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Present progress and future research in the relativistic klystron amplifier program at the Naval Research Laboratory
Moshe Friedman,
Victor Serlin,
Martin Lampe,
et al.
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This paper addresses the development of the relativistic klystron amplifier (RKA) which is a high power microwave (HPM) source. This source was invented at the Naval Research Laboratory and developed during the last ten years. The present RKA has a 50 db gain and is operated at a frequency of 1.3 GHz with a peak output power > 10 GW and with an efficiency > 35%. However this HPM amplifier is rather large and expensive for many applications. Moreover, extending the frequency of the NRL RKA to frequencies above 3.5 GHz was not fully successful. Recently, it was suggested that incorporation of two modifications to the RKA technology should improve the capabilities of the present NRL HPM source by orders of magnitude and extend the operational frequency to X-band. These improvements enhance the potential for successful and effective military and civilian applications. These modifications are described.
RF extraction issues in the relativistic klystron amplifiers
Victor Serlin,
Moshe Friedman,
Martin Lampe,
et al.
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Relativistic klystron amplifiers (RKAs) were successfully operated at NRL in several frequency regimes and power levels. In particular, an L-band RKA was optimized for high- power rf extraction into the atmosphere and an S-band RKA was operated, both in a two-beam and a single-beam configuration. At L-band the rf extraction at maximum power levels (>= 15 GW) was hindered by pulse shortening and poor repeatability. Preliminary investigation showed electron emission in the radiating horn, due to very high voltages associated with the multi-gigawatt rf power levels. This electron current constituted an electric load in parallel with the radiating antenna, and precipitated the rf pulse collapse. At S-band the peak extracted power reached 1.7 GW with power efficiency approximately 50%. However, pulse shortening limited the duration to approximately 50 nanoseconds. The new triaxial RKA promises to solve many of the existing problems.
High-current relativistic klystron research at Physics International
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We are developing an L-band (1.3 GHz) high-current relativistic klystron (5 kA, 500 kV) for repetitive (200 pps) pulsing. We have designed and tested an extraction cavity that removes energy from the modulated electron beam and radiates it into an anechoic chamber in the TM01 mode. Peak power in excess of 450 MW has been measured for a single shot and 275 MW for a sustained burst producing 3.3 kW of average power. This klystron is now being transitioned to a long pulse (> 500 ns), single shot facility.
System parameters germane to relativistic klystron amplifiers: how the utility of pulse energy depends on pulse duration, the target, and the atmosphere
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Relativistic klystron amplifiers (RKAs) at a variety of carrier wavelengths and pulse durations appear feasible to supply microwave pulses to an array of antennas acting as a beam weapon against targets at or above 100 km in altitude. In order to avoid voltage breakdown in the atmosphere, the array area must be large enough to converge the beam, producing a higher energy flux on target than at intermediate altitudes susceptible to breakdown. The area required depends on the physics of atmospheric ionization and on the pulse duration and the carrier wavelength of the RKA. A quantitative statement of the dependence of array area on relevant parameters is presented. The energy per RKA pulse that is usable without delay lines is determined here as a function of RKA pulse duration and wavelength. Changing the pulse length from 160 ns to 1 microsecond(s) and shortening the wavelength raise the energy usable without delay lines by a factor of 1000.
Self-consistent nonlinear theory of the resistive-wall klystron
Han Sup Uhm
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A self-consistent nonlinear theory of resistive-wall instability is developed for a relativistic electron beam propagating through a grounded cylindrical resistive tube. Because of the self- excitation of the space charge waves by the resistive-wall instability, a highly nonlinear current modulation of the electron beam is accomplished as the beam propagates downstream. A partial integrodifferential equation is obtained in terms of the initial energy modulation ((epsilon) ), the self-field effects (h), and the resistive-wall effects ((kappa) ). Analytically investigating the partial integrodifferential equation, a scaling law of the propagation distance zm at which the maximum current modulation occurs is obtained. It is found in general that the self-field effects dominate over the resistive-wall effects at the beginning of the propagation. As the beam propagates further downstream, the resistive-wall effects dominate. Due to a relatively large growth rate of the instability, the required tube length of the klystron is short for most applications.
Inductively loaded wide-gap designs for relativistic klystrons
Martin Lampe,
Richard F. Hubbard,
Moshe Friedman,
et al.
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As the power and the frequency of a relativistic klystron increase, design of both modulation and extraction cavities becomes increasingly difficult. If the gap is narrow, the rf electric field begins to exceed limits for electron emission or breakdown. On the other hand, if the gap width exceeds a small fraction of a wavelength, typical extraction modes do not couple well, and space charge potential energy reduces efficiency and limits the current that can cross the gap. We present theoretical investigations of a novel gap design that incorporates an inductively loaded return-current structure. This structure serves to neutralize the dc space charge of the beam, while sustaining the longitudinal rf field that extracts the microwave energy. With appropriate choice of resonant modes, it appears that gap widths exceeding a half wavelength can be used for modulation of high-current beams, as well as extraction of rf energy with high efficiency.
Simulation of beam-gap interactions and microwave extraction in the NRL relativistic klystron amplifier
Richard F. Hubbard,
Martin Lampe,
Moshe Friedman,
et al.
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Axisymmetric simulation codes have been extensively used to model relativistic klystron amplifier (RKA) devices. However, the inductively loaded wide gap cavity, which has been reported by Friedman, et al. and Serlin, et al. (these proceedings) contains several axial rods in the gap and thus is inherently nonaxisymmetric. An external circuit model has been incorporated into the axisymmetric simulation code MASK to treat the inductive structure. The structure is intended to allow much higher dc currents to propagate without the formation of a virtual cathode while allowing rf fields to penetrate. Simulation results confirm this picture and are in good agreement with preliminary experiments.
Low-frequency cavity oscillation driven by detuned current modulation
Han Sup Uhm
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A theoretical model of cavity excitation driven by a modulated electron beam is developed in connection with application to relativistic klystron amplifiers. The equations that govern the phase and amplitude of the excitation voltage appearing on the cavity opening are obtained in terms of time t and current modulation strength. Several points are noteworthy: (1) amplitude and phase shift of the induced voltage indicate a damping oscillation, whose frequency is proportional to the mismatch (Delta) (omega) between the modulation frequency (omega) and the cavity resonance frequency (omega) 0; (2) rise time of the cavity excitation amplitude decreases as the value of the frequency mismatch increases; (3) for a large value of the frequency mismatch (Delta) (omega) , the power transfer from the modulated beam to the cavity occurs at the beginning of the beam pulse; and (4) it is observed that the absolute amount of energy delivered from the beam to the cavity decreases drastically as the frequency mismatch increases.
Status of experiments at LLNL on high-power X-band microwave generators
Timothy L. Houck,
Glen A. Westenskow
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The Microwave Source Facility at the Lawrence Livermore National Laboratory (LLNL) is studying the application of induction accelerator technology to high-power microwave generators suitable for linear collider power sources. We report on the results of two experiments, both using the Choppertron's 11.4 GHz modulator and a 5-MeV, 1-kA induction beam. The first experimental configuration has a single traveling-wave output structure designed to produce in excess of 300 MW in a single fundamental waveguide. This output structure consists of 12 individual cells, the first two incorporating de-Q-ing circuits to dampen higher order resonant modes. The second experiment studies the feasibility of enhancing beam to microwave power conversion by accelerating a modulated beam with induction cells. Referred to as the `reacceleration experiment,' this experiment consists of three traveling-wave output structures designed to produce about 125 MW per output and two induction cells located between the outputs. Status of current and planned experiments are presented.
Design consideration of relativistic klystron two-beam accelerator for suppression of beam break-up
Hai Li,
Timothy L. Houck,
S. S. Yu,
et al.
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It is demonstrated in this simulation study that by using the scheme of operating rf extraction structures on the betatron nodes of electron drive beam in conjunction with adequate de-Q-ing, appropriate choice of geometries for the rf structures (reducing transverse impedence), and/or staggered tuning we can suppress the overall growth of transverse instabilities in a relativistic klystron two-beam accelerator to 4 e-fold.
Recent advances in Reltron and Super-Reltron HPM source development
Robert B. Miller,
Carl A. Muehlenweg,
Kerry W. Habiger,
et al.
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Reltron and super-reltron microwave tubes use post acceleration of a well-modulated beam and multiple output cavity extraction sections to generate high power microwave pulses with excellent efficiency. We have continued our development of these tubes with emphasis being given to four specific topics: (1) Recent experiments with our 1-GHz super-reltron tube have demonstrated operation at a peak power level of 600 MW. With pulse durations of several hundred nanoseconds, the microwave energy per pulse is about 250 J. (2) We have extracted significant power (several tens of megawatts) at the third multiple (3 GHz) of our 1-GHz super-reltron tube using output cavities designed for operation in S-band. (3) We have fielded a small S-band super-reltron tube on our 260 kV modulator. We have obtained lifetime data for this tube under repetitive (20 Hz), long pulse (2 microsecond(s) ec) operating conditions. (4) We have initiated feasibility experiments of the reltron concept by post accelerating the bunched beam produced by a SLAC XK-5 klystron. In this paper we report our experimental results and discuss relevant theoretical considerations related to each of these four topics.
Rep-rate explosive whisker emission cathode investigations
Marc S. Litz,
Jeffry Golden
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An experiment is underway to study the performance of several materials as field-emission cathodes for low voltage (<EQ 100 kV), repetitive (< 1 kHz) electron accelerators. A thyratron-switched blumlein line modulator with a 70 (Omega) characteristic impedance, and 1 microsecond(s) pulse width, is operated between 20 and 100 kV from single-shot to 300 Hz rep-rate. This provides a high average power (50 kW) test bed for the study. A comparison is made of cathodes fabricated from velvet, carbon, diamond coatings, niobium wire nanocomposite, and poly-crystalline tungsten. Surface emission is monitored by an array of Faraday cups. The `turn-on' time, uniformity of emission, and gap closure time are measured as a function of the spatially averaged, macroscopic electric field at the cathode. The carbon fiber cathode produces the largest current density and has the lowest threshold voltage for emission.
High-power microwave generation by virtual cathode oscillator
Weihua Jiang,
Katsumi Masugata,
Kiyoshi Yatsui
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A new configuration of virtual cathode oscillator has been investigated by two—dimensional simulation. In this configuration, a cylindrical cavity is located between the electron beam diode and the waveguide. Simulation results have shown that the reflected electron beam drives the cavity mode, which results in the modulation of the injected electron beam. As a result, the microwave output power obtained in the waveguide is higher than that without the cavity.
Recent results on vircator
Guozhi Liu,
Hongjun Wang,
Wenhua Huang,
et al.
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An initial experiment is underway to utilize the virtual-cathode-driven oscillator as a source of high-power microwave on a Flash-II accelerator. The most recent experiment launches 113 J per pulse, with frequency in the band around 9 GHz. The highest power, about 4.5 GW in a short burst, was obtained using axial extraction (in a TM radiation pattern) without applied magnetic field. The simple E-probes are applied to pick up the microwave signal inside a circular waveguide of 29.2 cm inner diameter. The microwave signal was power-divided to provide a time fiducial with the rest of the signal which was transmitted through a 61.5-meter BJ100 X-band dispersive delay line, both signals were then fed to an oscilloscope to provide a time-dispersed frequency spectrum. A 24.2 cm long conical titanium calorimeter was attached to the axial opening to measure microwave energy.
Experimental studies of the MIT 3.3-GHz relativistic klystron amplifier
Palmyra Catravas,
Wen Hu,
Chiping Chen,
et al.
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We report experiments on a 3.3 GHz relativistic klystron amplifier. The MIT RKA has a compact and flexible design. The central component of this experiment is a novel compact input cavity in which an explosive emission electron gun is embedded. The electron gun generates a 5 kA, 350 kV annular electron beam with 1.9 cm radius and 2 mm thickness. The beam is confined by a 10 - 17 kG solenoidal magnetic field produced by a superconducting magnet. Up to 5% of beam current modulation has been observed. Magnetic and electric probes placed along the drift tube are used to study beam bunching. Theoretical calculations and MAGIC simulations show good agreement with experimental results.
Critical issues in the design of two-stream relativistic klystron amplifiers
Chiping Chen,
George Bekefi,
Wen Hu,
et al.
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We examine several critical issues concerning beam formation, efficiency optimization, and short-wavelength effects in the design of recently proposed two-stream relativistic klystron amplifiers. A scheme is proposed for producing two annular relativistic electron beams. The optimal efficiency of two-stream RKAs is found to obey a favorable scaling law, and is estimated to be greater than that of nonrelativistic two-stream amplifiers by one order of magnitude. Short-wavelength effects are shown to be significant at high frequencies.
Feeding system for an RKA-driven antenna array
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In order to transfer electromagnetic (EM) energy efficiently from a relativistic klystron amplifier (RKA) to a steerable super-array, the issue of mode conversion must be considered, i.e., the transfer of electromagnetic fields from a coaxial-line waveguide to a number of rectangular waveguides. A special converter has been designed to solve this matching problem. The converter consists of several sections including a multi-fin converter, a fan- shaped converter, and a twisted-rectangular converter. The mode conversion in each of these sections has been studied analytically and experimentally. The optimal parameters for minimum energy loss have been found for the different converter sections and the power breakdown in some of the sections has been studied. Formulas for engineering design are presented.
Output section for high-power microwave sources based on quasi-periodic structure
Levi Schaechter
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An analytic method to investigate a quasi-periodic disk loaded waveguide, including input and output arm, is presented. We rely on Cauchy's residue theorem to formulate the transmission and reflection from a system composed of radial arms and grooves provided that the inner radius is kept constant; all the other parameters of the system can be arbitrarily changed. This method was successfully utilized to design the input and output section of a high power traveling wave tube which is very sensitive to reflections from both ends. We found this method particularly useful for the design of the output regions where rf breakdown imposes constraints on the geometry.
Second report on the experimental progress of the one microsecond, one kilojoule per pulse L-band relativistic klystron at Los Alamos
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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.
Recent experimental results from a long-pulse J-band relativistic klystron amplifier developmental effort
Keith G. Kato,
David D. Crouch,
David R. Sar,
et al.
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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.
Propulsion of small launch vehicles using high-power millimeter waves
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The use of microwave and millimeter wave beamed energy for propulsion of vehicles in the atmosphere and in space has been under study for at least 35 years. The need for improved propulsion technology is clear: chemical rockets orbit only a few percent of the liftoff mass at a cost of over $3,000/lb. The key advantage of the beamed power approach is to place the heavy and expensive components on the ground or in space, not in the vehicle. Early efforts to use microwaves in propulsion beamed at high average powers to heat rocket engine fuel for inter-orbital transfers from low earth orbit to the moon and Mars. In the past two decades, microwave sources have been developed to extraordinary peak powers over a wide frequency range and are now operating at repetition rates in excess of 100 Hz, giving average powers of -40 kW.1 Development of these sources has preceded in several parameters: a general movement to higher power, development of high power sources at increasingly higher frequencies and higher repetition rates at all frequencies. Fig. 1 shows the present state-of-the-art of peak power as a function of frequency.
High-power klystrons
John G. Siambis,
Richard B. True,
R. S. Symons
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Novel emerging applications in advanced linear collider accelerators, ionospheric and atmospheric sensing and modification and a wide spectrum of industrial processing applications, have resulted in microwave tube requirements that call for further development of high power klystrons in the range from S-band to X-band. In the present paper we review recent progress in high power klystron development and discuss some of the issues and scaling laws for successful design. We also discuss recent progress in electron guns with potential grading electrodes for high voltage with short and long pulse operation via computer simulations obtained from the code DEMEOS, as well as preliminary experimental results. We present designs for high power beam collectors.
Compact, high-average-power, injection-locked magnetron
Todd A. Treado,
Paul D. Brown,
Charles Antosh
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We are presently working to reduce the size and weight of the Varian-ARL, conventional- emission, high-power magnetron by redesigning the magnetron to incorporate a permanent magnet. As part of this effort, the average power capability of this 50-MW, S-band magnetron is being increased by an order of magnitude. This paper reports on the compact magnetron design and new results from an experiment which was performed with an all-tungsten anode magnetron at continuous, high repetition rates.
High-harmonic gyrofrequency multiplier
Adam J. Balkcum,
David B. McDermott,
Frederic V. Hartemann,
et al.
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The design of a sixth-harmonic gyrofrequency multiplier driven by a 300 keV, 1.7 A, prebunched axis-encircling electron beam produced by a 1 MW, 2.9 GHz gyroresonant rf accelerator is presented. A nonlinear simulation code predicts that the device, which is currently being tested, will emit 150 kW at 17.4 GHz with an efficiency of 31%.
Peak power tailoring and phase nulling of the AN/FPS-85 radar
Montgomery D. Grimes,
J. Mark Major,
Thomas J. Warnagiris
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The AN/FPS-85 radar is a large, fixed-position, phased-array radar located at Eglin Air Force Base, Florida. The radar has a peak radiated power of about 32 MW generated by a 72 X 72 matrix of transmitter elements that are individually rated for a maximum peak power output of 10 kW. The current transmitter modules have no automatic phase nulling capability and no remote output power adjustment capability. The result is a randomly varying amplitude pattern across the beam aperture. Southwest Research Institute is currently under contract by the U.S. Air Force to redesign the transmitter units and supply three operational prototypes. Part of the design and performance upgrade will be an automated, remotely controlled power and phase adjustment system which will utilize a centrally located host computer to send phase and amplitude adjustments to the individual transmitters via a digital serial link.
Reduction of bremsstrahlung recoil force on the second-order nonlinear dynamic polarization charge of a relativistic test particle
Howard E. Brandt
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The reduction of the recoil force on the second-order dynamic polarization charge of a relativistic rest particle participating in collective bremsstrahlung in a nonequilibrium beam-plasma system is presented. Contributions arise from interaction of the self-field of the test particle in second order with the particle's own induced second-order dynamic polarization, facilitated by prior Compton conversion and scattering off the second-order dynamic polarization.
Desktop subnanosecond pulser: research, development, and applications
Gennady A. Mesyats,
Valery G. Shpak,
Sergey A. Shunailov,
et al.
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The studies on production of high-power high voltage pulses of subnanosecond duration having been performed at the Laboratory of Electron Accelerators are reviewed. The subnanosecond pulser we have created is a supplementary device for the earlier developed nanosecond repetitive pulsed power source RADAN 303. The principle of operation of the pulser is successive formation of the leading and trailing edges of the high voltage pulse with the use of high-pressure gas switches. Design versions with a different number of electrodes in the peaking spark gaps have been examined. With a four-electrode spark gap, a pulse of peak voltage 170 kV and FWHM duration 150 ps was produced across a 50 (Omega) load. The processes occurring in transportation of high voltage pulses through the coaxial section of the pulser have been analyzed. The data on the control of the output pulse parameters and typical jitter values are presented. The fields of possible applications of the pulser are discussed. The pulser has been proved in short-run tests at a pulse repetition rate of 100 pps.
Short-duration positive and negative pulses caused by unequal phase velocities of coupled strip lines
Iwata Sakagami,
Satoshi Chino
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Short-duration pulses caused by the difference of even and odd mode phase velocities that arise in two strip coupled line networks are discussed on the basis of network functions derived from signal flow graphs. A ramp-waveform of rise-time 0.1 nsec is assumed as an input. Simulated waveforms are presented by using the fundamental transmission line equation in an inhomogeneous medium. With regard to the even and odd mode phase velocities, Ve and Vo, either Ve < Vo or Ve >= Vo takes place according to the cross-sectional structure. Referring to a microwave C-section, a directional coupler, and open/short terminal networks, several patterns of output responses are shown: The polarity of the short-duration pulses can be changed by the cross-sectional structure, or by network terminal conditions. The amplitude of the first short pulse increases up to one half of the amplitude of an incident ramp waveform as the coupling is weaker, and other pulses appearing after the first short pulse become so small as to be negligible.
Magnetic tunable filters for increased power handling capacity applications
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Magnetic tunable microwave band-pass filters on the basis of ferrite loaded resonators (FLR) have been considered. The field model of coupled FLR is suggested taking into account the particular conditions of excitation. The behavior of coupling coefficient and external Q are studied for various electrical and geometrical parameters of FLR on the resonance frequencies of principal mode for a particular rectangular waveguide. The experimental results showed a good agreement with theoretical results.
Novel design of ultrawideband conical TEM horn
Jianguo He,
Kecheng Liu,
Jiaxian Yin,
et al.
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The TEM horn for transmitting and receiving narrow pulse or impulse waveform has been discussed in this paper.A novel design concept and calculation method have been presented. According to this method, a ridged conical TEM horn has been designed and fabricated. The experiment results show that it has excellent performance, so that it is very valuable for the study oftransient field and ultrawideband(UWB) radar. Key words: TEM horn, Antenna, UWB radar, Impulse.
Investigation of the high-current linear induction accelerator operating with the relativistic magnetron
Alexander S. Sulakshin,
Nikolai M. Filipenko,
Gennadii P. Fomenko,
et al.
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The results of modeling and experimental investigations of a linear induction accelerator (LIA) operating with relativistic magnetron as a load are discussed in this report. The mathematic model of the LIA injector section is based on the substitution of all main LIA section parts by equivalent circuits. Satisfactory model quality and reliability of the simulation results are confirmed by good agreement of the calculation results with the experimental data obtained by LIA section operation on the electron beam and resistive load. During the simulation a relativistic magnetron model was used in which the self-consistent effective amplitude of the microwave field fundamental harmonic and, accordingly, the generated power are defined by forming voltage and input current, taking into account output current losses. Optimization of a `power supply -- microwave generator' as the unified system on efficiency and generated power will allow us to define the best characteristics of LIA elements and the relativistic magnetron parameters.
Recent multiwave Cherenkov generator experiments
Richard J. Adler,
Robert J. Richter-Sand,
Fred Hacker,
et al.
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The initial operating characteristics of the North Star Research Corporation multiwave generator experiment are discussed.
Backward-wave cyclotron maser oscillator experiment
Eli Jerby,
George Bekefi,
Avi Shahadi,
et al.
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A backward-wave cyclotron maser oscillator experiment conducted at Tel Aviv University is reported in this paper. The oscillator operates in the microwave regime (9.4 GHz) with a low- energy electron beam pulse (8 keV, 0.2 A, 1 ms). A frequency chirping effect observed by a heterodyne technique reveals the cyclotron resonance with a backward wave harmonic of the periodic waveguide.
Pierce-type dispersion relation for an intense relativistic electron beam interacting with a slow-wave structure
Chiping Chen
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A Pierce-type dispersion relation is derived for the interaction of an intense relativistic electron beam with a cylindrical slow-wave structure of arbitrary corrugation depth. It is shown that near a resonance the Pierce parameter can be expressed in terms of the vacuum dispersion function and the beam current. The dispersion relation is valid in both the low- current (Compton) regime and the high-current (Raman) regime. The dispersion characteristics of the interaction, such as the linear instability growth rate and bandwidth, are analyzed for both regimes.
Slotted third-harmonic peniotron forward-wave oscillator
Anthony T. Lin,
Chae K. Chong,
David B. McDermott,
et al.
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An experiment has been built to test the novel, efficient peniotron interaction at the third harmonic in a slotted eight-vane waveguide by using the axis-encircling electron beams produced by a gyroresonant rf accelerator. By configuring the oscillator so that the peniotron is excited as a forward wave at the cutoff frequency of a travelling-wave circuit, the peniotron can be made to dominate over the usually stronger gyrotron interaction. The 10 GHz peniotron will be driven by a 70 kV, 3.5 A axis-encircling electron beam with (alpha) equals v(perpendicular)/VZ equals 1.3 and is predicted by a nonlinear, self-consistent, multi-mode PIC code to yield 110 kW with 45% efficiency. The travelling-wave circuit is terminated on the upstream end by a load and by a transition to a TE41/TE11 mode converter on the collector end. PIC code simulation results and a description of the assembled experiment are presented.
Some practical aspects of the helical Cerenkov effect
Josip Z. Soln
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The helical Cerenkov effect as a novel radiation source, we believe, should be observable from microwave to visible wavelengths. From the kinematics point of view, microwave radiation with the wavelength of 10-1 cm could occur in a medium with the index of refraction of 1.4, beam energy of 3 MeV, and the uniform magnetic field of 4 Tesla. In the visible spectrum with the central wavelength of 5 X 10-5 cm, however, the observability of this effect is achieved with up to 3 MeV in beam energy, silica aerogel as a medium with the index of refraction of 1.075, and uniform magnetic fields from 5 to 10 Tesla. Specifically, for the 10 Tesla magnetic field, it is estimated that in the 250 - 750 nm visible region an electron generates a photon through an interaction length of 10 cm. As such, this effect could possibly be used as a detector of radiation by energetic electrons that are trapped in a medium by strong magnetic fields.
Suppression of beam break-up in a standing-wave free-electron laser two-beam accelerator
Hai Li,
Jin-Soo Kim
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Various schemes are examined in this study on the suppression of beam break-up (BBU) in a standing wave free electron laser two-beam accelerator (SWFEL/TBA). Two schemes are found to be not only able to effectively suppress the BBU but at the same time have minimum effect on the microwave generation process inside the SWFEL cavities. One is making the cavity-iris junction sufficiently gradual and the other is stagger-tuning the cavities.
Observation of frequency chirping and phase of a free-electron laser amplifier
Kenneth N. Ricci,
George Bekefi,
Brian Chen,
et al.
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We report observations of frequency chirping and phase of a free electron laser amplifier operating in the Raman regime. The FEL is driven by a mildly relativistic electron beam (750 kV, 25 ns) subjected to a combined axial magnetic field and a helical wiggler field. The input into the FEL amplifier is provided by a high power magnetron tuned to a frequency of 33.39 GHz. Phase and frequency shifts are measured both as a function of time and interaction length. It is found that in the Group I regime of FEL operation the output frequency is upshifted by approximately 100 MHz, but smaller upshifts are observed in the Group II or the reversed field configurations.
Ultrashort-pulse prebunched FEL
Gregory P. Le Sage,
Scott N. Fochs,
Frederic V. Hartemann,
et al.
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Currently under design is an X-band photocathode linac FEL system which produces ultra- short (picosecond), high-power (MW) pulses of coherent microwave and millimeter-wave radiation. In this system, the resultant electron bunches are considerably shorter than the wavelength of the emitted radiation, and these bunches coherently emit synchrotron radiation in the wiggler interaction region to generate subnanosecond, millimeter-wave MW pulses. The output radiation pulse length is essentially determined by the slippage between the electron bunch and the electromagnetic waves through the wiggler. The wiggler, which has already been constructed and characterized, produces a 5 kG helically polarized field, with an 84 mm period over a length of 2 meters. The device operates in the fundamental TE11 cylindrical waveguide mode. Close to grazing, where slippage is minimal, the pulses are further compressed because of the very wide instantaneous interaction bandwidth. In this regime, the output pulse duration is determined by the group velocity dispersion in the waveguide and the interaction bandwidth. A planned upgrade for the system is also described.
Operation of a dielectric-loaded wideband gyro-TWT amplifier
Keh-Chyang Leou,
David B. McDermott,
Frederic V. Hartemann,
et al.
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The bandwidth of a gyro-TWT has been broadened by partially filling the interaction circuit with dielectric to reduce its dispersion. Initial hot tests of the X-band frequency, proof-of- principle experiment demonstrated zero-drive stability and achieved a peak output power of 55 kW with 11% efficiency, 27 dB saturated gain, and an unprecedented constant-drive bandwidth of 11% for a 100 kV, 5A, (alpha) equals (upsilon) (perpendicular)/(upsilon) z equals 0.6 electron beam. As predicted by simulation studies, the amplifier's performance can be further enhanced with improved beam quality.
Scaled tests of Varian's 95-GHz slotted third-harmonic gyro-TWT amplifier
Chae K. Chong,
David B. McDermott,
Adam J. Balkcum,
et al.
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A high performance, low magnetic field, moderate voltage, 95 GHz gyrotron amplifier has been designed and then tested in a scaled experiment. The slotted, third-harmonic, three- section gyro-TWT utilizes an 11.6 kG magnet and a 70 kV, 5 A, (upsilon) (perpendicular)/(upsilon) (parallel) equals 1.3, axis-encircling electron beam with an axial velocity spread of 7% and is predicted by a self-consistent, nonlinear simulation code to yield a peak output power of 90 kW with an efficiency of 26%, a saturated gain of 61 dB, and a constant-drive bandwidth of 3%. The start-oscillation conditions for the absolute instability and the gyro-BWO modes were determined by analytical theory. A scaled 10 GHz, two-section slotted gyro-TWT has been tested with the axis-encircling electron beams produced by a gyroresonant rf accelerator. The preliminary results of the slotted amplifier are that it yields roughly 12 dB per section with 4% bandwidth and is not absolutely unstable at the cutoff frequency of the operating (pi) mode.
Theory and simulation of the cyclotron autoresonance klystron operating at Doppler-shifted cyclotron harmonics
Gennady T. Smirnov
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A linear analysis and numerical simulation are presented for Doppler-shifted cyclotron harmonics generated in the cyclotron autoresonance klystron 1,2 The harmonic-generation mechanism in the CARK is similar to the one in the ordinary klystron: First, an oscillation at the basic Doppler-shifted cyclotron frequency is excited in the main resonator of the CARK. Then, bunched electron beam excites harmonics in an additional quasi-optical resonator tilted to the main resonator. Because of shortening of wavelength both by harmonic-generation mechanism and Doppler effect very high frequencies are attainable in this device. For example, one could obtain radiation with wavelength of about 0 .I mm using an electron beam with the energy of electrons of about I MeV and a magnetic field of I 00 kG . It is essential, that there is no threshold condition for harmonic generation. Thus, it is possible to generate harmonics even with low electron beam current and/or poor Q-factor of a resonator. It is shown, that the efficiency of generation of the low harmonics (harmonic number 2÷5) can exceed I 0%. The efficiency of generation of the I 0th harmonic can be of the order I %. Comparative analysis of the CARK harmonic generator and the FEL harmonic generator is given
Simulations of the high-power Cherenkov device's superdimensional electrodynamical structures
Igor A. Chernyavskiy,
Viktor M. Pikunov
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The numerical research of superdimensional slow wave structures' (SWS) resonance properties is made. The features of dispersion in such devices are discussed. The conditions of high Q-factor of longitudinal resonances in one-stage and two-stage electrodynamical structures are investigated.