Output Cavity Research Articles

Study of gain in C-band deflection cavities for a frequency-doubling magnicon amplifier

An experimental study of the gain between two half-wavelength, 5.7-GHz TM/sub 110/ mode pillbox cavities, separated by a quarter-wavelength drift space, and powered by a 170-A, 500-keV electron beam immersed in an 8.1-kG magnetic field is reported. These cavities constitute the first section of a planned multicavity deflection system, whose purpose is to spin up an electron beam to high transverse momentum ( alpha identical to upsilon perpendicular to / upsilon /sub z/>or=1) for injection into the output cavity of a frequency-doubling magnicon amplifier. A gain of approximately 15 dB was observed in the preferred circular polarization, at a frequency shift of approximately -0.18%, in the opposite circular polarization, at a frequency shift of approximately +0.06%. These results are in good agreement with theory. >

Test results of X2242 and X2274 high power tetrodes with the JT-60 ICRF amplifier in a frequency range of 110–130 MHz

This paper reports the test results of newly developed Varian tetrodes, X2242 and X2274, using a JT-60 ICRF amplifier under the US-Japan collaboration program. The JT-60 ICRF amplifier was designed to deliver 0.75 MW at 110 to 131 MHz with the Varian EIMAC 8973 tetrode. Although the new tetrodes are similar to the 8973 in all dimensions, they have pyrolitic graphite grids for higher screen and control grid dissipation capability. The new tetrodes require only the following amplifier system modifications: (a) new filament, bias, and screen grid power supplies, (b) a second output port for reduction of rf voltage in the output cavity. The objective for the tests are to confirm 1.5 MW output at 130 MHz for 5 seconds, and to check the reliability of both the tube and the amplifier with a mismatched load which simulates power transmission to an antenna coupled to the plasma. The first test with an X2242 demonstrates that excessive screen dissipation limits the output power. The second test with an X2274, whose improved screen grid reduces rf heating to 50% of that of the X2242, achieves 1.7 MW at 131 MHz for 5.4 seconds. This is not only a power higher than the objective but also the highest long pulse VHF power level for fusion research above 110 MHz. The modified amplifier with the X2274 also shows good, stable performance in the mismatched load tests. As the theory predicts, the maximum anode dissipation is 1.4 times higher with a VSWR = 1.5 than with the previous VSWR ≅ 1.0.

Experimental study of efficiency optimization in a three-cavity gyroklystron amplifier

An experimental study quantifying the effects of both penultimate cavity tuning and magnetic field profiling on the saturated efficiency of a three-cavity gyroklystron amplifier has been performed at a frequency of 4.5 GHz. As predicted by theory, it is observed that the penultimate cavity detuning in the gyroklystron is in the opposite direction from a conventional klystron for enhanced efficiency operation. This is a result of the opposite energy dependence of the bunching mechanism for the two interactions. Magnetic field profiling has also been shown to have a very strong effect on the amplifier efficiency, both through beam loading effects in detuning the penultimate cavity and through enhanced energy extraction in the output cavity. The measured efficiency of the gyroklystron is observed to improve from 5% to 21% by profiling the magnetic field along the device, and then from 21% to 29% by penultimate cavity detuning with an optimized magnetic field profile.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Development of a long-pulse 1.3 GHz relativistic klystron amplifier

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. >

Analysis of the deflection system for a magnetic-field-immersed magnicon amplifier

A linear analysis of the electron-beam deflection system in a magnicon amplifier is presented. The system consists of identical cavities, one driven and the remainder passive, separated by a drift space and immersed in an axial magnetic field. The cavities contain a rotating TM/sub 110/ mode. The length of each cavity is pi nu /sub z// omega , and that of the drift space is pi nu /sub z// omega /sub c/, where omega is the RF frequency, omega /sub c/ is the relativistic gyrofrequency in the guide field, and nu /sub z/ is the mean axial velocity of the beam electrons. The linearized electron orbits are obtained for arbitrary initial axial velocity, radial coordinate, and magnetic field. The small-signal gain and the phase shift are determined. The special case where omega /sub c// omega =2 has unique features and is discussed in detail. For the NRL magnicon design, a power gain of 10 dB per passive cavity is feasible. Results from numerical modeling of a magnicon with two passive cavities are presented. Operation of the output cavity at the fundamental and higher harmonics of the input drive frequency is briefly discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pulsations in Optically Pumped Formic Acid 433 μm Far Infrared Laser

Pulsation behaviours of optically pumped Formic Acid (HCOOH) 433 μm laser are observed. Pulsation frequency varies continuously with tilting the output cavity mirror or changing the HCOOH pressure. Unstable pulsations are observed at a relative high pressure or by changing a cavity mirror with higher output loss.

Theory of a two-cavity gyrotron frequency multiplier.

A frequency multiplying device is analyzed that provides power at selected cyclotron harmonics in a quasioptical gyrotron. The device utilizes a prebunching cavity and an uptapered magnetic field that impose a double-resonance condition on the radiation generated. This selects among cyclotron harmonics in the output cavity even though the cavity structure may support all harmonics. The prebunching is accomplished by modulation of electron orbital magnetic moments by a low-frequency-input rf field. Two design examples are given for quasioptical gyrotron frequency multiplier operation at the third cyclotron harmonic (four or seven times the input frequency) with a 10--15 % electronic efficiency and greater than 33% overall rf conversion efficiency. The drawback of this method of prebunching is that high-input electric-field intensities are required for efficient operation. However, it is shown that the double-resonance method allows complete harmonic selection in the linear regime for zero detuning between the cyclotron and field frequency. When this detuning is nonzero, the selected harmonic still has a higher linear efficiency than all others.

Nose-cone removed pillbox cavity for high power klystron amplifiers

A simple pillbox cavity without the reentrant nose-cone is proposed as a new type of RF resonator in klystron amplifiers. It is advantageous in high-power applications, because of its lower electric field concentrations. The velocity modulation of an electron beam in the pillbox cavity is analyzed numerically by using the finite-difference method. The beam coupling coefficients are compared with the analytical solutions for a conventional reentrant cavity. It is shown that for a high-voltage electron beam case the pillbox cavity can provide almost as large a coupling coefficient as the conventional reentrant cavity. As an example, the analysis is carried out on a practical S-band pulsed high-power SLAC-5045 klystron. The maximum surface electric field in the output cavity of the pillbox type is shown to be reduced by 50% as compared to the conventional reentrant cavity used in the 5045 klystron. Discussed are several benefits of this nose-cone-less cavity, such as suppression of the multipactor, lower sensitivity of the resonance frequency to dimension errors, and ease of fabrication.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of the 425-MHz Klystrode

The results of analyses performed on a 500-kW peak power, 425-MHz Klystrode are presented. The Klystrode is an RF amplifier with three basic components; a gated-emission gun, a resonator (output cavity), and a spent-beam collector. The principal goal of the analysis is to understand the factors that limit the efficiency of the Klystrode and to suggest approaches to achieving improved efficiency. Numerical simulations were performed with the fully electromagnetic, two-and-one-half-dimensional, self-consistent particle-in-cell code, MAGIC. A resonator saturation theory was developed to predict Klystrode performance from startup to saturation. Based on this theory, a one-dimensional optimizing code KLYDE, was developed to address the design tradeoffs. These tools have been employed to investigate the Klystrode performance over a wide range of parameters with good agreement between MAGIC, KLYDE, theory and experiment. The analyses suggest that shortening the bunch length is the most effective way to achieve higher efficiencies. >

Phase-locked millimeter-wave two-port second-harmonic Gunn oscillators

Two-port harmonic oscillators have been developed which are suitable for voltage-controlled-oscillator (VCO) operation in frequency stabilized systems. Two oscillator designs are presented. The first has a fundamental frequency cavity located above the harmonic output cavity and the fundamental is coupled by means of the Gunn device bias-line filter. The second design is an in-line structure which uses a waveguide taper as the filter element separating the fundamental and harmonic frequency components. The latter design is a translation of the first oscillator concept onto a single plane, so that the prospect of an integrated monolithic version is conceivable. The performance of the oscillators is discussed, and a demonstration of their use in a heterodyne phase-locked loop control system is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mode conversion losses in highly overmoded waveguide cavities

A technique to quantify the effects of mode conversion losses in highly overmoded waveguide cavity resonators using the WKB method is presented. The WKB theory is accurate for weakly irregular waveguide cavities with small amounts of mode conversion. This approach to the problem has the advantage that the mode conversion losses in waveguide cavities can be quantified quickly and cheaply. The objective is to compute the amplitude and phase of a resonant mode in the cavity to zeroth order (i.e. without regard to mode conversion effects), and then use amplitude and phase of the computed zeroth-order fields to compute the amplitude of the converted mode. This approach is valid to the extent that the converted modes are not a major perturbation to the resonant mode. The results of the theory are presented in the context of the trade-offs associated with the design of an 85 GHz TE/sub 1.3/ phase-locked gyroklystron oscillator experiment. Mode conversion losses in the input cavity are a particular problem, and place limits on the amount of usable beam current (and therefore the output power capabilities of the gyroklystron). The isolation of the input and output cavities due to mode conversion must be carefully considered in any design. >

Millimeter-wave gyroklystron amplifier experiment using a relativistic electron beam

A fundamental-mode TE/sub 111/ degrees two-cavity intense-beam gyroklystron amplifier experiment, operating at an accelerating voltage of 1 MV, is reported. The two cavities that were tested are designed to serve as bunching cavities for a high-power output cavity. The two-cavity amplifier has demonstrated a linear gain of 15 dB and an unsaturated output power of approximately 40 kW, with the intracavity gain and power approximately 4 dB higher. The frequency of the second cavity has been found to track the frequency of the driven cavity over a range of 300 MHz around a center frequency of 35 GHz. Stable amplifier operation was achieved with beam currents as large as 150 A and a velocity pitch ratio of 0.36. The stable operating range was limited by spurious oscillation in the TE/sub 112/ degrees mode. Theoretical calculations indicate that higher gains might be attainable if this mode could be suppressed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Modification of a commercial fir laser system for efficient operation at 337 GHz

An Apollo Lasers model 122 far infrared laser system has been modified to allow sustained efficient operation at a wavelength of 890μm (337 Gigahertz). Modifications included installation of a circular metallic output cavity and redesign of the input and output couplers. Beam output profiles were taken to verify correct operation of the output coupler assembly. A simple automatic controller was implemented to provide unattended, automatic operation of the system.

Breakdown phenomena in high-power klystrons

In the course of developing new high-peak-power klystrons, high electric fields in several regions of these devices have become an important source of vacuum breakdown. In addition, a renewed interest in breakdown phenomena for nanosecond-pulse, megavolt-per-centimeter fields has been sparked by recent work in the area of gigawatt RF sources. The most important regions of electrical breakdown are in the output cavity gap area, the RF ceramic windows, and the gun ceramic insulator. The experiments and results on the breakdown in these regions are discussed, as well as the solutions to alleviate this breakdown problem. >

The lasertron rf generator for FEL applications

The lasertron rf generator uses an rf-modulated laser beam to generate a tightly bunched electron beam that is accelerated by a dc power supply to the fractional megawatt region. The tightly bunched beam then crosses an output cavity where most of the kinetic energy may be extracted. The bunching is so good that efficient microwave generation at 3 GHz and above is feasible, and three projects worldwide are under way for these frequencies. All three existing projects focus on the linear collider application of high peak power with good efficiency, short output pulses, and low average power. A short review of these projects is presented.The main thrust of this paper is to discuss the lasertron as the rf generator for free electron laser (FEL) rf systems. The author has developed a ring model of the lasertron device that shows excellent dc-to-rf conversion efficiency at high output power over the 400-1700 MHz frequency range. The issue of the lasertron advantages at higher average output power is also explored. First-order estimates of the weight versus frequency and power are also made. An outline of an experiment that uses the Los Alamos photoinjector to make a lasertron is also presented.

Very long pulse high power klystrons for FEL and their generating conditions

High power RF sources for major scientific machines (accelerators, plasma heating) are often either short pulse devices with very high peak power levels (e.g. 30-40 MW peak, 5μs pulses in S-band) or CW devices (e.g. 500 kW in S-band). Free electron lasers, on the other hand, require klystrons operating in a long-pulse mode (100 μs and 10 ms). In this case, the technology used in the two other cases (short pulse, CW) is not necessarily appropriate. When the peak power levels are very high in nearly-CW operation, new problems are encountered, for example thermal cycling and HV breakdown modes. The most critical parts are the output cavity, output window and the electron gun. We will discuss a family of 1·3 GHz klystrons as an example: 35-40 MW/10μs; 10-15 MW/200 μs and a few MW/1-10ms

Analyses of two-cavity gyroklystron with feedback

Making use of the linear kinetic theory and the perturbation approach, a gyroklystron with feedback, in which a low order (TE011) mode is presented in the bunching cavity by the feedback of the output cavity, and the low order mode in the bunching cavity is coupled by the electron beam to a high order (TE021) mode in the output cavity, has been investigated in detail. And electron beam to wave interaction in each cavity and electron bunching mechanism in the bunching cavity have been analysed. The formulas of electron beam to wave interaction power and starting current etc. have been derived. Then some calculations are carried out.

Some applications of particle-in-cell codes to problems of high intensity beams

The technique of using the “particle-in-cell” method was developed in the field of plasma physics [1]. Borrowed for problems of intense beams, it becomes an especially powerful tool because such problems frequently use single species “plasmas” and so pose a less severe requirement on the computer. We will look at several problems in which the PIC code method has been useful. The first is the classical Pierce gun in a transient or short pulse mode. Here the transverse beam optics is strongly affected by the time dependence of the current. The second is a study of high power klystrons searching for the source of an instability. The third is the high power rf source called the “lasertron” which is under development at SLAC. The interesting new development for the lasertron simulation is the introduction of a double gap output cavity for improved efficiency. The lasertron and klystron simulations are steady state solutions to rf problems with high- Q cavities. In order to limit the computation to a realistic time, these simulations use an external equivalent circuit which can communicate with the beam tunnel through ports placed at the locations of the rf cavities. Application for electron beams generally require using a fully relativistic electromagnetic code such as MASK. In some applications, the computation can be speeded up by limiting the solution of the fields to the electrostatic conditions. This can be especially helpful if the degree of precision required demands very large numbers of macroparticles. As a last example, we will show the use of this technique for a problem involving emittance growth for a high intensity beam for heavy ion fusion.

The Design and Performance of a 150-MW Klystron at S Band

Design considerations and performance characteristics of a 150-MW pulsed klystron amplifier at S band are described. One of the main problems encountered in generating such high power at microwave frequencies is the extremely high-voltage gradients developed across the output gap of the klystron which may cause RF breakdown. A double-gap 2π-mode output cavity is used successfully in the present case to generate a peak output power of 150 MW at a frequency of 2870 MHz with an efficiency of 51 percent, saturation gain of 59.3 dB, and a pulsewidth of 1 μs. A reduction in gap voltage of at least 25 percent as compared to a similar klystron with a conventional single-gap output cavity at the same voltage and at their respective maximum efficiency points is theoretically predicted. Examination of the gaps after an extended period of operation shows that they are smooth and without signs of erosion, indicating that the fields were uniform and not excessive. Extrapolating from the gradient which exists in this present tube, it appears feasible to generate still higher powers (of the order of 700 MW) efficiently with a single klystron at S band with a pulsewidth of 1 μs, if the need arises.