Super-Reltron high power microwave tubes use post-acceleration of a well-modulated beam and multiple cavity output sections to generate high power microwave pulses (100MW-1GW) with excellent efficiency (40-50%). In the past year we have continued our development of these tubes with emphasis being given to three specific topics: 1) long pulse operation. Recent experiments with our 1.3-GHz tube have demonstrated pulse durations in excess of one microsecond, producing energy per pulse in excess of 150 J. 2) Extended frequency coverage. We have developed a Super-Reltron-based HPM effects testing system which provides repetitive, high-power pulses over the frequency range of 0.7-11 GHz. 3) Extended lifetime designs. We have now developed Super-Reltron designs using thermionic cathodes, ceramic insulators, and gridless modulating cavities. These designs should give extended lifetime operation at reasonably high average power levels (> 100 kW). In this paper we report our experimental results and theoretical design considerations related to each of these three topics.

Design and testing of a magnitized, vacuum diode for use in a relativistic klystron amplifier is presented. This diode is to provide approximately 7 GW of electron beam power for 1 microsecond for partial conversion to rf energy by a 2 or 3 cavity klystron amplifier system. The present configuration will be shown. Results of calculations for the magnetic beam optics, electrostatic potential structure, and self-consisitent emission PIC simulations will be presented. Experimental results will be shown of the vacuum diode performance.

Development work is underway on a high current, mildly relativistic, L-band klystron source. The intense beam used in this tube presents unique design complications for the output circuit. Various considerations are discussed such as the trade-offs between beam potential and kinetic energies, optimum shunt impedances, and advantages of discrete versus distributed circuits. Three output circuit designs are presented. A description of the experimental hardware is given along with power extraction results from two output circuits.

The advantages of the radial klystron amplifier over the conventional klystron amplifier have been reported by Arman et al. Briefly, the radial structure of this design allows for much smaller impedances and thus higher power, the beam-cavity coupling is stronger because the beam travels inside the cavity, and the source is much more compact because there is no need for external magnetic fields. Here I report on possible advantages of the radial klystron oscillator over the radial klystron amplifier. The amplifying nature of certain HPM sources is often mandated by the requirement for synchronization and phase-locking of a number of sources in specific applications. In situations where amplification is solely adhered to for the purpose of achieveing higher powers, the oscillator will be a better choice if a mechanism can be found to grow the desired mode at the required frequency. By switching to the oscillator mode there will be no need for priming the cavity or maintaining the phase. This simplifies the design and reduces the operational and maintenance cost of the source. Here we report that an oscillator version of the radial klystron is possible and in fact more suitable for many applications. The mechanism for exciting and growing the mode will be transit-time effects thus providing all the beneficial features of the transit-time oscillators. The complications due to the presence of thin foils in the radial design still persist and will be dealt with in subsequent works. Numberical simulations using the PIC codes MAGIC and SOS indicate the radial klystron oscillator is a viable and efficient means of rf generation.

We first provide a brief review of the assumptions and approximations involved in H. S. Uhm's recent theory of current modulation in klystron amplifiers. We next employ this theory to obtain a solution to a cubic equation that approximates the klystron drift distance to the point of maximum current modulation, and also to obtain the corresponding current modulation amplitude in terms of relevant klystron parameters.

The bandwidth of a klystron output cavity scale (approximately) as...^0.8P0.2, where p is the beam perveance and P is the beam power. For high-perveance (p > 10 (mu) pervs), high-power (P > 10 kW) electron beams, it is relatively straightforward to design a broadband output cavity. However, the design of the input cavity of the broadband klystron is more difficult. The purpose of the input cavity is to produce a velocity-moldulated electron beam with a frequency-dependant modulation amplitude that will optimize the bandwidth of the entire klystron system, while providing a large enough magnitude of velocity modulation to minimize the length of the klystron. This paper shows how a multiplet (multiple cavity) buncher cavity can be designed to provide broadband (> 20%) operation while keeping the drift length of the klystron short.

The Electromagnetic Sources Division of the Advanced Weapons and Survivability Directorate at the Phillips Laboratory is dedicated to the development of HPM sources. This paper will report on the recent work done on a magnetically insulated line oscillator (MILO). The MILO is a linear magnetron type device with the bias magnetic field applied by the current flowing through the tube, hence no externally applied magnetic field is required. This work concentrates on the axial extraction of the microwave power in contrast to most of the previous work which concentrated on the radial extraction of power. Theory has predicted and experiments have shown that a number of parameters, including but not limited to, the boundaries at the ends of the slow wave structure, the location of the emission region, the method of launching the beam, the anode-cathode gap, and the number of cavities, all have a significant effect on the operation of the tube. A number of tests at diode voltages from 150 kV to over 500 kV have been conducted to gain a better understanding of the mechanisms controlling the operation of the device. To data RF powers over 1 GW for short pulses (< 100 ns) have been achieved with the goal of further experiments and corresponding simulations are reported here.

Preliminary experimental studies are reported of an L-band amplifier based on Hughes' plasma-assisted, slow-wave oscillator (PASOTRON) technology. The amplifier system utilizes a hollow-cathode-plasma electron-gun, and a plasma-filled slow-wave structure (SWS) to produce >= 100 microsecond(s) ec-long, 50 to 75 kV, 30 to 100-A electron beam pulses that propagate in a plasma channel without the use of any externally applied axial magnetic field. The electron-beam pulse coincides with a 100-microsecond(s) ec-long RF drive signal provided by a 2.6-kW TWT, which is coupled into the amplifier upstream of the SWS. The SWS consists of a ring-bar design which is novel to the PASOTRON family of devices and is used for its short length compared to a helix. Simulations of HP's high frequency structure simulator were used to optimize the ring-bar SWS. Preliminary data are reported showing the new L-band amplifier's gain, power, efficiency, and bandwidth. Methods of eliminating a backward wave oscillation, which was found to limit the performance of the tube, are also presented.

During the last two decades, in this country and in the former USSR, the military developed an interest in directed microwave energy, both as the means to disable the electronics of enemy assets and in defending against such a threat. The Russians, who are very clever microwave engineers, were probably ahead in the field, at least in the offensive aspects of it. With the collapse of the Soviet Union a good deal of the immediacy of the threat disappeared, but the potential of it remains and the work continues, albeit presumably attenuated.

We are developing a family of frequency agile relativistic magnetrons to continuously cover the bands from 1 to 3 GHz. We have achieved tuning ranges of > 33%. The magnetrons have been operated repetitively in burst mode at rates up to 100 pps for 10 sec. Power is extracted from two resonators, and is in the range of 400-600 MW, fairly flat across the tuning bandwidth. We are using a network of phase shifters and 3-dB hybrids to combine the power into a single arm and to provide a continuously adjustable attenuator.

Comaprison of two configurations of a novel high-power microwave generator is presented in this article. Coupling the beam-breakup instability with the transit-time effect of the electron beam in the cavity, rapid energy exchange between the electrons and cavity modes can occur. The dominant cavity modes in the axial and radial configurations are different but their growth rates are comparable. We found that the radial configuration can have a beam impedance of less than 10 (Omega) and are therefore more suitable for low-voltage and high power operation. Good agreements have been obtained between linear theory and simulation for both configurations.

A plasma-filled diode is used as a model of the interaction space in a virtual cathode oscillator (vircator). It is shown for a range of values of the positive/negative charge ratio at the entrance electrode (alpha) and the corresponding reduced electrode separation x_L the system is capable of generating oscillations. It is also shown that the amplitude of oscillations strongly depends on both (alpha) and x_L, although the frequency is largely governed by the value of (alpha) . Such investigations have been previously limited to a single value of (alpha) equals 1, when the positive and negative charge are in balance at the entrance electrode.

A 1D time-varying nonlinear theory based on the Duffing equation is applied to space-charge limited beams and specifically vircators. This theory classifies test-particle trajectories in a modulated nonlinear potential. Two predictions of the theory that can be directly compared to experiment are the final state of electron trajectories and the oscillation frequency of the electrons in the potential well. Experimental measurements of electron flux recorded along the vircator chamber wall correlates well with the numerically integrated final state of electron trajectory in the 1D theory. The oscillation frequency measured in the experiment is shown to be a better match to the oscillation frequency calculated from the nonlinear potential as compared to a parabolic potential (that results from a linear restoring force). In the experiment, random initial conditions arise from beam thermalization and nonuniform electron emission at the surface of the cathode. However, these characteristics alone do not explain the experimentally observed fluctuations in rf power and frequency. The predictions of the time- varying nonlinear potential theory clearly exhibits trends that were observed in the experimental results, in the form of classes of particle trajectories, fluctuations in particle asymptotic states, and particle motion sensitive to the shape of the virtual cathode.

This work details the continuation of the mathematical reduction of the recoil force on the second-order dynamic polarization charge of a relativistic test particle participating in collective bremsstrahlung in a nonequilibrium beam-plasma system.

A high-gain, prime-focus parabolic dish antenna system was designed and constructed for experimental use in the field. The antenna was designed to radiate in L-band at peak power levels exceeding 1 X 10⁶ watts. A 3.6 m diameter, commercial off-the-shelf parabolic dish antenna was modified with a custom-designed waveguide horn feed. The system was mounted on an antenna pedestal to allow for fine (approximately 0.001 degrees) elevation and azimuth control; the antenna and pedestal were mounted on a 4.3 m long trailer for mobility in the field. The antenna has a measured gain of 32 dBi and a 3-dB beamwidth of approximately 4.5 degrees. The system was successfully operated in the field in L-band at peak power levels exceeding 5 MW. The design, calibration, and testing of the antenna system will be presented.

The investigation of the interaction of E-beam with hybrid waves of nonhomogeneous plasma- cavity slow-wave structure have been carried out. It is shown that depression of external magnetic field at output part of plasma-cavity structure may be used for decreasing of phase velocity of active waves and space phase synchronization ones with space charge fields, induced in plasma. This mode of operation of plasma TWT was calculated. The investigations carried out theoretically have been supported by experiments with plasma TWT.

Plasma-beam generators of microwave oscillation, based on hybrid plasma-beam amplifiers with inertial back coupling have been created. The experimental research shows that this device generates wideband stochastic oscillations with E-efficiency up to 40%. The theoretical and experimental investigations of dynamics of transition to stochastic regime has been carried out. It shows that phase mechanism of stochastization allowed to generate microwave oscillations with the nonequality of frequency spectrum about 3 db and spectrum width up to 30%. Possibility of operating to spectrum characteristic by a small input signal has been considered. Compression of spectrum near the operating frequency has been obtained experimentally. The width of compressed spectrum is about 10-0.1% depending on power of input signal. The central frequency of the compressed spectrum corresponds to the operating signal frequency and may be changed in wideband following the changing of operating frequency. Utilization of plasma-beam generators for fusion and plasma-chemical devices plasma heating in the region of low-hybrid resonance is theoretically considered. It is shown that the rate of heating is proportional to spectrum width.

This paper reports experimental investigations of different coherent regimes in a system of two high-power strongly coupled relativistic magnetrons. The laboratory installation includes two relativistic magnetrons with intrinsic magnetic systems and a power supply from two linear induction accelerators. Two different types of connecting lines (symmetrical and nonsymmetrical) between magnetrons were used. Preliminary theoretical studies have shown that nonsymmetrical systems, due to their strong coupling, have shorter phase locking time (a few oscillation periods), a significantly wider locking range and are not so sensitive to the parameter variations as symmetrical systems. Regimes of power addition and subtraction in the common load have been investigated.

The advantages of linear induction accelerators (LIA) include high acceleration rate (1 Mev/m), high accelerated beam current values (up to 10 kA), capability of operating in a rep- rate (a few kilohertz) regime, high (up to 90%) efficiency of conversion of the power supply energy to the electron beam energy. The Tomsk Institute of Nuclear Physics (INT) has recently implemented the supply system and a few original LIA's. LIA's are meant to energize relativistic microwave oscillators with pulse power in the beam of 2-5 GW for a pulse duration of 50-100 ns. This paper presents the results of experimental investigations of two LIA's operating simultaneously. It has been done to realize coherent operation of two relativistic magnetrons. Locking of the accelerators is performed by the common start switch gap. As a load for each accelerator use is made of a relativistic magnetron with an intrinsic system of magnetic fields created by two coils forming Helmholtz couple. For a charging voltage of the forming lines of each accelerator of 50 kV there form the pulses on the cathodes of 400 kV with a pulse duration of 80 ns and a current up to 5 kA. The switch jitter is less than 5 ns.

An experimental device of special high impedance, low beam current vircator is designed by means of the pulse-line electronic beam accelerator at UESTC according to the principle of virtual cathode microwave oscillator. A grandient varial region is linked between the drift tube and the diode in order to reducing the impedance of the diode. A strong pulse longitudinal magnetic field is applied, on the diode-drift region. A special plate-cathode mesh-anode structure is applied, there are annular grooves with 2.5 mm spaced and 60 degree on the cathode. Numerical analysis on experimental system are taken out. The operation modes and output power of signal are determined by the method of cooperation cold and hot measurment. The output power in X-band is greater than 27Mw, and the efficiency is above 1.0%.

The results of theoretical studies of virtual cathode microwave devices are reported. The analytic calculation for the space-charge limit current of intense relativistic electron beam in arbitraty cross-sectional waveguide and resonator is derived in detail by the Green's function method. The space-charge field of single charged sheath in cylindrical cavity, and the behavior of virtual cathode phenomenon relevant to time in cylindrical drifting space are calculated and analyzed by means of Green function method and virtual displacement method. And finally, the phase and frequency locking on power combining of vircator are also presented and analyzed.

Past schemes for using beamed microwave power for space propulsion and providing power to space platforms have used microwaves below 10 GHz. Recent expansions of the high power microwave technology domain offer fundamental reassessment of the following missions: 1) location of orbital debris, 2) supplying power to loitering high-altitude airplanes, 3) satellite battery recharging, 4) imaging of asteroids, 5) orbit raising and transfer, 6) interplanetary probe launch to the outer planets and comets, and ultimately, 7) launch into Earth orbit. This group of applications may be done by a ground-based system. The system would start small, being built for the near Earth missions, and be enlarged incrementally as the technology matures and confidence develops. Of particular interest are sources in the millimeter range where there are low loss atmospheric windows and MJ pulses are available in quasi-CW operation. A development scenario for these missions using millimeter wave technolgy is described.

The objective of this research was to exploit wireless power transmission (microwave/millimeter)--to lower manned space transportation costs by two or three orders of magnitude. Concepts have been developed for lightweight, mass-producible, beam-propelled aerospacecraft called Lightcraft. The vehicles are designed for a 'mass-poor, energy-rich' (i.e. hyper-energentic flight infrastructure which utilizes remote microwave power stations to build an energy-beam highway to space. Although growth in laser power levels has lagged behind expectations, microwave and millimeter-wave source technology now exists for rapid scaling to the megawatt and gigawatt time-average power levels. The design exercise focused on the engine, structure, and receptive optics requirements for a 15 meter diameter, 5 person Earth- to-moon aerospacecraft. Key elements in the airbreathing accelerator propulsion system are: a) a 'flight-weight' 35GHz rectenna electric powerplant, b) microwave-induced 'Air Spike' and perimeter air-plasma generators, and c) MagnetoHydroDynamic-Fanjet engine with its superconducting magnets and external electrodes.

The transmission of impulsive signals through baluns and feed lines, between high-power, fast-risetime pulse generators and impulse-radiating antennas, leads to degraded system performance and increased pulse risetime due to transit-time dispersion, skin and dielectric losses, and electrical breakdown effects. These loss mechanisms are greatly reduced in system designs that eliminate feed lines and baluns by combining the antenna and generator in a single hybrid device that is compact, simple, and robust. This paper describes generators in which the antenna itself is pulse charged to hundreds of kV and subsequently shorted at the feed point by an oil spark switch. These Hertzian generators maintain conical symmetry to within a few millimeters of the feed-point switch, thus providing conditions for launching near-ideal spherical TEM step waves for driving impulse-radiating, focused-aperture antennas. Careful attention to symmetry, optical principles, and precise methods of measurement has yielded subnanosecond pulse risetimes that are more than ten times faster than predictions from spark- switch scaling laws.

This paper presents the results of measurements of RF surface currents induced on the outside of a hollow, conducting tube by conical horn antenna located completely or partially inside the tube and radiating out of one end. The frequency range of the measurements was 0.8-1.6 GHz. We investigated several antenna modifications which were effective in reducing the surface over a narrow band of frequencies was a cylindrical cage of strips of microwave absorber foam material placed over the horn and extending one wavelength or more beyond the horn edge. The narrow, rectangular absorber strips are oriented parallel to the horn axis and are separated by gaps equal to the width of the strips. Reductions in tube surface currents of up to 30 dB over a bandwidth about 5% of the center frequency were observed with this structure with minimal impact upon the antenna gain. Variations on this structure using tapered absorbing strips overlapping tapered conducting strips reduced surface currents over a much broader bandwidth. Pattern measurements revealed a significant reduction in backlobe levels with the cage device. Qualitative explanations of the results are given.

We examine the advantages of using photoconductive switches in the jitter-free linear mode of operation. We show that the jitter-free property of a linear-mode PC switch can be used to coherently add power in a free-space ultra-wideband radiation system. This approach allows PC switches to be operated at reasonable power levels where operation is very reliable.

Recent system developments that take advantage of avalanche-mode photoconductive switching have led to major breakthroughs in high power microwave (HPM) generator performance. The best example is the GEM 2 demonstrator system developed for the Air Force. The delivered system, using over 800 BASS^TM photoconductive switches arrayed in 72 identical modules firing with 10-psec accuracy, produces 1 GW of peak power and an effective radiated power of over 100 GW. We are now looking beyond GEM 2 at continuing major improvements. The simplicity and flexibility of modular designs, such as GEM 2, facilitates new configurations and encourages timely inclusion of additional innovations. Concepts under consideration range from miniaturized generators, taking full advantage of the solid-state approach, to generator arrays considerably larger than GEM 2. Power conditioning and antenna sizes dominate present systems. Optical triggering subsystems based on semiconductor lasers, though relatively small, are still much larger than the corresponding BASS-based microwave generators. Work underway is aimed at these limitations. Several candidate solid- state switching approaches for pulse charging, for example, offer many additional benefits to the optical trigger subsystem.

A data compensation routine using deconvolution technique for an ultra-wide-band (UWB) exposure facility is described. The single shot measuremetn system mainly includes a Tektronix SCD 5000 sampling scope and an EG&G D-dot sensor. The transfer function of the measuremetn system was evaluated with a reference impulse generated by a picosecond pulse labs 4050B picosecond pulse generator. An iterative Hilbert transform was applied to ensure the causality of the results of deconvolution. With software compensation, the bandwidth of the measurement system has been expanded from less than 2 GHz to about 15 GHz. The analysis shows that this exposure system can deliver UWB pulses with a rise time less than 150 ps, a magnitude from 50 to 120 kV/m depending on the location along the center line, and a pulse width about 5 ns.

We have explored high power microwave ((lambda) equals 1.5 mm) pulse amplification along a tapered undulator FEL using the 1D Compton FEL equations with slippage. For an appropriate taper, sideband instabilities are suppressed and a short (approximately 50 psec) Gaussian pulse will propagate in a nearly self-similar way as it grows in power, slipping through a much longer electron pulse (beam energy, 750kV; current, 100A; radius equals 2 mm; length equals 200 radiation periods). This is in contrast to the example of pulse propagation in a constant parameter undulator, where the Gaussian pulse breaks up into irregularities identified with sidebanding. Variation of initial pulse width shows convergence to a 50 psec wide output pulse. Because of the slippage of the radiation pulse through the electron pulse, the peak microwave pulse intensity, approximately 3GW/cm², is about three times the kinetic energy density of the electron beam.

A compact, high repetition rate (2.142 GHz in burst mode), relativistic (5 MeV) photoinjector facility is currently under construction at the UC Davis Department of Applied Science, on the LLNL site. Photoelectron bunches are produced by irradiating a high quantum efficiency Cs₂Te photocathode with a train of 100 UV (210 nm), ultrashort (250 fs) laser pulses. These bunches are accelerated in a 1-1/2 cell, (pi) -mode, X-band rf structure energized by a 20 MW, 8.568 GHz SLAC klystron. The peak current is 0.25 kA (0.25 nC, 1 ps bunches), with a normalized emittance (epsilon) < 2.5 (pi) mm-mrad. This prebunched electron beam is then transversally accelerated in a cylindrical waveguide by a 30-mm period, 10 period long helical wiggler. The peak wiggler field is adjusted to 8.5 kG, so that the group velocity of the radiated electromagnetic waves matches the axial velocity of the electron bunch (grazing condition). Chirped pulses in excess of 2 MW power are produced with an instantaneous bandwidth extending from 125 GHz to 225 GHz, and pulse duration of 15 ps.

Sheet electron beams and configurations with multiple electron beams have the potential to make possible higher power sources of microwave radiation due to their ability to transport high currents, at reduced current densities, through a single RF interaction circuit. Possible microwave device applications using sheet electron beams include sheet-beam klystrons, rectangular grating circuits, and planar FELs. Historically, implementation of sheet beams in microwave devices has been discouraged by their susceptibility to the diocotron instability in solenoidal focusing systems. However, recent theoretical and numerical studies have shown that stable transport of sheet beams is possible in periodically cusped magnetic (PCM) fields. The use of an offset-pole PCM configuration has been shown analytically to provide side- fields for 2D focusing of the beam, and this has been recently verified with PIC code simulations. We will present further theoretical studies of sheet and multibeam transport and discuss results from an experimental investigation of the formation, stability and transport of PCM-focused sheet electron beams. This includes a laboratory method of forming an elliptical sheet beam using magnetic quadrupole pair and a round-beam Pierce gun.

A free-electron laser (FEL) oscillator experiment operating with a low-energy, low current (< 4 keV, < 0.5 A) pulsed electron beam is presented in this paper. In the table-top FEL oscillator constructed in our laboratory, the electrons travel along a planar undulator ((lambda) _w equals 2 cm) and an axial magnetic field, and interact with TEM-mode electromagnetic waves. The FEL output signal is observed at approximately 40 cm wavelength in the UHF regime.

Modifications caused by the nonmonoenergetic beams to the recently described helical Cerenkov radiation (resulting from electron helical motion in a medium under the influence of magnetic field) are studied in a visible portion of the spectrum. The analysis is greatly simplified by utilizing a new analytical approximate expression for the number of emitted photons per unit path length for the usual 'monoenergetic' helical Cerenkov radiator. The effect of the nonmonoenergetic beam on the helical Cerenkov radiation is that, for the same visible frequency and the same radiation angle, the radiation may occur, in addition to through the the usual helical Cerenkov effect, also through harmonic radiation above and below the helical Cerenkov threshold. Harmonic radiators may enhance the usual helical Cerenkov radiation. For the medium of silica aerogel with the index of refraction of 1.075 and the beam energy in the 2-3 MeV range interacting with the magnetic field of about 10 tesla, one estimates that in the visible portion of the spectrum an electron from a nonmonoenergetic beam will generate between one and two photons at the end of the 10 cm interaction length.

The initial-boundary problem for relativistic electron beam injected into a plasma system is considered without any restrictions on beam-to-plasma densities ratio value. Brief analysis of physical processes under beam current increasing is given. Space-time structure of the fields induced by beam front is obtained and investigated. These fields form transient phenomena in plasma-filled microwave sources. In particluar, they can shorten the transient time necessary to attain the operation state.

For generation of high power supershort directed electromagnetic pulse it is proposed to use faster-than-light electron current that is produced by the obliquely incident ionizing radiation illuminating plane metallic surface. The characteristic property of such source is the equality of the ionizing radiation incidence angle to the electromagnetic pulse emission angle. This fact allows to use the ordinary focusing optics methods for directed pulse generation. In the case of space-charge limited current the dominant wavelength of the electromagnetic pulse decreases with the intensity raise of ionizing radiation that leads to decrease of the electromagnetic radiation angular spread. For sufficiently high ionizing radiation intensity generation of directed, slowly spreading, beamlike electromagnetic pulse may be possible. By applying this principle of generation the limitation on accumulated energy area that is common to traditional technologies can be removed. Accordingly the power of microwave device may be increased by ten up to thousand times in comparison with existing sources, the duration of electromagnetic pulse may be reduced to tens nanoseconds or less and the spread of EMR beams may be realized less then 10^-2. The increasing of produced radiation can be obtained by simple increasing emitted surface. In relation to this fact large-scale device with large radiated energy may be based on small 'elementary' sources just as house built from bricks. A compact form a small weight are the characteristic properties of microwave device with superlight electron current.

Using a subnanosecond high-voltage modulator with a pulse repetition frequency of up to 100 pps, we have carried out experimental research into the excitation of ultrabroadband TEM antennas. The modulator consisted of a RADAN-303B nanosecond driver and a pulse sharpener based on gas spart gaps. The device was capable of generating single-polarity and monocycle pulses with readjustable duration and amplitude. The amplitude and power of the pulses supplied to the antenna ranged up to 100 kV and 200 MW, respectively. Preliminary data have been obtained on the stability of modulator subnanosecond pulse parameters under conditions where the amplitude dispersion of nanosecond-driver pulses is on the order of 5%. Results are provided for the electric strength of the matching junction between the modulator and the antenna. The ultrabroadband pulse was recorded at distances of up to 25 m. Its characteristics permitted spatial resolution of reflections from conductive objects with a shape nonuniformity scale of no more than 25 cm. Radiators with increased directivity in the E or H (E and H) plane were explored. Such radiators incorporates two (four) TEM antennas energized by a split modulator pulse. Consideration is given to versions of phased and opposite-phase antenna connection.

The starting point for this investigation is a suggestion that it is possible to get a clearly defined shock electromagnetic wave in a quite short modified magnetically insulated transmittion line (MITL). The line modification resides in an inner coaxial dielectric insert. One may consider the insert as a distributed matched spark-gap. The parameters of the high voltage pulse supplying at the line input are as follows: 1.5 MV, 20-30 kA, 90 ns, front duration--30 ns. A current pulse with a front duration less than 1 ns has been observed at the line output. The output current has ranged up to about 10--12 kA. Efficiency of the sharp front formation grows owing to its dependence on the speed of the flashover front propagation. So, an estimation of this dependence is a step of great importance is solution of the problem. The flashover front speed has been estimated on physical grounds which can be derived from experimental data for the early phases of dielectric surface flashover in vacuum.

This paper introduces a new cooperative research program of national scale that is focused on crucial research issues in the development of high energy microwave sources. These have many applications in the DOD and industry. The Air Force Office of Scientific Research (AFOSR), in coopertaion with the Phillips Laboratory, the Naval Research Laboratory, and the Army Research Laboratory, has established a tri-service research consortium to investigate novel high energy microwave sources. The program is part of the DODs 'Multidisciplinary University Research Initiative' and will be funded at a rate of $DLR3.0M per year for up to five years. All research performed under this program will be unclassified. Under its auspices, HPM scientists at nine US universities will be attacking twenty-two separate research projects under the leadership of Neville Luhmann at UC-Davis, Victor Granatstein at Maryland, Magne Kristiansen at Texas Tech, Edl Schamiloglu at New Mexico, John Nation at Cornell, Ned Birdsall at UC-Berkeley, George Caryotakis at Standord, Ronald Gilgenbach at Michigan, and Anthony Lin at UCLA. To facilitate the rapid transition of research results into the industrial community, formal collaborative subcontracts are already in place with James Benford at Physics International, Carter Armstrong at Northrop, and Glen Huffman at Varian Associates. Although this new program officially only came into existence in mid-March of this year, it builds on over a decade of microwave research efforts funded by the plasma physics office at AFOSR. It also is synergistic with the ongoing Tri-Service Vacuum Electronics Initiative led by Robert Parker of NRL as well as with the AFOSR's and Rome Laboratory's long standing Advanced Thermionic Research Initiative. An overview will be given of the broad spectrum of research objectives encompassed by NUCOMR. Areas of collaboration and technology transfer will be highlighted. The areas in which the three university consortia will conduct research are described, and the connectivity to industry and to the DOD laboratories are discussed. There are a number of critical technical barriers to reaching the desired goals for high power and high energy sources. These are discussed and the planned focus of research to resolve them is also presented.

Backward-wave oscillators driven by high current relativistic electron beams are capable of producing high power coherent radiation in the ceentimeter and millimeter wavelength regimes. However, the efficiency of these devices is usually limited to 15-20% when a homogeneous slow-wave structure is used. Utilizing a two-section slow wave structure, where the spatial period of the second section is larger than that of the first section, a BWO efficiency of greater than 50% was calculated. A conceptual design of a high efficiency S-band backward-wave oscillator driven by a 500 kV, 5 kA electron beam has been developed and analyzed.

Results are presented of microwave spectral measurements on a gyrotron-backward-wave- oscillator (gyro-BWO) driven by a long-pulse, relativistic electron beam. The Michigan Electron Long Beam Accelerator (MELBA) generates 800 kV, 1-4 kA, 0.5 microsecond(s) electron beams from a cold cathode. Gyro-BWO microwave emission has shown high-power fundamental-cyclotron (4.5-5 GHz) emission (peak of approximately 40-50 MW tube power). Second cyclotron-harmonic gyro-BWO microwave emission spectra are reported here in the frequency range from 5-6 GHz, at lower tube powers (MW range). The effects of tapered magnetic fields with uniform or tapered interaction radii are to broaden the microwave spectrum over the uniform tube-field case.

The Air Force Phillips Laboratory Gyro-BWO experiment is utilizing the RAMBO pulser, with electron beam parameters of: V_D equals 300-800 kV; I_D equals 1-50 kA; pulselength equals 1-3 microsecond(s) . An annular electron beam of approximately 1-3 kA is produced by an annular aluminum cathode. The interaction cavity is designed to radiate in the frequency range of 4.2-5.5 GHz in a TE₀₁ mode. The interaction cavity has a radius of 4.37 cm and a length of 15 cm. A diode magnetic field is used for a field immersed diode, and is tunable from 0-2 kG. The interaction region magnetic field is tunable from 2-7 kG. The diode and interaction magnetic fields are used together to provide a magnetic compression of the electron beam. C-band bevel-cut antennas located at the diode end of the experiment are used to extract the backward wave. Experiments have given microwave output pulselengths of 40-500 ns, on a voltage pulselength of 200-1400 ns. Extracted power range from 0.1 to 4 MW, and is voltage dependent. Nearly constant rf power can be extracted from 4.2-5.0 GHz in small passband filters. Breakdown of the air in the C-band waveguide has been observed during operation in the TE₂₁ mode. Evidence of mode competition exists in the form of two different frequency values appearing at the same time. A (lambda) /4 slotted cavity has been designed, as well as a helical slotted cavity, in an effort to suppress the TE_n1 modes, n does not equal 0. Analysis and numerical simulations from the 2 1/2-D PIC code MAGIC are presented, as well as the latest experimental results.

Cyclotron-resonance maser (CRM) oscillator experiments in a nondispersive (TEM-mode) waveguide are reported in this paper. The table-top CRM oscillator constructed in our laboratory operates with a low-energy (< 5 keV), low-current (< 1 A) electron beam. The electron beam is rotating in the cyclotron frequency due to an axial magnetic field produced by an external solenoid. The large electron transverse velocity, needed to obtain amplification in a TEM-CRM, is achieved by a strong kicker coil. The coplanar waveguide used in this experiment supports odd and even TEM-modes, and enables cyclotron interactions with both first and second harmonics. Microwave output power at the first cycoltron harmonic is observed in the range of 3-6 GHz, where the frequency is tuned by the axial magnetic field in this range. A considerable second harmonic emission is observed around 7 GHz frequency. This experiment may lead to the developement of a new compact high-power microwave source.

A TE_10,4 gyrotron oscillator with advanced built-in quasi-optical mode converter and radial output coupling into a Gaussian mode (94.5% mode purity) has been operated at 0.46 MW with 200 ms pulse duration and total output efficiency 32% (38% electronic efficiency). The maximum output power was 0.6 MW (12 ms) at 27% output efficiency. In first proof of principle experiments this total tube efficiency was improved to 51% by the use of a single- stage potential depressed collector. In short pulse experiments (10 ms) with a TE_22,6 mode cavity about 1 MW output power was generated at 140.1 GHz with U_b equals 85 kV and I_b equals 53 A. Modes of type TE_m,6 with m equals 20 to 26 were excited in the frequency range between 136 GHz and 155 GHz with output powers of 500-600 kW at I_b equals 40 A. The measurements were carried out with a single disk output window and a nonimproved quasi-optical mode converter. First short pulse experiments with a coaxial cavity gyrotron designed for 1.5 MW output power gave 1 MW at 140 GHz (TE_28,16) and 1.3 MW at 166 GHz (TE_27,15) with efficiencies of 23% and 29% respectively.

Phase locking of a high power pulsed gyrotron oscillator through the use of a quasi-optical circulator was investigated. A second harmonic gyrotron which features a novel complex cavity, operating at 34.5 GHz, was used in the experiment. The quasi-optical circulator consisted of a 5.75 inch diameter ferrite disk biased with a one kilogauss permanent magnet. A polarizing grid was used to separate the input and output signals in the circulator. In order to couple the gyrotron oscillator output efficiently to the quasi-optical system, a number of mode converters, TE₀₃-TE₀₂, TE₀₂-TE₀₁, TE₀₁-TM₁₁, and TM₁₁-HE₁₁, were required. The insertion loss of the circulator and mode converter chain was approximately 1 dB, and an isolation exceeding 25 dB was achieved. In addition, a low power WR28 waveguide isolator was inserted in the injection signal path, providing an additional 35 dB of isolation, for a total isolation of 60 dB. The injection signal was provided by a synthesized signal generator and a 100 Watt traveling wave tube amplifier. A sample of the gyrotron output signal was obtained through an additional horn and mixed with a sample of the injection signal, producing a difference signal. The injection signal was swept slowly through a known frequency range while the difference signal was recorded. The recorded signals were analyzed off-line, and the locking bandwidth was determined. Experiments were performed for injection powers from 0-60 Watts, and a gyrotron output power of approximately 80-100 kW. Phase locking was observed for all non-zero injection powers.

Nonlinear, time-dependent calculations have been carried out for closed-cavity gyrotron oscillations using a strongly modulated electron beam. It is found that radiation pulses of width 200 ps can be generated with carrier frequency of 18 GHz at over 10% efficiency. The gyrotron features a tapered wall radius to allow an equidistant spectrum of cavity modes. Evidence of a disruption in locking is found at electron beam currents several times the start oscillation value. Applications for short pulse radiation in the millimeter and submillimeter wavelength range include radar, Fourier transform spectroscopy, plasma diagnosis, and time domain metrology. It is shown here that extremely short radiation pulses, at kilowatt power levels and high efficiency, may be obtained by mode locking a tapered cavity gyrotron using an electron beam with a modulated current density. A nonlinear time-dependent theory and particle-in-cell simulations are used to analyze the device. The two theoretical approaches assume TE_11n and TE_01n modes, respectively, which are excited by an electron cyclotron electron beam wave which is matched in phase and group velocity to these modes. The design of the oscillator cavity is outlined, as well as mode locking results as a function of beam current pulsewidth, pulse repetition frequency and amplitude.

A two-cavity gyrotron has some advantages in comparison with a traditional gyrotron. In particluar, the use of relatively short cavities with properly chosen length of the drift tube between them allows to enhance output power retaining the maximum efficiency, and to realize a tunable oscillator with high efficiency and power. The use of feedback between the cavities is proposed to solve the problem of mode competition in such an oscillator. The results of an experimental study of a Ka-band two-cavity gyrotron operating in the TE₀₂₁ cavity mode are presented. The maximum power 300 kW was achieved with efficiency of 23%, the maximum efficiency was about 30%. Three zones of generation corresponding to different phase length of the feedback loop were observed. Experiments showed that the use of feedback in a two-cavity gyrotron subdues spurious oscillations.

An experimental study was carried out to test high-harmonic gyroklystron amplifier concept. A two-cavity 35-GHz second-harmonic gyroklystron with the TE₀₂₁ cavity mode has been built and tested in pulse operation. Output power of 258 kW with efficiency 18% and 17-dB gain have been produced at 72 kV beam voltage and 20-A beam current. Bandwidth of about 0.3% have been observed. The restriction of the output power, efficiency and gain was caused by the spurious oscillations excited in the second cavity in the TE₀₁₁ mode at the fundamental cyclotron frequency.

Both a small signal approach for ac space-charge effects on beam bunching in the gyroklystron drift tube and nonlinear self-consistent field slow-time scale ballistic theory are applied to investigate the wideband gyroklystron gain including the beam velocity spread effects. The analysis of ac space-charge effects on beam bunching, based on the parameters of existing electron guns, results in the possibility of the wideband amplification in the tremendously high gain gyroklystron using the drift tube of the approximately 20 (lambda) length or more. The test results of the IAP two-stage, 16kV, 3A, 20dB gyroklystron operating in TE₀₁₁ mode are presented and discussed. Also, the test results of the Tory TE₀₁₁ three-stage, 50kV, 14A gyroklystron are discussed. Having the first drift tube of the approximately 10 (lambda) length, this gyroklystron demonstrates the 40dB gain, the 1.3% bandwidth, and the 230kW output saturated power.

In this paper we propose certain ways to construct coherent systems with different properties. They are based on the systems with strong coupling. Strong coupling is realized by direct junction of the oscillators with a common load. We investigated continuous and pulsed regimes in symmetrical and nonsymmetrical systems of oscillators. In such systems the regimes of power addition and subtraction in the common load may occur. The model theoretical and experimental studies have shown a short time of phase locking and high stability of coherent regimes with great difference between the parameters of oscillators. The experiments have been carried out in the S and K bands. These served the basis for the experiments on relativistic magnetrons coherent power addition. The results will be presented at this conference.

A coaxial dielectric-lined waveguide is considered as the slow-wave supporting structure of the dielectric Cherenkov maser amplifier. Its basic 'quasi-TEM' mode has very weak dispersion at phase velocities closed to the speed of light. This feature is attractive since it may provide the amplifier with very large bandwidth values. The system dispersion relation is derived for an infinitely thin hollow electron beam within a coaxial waveguide loaded with one dielectric liner located at either inner or outer conductor. Spatial growth rates are calculated numerically at various parameters of the slow-wave structure and electron beam. Bandwidth dependences on parameters are examined campared to those of the conventional dielectric Cherenkov maser configuration. Constructive peculiarities of the coaxial configuration allowing original approaches to the problems of electron beam dumping and RF input and output matching are discussed.

The broadband microwave radiation of electron beam-plasma interaction has gained more attention in recent years for its potential of frequency tunable. The radiation mechanisms of this interaction process are main plasma radiation and electron beam radiation. In this paper, the harmonic radiation mechanism due to nonlinear plasma current is studied. In the case of short beam-plasma interaction, only longitudinal Lamguir wave excited by the beam in plasmas is considered. The current of plasma electrons corresponding to the Langmuir wave is nonliear, and the Fourier analyses show that the current includes higher harmonic components of plasma frequency (omega) _p. Under the far-distance radiation approximation, the radiation angle distribution and the radiation power caused by the higher harmonic components of the nonlinear source current are derived. And the radiation fields have higher harmonics of (omega) _p. And the dipole radiation may play an important role in plasma radiation.

Recent backward wave oscillator (BWO) studies show the importance of the reflected forward traveling wave, end reflections, and the initial phase difference between backward propagating waves on microwave generation efficiency. A high power microwave generator using efficient interaction of a relativistic electron beam with backward and fast forward waves was investigated both theoretically and experimentally and is described in this paper. The effects of variations in phase velocities on BWO efficiency was also investigated. Theory predicts efficiencies of 35-40% for uniform and up to 60% for nonuniform slow wave structures (SWSs), in the absence of beam space charge. Furthermore, it was found that the interaction with the forward wave and the SWS inhomogeneity can counter the effect of beam space charge in reducing BWO efficiency. Experiments were performed using 'Sinus' family relativistic electron beam accelerators. Uniform relativistic BWOs with microwave power exceeding 650 MW at a radiation efficiency of 30% were achieved. Efficiencies of nonuniform BWOs reached 40% with corresponding microwave powers of 500 MW.

The several types of the relativistic klystron amplifier output circuits are discussed. The composition and properties of the electromagnetic fields in the klystron electrodynamics structures filled by a relativistic electron beam are described. The calculation results of the electron wave propagation in the relativistic klystron TW output circuits are presented. The linear theory numerical data are compared with the 'cold' experimental results. 2D investigations results of the KMT-3 relativistic klystron with the single and double gaps are analyzed.

The results of numercial modeling and experimental investigations of the linear induction accelerator operation where relativistic clystron is applied as a load are presented. The electron gun with the dielectric emitter (DE) is employed as the injector for this system. As a result of this investigation, the electro-optical system has been successfully realized allowing us to form electron beams sufficiently homogeneous in cross-section with current level of no less than 150 A. Compression of the beam from DE at the first stage of moving is supported, essentially, due to a system of focusing electrodes, similar to Pierce optics. Then, compression of the beam to the size required for its free motion in the anode tract and clystron's drift tube occurs in increasing external magnetic field. In this purpose, the configuration of tracking magnetic field was calculated and suitable magnetic system has been made. The results obtained experimentally are in good agreement with calculated data. With emitting dielectric surface of 50mm in diameter the laminar electron beam of 8mm in diameter was obtained. At accelerating voltage of 400kV and pulse duration of 120ns, required for the excitation of the X-band clystron amplifier the value of current was of the order of 200 A. Prints of the beam on targets allow us to make the same findings.

An analysis of a plasma-filled backward-wave oscillator, in which the axial magnetic field is not employed, is presented using fluid model. The operating modes, output frequencies, and linear instability are, respectively, investigated. The results show that the background plasma affects the output frequency obviously, and that the growth rates are closely related to the electron beam radius, modes of the rippled-wall waveguide, and the background plasma density. The growth rate can be increased as long as the beam radius is made larger. In addition, the present investigation also indicates that the results, given by adding an infinitely strong guide magnetic filed in the device, do not completely apply to the case of no guide magnetic field; thus the results obtained in this paper are significant for the backward-wave oscillator in which the background plasma could be used instead of the guide magnetic field to confine the electron beam against the space-charge effects.

The results of the experimental investigation of a relativistic backward-wave oscillator with an electron beam formed by a thermionic injector, are presented. The influence of vacuum conditions on the device operation was studied. Maximum radiation power 5 MW (efficiency not less than 10%) at pulse width 8 microsecond(s) that corresponded to electron current pulse duration was obtained in the X-band experiments. The microwave radiation band was near 10 MHz that corresponded to frequency change depending on voltage.

This paper represents the results of theoretical and numerical investigations on studying an amplification microwave in the vircator triode in coaxial making. The range of a parameters of the greatest higher amplification was defined for TH and TE-modes.

The investigation of electromagnetic oscillations excitation in the vircator with an external magnetic field is carried out. The conditions of producing a cyclotron wave parameter resonance with virtual cathode oscillation harmonics have been obtained where the radiation power resonance character depends on the magnetic field value.

This paper presents the experimental results of a Cherenkov free-electron laser that employs an annular relativistic electron beam with energy of (gamma) 0 <EQ 2.0 and current of I <EQ 2.5kA, passing through a dielectric-lined waveguide, to generate coherent electromagnetic radiation. Powerful Cherenkov free-electron laser radiation has been observed at a frequecny range from 8 GHz to 12 GHz. The peak power is 2MW, and the operating efficiency is about 0.6%.

The electron waves resonance properties in the one-stage and two-stage microwave Cherenkov devices based on the superdimensional slow-wave structures are investigated by the linear theory methods near (pi) -cutoff of E₀₁ waveguide mode. The variation of the generation frequencies and its starting currents, when diode voltage changes in a wide range of values, are investigated. A concept of a longitudinal electron oscillations is defined for a one-stage device. The dependencies of the generation frequency and starting current versus the drift tube length are investigated for the two-stage device in detail.

Possibilities of efficiency improvement in relativistic electron devices with initially linear electron beams by providing resonance condition for Cherenkov and cyclotron interactions simultaneously are studied theoretically and by computer simulation. The main difference from previous research is the investigation of dynamic cyclotron resonance. The dynamic cyclotron resonance occurs in strong nonlinear regimes when the electron beam is separated by electromagnetic field to groups of accelerated or decelerated particles. For strong nonlinear regimes of interaction we can select some groups of electrons that interact with electromagnetic field under the same Cherenkov resonance condition but under different cyclotron resonance conditions. It was shown that efficiency can be improved from usual value for Cherenkov interaction (25%-28%) up to 50% by choosing appropriate value of focusing magnetic field.

Ultra-wideband emitters are of interest for a variety of potential applications that range from radar transmitters to communications applications. This technology is of current interest to the USAF Phillips Laboratory where theoretical and experimental efforts have been underway for a number of years. Research into the production of ultra-wideband sources at the Phillips Laboratory has been accomplished along several different technology lines. The approaches include three main thrusts: 1) very powerful hydrogen spark gap pulsers, 2) compact hydrogen gas switches in conjunction with high gain ultra-wideband antennas and, 3) solid state switched array antennas. This paper reviews the progress-to-date along these lines and identifies some pacing research obastacles that limit further improvements.