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Grid-Controlled Power Tubes in Particle-Accelerator Applications

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Grid-Controlled Power Tubes in Particle-Accelerator Applications © 1965 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. 76 GRID-CONTROLLED P&'ER TDBES IN PARTICI&ACCEIERA?~ APPLICATIONS PT. V. Hoover Radio Corporation of America Electronic Components and Devices Lancaster, Pennsylvania Summary Shielded-Grid Triodes Tvpical illustrative examples are cited The shielded-grid triode, shown in Fig. 1, for the use of grid-controlled power tubes has been employed in particle-accelerator in the various classes of particle acceler- service for more than a decade, dating from ators, e.p., cyclotrons, synchrocyclotrons, the beginnings of the Livermore LINACS (e.g., proton synchrotrons, electron synchrotrons, A-La). The original prototype sample of a and linear accelerators, A state-of-the- shielded-grid triode has provided the rf art review of triode and tetrcde power-tube power to the Derkeley Bevatron2 for the past technology is nresented, together with some eleven years and has operated for more than anparent trends in the design and develop- bh,OOO hours. ment of new tubes. The use of triodes and Shielded-grid triodes, described earlier tetrodes with integral cavity-resonators in the literaturej, are capable of producing enhances the feasibility of their use at continuous power (cw) up to 700 kilowatts and increasingly higher freauencies and/or power pulsed peak power up to 2 megawatts. The levels. In addition to their use as radio- single-ended version of the shielded-grid freouency generators in <article acceler- triode has an upper frequency capability of ators, grid-controlled power tubes are also about 100 megacycles; the use of a double- applicable as pulse modulators and amplitude ended envelope configuration on the tube regulators. The unique advantages of gridded permits operation at freouencies somewhat power tubes for particle accelerators are above 200 megacycles. At the lower frequen- discussed. cies, where input-circuit losses are negli- gible, the shielded-grid triode can provide Introduction 30 db of power gain. In the 69h9 triode shown in Fig. 1, 2h Developments in particle accelerators- individual shielded-grid triode units are have been inexorably contingent upon arranged cylindrically and, together with the developnents in grid-controlled power tubes anode as the outside cylinder, are housed in from the first model of the cyclotron to the a ceramic envelope. The radial-compression giant synchrotrons of today. Since the out- ceramic vacuum seals used in the tube reore- look is bright for continuing progress in sent a significant development in vacuum-tube the development of grid-controlled power envelope design. An unusually strong tubes, particle-accelerator scientists can aluminum-oxide ceramic cylinder is the in- anticipate continuing contributions to the sulating envelope. The mechanical compressive success of new generations in machine de- force necessary to achieve radial compression sign. is obtained from hoop tension in the strong Experience with grid-controlled tubes copper-plated metal band into which the at very-high and ultra-high freauencies ceramic insulator is forced. The resulting has demonstrated that electron transit-time radial compressive force is localized in a effects can be obviated by appropriate tube narrow region so that the stress is suf- and circuit design. Further progress in the ficient to cause plastic flow of the metal development of &d-controlled tubes is and effect a vacuum-tight seal. Radio- being demonstrated by the continuing de- frequency displacement currents flow through velopment of devices in which the electron- the seals without encountering intermediate control systems and circuit elements are high-resistance oxides or other semiconductor integrated within a common vacuum envelope. layers found in brazed types of ceramic-to- Compact high-gain "totem-pole" amplifiers metal vacuum seals. Dielectric losses in the are being developed as driving stages for aluminum-oxide insulators are very low, even super-power triodes and tetrodes. The de- at ultra-high frequencies. velopment of high-speed fault-protection The Berkeley 88 inch Cyclotronh and the systems has been of ke,ystone importance in Livermore 90 inch Cyclotron have been oper- the operation of power tubes at increasingly ating with shielded- grid triodes for a number higher power levels.1 of years. More recently, the tube has been 1965 HOOVER: GRID-CWTROLLEDFOWER TUBES IN PARTICLE-A~~ELERAT~RAPPLICA'M~NS 77 providing rf power for the Oak Ridge tube type is intended for operation at high Isochronou cyc1otrors. The Berkeley and peak pulse power and more modest average power, Yale HILAC F (Heavy Ion Linear Accelerator) the tube is equipped with filament strands machines are powered at 70 megacycles by five containing a matrix-oxide emitter; the filament shielded-grid triodes operatin& at pulse du- heating power is then only 2.7 kilowatts (1.5 rations of 2.5 milliseconds. Four shielded- volts at 1800 amperes). prid triodes have operated for more than Fig. L shows a cross-sectional sketch of 20,000 hours in HILAC machines and one tube triode units for use in uhf tubes. Integral has operated for nearly 30,000 hours. Pulsed wire-support fins extend from the control-grid rf power for the Los Alamos PHERMEX7electron cylinder on both sides of the directly-heated accelerator has been supplied at 50 megacycles filament strand. Active 0.0033-inch-diameter by six shielded-grid triodes. The PHERMEX tungsten wires are wound laterally in grooves machine has been in operation for two years across the fins in a circumferential direction without a failure in the power-amplifier tubes. on the outside diameter of the grid cylinder at a ptch of 72 turns per inch; the wires are then Super-Power UHF Triodes covered with copper by a firm rolling operation. This construction provides a short thermal- The 205& super-power triode shown in conduction path for the flow of grid heat to Fig. 2 is believed to be the most powerful the fluid-cooled, copoer grid-cylinder, and triode in the world. It is capable of effectively overcorrr,s the problems of excessive producing at least 10 megawatts of pulse power grid temperature sometimes encountered in and can operate at frequencies up to 600 mega- triodes of classical design. The possibilities cycles. It has generated 450 kilowatts of of primary grid emission are thus minimized. continuous power at hs0 megacycles, and is Grid-wire-to-filament-strand spacing is 0.015 probably capable of considerably higher power inch. Thorough fluid cooling and individual at uhf - possibly in excess of 1000 kilowatts. pantographic suspension of the filament strands At lower freouencies, technical projections provide'a thermally stable structure. Grid- indicate that this tube should be capable of anode spacing is 0.29 inch. Intense-cooling producing about 2300 kilowatts of continuous technioues are applied to the copper anode power output. When operated as a grounded- structure for the purpose of providing high grid (cathode-driven) uhf amplifier8, this anode-dissipation capability. triode has a power gain of 13 db and a plate- The first application of the uhf super- conversion efficiency of 50 per cent. power triode in particle accelerators has been k shown in Fig. 2, the tube has a in operation for several years at Argonne double ended-configuration3, i.e., the vacuum National Laboratories. A single triode pro- envelope is desiened so that the tube is vides all of the rf power necessary or the positioned at the middle of a half-wave cir- operation of a SO-Nev LINAC Injector $ to the cuit when it is operating in the fundamental- Argonne Zero-Gradient Synchrotron (ZGS). The order circuit mode. The anode electrode at krgonne radio-freouency system shown in Fig. 5 +,he center of the tube is water-cooled by a was built by Continental Electronics. The flow of 150 eallons per minute. Ceramic cylindrical vessel in the foreground of Fie. 5 bushings on either side of the anode insulate contains the super-power triode and associated it from the grid-flange terminals at each end radio-freouency circuitry, together with of the tube (two grid-terminal connections coupling circuits to a rectangular waveguide are necessary for double-ended circuitry). which conducts the power to the LINAC tank The ceranic-to-metal vacuum-seals are of the visible in the background of the photograph. radial-compression type described previously. This eauipment operates at a frequency of 200 This giant t.riode contains a total of 90 megacycles with SOO-microsecond pulses, and injividual units combined in a common vacuum has been tested at peak power output in excess envelope. Fig. 3 shows the grid-cathode of 5 megawatts. The initial sample of the mount assembly with its plurality of in- super-vower triode has operated in the Argonne dividual triodes arrayed on a 6.6-inch-diame- system: for more than 5500 hours over a period ter cylinder; the active-region length of of four years. Circuitry to operate a super- each triode is IL inches. Cathode construction power tr'iode is currently being procured by is of the individual directly-heated fila- Brookhaven National Laboratories for use in mentary type. Two different types of emitters conjunction with the LINK Injector to the are ootional, depending on the specific servile Alternating-Gradient Synchrotron (AGS). in which a particular tube type is to be A single super-power uhf triode is also employed. Thoriated-tungsten filament strands employed to supply radio-frequency power are utilized in tube types intended for the Cambridge Electron Accelerator (CEA) 18" services reauirinfr high average power or long- at Karvard&IT.
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