 304L stainless steel is a better Choice than OFHC copper in

reducing the conduction of heat.

21

 

4000 I I I I I 20

    

Type304L I

A 3000 P Stainless Steel "15 A

E i E8
53 I 565
x0 x m
30 . 3‘8
5E ' 2?
‘90 2000» «10 gal;
130 I t

C'- C_I
O& I 8%
3.3% I a?
g I E 9
a) I 0)]—
:5 IS

1000 *

 

RRR=10 OFHC Cu

 

I I

0 50 1 00 1 50 200 250 3
Temperature T (K)

o ._____
O

0

Figure 7: Thermal conductivity of select materials.

Another source of heat in the outer conductor is the ohmic losses in the
conductor material. The electrical resistivity, shown in Figure 8 [11], is another
dominating factor in the choice of material to suppress large ohmic losses in the

outer conductor. Ohmic losses are characterized by [12]

 

1 2 PX
=—I —. .2
q’f 2 2W5 3
where
5= p 3.3
”/10/

is the skin depth, p is the resistivity of the material, r is the radius of the outer

conductor and x is the length of the coupler. Hence, the ohmic losses are less

22

for a material with a lower resistivity. The OFHC copper is the better Choice for

reducing ohmic losses.

 

 

 

 

 

1o“ . -..f-.., . ......,
: /
’ Type 304L
Stainless Steel

1? _
c 10 7r 1
Z I I
a i .
IE I
Z)
8
I:
76
.9
78‘ 10’s: 1
E ; .

I

t RRR=10 OFHC Cu

-9
10 I I ILIIIII I I I .IIIII L I I .III
10° 101 102 103

Temperature T (K)

Figure 8: Electrical resistivity of select materials.

A decision must be made between the stainless steel and copper to
construct the outer conductor. The ohmic losses in the stainless steel are
unacceptable since the heat delivered to the helium system would be larger than
one watt (the design maximum). The thermal conduction of the copper conducts
too much heat from the 300 K end of the coupler into the 2-4.5 K helium system,

again larger than one watt. Since neither material by itself will satisfy the design

23

criteria, a combined material solution was developed using a thin walled (.020

inch wall) 304L stainless steel tube with the inner surface plated with an 8 —16,um

layer of OFHC copper. The thin copper layer reduces the ohmic losses from the
stainless steel as well as reduces the thermal conduction from an equivalent pure
copper outer conductor. By adjusting these parameters an optimum design
solution was found minimizing ohmic heating and conduction heat transfer.

The inner conductor can be made from all OFHC copper since there is no
conduction path to the helium system and also the vacuum window of the coupler

already has a copper inner conductor to mate to.

3.2 Thermal Analysis

The cavity temperature of the 322 MHz cavities is 2 K and 4.5 K for the
80.5 MHz cavities [1 ,3—4]. The outer conductor of the power coupler provides a
direct conduction path for heat transfer between the outside of the cryomodule to
the cavity, which are at 300 K and 2-4.5 K respectively. To maximize the thermal
efficiency of the cryomodule, this conduction path should be minimized.

The heat dissipated into the liquid helium bath of the cavity is specified to

be less than one watt per coupler. The total heat q to ta 1 is the sum of the rf

losses in the outer conductor, the power radiated by the warmer inner conductor

and the conduction along the coupler axis [13].

qtotal = qcond + qrf + qrad ' 3-4

Where equation 3.1 utilizes

24

k(T)/1 : ks.s‘(T)Ass + kcu (T)Acz ’ 3'5

l
for the parallel conduction paths of the stainless steel and the copper plating.

The radiated power qm d from the inner conductor is [14]

 

4 4
q = OSBAic(7Tc Too)
rod 1 [ 1 ] A. 3.6
____+ _____.l _J£_
e. a A
[C 0C 0C
where a is the emissivity and 0' SB is the Stefan-Boltzmann constant. The rf

losses are as described in equation 3.2 where the current is assumed to be

constant along the conductor and is calculated as

1:,I§5. 3.7
2

This is the maximum peak current when a standing wave is present. This
approximation for the current will result in larger calculated rf losses than what
will appear in reality.

A reverse difference model was constructed to calculate q to M l and the

temperature profile along the outer conductor. The temperature is defined at the
cavity as 2 K or 4.5 K and again at the 77 K thermal intercept. In Figure 9 and

Figure 10, the total heat qtomI is plotted for a given copper plating thickness as a

1
function of length from the cavity to the 77 K intercept. The 77 K intercept is

chosen to minimize the conduction and rf loss heating.

25

18pm

Heat 0 (W)

9
co
f

 

0.6 -

0.2 r

 

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18
Length (m)

Figure 9: Heat dissipated in the helium system for select copper plating thicknesses due to
the power coupler for the 322 MHz half-wave cavity.

 

Heat 0 (W)

.0
a)
T

 

.0 .o .o

N A O)
I I I
I I

 

 

I I I

 

G l I I l I
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Length (m)

Figure 10: Heat dissipated in the helium system for select copper plating thicknesses due to
the power coupler for the 80.5 MHz quarter-wave cavity.

26

3.3 Plating Measurements

Verifying the thickness of the copper plating on the outer conductors is a
critical step in the construction of a superconducting cryomodule. If the copper
plating is too thick, a static heat load would be added to the cryomodule causing
added stress on the cryogenics system. If the copper plating is too thin the
electric field will penetrate into the stainless steel inducing currents causing large
ohmic losses in the outer conductor. This again will dramatically increase the
heat load on the cryogenics system.

Measuring the thickness of the copper plating, which is on the order of 8 to
16 micrometers, can be a tricky task to handle accurately. The most accurate
method would be to cut a cross section and measure the thickness under a
microscope, but that is not a possibility due to its destructive nature. A second
method would be to measure the mass of the conductors before plating and then
again after the plating and calculate the thickness due to the difference in mass.
The technique used was to apply a known DC current I thought the conductor
and measure the voltage drop V across a measured distance I. From this, the
total resistance of the conductor can be calculated.

_ at:
V" Rtotal’ 3-8

where Rtom is the parallel combination of Rss (the resistance of the stainless

1

steel) and Rcu (the resistance of the copper plating).

27

RSSRCu
Rm, =————R +R (0). 3.9
SS cu

After solving equation 3.9 for RC“, the thickness of the copper plating can

 

calculated.
length I
R = resistivit = -— Q .
C“ yI area I pCHIA ]( ) 310
cu
A =7zr2—7r(r—t )2z271rt . 3.11
cu cu cu

By solving equation 3.10 for Am and substituting in equation 3.11, tcu (the

copper plating thickness) can be determined.

,0 l p 1‘
t =r— rz- 6“ z c" . 3.12
C“ III? 2727'R
cu cu

 

 

where r is the inner radius of the outer conductor, or in other words, the radius at
the stainless steel / copper interface.

To minimize the error, the measurements were done at a temperature of
77 K. The conductors were submersed in liquid nitrogen during the
measurements to obtain a temperature of 77 K. For the specified copper
thickness of 12 micrometers, Figure 11 shows the contribution of the resistance
of the copper and the resistance of the stainless steel to the total resistance,

which is measured.

28

 

 

0
U!
f

CU

Resistance (ohms)
N

 

 

 

1.5 " R '
total
1 " II
0.5 "
o L L l I l
O 50 100 150 200 250 300

Temperature (K)

Figure 11: Resistance per unit length.

From Figure 11 it is easy to see that the total resistance is dominated by
the copper plating resistance at 77 K. Since the measured resistance is
dominated by the resistance of the copper plating, the error in knowing the

stainless steel resistance has less effect on the measured plating thickness.

3.4 Vacuum Window, Conductor Size and Multipacting

The vacuum window of the coupler, which isolates the cavity vacuum from
room air, can be an expensive item to develop from scratch. For this reason a
commercial window was sought. The coupler requires the window to be non-

magnetic, able to withstand bake out temperatures, low loss, well matched at the

29

cavity frequency and be able to handle the appropriate power. An If feedthough
from Insulator Seal was Chosen for the window assembly [15].

The window is made from >94% alumina ceramic insuring low loss at 322
MHz and 80.5 MHz and a high temperature threshold. The inner conductor is
made from 0.25 inch OFHC copper, and is mounted on a standard 2.75 inch
diameter stainless steel conflat flange.

The diameter of the inner and outer conductors is not only critical to the
thermal analysis but also determines the impedance Of the coupler and can
determine the power levels at which multipacting will occur. Since the vacuum
window already has a 0.25 inch diameter inner conductor that will become the
starting point for the design. To reduce cost and simplify the manufacturing, an
attempt was made to use standard tubing sizes for the inner and outer conductor.

To determine the outer diameter of the coupler, a characteristic
impedance for the coupler must be specified. The transmission line mating to the
coupler will have a characteristic impedance of 50!), therefore making the
impedance of the coupler 500 would be the practical choice. Other impedances
are possible but would require the addition of an impedance transformer to match
the coupler to the transmission line further complicating the design. For an inner
conductor diameter of 0.25 inches and a characteristic impedance of 500, the

outer conductor diameter can be calculated from [16]

d.
16

r] d
__0 _ec_
ZO —2”In[ ]. 3,13

and is about 0.576 inches. Here the wave impedance of free-space is given by

30

,u
’70: I_0z377§2. 3,14
60

The inner diameter of a standard 0.625 inch diameter, .020 inch wall 304L
stainless tube is 0.585 inches, which corresponds to a characteristic impedance
of 510. The reflection coefficient between the 500 transmission line and the

510 coupler can be calculated as

 

trans _ Zcoupler _ SO - 51 _

F = — -.010 .
- Z 50 + 51 3'15
trans coupler
The reflected power then becomes
P = |r|2P z 3 16
r f — ' '

So an inner conductor made from 0.25 inch diameter copper rod and a outer
conductor constructed from standard 0.625 inch diameter, .020 inch wall 304L
stainless tubing will satisfy the impedance criteria for the coupler.

Multipacting is another driving force behind the diameter of the coaxial
coupler. Multipacting is a resonant condition that occurs in rf structures when an
electron is emitted from the surface of the conductor and accelerated by the rf
field. If the resonant condition is met when the electron hits the surface several
more electrons may be emitted and accelerated by the rf fields. This can lead to
large power losses in the conductor and cause additional heating to the coupler
as well as unwanted outgassing into the cavity. For coaxial transmission lines

the power levels at which multipacting can occur is proportional to the frequency

31

and the diameters of the inner and outer conductors. For one-point multipacting

(from inner or outer conductor back to itself), the multipacting power is

~ (flII4Z. 3.17

P .
one — pomt

and for two-point (from inner to outer and back) this becomes

~ 4 2
tho_poim (fd) 2 . 3.18

From the analysis done in [17], it was estimated that the power levels at which
multipacting is most likely to occur are relatively low compared to the operating
power levels of the couplers. An estimate of <10 watts for the 322 MHz coupler
and <1 watt for the 80.5 MHz coupler compared to their operating power levels of

1012 watts and 344 watts, respectively.

32

CHAPTER 4

PROTOTYPE ASSEMBLEY, TESTING AND CONDITIONING

The assembly of two prototype couplers has been constructed to verify
their performance. The two-coupler setup must be thoroughly cleaned to prevent
contamination of the accelerating cavity. The coupler will endure a bake-out
process to drive out any gasses within the material. The couplers will also be

tested to verify that they can meet the power capacity required of them.

4.1 Prototype Assembly and Cleaning

A test setup was designed to test and condition two couplers
simultaneously. For the 805 MHz couplers, a waveguide was used to join the
couplers together; however, that is not practical since the size of a 322 MHz and
80.5 MHz waveguides would be extremely large. The connection between the
two couplers was instead made with a cylindrical copper sleeve which shorted
them together as shown in Figure 12. The outer conductors were extended
through the vacuum assembly needed to pump out the couplers. During high
power testing the inner conductor joint limited the power capabilities. When high
power was applied the rf losses on the inner conductor heated the joining sleeve
causing it to expand and loosen the inner conductor union. A slotted sleeve with

tighter tolerances would be a better choice for the inner conductor connector.

33

Inner conductor Connecting sleeve

  
  

 

a
\V

 

 

 

 

 

 

I
i
|
I
i
i
i
i
i

/ e I
— .a\\\\\\\\q\\\x\\\\\\\\\\$

 

Zfi/r’z/ygé/fi/fléV/xé/fléé/y/z/yl/B- - -

' 8’”.

O
a a h
- . - I

 

 

 

 

 

 

 

 

 

Outer conductor

Figure 12: Two-coupler test setup conductor joints.

All cavity vacuum components must be assembled in a class 100 Clean
room to prevent particle contamination to the cavity. Any particle contamination
can cause a devastating reduction in performance of the cavity due to field
emission. The power couplers share the cavity vacuum and therefore require
Cleaning and assembly in the clean room. All coupler components exposed to
the cavity vacuum including the inner conductor window assembly and the
copper plated outer conductor must be cleaned with a micro-90 solution in the
ultrasonic cleaner and rinsed with ultra pure water. The two-coupler test setup

requires the same Clean room processing and assembly.

4.2 Bake-out Procedure and Results

A procedure referred to as a bake-out is used to drive out absorbed
gasses from the coupler walls. By heating the coupler we can expel the water
remaining from the cleaning process and other gasses within the walls. Once the
materials are evaporated out of the coupler walls into gaseous form the vacuum
pump can remove them from the setup.

The bake-out procedure is quit simple. Thermocouples were placed on
the outer and inner conductors of each coupler to monitor the temperature. A
heat tape was then wrapped around the two-coupler test assembly and encased

with aluminum foil to retain the applied heat. The temperature was then ramped

up until all parts of the assembly reached a temperature of >100 0C. The
temperature was maintained until the vacuum pressure stopped showing much
improvement. The temperature profiles Of the bake-out are shown in Figure 13

and the vacuum pressure can be seen in Figure 14.

35

Temperature (C)

Pressure (mbar)

160

 

 

 

 

 

 

 

 

 

35

 

 

 

 

 

35

outer conductors
140 ~
120 - -
‘—‘l
100 - inner conductors -
80 ._ _
60 F -
40 ~ ~
20 I l I L I
0 10 15 20 25 30
Time (hrs)
Figure 13: Temperature profile during the two-coupler test setup bake-out.
10‘s.
10'7. ‘:
10‘8 -
1 0'9 I l I J I
0 10 15 20 25 30
Time (hrs)
Figure 14: Vacuum pressure of the two-coupler test setup during bake-out.

36

The temperature profile in Figure 13 shows a difference between
temperature sensors. This is due to there placement in the test setup. The outer
conductor thermocouples were positioned between the heat tape and the outer
conductor so the actual temperature of the outer conductor may be lower than
the measurement. The inner conductor thermocouples were positioned inside
the inner conductor where a good thermal contact could not be made therefore
the inner conductor temperature was likely to be higher than the measured value.

In conclusion the temperature of all the vacuum surfaces of the coupler reached

a value of >100 0C.

4.3 Matching Parameters

The reflection and transmission coefficients are two important parameters
in evaluating the rf performance of the couplers. If the couplers are not properly
matched a large portion of the power being sent to the cavity will be reflected
back to the source. The measurements were conducted by attaching the ports of
the network analyzer to the two-coupler setup via 1-5/8 EIA to type-N adapters.
The reflection measurements were conducted by placing a 50!) load at one port
and measuring the reflection coefficient from the other port. Reflection
measurements were conducted from both ports and their results were identical.
The transmission measurement was conducted by attaching both ports of the

network analyzer to the two ports of the couplers via the 1-5/8 EIA to type-N

37

adapters. The measured results of the reflection and transmission coefficients

are presented in Figure 15 and Figure 16, respectively.

 

811 (dB)

 

 

 

 

-60 -
—22.3 @ 80.5 MHz
-70 -18.6 @ 322 MHz 2
-80 I I I I
0 200 400 600 800 1000
Frequency (MHz)
Figure 15: The reflection coefficent through the two-coupler test setup.

38

 

821 (dB)

 

 

-0.07 @ 80.5 MHz
—6 r -0.12 @ 322 MHz

 

 

0 200 400 600 800 1 000
Frequency (MHz)

Figure 16: The transmission coefficent through the two-coupler test setup.

The measured results do not account for the reflections and losses within
the 1-5/8 EIA-to-type-N adapters used for the measurements or the coupler to
coupler union. Therefore the true reflection and transmission coefficients of the
couplers are speculated to be lesser and greater, respectively. The matching
between the 500 transmission line and the couplers at the design frequencies

are acceptable for their application.

4.4 Power Testing and Conditioning
The generator power currently available produces significantly less than the
design power for the couplers. A standing wave cavity was constructed to

overcome the lack of generator power. With the use of a resonant cavity, the

39

equivalent voltages and currents that the coupler would endure at the full power
levels can be achieved.

The resonator cavity is a coaxial cavity partially constructed from the two-
coupler test setup to expose the couplers to the standing wave fields of the
cavity. One end of the cavity is an open coaxial termination at which rf power
can be easily capacitively coupled. The other end of the cavity is a coaxial
sliding short that can easily adjust the length of the cavity and therefore the
frequency and standing wave pattern as well. The setup is shown in Figure 17.

The goal of the high power test is to expose the coupler to larger voltages
and currents than the design will require during operation. The most vulnerable
part of the coupler to fail is arching at the vacuum window. The electric field of
the coupler is shown in Figure 18. To accomplish this, the standing wave pattern
can be adjusted to maximize the current or the voltage at the vacuum window
location. The standing wave patterns for the voltage and current are assumed to
be sinusoidal since the length of the resonator is dominated by free space, only a
small fraction of the length is alumina. For the open termination at the input
coupling end the boundary conditions are that the voltage will be maximum and
the current zero. At the sliding short end of the cavity the current will be

maximum and the voltage zero.

40

Sliding Short

Bi-Directional Coupler DUTs

Input Coupler

Figure 17:

m Iv. 4.2%. -

‘myzi'x‘uJ

 

Two—coupler test setup for high power testing and conditioning.

41

 

 

 

E FieIdIV/m]

1.78368+05
1.67218+@5
1.5606e+05
1.99928+85
1.3377e+05
1.22628+85
1.11H7E+BS
1.80333+85
8.9179e+89
7.8232e+09
6.6885e+8%
5.5737e+DM
H.45988+BH
3.3892e+89
2.22958+BH
1.11H7e+BH
B.BBODE+DD

 
    
  
 
 
 
   
  
   
   

 

 

 

Figure 18: Electric field simulation for the coupler at 322 MHz with 1 kW applied to the
1-5/8' EIA input of the coupler and the cavity port shorted to produce the
equivalent maximum standing wave fields the coupler will endure during
operation using Ansoft’s HFSS software package.

42

The peak voltage of the FPCs can be estimated from the impedance of
the coupler and the required power. The peak voltage that could occur will be
the standing wave voltage when the beam is not present and the coupler is no
longer matched to the cavity. When this occurs all the power is reflected from

the cavity and the peak voltage on the coupler becomes

Vp =2 PangR. 4_1

The corresponding voltages for the quarter-wave and half-wave FPCs are
approximately 370 V and 640 V, respectively. In the test setup the peak voltage

can be calculated the same way but using the line power instead.

The two-coupler test setup is a multi resonant cavity. There are an infinite

number of resonant frequencies for the setup but only the first few have
relevance since they are in the range of the F PCs frequencies. The first four
resonant frequencies are shown in Figure 21 and their corresponding voltage

standing wave patterns are shown in Figure 22.

43

 

[)4 57.14 77:74

Magnitude S(21)
I
(D
o

—110—

 

—120r

 

 

-130

 

0 1 00 200 300 400 500
Frequency (MHz)

Figure 19: The first four resonant frequencies of the test setup of length 1.02 (m).

 

 

 

 

 

 

 

 

 

 

5N4
E
a)
z:
E
9 N4
(0
3 \
o:
0
g \
C
s \
(I)
S 3M4
S
6 /
>
71/4
_1 I I l I
0 0.2 0.4 0.6 0.8 1 1 .2

Length (m)

Figure 20: The voltage standing wave pattern for the resonant modes of the test setup.

To condition the FPC, the entire vacuum section of the coupler needs to
be exposed to the full range of fields it will endure in actual cavity Operation. By
conditioning the FPCs any outgassing that is a result of ohmic heating will occur
in the conditioning setup were the contaminants can be easily removed by the
vacuum system without risking contamination to the accelerating cavity.
Conditioning is also used to breakdown any multipacting barriers in the coupler.
If the coupler was installed on the accelerating cavity and multipacting occurs, it
could be difficult to distinguish whether it is the coupler or the cavity. Finally it is
a much simpler to fix a bad coupler before installing it on a cavity.

The two-coupler test setup is able to condition the couplers by varying the
short position to move the peak fields down the couplers. By varying the short
position the resonant frequency of the setup is also varied. .The peak voltage
standing wave pattern is plotted in Figure 23 for the range of frequencies needed
to condition the couplers; in this scenario the couplers must be switched to

entirely condition them.

45

 

 

\ 395 MHz to 330 MHZ

 

Standing Wave Pattern
O

 

\

_1 I 1 I I l
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Length (m)

 

 

 

 

 

 

Figure 21: Voltage standing wave pattern for coupler conditioning spaning 330-395 MHz.

A standing wave method that can be used were the frequency is fixed is to
construct the test setup with both ends as sliding shorts. By having both ends
adjustable the frequency can be kept constant as the field patterns are varied.
This method is favored for the reason that multipacting is a function of frequency
so if the entire coupler is not conditioned at the operating frequency. multipacting

barriers may have been skipped over.

4.5 Test Results
The two-coupler test setup was used to verify the power handling of the
couplers. During testing soft multipacting barriers were encountered around 10

watts and up. The barriers were quickly and easily conditioned away by

46

sweeping the frequency and pulsing the generator at higher power levels. A
power level of 1.27 kW was ultimately reached at 321 MHz where the design
requires 1.01 kW. The power level at 321 MHz was limited by the generator
power available.

The frequency of the test setup was then adjusted to 161 MHz where
more generator power was available. A power level of 6.3 kW was reached by
pulsing the generator power on and off. The limiting factor in the 161 MHz test
was the inner conductor connection in the test setup. As the power level was
increased, the inner conductor temperature increased, as expected due to rf
losses in the conductor, causing the connecting sleeve to expand and loosen the
rf connection. At first the loose connection appeared to be multipacting when
only monitoring the line power of the test setup. When multipacting occurs, the
vacuum pressure will Change due to the electrons emitted into the vacuum. This
was not the case since the vacuum pressure did not change. Another
observation made was the suspect multipacting was dependent on the
temperature of the inner conductor and not the power level. At this point it was
easy to verify the loose connection by shaking the test setup which produced the
same results as the suspected multipacting.

Figures 19 and 20 show the standing wave voltage approximation for two
different tests were a line power of 1.27 kW at 321 MHz and 6.3 kW at 161 MHz
was achieved. Based on the power levels achieved at 322 MHz and 161 MHz
the coupler will be able to handle the lesser power required at 80.5 MHz without

anyissues.

47

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1000 I l I I l 20
—> Window B
§ 2
e 500 ~ V°”age « 10 E
3 e
g 5
> \ / o
x x
(0 (0
Q) 0)
0- 0 O D.
0) a)
> >
t0 to
E 3
O) O')
E .E
“O ‘0
E —500 - ‘ "O E
a) a)
\ Current
Window A —) <—
_1000 I l I l I _20
O 0.2 0.4 0.6 0.8 1 1.2 1.4
Length (m)
Figure 22: High power standing wave voltage and current pattern approximation for tests
conducted at 321 MHz.
2000 I I I I I 40
—-> (— Window B
1500 — " — 30
2 <3
53’ 1000 — urrent “ 20 S
9 500 — — 10 5
x x
(0 (0
m \ ..
o. 0 0 a.
Cl.) \ (D
> >
(0 £0
2 —500 » \ ~ —1 0 3
O) C!)
.E E
“O ‘0
g —1000 ~ . —20 g
to (I)
—1500 ~ Window A _) <— Voltage V I '30
_2000 I I I I I —40
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Length (m)
Figure 23: High power standing wave voltage and current pattern approximation for tests

conducted at 161 MHz.

48

CONCLUSION

The growing number of applications for superconducting accelerators has
created a need for affordable accelerator designs. A low cost power coupler
solution based on commercially available and affordable vacuum windows has
been demonstrated for use with the Rare Isotope Accelerator ,6 = 0.085 80.5
MHz quarter-wave, and the ,6 = 0.285 322 MHz half-wave cavities. The F PC
designs are able to achieve the coupling required to control the amplitude and
phase of the beam. Two prototype FPCs have been tested to above the power
levels required for their use in the RIA. The thermal and electrical properties of
the FPCs have studied to create the best possible design with the best of both
properties.

Construction of FPCs for the ,6 = 0.085 80.5 MHz quarter-wave and the

,6 = 0.285 322 MHz half-wave cavities are currently underway for use in a low

beta test module. The module will be a functioning cryomodule containing the

,6 2 0.085 80.5 MHz quarter-wave and the ,6 = 0.285 322 MHz half-wave cavities
along with the FPC designs that have been demonstrated. The couplers will then
be tested in an actual module encompassing all the design goals of the low beta

modules required for the RIA.

49

APPENDIX

 

Rm

 

4 fi\¥\*\—\¥A<\W —

ISI Vacuum Window

   

 

 

 

 

 

All

 

 

  

1 5/8" El

Figure 24: 322 MHz FPC cross section.

50

APPENDIX

lm/r Mr l/il

/.

 

=W-/i

.\_

 

iiflll l I/

/V

Null/ma

 

 

>>OUE>> E330m> _m_

   

‘7‘»‘\‘.‘| t.‘.|...|'b.y'||. l I l TU

A

<_m_ EB V

Figure 25: 80.5 MHz FPC cross section.

51

t ‘K " "L

 

[1]

[2]

i3]

[5]

[6]

[7]

[8]
[9]

REFERENCES

National Superconducting Cyclotron Laboratory, ”The Rare Isotope
Accelerator Conceptual Design Report”, NSCL, East Lansing, MI, to be
published 2005.

“Completion of the LNL Bulk Niobium Low Beta Quarter Wave Resonators”,
in Proceedings of the 9th Workshop on RF Superconductivity, Santa Fe, NM
1999.

J. Bierwagen, S. Bricker, J. Colthorp, C. Compton, T. Grimm, W. Hartung,
S. Hitchcock, F. Marti, L. Saxton, R. York, and A. Facco, “Niobium Quarter-
Wave Resonator Development for the Rare Isotope Accelerator”, in
Proceedings of the 11th Workshop on RF Superconductivity, Travemunde,
Germany, 2003.

T. Grimm, J. Bierwagen, S. Bricker, W. Hartung, F. Marti, R. York,
”Experimental Study of a 322 MHz v/c=.28 Niobium Spoke Cavity", in Proc.
Of the 2003 IEEE Particle Accelerator Conference, Portland, OR, 2003.

W. Hartung, C.C. Compton, T.L. Grimm, R.C. York, G. Ciovati, P. Kneisel,
”Status Report on Multi-cell Superconducting Cavity Development for
Medium-velocity Beams”, in Proc. Of the 2003 IEEE Particle Accelerator
Conference, Portland, OR, 2003.

P. Kneisel, G. Ciovati, K. Davis, K. Macha and J. Mammosser,
”Superconducting Prototype Cavities for the Spa/Iation Neutron Source”, in
8'” European Particle Accelerator Conference, Paris, 2002.

H. Padamsee, J. Knoblock, T. Hays, ”RF Sgpercondictivity For
Accelefitors”, John Wiley & Sons, Inc, 1998.

J. Slater, ”Microwave Electronics”, D. Van Nostrand Company, Inc, 1950.

J. Moore,C. Davis, M. Coplan, ”Building Scientific Apparatus 2nc_f’, Perseus
Books Publishing, L.L.C., 1991.

[10] J. Lienhard N, J. Lienhard V, ”A Heat Transfer Textbook 3rd", Phlogiston

Press, 2002.

[11] ”Cryogenics Technology Group Material Properties ”, National lnstatute of

Standards And Technology,
http://cryogenics.nist.gov/NewFiles/material_properties.html

52

[12] D. Cheng, ”Field and Wave EIectromaqnetics”, Addison Wesley Publishing
Company, Inc, 1992.

[13] R. Rabehl, "Thermal Analysis of a Conductive/y Cooled Spa/lation Neutron
Source (SNS) Fundamental Power Coupler" FNAL, Batavia, IL, 2002.

[14] F. lncropera, D. DeWitt, ”Introduction To Heat Transfer”, Addison Wesley
Publishing Company, Inc, 1992.

[15] Insulator Seal, |nc., ”High Vacuum Electrical And omen Components”,
Insulator Seal, |nc., Sarasota, FL, 1995.

[16] R. Harrington, ”Time-Harmonic Electromagnetic Fields”, IEEE Press,
Wiley-lnterscience, John Wiley & Sons, Inc, 2001.

[17] E. Somersalo, P. Yla-Oijala, D. Proch, "Analysis of Multipacting in Coaxial
Lines," in Proceedings of the 1995 PAC, Dallas, TX, 1995.

GENERAL REFERENCES

[18] Y. Kang, S. Kim, M. Doleans, I. Campisi, M. Stirbet, P. Kneisel, G. Ciovati,
G. Wu, P. YIa-Oijala, “Electromagnetic Simulations and Properties of the
Fundamental Power Couplers for the SNS Superconducting Cavities ”, in
Proceedings of the 2001 Particle Accelerator Conference, Chicago, IL,
2001.

[19] T. Grimm, W. Hartung, M. Johnson, R. York, P. Kneisel, L. Turlington,
“Cryomodule Design for the Rare Isotope Accelerator“, in Proc. Of the 2003
IEEE Particle Accelerator Conference, Portland, OR, 2003.

[20] T. Grimm, W. Hartung, J. Popielarski, C. Compton, M. Johnson, Personal
Communication, NSCL, East Lansing, MI.

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