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AAC'04 Agenda
AAC'04 Poster
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MM Wave Sources Working group
Lawrence Ives: Comparison of Multiple Beam and Sheet Beam Klystons
  There is considerable interest in distributed beam RF sources for the next generation of high energy accelerators. Distributed beam sources utilize unique circuits to extract energy from electron beams with considerably increased transverse area. This allows high power extraction with significantly reduced current density beams than from existing round beam sources. This allows reduction of the voltage which drives much of the cost of the power supply systems. With these distributed beam sources, however, comes a number of issues. These include considerably more complicated electron guns and complex RF cavities and increased susceptibility to parasitic oscillations. Coupling into and out of these devices also becomes more complicated. This presentation will explore the different designs being used and proposed for these new sources and examine the relative merits of each. CCR recently completed a Phase I program to explore the feasibility of a high power sheet beam klystron and is currently developing a multiple beam klystron through a Phase II grant. The design parameters of each will be presented, and the predicted performance of each will be compared. A key issue, of course, will be manufacturing cost, and this will also be explored.
Lawrence Ives: Operation of a Confined Flow Multiple Beam Gun
  CCR has completed initial testing of a confined flow multiple beam gun for a 50 MW klystron. The successful testing of this gun provides the oportunity to employ a new class of RF sources for high power accelerators. The gun operates below 200 kV, which will allow utilization of solid state power supplies rather than pulse modulators. This will have dramatic impact on the cost of RF sources and power supplies for large systems. The voltage reduction also provides improved efficiency, reduced radiation shielding, and reduced electrical stress on the entire RF source system. These improvements could dramatically reduce the cost of accelerator systems.

The gun was tested in a beam analyzer specifically designed for distributed beam sources. The analyzer provides transverse profiles of the electron beam at various distances from the cathode. This allows precise measurement of the beam quality, including scallop and spiraling. It also provides information for confirming the accuracy of the computer codes and for improving the performance of the gun itself.

The gun in test also allows application of voltage to the focus electrode. This could be extended to allow gridded operation of a multiple beam gun, which could allow short pulse operation to eliminate the requirment for pulse compressors.This presentation will describe the gun design, construction, test, and planned modifications for implementation in a full power klystron, currently scheduled for testing in 2005.

Michael Read: Generation and Focusing of Sheet Beams for High Power Klystrons
  Sheet beam klystrons are a promising method for generation of high power at high frequency. However, the generation and focusing of the beam for this type of device requires 3D design methods available only recently. We discuss the design of the electron gun and ppm focusing for a 100 MW beam for an X-Band klystron. The gun has a beam voltage and current of 415 kV and 250 A, respectively. The cathode is 10 cm x 10 cm while the beam is 10 cm x 0.8 cm. The cathode is non-immersed. The gun is gridded, allowing the use of new hybrid modulators being developed for the Department of Energy. These modulators have a significant amount of stored energy, and the gun has been designed to reduce the possibility of a cathode-anode arc.

The ppm focusing is of the "cusp" type with focusing in both transverse planes. In an initial design, 96% of the beam was transmitted over a distance of 80 cm.

Details of the gun and ppm design will be presented, and prospects for designs at lower impedance and higher frequency will be discussed.

Vyacheslav Yakovlev : FAST X-BAND PHASE SHIFTER
  A phase shifter to be the key element of an active high-power switch is described. This phase shifter employs ultra-fast, electrically–controlled ferroelectric elements. This high-power switch will allow one to build an active Delay Line Distribution System (DLDS), which would provide substantial reduction in the length of waveguide, compared to what would be required for the traditional passive DLDS design for NLC. The results of preliminary optimization of the phase shifter at the NLC frequency of 11.424 GHz are presented showing the feasibility of building the switch to control a power of 500 MW. Initial tests at a power of up to 50 MW are planned using the Omega-P/NRL X-band magnicon. ______________________________________________________Research supported by the Department of Energy, Division of High Energy Physics.
Anatoly Vikharev: Plasma Switch for X-Band Active SLEDII RF Pulse Compressor
  As proposed by SLAC [1] the efficiency of a pulse compressor of the SLED-II type could be increased by changing both the phase of the microwave source and the coupling coefficient of the delay line. In the existing SLED-II system the resonant delay line is coupled with the source via an iris with a constant reflection coefficient. Replacement of the iris with an active component makes it possible to create an active SLED II system. In this paper use of a plasma switch as the active element is discussed. This plasma switch is a similar to that which has been developed for active Bragg compressors [2]. Designs and low-power tests of some versions of the plasma switch are described. Low-power tests showed that such plasma switches could provide variation of the reflection coefficient in the range from Ro = 0.89 – 0.92 to Rd = 0.4 – 0.2. Experiments with a compression system that consisted of the plasma switch and a 6 m resonant delay line with round trip power losses of 1.5% showed that it is possible to achieve the following parameters for an active SLED II compression system: compression ratio C = 10 – 16, power gain M = 7 – 12, and compression efficiency h = 65 – 75%.

[1] Tantawi S.G., Ruth R.D. and Vlieks A.E., Active radio frequency pulse compression using switched resonant delay lines, Nuclear Instruments and Methods in Physics Research A, 1996, v.370, pp.297-302. [2] Vikharev A.L., Gorbachev A.M., Ivanov O.A. et al., Active microwave pulse compressors employing oversized resonators and distributed plasma switches, AIP Conf. Proc., 1999, v.472, p.975.

*Research supported by US Department of Energy.

  A high efficiency, high power magnicon at 34.272 GHz has been designed and built as a microwave source to develop RF technology for a future multi-TeV electron-positron linear collider. To develop this technology, this new RF source is being perfected for necessary tests of accelerating structures, RF pulse compressors, RF components, and to determine limits of breakdown and metal fatigue. After preliminary RF conditioning of only about 2´105 pulses, the magnicon produced an output power of 10.5 MW in 0.25 ms pulses, with a gain of 54 dB. Slotted line measurements confirmed that the output was monochromatic to within a margin of at least 30 dB. __________*Research supported by the Department of Energy, Division of High Energy Physics.
Jay Hirshfield: Whispering gallery pulse compressor
  If the classical SLED [1] were scaled immediately from the S-band to a higher frequency, the power capacity of the rf pulse compressor would be drastically reduced. For the X-band, the most adequate equivalent of the SLED seems a barrel-like cavity operating at a whispering gallery mode: with azimuthal index [2]. The rf breakdown threshold of the compressor can be raised to a still higher level, if the perforation coupling of the cavity with the input-output waveguide [2] is changed for a coupler based on a wave tunneling effect [3].The pulse compressor of the latter type can be designed by simple analytical and numerical methods. An 11.4 GHz / model of the compressor tested at a low power had a performance consistent with the theory and similar to that of the classical SLED: depending on combinations of parameters, the efficiency was between 55% and 80 %. A preliminary technical design for an evacuated high-power compressor has been developed. According to a theory, a twin-cavity version of the device can efficiently compress microwave pulses produced with sources of a limited frequency band, in particular, chirped pulses.Such robust and relatively compact compressors can be used to test components of novel electron-positron colliders and to feed relatively small medical electron accelerators. The work was supported by a DoE SBIR grant.

References1. Farkas Z.D. et al, SLED: A Method of Doubling SLAC’s Energy, Proc. 9th Conf. On High Energy Accelerator, SLAC, Stanford, CA, USA, May 2-7, 1974, p. 576, SLAC-PUB-1453.2. Balakin V.E., Syrachev I.V. VLEPP RF Power Multiplier // Proc. III-rd Int. Workshop on Next Generation Linear Collider, Branch INP, Protvino, Russia, 1991. P. 145–156. 3. M. I. Petelin, J. L. Hirshfield, S.V. Kuzikov, A. L. Vikharev. High power microwave pulse compressors: passive, active, and combined. SPIEs 14th Annual Symposium on Aerosense, 24-28 April 2000, Orlando, Florida USA.

  Performance and design features of metal PBG and rod-loaded cavities for single-beam and multi-beam rf accelerating/generating devices are considered. Fundamental differences of the performance between single-defect and multi-defect structures are identified. A six-beam cavity design with external coupler is optimized for a multi-beam PBG klystron. Preliminary design of a compact, 6-beam, X-band MBK demonstrates feasibility of generating high power with high efficiency.

*Work supported by DOE SBIR grant number DE-FG02-03ER83845