. . . . , ?'. A . L.. :. T. TOFI ORNL P 2669 . I 11 * 11, + . 1 . : - V w EEEEEEEE MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS – 1963 م 0 / 7 را ع ها ما CONF 661016-22² H.C. $ 7.00; 27,50 . CW MICROWAVE VSTEMS FOR ELECTRON-CYCLOTRON HEATING EXPERIMENTS H. 0. Eason, Jr. Oak Ridge National laboratory Oak Ridge, Tennessee RELEASED FOR ANNOUNCEMENT |' IN NUCLEAR SCIENCE ABSTRACTS . Over the past several years experiments have been under way at ORNL which involve the formation and heating of plasma in a non-uniform magnetic field by means of the application of power at the cyclotron resonance fre- quency for electrons. This frequency is directly proportional to magnetic field strength and equals approximately 2.83 megacycles per gauss. For mag- netic field strengths which are of interest in CTR, i.e., several kilogauss, the frequencies required therefore fall within the microwave and millimeter wave portions of the frequency spectrum. Since the ORNL approach to CTR has historically been on a dc basis, and due to the potential application of the technique to other plasma devices, all of the microwave power sources previ- ously used or now contemplated are CW (continuous-wave) devices. A typical experimental arrangement is shown in Fig. 1.9 This arrange- ment requires the generation and transmission of several tens of kilowatts of CW microijave power at the highest available frequency (since capabilities for producing magnetic field almost always exceed the frequency capabilities of suitable available tubes). The power is coupled into a cylindrical low-loss reflecting chamber which is commonly called a cavity although all of its di- mensions are multiple wavelengths and it has no discrete resonances. This cavity is placed within an evacuated chamber and is symmetrically located with respect to a non-uniform confining field - in this case a simple magnetic Santa - - 1 .. .. M . *Research sponsored by the U. S. Atomic Energy Cormission under contract with the Union Carbide Corporation. . Pt2 20 4, WOW " ! :1 -2 .5 . : - martinamente p rendere mirror. The coil currents are adjusted such that the proper constant-B con- .. tour for electron-cyclotron resonance, as shown by the dotted lines, exists within the cavity. Gas, usually deuterium, is bled in at the proper rate to increase the pressure to the typical operating region of 1 x 20° torr. The microwave power ionizes the gas to form a plasina which is trapped by the de magnetic field and simultaneously heated by means. of cyclotron resonance ab- sorption of the microwave energy by the electrons. Many of these electrons reach energies of several hundred keV and thus produce large quantities of .::. high energy X-rays upon striking the walls of the chamber. This large x-ray f.lux and the associated shielding usually requires that the microwave power source be operated from a remote location since transmission losses are severe at the higher microwave frequencies. This requirement for remote operation therefore implies some combination of electrical and mechanical accomplishment • of all control and munitoring operations associated with the microwave power source. .:.:.:..:.: The CW microwave tubes presently available which have potential applica-....... tion in this program, i.e., CW power outputs of 1 kilowatt cr greater, fall into two general categories - the klystron amplifiers, which are available at frequencies ranging up to 18 Gc., and the traveling-wave tubes employing coupled-cavity interaction structures which are available at the higher fre- quencies. Although oscillator tubes which produce large CW outputs are avail. ; able in some frequency ranges, and, equipment-wise, should be simpler to use,. it has been found that amplifiers are much to be preferred, in spite of the added complexity of the driver, because of their improved power output char-"; acteristics with a load mismatch and the ease of control of the output by variation of the few milliwatts of drive power as compared to control of the oscillator output by variation of the beam supply. - 3 Virtually all microwave tubes have the common requirement of a negative dc power supp].y for the electron beam, since the output waveguide is electri- cally connected to the body of the tube and must be operated at ground poten- tial. In addition, one or more floating supplies for the heater and auxiliary electrodes are normally required. The ac to rt conversion efficienty of a typical klystron amplifier is in the range of 25% to 30%, while the efficiency of a traveling-wave tube in the millimeter-wave region may be as low as 7% to 10%. This efficiency may sometimes be improved by the use of collector depres- sion techniques, although at the expense of increased power supply complexity due to the requirement for an additional floating power supply. For operation of any high-power CW tube, however, large dc power supplies are required which have the capacity to destroy the delicate interaction structure in the tube in VA :::::::::.... ...... ..: :::...: . :. .. . .. ... ... ....:,.,. ::.... the event the electron team is defncussed or deflected so as to intercept the :.. structure and cause excessive "body current." For this reason high-speed thy- ratron or spark-gap crowbars are normally connected across the output of the .. beam supply and sictuated automatically by a body-current signal to discharge......... the filter when a fault occurs and to divert the output of the supply until the primary power is removed.? A typical specification for crowbar operation limits the energy dissipated in a short-circuit to a maximum of five joules with operation within five micrcseconds maximum after receipt of a signal. :..!.......... ... ... : . :.: . ; . . . : :::.:. :.::::: 8. ::: !.:::......... .: As noted above, the tubes must generally be operated as closely as possi- Mr. A 1 .. "...; b le to the plasma. device in order to minimize :waveguide losses. An additional consideration therefore is the fringing magnetic field surrounding such a de- vice and its effect in defocussing or deflecting the electron beam within the ** tube, causing excessive interception by the structure. While most tubes are inherently rather well shielded from weak exterial fields by their own bulky magnetic focussing structures, it is obvious that difficulties must eventually . .... . . ... .... . . . .. .. ..... .... ..... ... . . :.:. : . .. .. . . . . ..... . . -- - ..4 . . : be encountered as the interfering external field is increased. Tube manu- facturers are somewhat reluctant to specify an environmental limit due to their lack of facilities for testing the effect of such an influence. How- ever, several types of tubes have been operated at ORNL in a 20-gauss fring- ing field with no observable effects. Recently onė X-band klystron amplifier was operated in a field up to 50 gauss perpendicular to the axis before ex- -hibiting an increase in body current. Most high-power CW tubes require water cooling, output waveguide pres- surization, and electromagnétic focussing of the electron beam. Applications recommendations of a general nature as well as specific applications require- ments and ratings for the tube type involved are available from the tube manu- facturer. It is desirable, in the interests of reliability and simplicity of operation, to insure that these ratings anä requirements are met by the use of **** tail-safe permissive interlocks wherever possible. Filtered demineralized water is desirable for cooling, and parallel cooling circuits should have in- --dividual flow-switch protection for reliability. Pressure-svitch interlocks are desirable on the output waveguides to not only insure that the guide is: pressurized but also to shut down the system in the event that a flange joint is inadvertently disconnected or the guide is damaged. It is also desirable to interlock the operation of the system in a sequential manner in order to reduce the possibility of operator error and provide maximum protection to the equipment. This also permits safe operation of the equipinent by. persons having ...... less training or familiarity with the equipment. Perhaps the part of a high-power CW tube which is most delicate is the out- put waveguide vacuum window. This part is highly susceptible to damage from waveguide arcs which occur at power levels exceeding 2 kilowatts at X-band and at correspondingly lower power levels at higher frequencies. These windows are :: . . Isi :::.. .. ::::::.. ..:..,.;,..:: .N ET :. . 1 • 0 . . -------... ........................ typically circular slabs of aluminum oxide or beryllium oxide ceramic or half-wavelength thickness brazed into a short cylindrical cavity which is connected to the output waveguide, Beryllium oxiảe is a superior material for most applications but has not been in comnon use due to metallizing difficulties and to the physiological hazards associated with beryllium and its compounds. Since the nature of a waveguiae arc is to propagate toward the power source, such an arc can destroy the output waveguide window even though it is initiated elsewhere in the system. · Protection from waveguide arcs is accomplished by senşiris the presence of the arc with a reverse-power directional coupler or an optical sensor and hoe . .:: using the signal thus derived to remove the drive power or crowbar the beam supply within a few microseconds. Figure 2 illustrates window failures due :: iros;..:.:*:;:::::. ..:::.';::: : ::: : :: : ::.- 7.7... ... . ....... to arc damage. These windows did not come from tubes but instead were used : : for entry of the waveguide into the vacuum system of the plasma experiment. The failure example in Fig. 2 characterized by a crack parallel to the narrow dimension of the waveguide is more typical, although in an extreme case the **** .:: arc may melt the ceramic material of the window as illustrated by the other example. The disastrous effects of such failures upon tubes or vacuum systems are obvious in ........ .... . The question of arc protection leads naturally to the question of arc. .. prevention in the waveguide system which connects the tube to the plasma load. Since the tubes are generally "state-of-the-art" devices which have not been : ... widely applied to other areas, there are usually very few commercially avail- able components which are tailored to the high-power application. Flange- mounted waveguide windows for entry into the plasma device similar to those shown in Fig. 2 are usually available from the tube manufacturer. Ferrite circulators or isolators are available at the lower frequencies of interest ***Popus . m***** ***** *** *** ...:.:. ::. .. . .. .. . .. ... ..::::::: : : . -6-• . , and are very desirable for isolation of the tube from the effects of load mismatch whish can alter the power output, damage the waveguide windov, ox initiate waveguide arcs. Water cooling is desirable for all waveguide sec- tions and components due to the rf losses. In general, dielectric materials are to be avoided, and the components should have a low VSWR, low losses, and be designed such that microwave electric fields within the component are mini- mized. This requirement implies the use of sidewall types of dirėctional cou- plers and hybrid junctions and the avoidance, wherever possible, of short- radius "E" benäs and capacitive matching structures. In the X-band range, where the waveguide size is such that accurate bending is difficult, extensive use has been made at ORNL of cast tends and other components as illustrated in Fig. 3. These castings are made of a beryllium copper alloy and are essem- bled in combination with straight waveguide sections by brazing. This permits. zapid fabrication of rather complex waveguide sections in a very compact manner because of the elimination of a large number of flange joints which are often t troublesome and produce a source of arcing due to imperfect contact. Standard 1 1 1 treatment for high-power flanges consists of hand-lapping on a flat surface with the final lapping strokes being made parallel to the wide dimension of the waveguide. This treatment produces a slightly convex surface which helps in- . sure contact around the periphery of the opening. Also shown in Fig. 3 is an ... example of a high-power water-coo. Led sidewall directional coupler which was *... *. fabricated using cast bends to reduce the insertion length. The straight arm is used in't anpower section - พน VE and has an insertion length 11 M ....of only 4-1/4 incires. The coupler has six coupling holes in a binomial array. and; when used with a standard test-benchi termination on the side arm, has a directivity of greater than 30 db over a 200-megacycle bandwidth centered at 10.6 Gc. inis directivity illustrates the feasibility of fabricating low-VSWR VU 20 ..... .. .. : the ..-......... ..... ... . . ..... .... ..... . . . .... .- ... .. components and waveguide sections using such castings, since no unusual ma- chining or fabrication techniques were used in the construction. These cou- plers have been built and used in high-power CW systems with coupling values ranging from 30 to 60 db. Figure 4 is a photograph of a cavity which has been used with a 2-kilowatt input at 35 Gc. In order to minimize rf losses, the material used for cavity construction has been copper in most cases, although copper-plated stainl.ess steel and aluminum have both been used on occasion. The cavity must be water- cooled since it must eventually absorb the total, microwave input power either directly or by way of charged particles striking the walls. Extensive use of perforated copper materials and waveguides-beyond-cutoff has been made in orier to permit optical viewing and connection to the vacuum pumping system. The interior surfaces are made smooth and iree from points and projections which tend to produce high electric field gradients. Tight metallic joints are used in the assembly in order to prevent arcs and to minimize leakage of. microwave energy. Leakage is especially important because of its interference with low-level diagi.ostic instrumentation. Since the leakage power is usually "..;,.., ..;'.:.:. :.:......::::......::::.:.:.:.:.:.:..:.:.:.:.:.:.:. :.:.: . : ... modulated by variations in the plasma, its effect is difficult to isolate if rectification occurs, for example, in an obscure junction in the input stages of a low-level amplifier or other dc device. The potential personnel hazard must also be considered, since the accepted. physiological tolerance level for **** microwaves is only 10 milliwatts per cm"Diagnostic probes which must be :, movable are inserted into the cavity either through a metallic bellows or a ::....::::: close-fitting coaxial filter consisting of several alternating Jow- and high- i . impedance sections of quarter-wavelength transmission line. This area deserves special design attention since a loose-fitting entry of a probe through the cavity wall may result not only in arcing but in coupling large amounts of ***** power out of the cavity,%., 3.. . 0 PT . T..... .... . . ... -. -- ** ...... h ... ' '!'"' T T1 Wi.. . - - .. . - ' ' I '. 1. . - . - ... . .. -17 N A . M : : ...... 8. : :: .... ....... . . Besides the problems of generation and transmission of large amounts of microwave power, there remains the problem of coupling this lower efficiently into a plasma which presents a highly variable loač impedance as the paren- eters of the plasma experiment are varied, e.g., the magnet coll currents, mirror ratio, gas feed rates, pressure, or the introduction of obstacles with- in the plasma chamber. Early attempts at matching the microwave source to the plasma using conventional tuners were unsatisfactory due to tie unreliability of sliding short-circuits at high power and the time involved in their mani- pulation. A more satisfactory method of matching the microwave source to the plasma uses sidewall hybrid junctions as shown in Fig. 5. This component was assembled from castings nf the type shown in Fig. 3. The component is well adapted to high-power use since its construction tends to minimize inter- nal electric field gradients. Its operation is similar to the familiar magic-T 9. in that when power is fed into port 1, it divides equally between ports 2 and 3 with excellent isolation to the fourth port. The important difference is that à 900 phase difference exists between the voltages at terminals equidistant was from ports 2 and 3. If ports 2 and 3 are terminated at equidistant terminals in equal impedances the reflections then combine to exit at port 4. This com- . ponent may therefore be used to isolate a power source from effects of load :::...mismatch so long as the two load impedances can be maintained equal in both magnitude and phase. This requirement is met in the plas.na-feed application by connecting both output ports" to the cavity, making the arms of equal length, and locating the open ended waveguide entries to the cavity synmetrically with ...::::: . ... ... ...... ...... .......: ::: :..................:.:.: .::.. ..... .. respect to the walls and the magnetic field. This tends to make the impedances equal for both static and dynamic conditions. Adequate impedance matching and increased efficiency have been oitained by short-circuiting the fourth port of 1 64 .in + the hybrid to re-direct the reflected power back into the cavity, although better matching coull be achieved, if necessery, by terminating the fourth port with a load. This type of coupling has been employed successiuily at X-band fre- quencies and at 35 Gc.5 The sidewall hybrid junctions and the method of locating and connecting them to the cavity are illustrated in Fig. 4. The typical reflected power observed in an X-band system operating at the 19- kilowatt CW level is approximately 100 watts. The maximum power return ob- served in the output waveguide with three of these tubes feeding a single cavity has been 600 watts, of which approximately 200 watts vas due to mutual coupling through the cavity ficom the other tubes. Oversize waveguide feeds . :;;: ........ .. employing "horn-type" tapers have been used with fair success at 35. Ge, and this type of feed will be used in a 5-kilowatt, 55 Gc. system now being designed. No difficuities have been encountered due to the operation of up to three X-band klystron amplifiers connected separately to a single cavi, without first combining the outputs into a single waveguide. The isolation provided by ferrite devices prevented, of course, any serious interaction bet:een the tubes. Serious interaction effects were encountered in the oper- ... ation of two 35 Gc. oscillators in this manner, however, since high-power ferrite devices were not available. It is felt that the basic limitation in 3.*** this use of multiple sources lay with the oscillators themselves instead of ..... .. ..the lack of isolation, since the feedback for the tubes used was derived : largely from load reflections and since there was obviously some cross-- coupling of the outputs through the cavity. Operation of multiple amplifier tubes without individual isolators should be much improved, especially if driven by a common phase-coherent source with phasing controls available at ,Freita .EX .7 . 2* 4. :. :.*...... 4 .... .. .. . . 11AM . . 1 ' UL T IMA 11. 7 . mu - , ITS A - TTXV ... . - 10 - 3 It has been found that one very critical factor in coupling microwave power into a plasma is the location of the waveguide vacuum window at the plasma device with respect to the magnetic confining field so as to insure that plasma following field lines does not strike the window or cause a seri- ous pressure rise in the evacuated waveguide. One must also insure that the evacuated waveguide does not pass through an electron-cyclotron resonance field. For this reason, as illustrated in Fig. 4, the vaveguide entry into a mirror device is always located in the central region where the field is lower than the resonance field, rather than at the cavity ends. A satisfactory window location is less obvious for other field configura- tions as shown by the example in Fig. 6, which illustrates the shape of the flux lines and constant-B contours in the 'EIMO "folded-cușp" field.* The con- tours of constant magnetic strength, as shown by the 'dotted lines, are closed .., surfaces which do not intersect the walls. The desired contour for electron- cyclotron resonance, normalized to 1.0, is everywhere enclosed by stronger ... fields, which reach a maximum value in excess of 1.5 times Bresonance as one proceeds outward in the r or 2 direction. Obviously, then, as one proceeds still further in any direction, there exists another region of cyclotron reso- nance, no matter which direction is choser. The zones marked "x" are "forbid- den regiuns" for window location, si'ice plasma following flux lines from the - - - - - - central region would strike the window or cause an undesirable pressure increase if these zones were used. Since it is desired to couple to the plasma oniy in the inner resonance zone, the cavity walls must be located at or near the field 1...,:::. . : : : : '.'. '....... . ....' ;......... ......... máximum: - The waveguide vacui window must then also be placed near the field maximum, as indicated, to prevent cyclotron resonance absorption inside the evacuated waveguide with the resulting gas discharge, power loss, vnd mismatch. Similar considerations apply to the location oã the waveguide entry and vacuum window in other field configurations. . *. 1 . : ! one.. ." It should be mentioned that some difficulty has been encountered with breakdown in long lengths of evacuated waveguide used with one of the devices even though no region of cyclotron resonance existed within the guide. These difficulties have been traced to a pressure lise inside the waveguide due to outgassing, lack of cleanliness, and low pumping speed. Obviously the pru- vision of some pressure relief along the waveguide to improve pumping, and the use of a bakable vacuum system should overcome these difficulties. Several high-power CW microwave systems have been assembled and used for electron-cyclotron resonance heating experiments. at ORNL, including those with capability of various power levels up to 50 kilowatts CW at frequencies around 10.6 Gc., two systems at lower frequencies, and a system producing 2 kilowatts CW at 35 Gc. CW power sources presently in the design stage include one with a 30-kilowatt output from two klystron erplifiers at 18 Ge, and one with a 5-kilowatt output from a TWT amplifier chain operating at 55 Go. LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, por the Commission, nor any person acting on behalf of the Commission: A. Mikes any warranty or representation, expressed or implied, with respect to the accu- racy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumer any llabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed la 18 report. As used in the above, "per:30n acting on behalf of the Commission" includes any em- ployee or contractor of the Coromission, or ex.ployee of such contractor, to the extent that: such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any Information pursuant to his employment or contract with the Commission, or his employment with such contractor. . tor: ------- 29t LWA . . .. . :::::: . - 12 :::::... ..... ... REFERENCES 1. Thermonuclear Div. Semiannual Progress RepoziiJanuary 31, 1961, ORNL 3104, Section 3.1. 2. Thermonuclear Div. Semiannual Progress Report. October 31, 1962, ORNL 3392, Section 3.1. 3. Thermonuclear Div. Semiannual Progress Report. October 31, 1963, ORNL 3564, Section 3. 4. Thermonuclear Div. Semiannual Progress Report: October 31, 1964, ORNL 3760, Section 3. 5. Thermonuclear Div. Semiannual Progress. Report. October 31, 1965, ORNL 3908, Section 3. emiannual Progress Report. April 3 1 ORNL 3989, Section 3. . 7. Morris, A. J., and Swanson, J. P., "The High-Speed Protection of i Microwave Tubes and Systems," The Microwave Journal, Vol. 5, No. 11, pp 78-85, Nov. 1962. 8. Nelson, B. E., Introduction to Klystron Amplifiers, AEB #19, April 1963, Varian Associates, Palo Alto, Calif. 9, Beust, W., and Ford, W. L., "Arcing in CW Transmitters," The Microwave 3......0, Journal, Vol. 4, No. 10, pp 91-95, October 1961; : .....: :: ::::::: : 10. Southworth, G. C., Principles and Applications of Waveguide Transmission, .:: Do 346-352, D. Van Nostrand, 1950..:** ******** * ..........11. Henspergez, E. $.. "The Design of Multi-Hole Coupling Arrays," The Micro- .::wave Journal, Vol. 2, No. 8, pp 38-42, August 1959. 12. Riblet, H. J., "The Short Slot Hybrid Junction," Proc. Inst. Radio Engrs., Vol. 40, pp 180-184 (1952). ..| FIGURE CAPTIONS Figure 1 Schematic Diagram of the EPA Experiment Figure 2 Photograph of Waveguide Vacuum Windows Which Have Failed Due to Arc Damage Figure 3 Photograph of Cast Waveguide Components and a Sidewall Direc- tional Coupler Employing Cast Bends . ... . .... ...... . Figure 4 Photograph of a Cavity Used with a 2-Kilowatt CW Input at 35 Gc. Figure 5 Photograph of a Sidewall Hybrid Junction Used for Coupling to the Plasma Chamber Figure 6 Half-Section Diagram of the ELMO Folded-Cusp Fiela Configuration. : .... .... This drawing illustrates the croice of a satisfactory region for waveguide vacuum window location. The regions marked "x" are unsatisfactory due to the effects of plasma flow along magnetic ::............ flux lines........... .............. ...... ........ ......!:. liv 11 :. : .·,., i v.,,., . .. ty .. . A ETTER .. ORNL DWG 63-7563 .........:::::. CAVITY VACUUM TANK CONSTANT B CONTOUR WAVEGUIDE INPUTS 50Kw CW-10.6 GC :-D GAS INLET . YU K A ivind remman ... be ..::::..... PLASMA PUMPING · TUBES ......... INCIU TUBIITUNUMI ... . PLASMA TARGETS ...:::: : 01" MAGNETIC FLUX LINES . :::::::.... : ... TO S: TO ... VACUUM VACUUM PUMPS PUMPS VACUUM BARRIER . . TO VACUUM PUMPS MIRROR: COIL : . U .. : ....... Fig. 1. Schematic Diagram of the EPA Experiment . LA . : : ، ماء لجنة 2' لم ا . ا بعد . دل ، ده سال ا ي : 1 : ،،، ... - ۱۱٫۰ . " , .... . . ... ... ... . . . . . ؟ اک ... . . لا :- . ! 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LA . ! . 1 1 * - . - . . . WWW ::: :: . 27 . + .. . 7 . M 4 ANS 1 . X . . A 1 . . . IN . . . . . TANS .. . I!. - . . . wr . W N . ht . . . "* si ... .. . FLAT FIEL FLAT FIELD . : . ...::: ": 3::.... - SATISFACTORY LOCATION : FOR WAVEGUIDE : VACUUM WINDOW .. . . . .is . : . :::: ....:::: M $ 1.56 . acuum Pump Vacuum Pump L MIRR MIRROR COIL ---L2:52 . wwwww: ww IN 100MM . 0 Vacuum Vacuum Pump Pump Magnetic flux : lines iii i :1 i Ilo-Surfaces of constant 1 W , magnetic field 20" 6" 4" 2 ? 4" 6" 8" 10 _ 6. Hall-Section Diagram of the ELMO Folded-Cusp Field Configuration. This drawing illustrates the choice of a satisfactory region for waveguide Vacuum window location. The regions marked "x" are unsatisfactory due to the effects of plasma flow along magnetic flux lines. . 1 0 - . : ......... S S * +- . ..- . M . END DATE FILMED 12/ 21 / 66 . TEL . 2 2 ". .. .. ..... - .'. " . 49 7. 1.1 3. . : L. $ 24