\| w i ___—_—— __._'—- #1 I , ’——- —____4_ __—_—— __’_———— __—_—— f —_—d ’— #— _____—— ____— — _—_—. \ I WIWINHMWHI TH‘S W anaawmmu ELECTRON GUN“ "Tfissés for ”sizes Degree. of M. 3‘ MECHIGAN .‘E‘M‘E COLLEGE "3* 2 r‘ M: 1. .{SJSz'i use-"gs: mags 294;? L - _-__-4. _.-_ ‘_._-o-‘o-~ .- *‘o- “ho—.— |II|IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIJIIIIIII 3 1293 01774 980 . I l . “ ‘IU; V . I .. This 1. to certify that the I - . thesis entitled 5’ ’f. A! 1mm WORD! Gm - presented by I Robert George mu. i ‘ has been accepted towards fulfillment I. - ~ of the requirements for e 4 . |‘ Major prof Date—ZmJZnMJQLLL—__ [-705 V in}, ‘. c.0913 OF THIS WIT ARE AVE; I TIT . on rgmxm BY H5" "J“ '5 1.1.- Dommvsmvxs'ron m'I'BLLIG; .NGE T-Z: AIR mm new Du? AND m'rmo LTI No. 15181; ._ Lifilfl £151.19 ' , DAYTON’ DAV/o PLACE IN REWRN Box to remove this checkout from your record. To AVOID FINE remm on or before date due. MAY BE RECALLED with earlier due date if requested. AN EXPERIMENTAL ELECTRON GUN by Robert George Ni ele A basis Submitted to the Graduate School of Michigan State College of Agriculture and Applied Science in partial fulfilment of the requirements for the degree of MASIER 01' SCIENCE Department of Physio! 19“? momm I ehould like to take this opportunity to expreee my appreciation for the new helpful suggeetione of Profeeeor ‘I'homae H. Osgood under MW“ whose direction this work was done. [3W Ii :39! TABLE OF CONTENTS Page IntrOduCtion0.0000000000000001 Diecueeion...................1 Deecription of.Apparatue . . . . . . . . . . . . 2 Reenltl....................JI Conclusion . . . . . . . . . . . . . . . . . . . ll Appendixl....................l3 AppandlxII 00000000000000.0001? HERODUCTICN Two quantities are of interest in the study of the ionization of gases by means of electrons:(a) the average number of ions per centi- meter of path, and (b) the total number of ions produced by an electron before its energy is reduced below the minimum required to ionise a gas molecule. In either case the measurement of these quantities require! the introduction of a beam of electrons into an ionisation chamber. Ihe production of a suitable beam of electrons is. therefore. one of the problems involved in such measurements. The ideal beam would be such that it would be possible to direct the electrons. all of which would have the same velocity, through a small aperature. It is not essential that all electron paths be parallel. They may converge to a focus at a small angle of convergence. Further, it is desirable, but not neces- sary, that the axis of the beam coincide with the axis of the aperture. and that the beam have a small cross sectional area at the focal point. he electron gun discussed here was designed to produce a beam of electrons capable of being focused and aimed in the desired direction. DISCUSSION The method of obtaining a beam employed by early workers in this field usually consisted in using a pair of beam limiting mrtnrew (Ref. 1 - 5). Several variations of this arrangement were used without any attempt to utilise them for focusing the beam. The present use of electron lenses in cathode ray tubes and in the electron microscope suggests the possibility of employing a similar 2. device in order to produce the desired type of beam. .1 large amount of material has been.published on the design of electron guns and lenses (Ref. 6 - 12), and the device here described was designed and built in accordance with procedures described in Ref. 12, pm. 100 - no. and Ref. 11. p. l+50. The present device is illustrated diagramatically in.Fig. l. The small cross sectional area of the beam at the focal point is ob— tained by limiting the beam emerging from an emitter with a small aperture, and by focusing. The small angle of convergence at the focus is obtained by making the image distance (q) large compared with the object distance (p), as in Fig. 1. Since this can be done only at a sacrifice of beam diameter at the focus. a compromise mumt be made. The best compromise depends on the nature of the aperture through which the beam is to be aimed. It might. for example, be a.hole in a thin foil. or it might be a capillary tube of great length in comparison with its diameter. In the former case. a very sharply converging beam would be permissible. The intensity of the beam is obtained by measur- ing the current to the Faraday cylinder. The energy of the electrons in the beam is determined from the total fall of’potential through which the electrons move. DESCRIPTION or APPARATUS The electron gun.pr0per was housed in a glass cylinder 3-1/2” inside diameter, l/h” wall thickness. One end was sealed with wax to a 1/2' thick brass plate provided with a glass window. The vacuum connection was made at this end. A brass ring 1/2' thick was waxed to .N mkhmrxiux 33:, «emEcEK .Q MEEcwwV :2:er N. locates 363 .w unaltered. .K routersou Ezzueh .h \mthmQ %U~OB\U.\ .N 330‘ \etm .% «mtmewmxeefii teeth MMBEN .Q kmEEQEU 3.3% .m. \uhtimc mtxweeek Emcee“, .0 55% mt.“ .N $33..th mt.\w3.ee\ maxtk .Q MNQQ Faiths: chukxueta .\ macekwbmFV wuwbwb .Y $38K answer.“ pubexmumtfi J rwl , IJ, _|L_i c IIMT TIL Jmell. a». h. the other end. This ring was provided with a circular land of rec- tangular section which matched a groove on the solid end plate on which the electrodes were mounted. This and plate was clnped to the ring with a rubber gasket between them. The electrical connections were led through the end plate by means of fiber bushings threaded and waxed in. The electron source consisted of a filament coated with an emitting mixture and mounted on a pm: standard taper fitted into a matching brass tapered connection at the back of the solid brass plate. figs. 2 and 3 illustrate the arrangement. The filament power was supplied by batteries and the focusing electrode voltages were obtained from an adjustable rectifier power supply with appropriate potential dividing and voltage dropping resistors. Protective resistors were also placed in series with each electrode in case there should be a tendency to arc. Figure 1I is a schematic drawing of the complete system. The design calculations for this electron gun are given in Appemiix I. The references are given in Appendix II. RESULTS A preliminary test was made using only the object aperture and a limiting aperture between the source and the Faraday cylinder. The pressure in the system was of the order of 10"5 mm. of mercury. but in- creased to 10.3 mm. as the electrodes begin to evolve gas. As the pressure increased an arc discharge set in at voltages of the order of 50 volts on the object aperture electrode. The total beam current at the beginning of the tests was of the order of 5 milliamperes. This current progressively Figure 2. Figure 3a. figure 3b. LIL. 0...: 0.3% so: set \xkkeu. i u _ LOBOK A... Lexxxxeek ii vetex. 2...... 8. FIGURE 1|- - IEGEND A. Object aperture electrode 3. First focusing cylinder 0.. Second focusing cylinder D. Image screen (llucrescent) I. Faraday cylinder 1‘. l‘ilament If. Filament current in amperes. ‘A,B Current to electrodes A.and B in milliamperes. In Current to Iaraday cylinder in microamperes. I"! Potential on A and 3 relative to filament. so Potential on c. D, and r. relative to filament. R1.;32 Protective resistors, 2200 ohms. R3 Protective resistor, O.h7'Meg. Rh. Potential control resistor. 21.000 ohms. 35 Potential divider resistor, 75.000 ohms. 36 Potential divider resistor. 10.000 ohms. a Filament rheostat, 10 ohms. (I) Trap - 002 ( 2) McLeod Gauge (3) Mercury diffusion pump (‘4) CaClz 5171118 tube (5) Gaelz drying tube (6) Mechanical fore pump (7) Electric heater for diffusion pump 9. decreased as the tests progressed. This decrease may have been due to aging of the filament, or it may have been due to a decrease in current arising from the arc discharge as the pump began to remove the evolved gases faster than they were released. No tendency to are was noted at voltages of the order of 1000 volts on electrode C(rig. l) and 120 volts on electrode 3 when the pressure was of the order of 10'5 mm. of mercury. The current to the Faraday cylinder in the absence of an arc discharge was of the order of 5 - 10 micro amperes. This value cannot be taken as the current in the beam proper, however, since the Faraday cylinder was not adequately shielded from stray electrons in these preliminary tests. According to the calculations given in Appendix I for the configuration shown in figures 1 and 5, a beam of electrons ought to be focused at 15 centimeters with the object at n.52 centimeters when the ratio of the voltage on O to that on 3 lies between 6 and 7. Experiment showed, however. that a sharp spot appeared on the fluorescent screen D, Fig. l, with 1000 volts on C and 200 volts on D. This 200 volts on B was not critical, since a variation of :t.lO volts made little difference in the sharpness of the spot. For this configuration, therefore, the voltage ratio necessary to produce a focus at 15 centimeters was 5, not 6 or 7. During these tests considerable difficulty was experienced with gassing of the elements within the tube. Whenever the filament current exceeded 6.5 amperes. the pressure in the system increased from about 10""5 mm. of’mercury to pressures of the order of13 - 8 x 10"h am. .At these pressures a mild arc type discharge occured at voltages of the order of N0 - 50 volts cash and 200‘- #00 volts on C. Table I gives some typical operating voltages and currents for this electron gun. 10. The current to the Faraday cylinder was of the order of 25 microamperes under these conditions an}. could be increased to 100 microamperes by moving a horseshoe magnet close to the glass envelope of the tube. Observation of the blue glow of the are dis- charge showed that the action of the applied magnetic field was to direct the electrons in the discharge down the axis of the tube toward the Iaraday cylinder. his accounts for the increase of current to the Faraday cylinder. TABIIEI Typical operating Voltages and Currents 1&3 ”c IA.B 1, Remarks 6.6" loo" 600' 0.0 0.0 150' 950' 0.1:» 2. rd- 6.?. ”2' “00' 0.3 m. 25, Isa. 6.3a 195' 1000' 0J4... 20.;14 Spot appeared on screen: no noticable glow No spot on screen; marked arc discharge. Sharp spot on screen; marked arc discharge: pressure in system 8 x 10"“ Is. Hg. See Fig. 14 for a diagram of electrical connections. If : filament current. T's.) I 1.3 Potential on electrodes A and 3 relative to the filament c _ Potential on electrode 0, relative to the filament Current to electrodes A and B. In 3 Current to electrode 3 (Faraday cylinder) ll. Observation of the blue glow in the absence of the magnet showed the electrons in the beam emerging from the electron lens to be deflected awey'from the axis of the tube. This observation‘in— dicated the presence of a strong disturbing magnetic field in the neighborhood of the electron gun. In order to check this suspicion. a compass needle was moved about and the direction of the field thus indicated was observed. The compass needle showed the field to be badly distorted. .A check on a steel column about two feet from the apparatus showed this column to be strongly magnetized. The location of the focused spot on the screen was about three centimeters above and about a centimeter to one side of the place it should have been if the beam.lay on the axis of the electrode configuration. This indicated the presence of a.magnetic field of the order of 3 oersteds. when this beam of electrons was directed onto the aperturea in this screen by means of a magnet; no increase in current to the Faraday'cylinder could be observed on the meter used. This meter had a 0-100 microampere scale. the smallest division was 1 microampere. and an increase of about 0.2 microamperes could have been detected. 7 Since the current to the Faraday cylinder decreased as the glow of the arc discharge decresled and became sero when the discharge stepped. it may safely be assumed that part of the current received by the Faraday cylinder was due to ionisation occuring in the arc discharge. CONCLUSION On the basis of the results discussed above. it may be con- cluded that in order to obtain a measurable current to the Faraday cylinder by the means employed here, it will be necessary to increase 12. the emission from the source and to increase the size of the entrance aperture to the lens. These results also indicate that the electrodes ought to be thoroughly degassed and the whole electron gun ought to be well shielded magnetically. The electron lens described here is of the immersion type. so-called because of the analogy with the optical case in which the obJect and image are immersed in media of different indices of re- fraction. as in the oil immersion microscOpe. This is the type used in electron guns for cathode ray tubes and television tubes. APPENDIX I The differential equation of the trajectory of an electron is an axially symmetric electrostatic field as given in Ref. 12. p. so is c 40* L 0,7, ”(5%) 9.2.4.: __ ____.._../+(fi) a! _._.. 0 where r'g distance from axis of symmetry 8 3 distance along axis of symmetry V’g potential distribution as a function of r and s. It should be noted that: (a) e/m does not appear in this equation, hence it applies to any charged particle: (b) it is homo- geneous in V, hence the absolute value of the potentials of the elec- trodes does not influence the trajectory. The relative potentials only need be changed to change the traJectory. (c) It is homogeneous in r and s. hence if the electrode sise is changed by a constant factor through- out. the traJectory is changed by the same amount. This equation is t00'complicated for practical use, however, and a simplification is. therefore. introduced. The electron beam is assumed to be confined to a small region.near to the axis of symmetry, such that only the first terms in the expansions of .9 V0,!) :49 V03?) ’ 3r ’ 92- need be considered. With this simplification the equations of motion 11+. become. for V032) z: /(o,?) = ”(3) $ I" u d12- , ,1 .. .. - V a —— e .( Wit”. , e: 6() ’ rmdf’ 8/2) The energy equation is J % ‘I. Law—- = %“(77) = e we Z and the equation for the trajectory becomes .,, I .. ,, cl r + V. J r :4 07?» 2V, 0.2.. +W/ :— 0 where L, V... and Y.” are the axial distribution of potential and its first and second derivatives with respect to 5. If the potential distribution and its first and second derivatives are known. the trajectory may be determined by means of this equation. If a trajectory is given. the required axial distribution of potential may be determined. The usual procedure is to determine the potential distribution (Ref. 9. 11 and 12) and then using the approximate equation determine the cardinal points of the electron lens for a number of different electrode configurations. Data for the construction of various types of electron lenses are given in Ref. 11 and 12. The locations of.the cardinal points for a typical electrode configuration are shown in Hg. 5. The equations connecting these quantities are given in Ref. 12. p. 108. They are. with a slight change of notation to conform with the notation of Ref. 11. p. ’450. fig. 13.21. . ‘P 2 11+ 21- s = 1:2+ 22. x1% = fife: I a 41/11 where m is thesmgnification and the other quantities are as shown in fig. 5. 15. Calculations for the design of the present electron gun were based on the graph given in Ref. 11. p. 1#50. These calculations are given below. Diameter of electrodes (D) g 1.65 cm. p 3 ~10 cm. V2] 71 g 5 :1/13 3 -1.6 :2/13 . 3.8 21/13 . -2.s 22/1) . 2.5 fl : -2.51+ cm. f2 3 6.27 cm 21 3-“.62 cm. 22 = l$.13 cm. x1 3 -10 + n.62 a -5333 cm. -3.6u) (6.212 g 3.08 c“. -5.38 q I 3.08 + “.13 a 7.21 cm., l a -0.)t7 JP I Let p a 4.62 cm., and 73/ 71: 6. Then fl = “2011‘ cm. ‘2 : 5.1%“ cm. Z1 : “303 22 3 3.3 x1 : -0.82 x2 3 like q : 17.5 m = -2.6 Or. let p g -’4.62 cm., and 72 [V1 3 7. Then :1 - -i.65 :2 : n.95 cm. 21 : ~3.3 22 g 2.8 x1 3 -1.) x2 3 6.27 q : 9.07 m : 4.27 Therefore, if a focus is desired at a distance of 15 cm. from the plane of separation of the electrodes, 72/71 should lie between 6 and 7, while a parallel beam should result if 72/71 is made equal to 5. 16. m Miami continue}. MS»:oo\ t8 $09.»on \txgeex .0203“. m toxxexeocmw \o 2.30. Li «confine? mt.§:oo\ .3.ka N 353ch ewes; m mmucex. Mnex confines motive o\ mum.\QQ .QKi «Rant Eocene mmeEN wx notodx £RCEKR at . .1 «Nate 35 here as exam \BQ w. he .32on \BEEXBU APPENDIX I I A. Ionization of Gases by Electrons 1.) The Passage of Electrons through Small Apertures. J. 1'. Lehmann and T. H. Osgood. Proc. Gamb. Phil. Soc. Vol. XXII, Part 5: July 1925. p. 731 2.) The Tbtal Ionization Due to the Absorption in Air of Slow Cathode Bays. J. 1'. Lehmann and T. H. Osgood. Proc. Roy. Soc. 115 A (1927) p. 609. 3.) Uber die Ionisierung von Luft durch Kathodenstrahlen von 10 «- 60 av. Anton 11.1. Ann. der smut. 5 Folge, Band. 3. p. 277 (1929). 1+.) The Variation of Ionisation with Velocity for the p -Particles, 1:. Wilson. Proc Roy. Soc. Ser A, 85, 1). 21m (1911) 5.) The Variation of Ionizing Power with the Velocity of Cathode Rays. J. L. Glasson. Phil. Mag. 22. p. 6117 (1911) 3. Electron Optics and The Design of Electron Lenses. 6.) Ilektronen - Ubermikroskopie, Manfred von Ardenne. Julius Springer, Berlin. 191+O. 7.) Bietrag sur Geometrischen ElectronenOptik. M. Knoll u. 1. Ruska. Ann. 4. Phys. 5 l‘olge, Band 12' (1932) Heft 5. s. 607. 8. Geometrischen Electronenoptik. 3. Bruche u. 0. Scherser. Julius Springer, Berlin. 193,4. 9. Electron Optics-Theroetical and Practical. L. M. Myers. D. Van Nostrand 00,, New York. 1939. 10. The Fundamental Equations of Electron Motion, L. A. HacCan. Dell Syst. Tech. Jour. Vol. XXII, July 1918, p. 153. 11. llectron Optics and the Electron Microscope. V. I. Zworykin, G. A. Morton. I. G. Bamberg, J. Hillier, and I. V. Vance. HoGraw- Hill, New York, 19145. 18. 12.) Elect ron Optics in Television. I G Haloff d D . . an . W. Epstein McGraw-Hill, New York, 1938. ., .1 ill? Eli’s. E'u‘ .. MICHIGAN STATE UNIV. LIBRQRIE llIIIHLIIIIIIIIIHLIl||l|l|||1llllllHllllllllllllllmllllllHI 017749809