An overview is provided for the novel relativistic klystron amplifiers which are under active study at the Naval Research Laboratory. These amplifiers are driven by an annular intense relativistic electron beam (500-kV, 10-kA range), which is modulated by an external RF source (1.3-GHz, 100-kW range). Experiments, theory, simulation, and simple models are presented to illustrate the unusual properties of such devices which result from the intense space charge of the beam. Chief among them are electrostatic insulation against vacuum breakdown at high power levels, efficient current modulation, short bunching length, and amplitude and phase stability of the output signal. Many of these unexpected features were revealed in two separate experiments: one with a lower current beam (5 kA, 2-cm beam radius), and the other one with a higher current beam (16 kA, 6.6-cm beam radius). Three gigawatts of RF power at 1.3 GHz were generated with the large diameter beam at an efficiency of 35% with 37-dB gain. These experiments are reviewed, along with a combination of particle simulation results and analytic models which facilitate the interpretation. Special attention is paid to the unfamiliar features of these amplifiers, and the critical problems which must be solved before such amplifiers can fulfil their potential in a wide range of applications are addressed. >

Relativistic klystron amplifiers driven by modulated intense relativistic electron beams

Relativistic Klystron amplifiers driven by modulated intense relativistic electron beams

The operating characteristics of a two-cavity X-band gyroklystron experiment are reported. Beam voltages and currents up to 440 kV and 200 A, respectively, are generated in 1 mu s pulses by a thermionic magnetron injection gun. Velocity ratios ( nu /sub perpendicular to // nu /sub z/) near one in the output cavity are used to achieve peak powers of 24 MW near 9.87 GHz. The maximum saturated efficiency of more than 33% occurs at a beam voltage of 425 kV and a current of 150 A. A large signal gain in excess of 34 dB is realized by operating the input cavity just below the start oscillation threshold. Details of tube stability and the dependence of amplification on magnetic field profile, input signal parameters, and various beam quantities are presented. >

Performance characteristics of a high-power X-band two-cavity gyroklystron

The design of a gyroklystron experiment capable of fulfilling the expected requirements of future electron-positron colliders is presented. The design is an extension of previous work that utilized the interaction of a 500 kV beam and TE/sub 01//TE/sub 02/ circular cavities. Details of an electron gun design which is capable of producing an 800 A, 1 /spl mu/s beam are given. Modifications to an existing code which enhance the system modeling are described before the circuit simulation results are summarized. >

The design of a 100 MW, Ku band second harmonic gyroklystron experiment

We have used relativistic klystron technology to extract 290 MW of peak power at 11.4 GHz from an induction linac beam, and to power a short 11.4-GHz high-gradient accelerator. We have measured rf phase stability, field emission, and the momentum spectrum of an accelerated electron beam. An average accelerating gradient of 84 MV/m has been achieved with 80 MW of relativistic klystron power.

/pdf/high-gradient-electron-accelerator-powered-by-a-3hn32qfsr3.pdf

High-gradient electron accelerator powered by a relativisitic klystron.

Experimental work is underway to investigate the feasibility of using relativistic klystrons as a power source for future high-gradient accelerators. The aim is to develop a high-power (500-MW) short-wavelength (2.6-cm) relativistic klystron with beam kinetic energy greater than 1 MeV. Two different relativistic klystron configurations have been built and tested: a high-gain multicavity klystron at 11.4 GHz and a low-gain two-cavity subharmonic buncher driven at 5.7 GHz. In both configurations power is extracted at 11.4 GHz. In order to understand the basic physics issues involved in extracting RF from a high power beam, both a single resonant cavity and a multicell traveling-wave structure were used for energy extraction. A previously reported problem of high-power RF pulse shortening was overcome, and peak RF power levels of 170 MW have been achieved with the RF pulse of the same duration as the beam current pulse. >

http://ieeexplore.ieee.org/iel5/802/2472/00073371.pdf

Relativistic klystrons

A high-power 11.4-GHz RF generator which consists of a 5.7-GHz chopping system and two 11.4-GHz traveling-wave output structures has been tested. The device was designed to generate about 500 MW of pulsed RF power at 11.4 GHz when driven by a 1-kA, 3-MeV induction beam. Problems with beam breakup in the output structures have limited the width of the RF output pulse for currents above 600 A. Short RF pulses up to 420 MW have been produced. Modifications are being made to decrease the growth of the beam-breakup fields in the output structures, and the chopping section will be used to study various extraction structures and the reacceleration of a bunched beam by induction cells. >

/pdf/a-chopper-driven-11-4-ghz-traveling-wave-rf-generator-345xdzff25.pdf

A chopper driven 11.4-GHz traveling-wave RF generator

Experimental work is now under way by collaborators at LLNL, SLAC, and LBL to investigate relativistic klystrons as a possible rf power source for future high-gradient accelerators. We have learned how to overcome our previously reported problem of high-power rf pulse shortening and have achieved peak rf power levels of 290 MW. We have used the rf from a relativistic klystron to power a short, 11.4-GHz high-gradient accelerator. The measured momentum spectrum of the accelerated electron beam corresponds to an accelerating gradient of 84 MV/m. 5 refs., 7 figs.

/pdf/recent-progress-in-relativistic-klystron-research-suize6yb77.pdf

Recent progress in relativistic klystron research

We present experimental results of a version of the Choppertron microwave generator designed to work with the high emittance beam of the Advanced Test Accelerator (ATA). Simulations showed that a 800-A, 120 /spl pi/ cm-mrad beam (typical of ATA), could produce 800 MW of RF (11.4 GHz) power using two 12-cell, traveling-wave output structures. Funding contraints prevented final tuning of the modulator system and limited the experiment to 530 MW in narrow pulses. Over 400 MW were extracted from a single output structure through fundamental waveguide. Beam breakup was successfully suppressed with >800 amperes of current transported through the extraction section.

http://ieeexplore.ieee.org/iel3/3749/10950/00505273.pdf

B. Mecklenburg

Papers

High-gradient electron accelerator powered by a relativisitic klystron.

Relativistic klystrons

A chopper driven 11.4-GHz traveling-wave RF generator

Recent progress in relativistic klystron research

Chopperton II