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 report the operating characteristics of a sequence of two-cavity second harmonic gyroklystrons which are derived in part from a previous fundamental tube and utilize output cavities which resonate at twice the drive frequency. We present results from the design simulations as well as details of the stable range of operating parameters. While the harmonic tube is somewhat more susceptible to spurious oscillations and more sensitive to parameter variations than the fundamental device, there is still considerable parameter space available for amplifier operation. Peak powers above 30 MW are obtained with efficiencies greater than 28% and large signal gains of 27 dB. These results depend critically on the magnetic field profile which has a slight up-taper at the optimum operating point. The nominal beam parameters include a pulse length of 1 /spl mu/s, a voltage near 450 kV, a current in the range 235-245 A, and a perpendicular to parallel velocity ratio (/spl alpha/=v/sub /spl perp///v/sub z/) near one. >

Experimental studies of stability and amplification in a two-cavity second harmonic gyroklystron

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.

This paper is a progress report on studies carried out at SLAC to assess the high-gradient behavior of 11.4 GHz copper accelerator structures for future linear colliders. the structures which have been examined in the last year are a 7-cavity standing-wave (SW) section and a 30-cavity traveling-wave (TW)section. Both structures are of the constant-impedance uniform-aperture type with a 2{mu}/3 phase shift per cavity. The results presented here included new information on RF breakdown, field emission, RF processing and dark current. the captured dark current depends on the rise time of the RF pulse.

High-gradient studies on 11. 4 GHz copper accelerator structures

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

Experimental work is being performed 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 or previously reported problem of high power rf pulse shortening and have achieved peak rf power levels of 330 MW using an 11.4-GHz high-gain tube with multiple output structures. In these experiments the rf pulse is of the same duration as the beam current pulse. In addition, experiments have been performed on two short sections of a high-gradient accelerator using the rf power from a relativistic klystron. An average accelerating gradient of 84 MV/m has been achieved with 80-MW of rf power.

Relativistic klystrons for high-gradient accelerators

We are developing relativistic klystrons as a power source for high gradient accelerator applications such as large linear electron-positron colliders and compact accelerators. We have attained 200 MW peak power at 11.4 GHz from a relativistic klystron, and 140 MV/m longitudinal gradient in a short 11.4 GHz accelerator section. We report here briefly on our experiments so far. 5 refs., 1 fig., 1 tab.

/pdf/relativistic-klystron-research-at-slac-and-llnlt-2s43eh28kl.pdf

G. A. Loew

Papers

High-gradient electron accelerator powered by a relativisitic klystron.

High-gradient studies on 11. 4 GHz copper accelerator structures

Recent progress in relativistic klystron research

Relativistic klystrons for high-gradient accelerators

RELATIVISTIC KLYSTRON RESEARCH AT SLAC AND LLNLt