Proceedings of the 1981 Linear Accelerator Conference, Santa Fe, New Mexico, USA

RF SOURCES FOR PARTICLE ACCELERATORS - A PROGRESS REPORT M.B. Shrader and D.H . Preist Varian, EIMAC Division, San Carlos, CA

Introduction The continuing need for efficient high-power RF sources for the particle accelerator and fusion reactor programs being sponsored worldwide has been and still is a strong influence on the R&D programs at EIMAC Varian. In the recent past, for example, in a cooperative program with LASL1, the EIMAC X2170 has generated over 1 MW CW power at 80 MHz. This paper will describe very briefly two EIMAC sponsored developmental programs that attempt to go beyond current practice. One is a modular radial strip beam tube with an objective of several MW of CW power at frequencies up to 100 MHz. The basic advantages of a modular design will be dis­ cussed, and examples of modular tubes for low power and for fusion will be shown. The other is a e ~CM gridded density modulated device which should be o 1 2 IN. capable of a MW of CW power at 100 MHz, falling off to perhaps 100 KW CW at 1000 MHz, thereby bridging the gap between 100 and 300 MHz which has so far not been completely filled at high power levels by FIGURE 1 either conventional tubes or klystrons. Develo pmental X2224 Modular Tetrode Multi-Megawatt Modular Tetrode Program key parameter Gm /A , or transconductance per unit area is plotted against overall tube size. Existing Figure 1 is a photo of the developmental X2224 conventional tubes show transconductance per unit modular tetrode with the anode removed. Eighteen area varying inversely with size . The X2224 modu­ modules, each consisting of a cathode, control grid lar tetrode is seen to be superior, and a larger and screen grid, are mounted on a water-cooled stem. version would have the same transconductance per The cathode is directly heated Thoriated Tungsten unit area . As future accelerators demand larger in strip form. The grids are made of pyrolytic amounts of RF power, for example, several MW CW, graphite. The advantages of this type of construc­ from one tube, the modular approach would clearly tion for large power tubes is t hat the interelectrode be preferred. spacings which determine the performance can be made much small er t han in a tube of conventional design. This results from the fact that the desired close CONVENTIONAL TETRODES CO MPARED spacings between active elements can be determined by suitable precision insulators located close to the active electrodes. Connections to a relatively non-precision stem are made with flexible straps. In conventional tubes using monolithic grids and cathodes made of thin wire meshes or cages the interelectrode spacings must be increased in propor­ tion to the overall size, or the variations in spacings over the structure will be unacceptable largely due to the fact that the interelectrode spacings are dependent upon the entire tube mount structure. Local overheating of grids and anode will inevitably occur . To prevent this the spacings must be increased and the performance especially the power gain of an RF amplifier will be reduced . On the other hand, a modular tetrode can be made indefinitely large without requiring an increase in FIGU RE 2 spacings by simply addin g more modules. This situ­ ation is pictured graphically in Figure 2 where the In addition to the superior electrical perfor­ mance there will be a corresponding decrease in the mechanical comple xity of manufacturing such a tube. With a relatively large number of standard modules required for a given manufacturing run, one can lFazio, Hoffert, Patton, and Sutherland, "A l-MEGA ­ afford to put extra effort into tooling the rela­ WATT CW RF POWER SOURCE FOR 80 MHz," Particle tively'small parts. This enhances further the Accelerator Conference, March 1981.

326 Proceedings of the 1981 Linear Accelerator Conference, Santa Fe, New Mexico, USA preclslon obtained and will reduce the cost of Figure 5 shows the tube in its present form, manufacturing large power tubes compared to merely using ceramic-metal construction, with the circuitry extending present technology. attached. This tube is presently under test at 18 K\~ CW output at about 800 MHz with 30 KW CW as At present the EIMAC X2224 is entering the test :he objective. Other parameters of interest are phase with the objective of about 1 MW CW output. set out in the Table below. The physical size of this tube is approximately the same as our new 300 KW Pyrolytic Graphite tetrode. Future development will concentrate on simplification and cost reduction of the modules themselves.

Inductive Output Tube We have taken the lOT invented by A.V. Haeff and described by him in 19392 and by Haeff and Nergaard in 19403 to higher power levels and higher frequen­ cies . Haeff's tube produced over 35 watts CW output at 500 MHz, a remarkable performance at the time. An early model of this glass envelope tube is shown in Figure 3. ~y ,\\ _~...... \ FIGURE 5 ~ . lfi~ ll! ' - 1\. . ~ . ~!>c~ o ,,"" !> ..!> 0'1 3 eo~ ~ FIGURE 3 Experimental Haeff Tube Figure 4 is a schematic cross section of the TABLE I - TEST DATA inductive output amplifier as shown by Haeff and 3 Achieved Objective Nergaard in reference . The electron beam is focused Frequency (MHz) 771 820 by a magnetic field and is density modulated at the CW Output (KW) 10 30 input circuit RF frequency. RF power is extracted Power Gain (dB) 21 24 from the density modulated beam by a simple resonant Efficiency (%) 52 55 output cavity. Beam Voltage (KV) 18 30 Pulse Power Output (KW) 32 50 Pulse Power Gain (dB) 24 27

The data shown by no means represents the limits for this tube type. A Theoretical analysis indicates power vs . frequency from this first design, an improved version of Haeff's, to be as shown in Figures 6 and 7. Further improvements can be expec­ ted in the future. The improvement in performance over the original Haeff tube, about 1000 times in power output, i s due to the application of modern materials and microwave beam tube technology as well as certain proprietary improvements in design . It closely resembles the advances made in klystrons, (KW before World War II, MW after). FIGURE 4 The fact that this interesting tube type lay dormant for 40 years is undoubtedly due to the tremendous emphasis on velocity-modulated tubes in 2An UHF Power Amplifier of Novel Design by A.V . the fifties and sixties. It is now possible to see A.V. Haeff, Electronics, February 1939 . how its ultimate performance compares with other tube types, or how it fits into the total picture. 311A Wide-band Inductive Output Ampl ifier" by A. V. Being basically a gridded density-modulated tube, Haeff and L.S. Nergaard, Proc. I . R.E . , March 1940. its power output is limited by the grid and its

327 Proceedings of the 1981 Linear Accelerator Conference, Santa Fe, New Mexico, USA

SIMPLE l.OT. Figure 8 is a projection of where the modular L O"" DV"-Y tetrode and the lOT may fit in the power versus frequency spectrum. Both devices have interesting possibilities in filling gaps in the frequency spectrum which have existed for a long time. More compact RF qenerators in the 100 MHz to 300 MHz region could simplify some of the constructional problems involved in large storage ring tunnels. More power per generator in the 100 MHz region and below will simplify RF equipment design in accelerators as well as fusion reactors. The potential features of these new tubes suggests that FIGURE 6 in today's world of increasing energy costs and tight money they should have a substantial place.

'T' 100 '\ 000 f ( MHz) KlYS R 5

SIMPLE l.O:r: ! ~ - I MA)/.. , C .W . RF ou.,-PU "" PowER "5. FREQ. . F1 G U R E 8 f--·_-f-+.1f'-+-H+f+t-+-t-t+-Irl-Ti I

-I .

1,0 0 0 FIGURE 7 10,0 0 0

Discussion The cathode grid arrangement described is not fully modular in the BOO-MHz design, but it evolved from our earlier work on modular grids and cath­ odes. Our present Inductive Output Tube (lOT) dev ices are not at high vo ltages yet. At 18 to 20 kV, we reach about 55% efficiency. If we can get up into the 100-kV region, we. s.hould. approach 60 to 70% which would be competltlve wlth klys­ upper frequency is limited by electron tran~it time. trons. The major advantage of the lOT is for AM The klystron is not limited by either , and lts per­ linear applications, for example UHF TV, where the formance is accordingly more impressive. The attractiveness of the Inductive Output Tube lies in klystron is left way behind. . its much smaller length, so that at frequencies We think pyrolytic grids will eventually flnd between 100 and 300 MHz it has manageable propor­ their way into the 2170. We are working our way tions where a klystron would be unwieldly, and its up from the smaller types. We are at 300 kW now, higher efficiency especially as a linear power and have the facilities to grow the bigger cups; amplifier in AM service. In this tube the beam in a couple of years one could expect them in the current varies with drive level as in a classical 2170. tetrode. In a conventional klystron the beam current We are considering trying an lOT at lower is invariant with drive level, making it very frequency--500 MHz; it will have solenoid focusing. inefficient at low signal levels. Compared with On the quest ion of higher peak powers at low duty factor, the chart shown ind icated that 10 MW, conventional tetrodes, the lOT has more power gain, 3 wS should be achievable at higher voltages , a simpler structure, an output circuit (cavity) all which we think we can reach. The input circuitry at d.c. ground potential, and a separate collector is complex, requiring isolation of the voltage and which can easily be made large enough to handle the waste beam power, as in the klystron. Other advan­ rf but it is easier to handle than a tetrode's tages include the possibility of operating the output circuit. collector at a lower potential and the attainment of interesting bandwidths by optimizing cavity design.

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