This paper describes the experimental development of a long pulse, high current, annular beam relativistic klystron amplifier. The desired performance parameters are 1 GW output power and 1 μs pulse length with an operating frequency of 1.3 GHz. The electron beam voltage and current are nominally 600 kV and 5 kA. Peak powers approaching 500 MW have been achieved in pulses of 1 μs nominal baseline‐to‐baseline duration. The half power pulse width is 0.5 μs. These pulses contain an energy of about 160 J. The design of this class of tube presents some unique challenges, particularly in the output cavity. The output cavity must exhibit a very low gap shunt impedance in order to obtain reasonable conversion efficiency from the low impedance modulated electron beam to microwave power, while still maintaining a reasonable loaded Q for mode purity. The physics of this device is dominated by space charge effects which strongly impact the design. Current experimental results and theoretical design considerations for this class of tube, and scaling to higher frequency operation, suitable for the Next Linear Collider are discussed.

This paper describes the development of an L-band annular beam, high-current relativistic klystron for producing a peak power of 1 GW at a pulse length of 1 µs. The tube consists of three cavities: the input cavity driven by a 300 kW magnetron, an idler cavity, and an output cavity. The tube has produced 475 MW and an energy per pulse of 160 J. The microwave output pulse terminates prematurely, before the peak power in the electron beam pulse is reached. The pulse shortening is believed to be caused by too high a voltage across the gap of the output cavity. The result is electron reflection in the output gap leading to beam disruption and high voltage breakdown across the gap. Current experimental results are presented with an emphasis on the design of output cavities for coupling microwave power from high current, very low impedance modulated electron beams.

Work is continuing on a high-current relativistic klystron amplifier (RKA) with the goal of producing 1 kJ per pulse with a 1 microsecond(s) pulsewidth and a peak power of 1 GW. The three cavity tube has already produced pulses with more than 150 J and over 450 MW peak power. The original output cavity was thought to be limiting the performance, and a new cavity has been designed, built, and is now on-line and being conditioned up towards high power. Current experimental results are presented.

We examine beam-cavity interaction physics relevant to mildly relativistic, intense-beam klystron amplifiers. This is an interesting but difficult regime of operation, because of the combination of high beam current and low voltage. The advantage of this regime is that it is relatively easy to access high beam powers (and potentially high microwave output powers) at relatively low beam energy. We calculate the effect of the extremely high beam loading in the input and idler cavities. The output cavity's shunt impedance must match the low beam impedance in order to prevent high output gap voltages that will reflect electrons back upstream. This leads to very low cavity Q factors (< 10). We derive expressions for mode purity for such low-Q cavities and show that acceptable cavities can be designed,

We discuss basic Relativistic Klystron Amplifier (RKA) physics. We show that, for pulse lengths of one microsecond or longer in the intense space-charge regime, the maximum power extraction from the beam does not coincide with the maximum harmonic bunching. We also discuss the beam-cavity interaction model and present experimental and theoretical single cavity modulation results. Finally, we discuss alternatives to the basic RKA design that enhance beam to microwave power conversion efficiency, particularly at high frequency.

Summary form only given, as follows. The development of a high-current relativistic klystron amplifier (RKA) is continuing at Los Alamos. The goal is to produce 1 kJ per pulse, with a one-microsecond pulsewidth, and a peak power of 1 GW. The three cavity tube has already produced pulses with more than 150 J and over 450 MW peak power: The output power has been limited by breakdown In the output cavity. A novel scheme for ex-situ conditioning of the output cavity gap pieces is being developed. A specialized cavity with a high Q, that holds the gap pieces in the same geometry as in the tube, is used in conjunction with a high power commercial klystron to field condition the surfaces. Results from this work and recent PIC code simulations will be presented.

This paper describes the experimental development of a long pulse high current relativistic klystron amplifier (RKA). The desired performance parameters are 1 GW output power and 1 mu s pulse length with an operating frequency of 1.3 GHz. Peak powers approaching 500 MW have been achieved in pulses of 1 mu s nominal baseline-to-baseline duration. The half power pulse width is 0.5 mu s. These pulses contain an energy of about 160 J. RF output rises linearly in concert with the beam current pulse, and terminates abruptly just before the highest part of the pulsed voltage curve is reached. A possible explanation, not yet experimentally confirmed, for the premature termination of the RF pulse is an output cavity gap voltage that is too high, causing electron reflection at the gap and RF breakdown across the gap. A new output cavity has been designed with a much lower shunt impedance and a loaded Q of 4.

Recent experimental results, supporting simulations, and design modeling are presented from a developmental effort to produce a long pulse (approximately 1 microsecond(s) ) J-band (5.85 - 8.2 GHz) relativistic klystron amplifier (RKA) of the high current NRL genealogy. This RKA is designed to operate at approximately 6.6 GHz, with a desired rf output approximately 700 MW. Conversion of electron beam energy to microwave energy is obtained by a mock magnetically insulated coaxial converter which, in various incarnations, can be made to be either a cavity gap extractor or an inverse cathode.

We have designed and are testing a large orbit gyroklystron amplifier for 1.3 GHz operation in 65 ns pulses. The ultimate power output goal is 500 MW with a gain in excess of 20 dB. This initial investigation is intended to lay the groundwork for operation at 11.4 GHz for particle accelerator applications, and also at frequencies of up to 35 GHz for other uses. Computational design has been performed with the resonant cavity code MAFIA and the particle in cell codes MERLIN and ISIS. Electron beam optics through a magnetic cusp was also studied with ISIS and MERLIN, and verified experimentally, to develop a suitable electron beam trajectory from the diode into the resonator region. Performance tests of a single stage device have been performed. An unsaturated gain of 43 dB has been observed using 4 kW of input drive, yielding an amplified output of 100 MW. >

A high-current relativistic klystron amplifier (RKA) is being developed with the goal of producing 1 kJ per pulse with a 1 microsecond(s) pulsewidth and a peak power of 1 GW. The three cavity tube is fully assembled and is undergoing high power testing. Peak power levels as high as 400 MW have been produced so far. Current experimental results are presented.

This paper discusses the pulse power and explosive emission electron beam diode development effort we have undertaken to power a relativistic klystron amplifier (RKA) microwave source. The pulsed power and electron beam must enable the RKA to produce one kilojoule of 1.3 GHz radiation per pulse at a 5 Hz repetition frequency. These efforts include tests and improvements of a 1 μs pulse length thyratron switched modulator, and the computational and experimental design of a 1-μs-pulse-length explosive emission electron gun. The one microsecond pulse length is almost an order of magnitude beyond what has been achieved heretofore with an RKA. Achieving a peak power approaching 1 GW for 1 μs requires a well behaved electron beam on that time scale. An electron beam diode has been developed that delivers a peak current of 4 to 5 kA for a pulse duration exceeding 1 μs, at a beam kinetic energy above 600 keV. BANSHEE is the high voltage modulator designed for use as an electron beam driver for high power microwave tube development. The BANSHEE output pulse design parameters are 1 MV and 10 kA, with a 1 μs pulse width at a repetition rate of 3-5 Hz, driving a load of impedance of 100 ohms. BANSHEE is a thyratron-switched line-type modulator with a pulse transformer output stage. The modulator design is pushing the state of the art in thyratron technology and capacitor lifetime. The results of the BANSHEE modulator testing are described.

The goal of this research effort is to develop a long-pulse relativistic klystron amplifier (RKA) by extending the pulse length of this gigawatt-class device by an order of magnitude beyond the current state-of-the-art (100 ns) to one microsecond. A research approach is described for obtaining kilojoule microwave pulses at 1.3 GHz. Achieving kilojoule microwave pulses requires extending the electron beam pulse duration beyond one microsecond without diode closure, and maximizing the microwave extraction efficiency at the fundamental frequency. Our earliest experiments have produced a modulated electron beam for one microsecond with a peak rf current of 0.9 kA and a voltage of 350 - 400 kV. In some cases we have observed beam modulation in excess of 2 microseconds. The component of beam power at the microwave drive frequency (1.3 GHz) was approximately 350 MW. Although only a small effort has been put forth to address the output coupling issues, approximately 50 - 70 MW was coupled into dominant mode rectangular waveguide. Recently, an electron beam diode has been tested that delivers peak currents in excess of 5 kA for a monotonically increasing current pulse exceeding durations of 1 microsecond(s) at beam kinetic energies above 500 keV. to achieve this result close attention was given to minimizing the current losses from the diode and maximizing the beam current transmission through the RKA.

We discuss basic Relativistic Klystron Amplifier physics. We show that in the intense space- charge regime the maximum power extraction does not coincide with the maximum harmonic bunching. In addition, we show that as the beam is bunched, the additional power stored in the Coulomb fields does not add significantly to the overall power extraction. Because of these effects, the power extraction at 1.3 GHz for a 500 kV, 5 kA beam with reasonable beam-to- wall spacing is limited to around 35%.

A research approach for obtaining kilojoule microwave pulses of microsecond duration at 1.3 GHz from the relativistic klystron amplifier is described. Achieving kilojoule microwave pulses requires extending electron beam pulse durations and maximizing the microwave extraction efficiency at the fundamental frequency. An electron beam diode has been constructed that delivers peak currents in excess of 5 kA with a monotonically increasing current pulse exceeding durations of 1 mu s at beam kinetic energies above 400 keV. Close attention has been given to minimizing the current losses from the diode. Maximum microwave extraction efficiency at the fundamental frequency has been related to the beam bunching amplitude and output cavity shunt impedance in terms of a simple circuit theory. The circuit theory predictions have been tested by particle-in-cell code calculations of the electron beam interactions with the proposed cavity structures. The successful cavity structures have been constructed and are awaiting testing. >

The performance of the Friedman-type high-current relativistic klystron amplifier (RKA) is being extended to the microsecond regime while attempting to achieve the gigawatt-level peak power capability that has been characteristic of the RKA at shorter pulse lengths. Currently the electron beam power into the device is about 1 GW in microsecond duration pulses, with an effort underway to increase the beam power to 2.5 GW. To date the device has yielded an RF-modulated electron beam power of 350 MW, with up to 50 MW coupled into waveguide. Several aspects of RKA operation under investigation that affect RKA beam bunching efficiency and amplifier gain include cavity tuning, beam diameter, beam current, and input RF drive power, and the development of an output coupler that efficiently couples the microwave power from the low-impedance beam into the rectangular waveguide operating in the dominant mode. Current results from experimental testing and code modeling are presented.<>

Summary form only given, as follows. A two-cavity relativistic klystron based on the design of M. Friedman et al. (J. Appl. Phys., vol.64, p.3353, 1988) has been under test for several months. The field emission foilless diode, which generates the relativistic electron beam, has been designed for long-pulse 100-Ω operation for beam energies ranging between 300 and 600 keV. An attempt has been made to extend the duration of high-power operation of this klystron to the microsecond regime. Data on diode operation, beam bunching, cavity RF powers, and extracted RF power into a load were obtained