This paper describes the progress made in recent years in addressing the technological challenges that must be met to achieve high power microwave sources for a variety of purposes. Efforts are underway to increase the pulse width and power of high power microwave sources. Research in narrow band sources has progressed toward the junction of high power and conventional tube technology. Ultra-wide band technology is at present focused on a number of promising avenues that will improve switching and antennas that promise to increase substantially the field strength projected at substantial range from the antenna. Efforts in both narrow band and ultra-wide band technology also seek to explore the potential for operating at increased pulse repetition frequency. This paper discusses these efforts and gives recommendations for future investment of research talent to these ends.

This research program investigates high power microwave generation utilizing a microsecond electron beam accelerator to study means of eliminating microwave pulse-shortening. The particular device under study is the coaxial gyrotron oscillator in the S-band frequency range. Experiments have concentrated on three types of gyrotron cavities: (1) coaxial, unslotted, (2) coaxial, slotted, and (3) noncoaxial, unslotted. The first major result is that the coaxial rod raises the limiting current in the e-beam diode, permitting reliable, higher current extraction into the microwave tube. The second major finding is that the slotted cavity gives the highest peak powers (approximately 90 MW) but very short pulselengths (approximately 10 - 20 ns). The unslotted coaxial gyrotron emits power levels of 20 - 40 MW with longer pulselengths (up to 40 ns). The noncoaxial gyrotron radiates lower peak power levels (approximately 20 MW). All of the gyrotron types exhibited signs of the pulse shortening mechanisms of mode hopping and mode competition as diagnosed by time-frequency-analysis (TFA). TFA also shows that lower power microwave oscillation is maintained over some 300 ns, but the power level may be modulated due to e-beam voltage fluctuations.

A new method of electron cyclotron emission is reported in which electrons emit stimulated radiation at frequencies that are greater than the usual Doppler-shifted electron cyclotron frequency by orders of magnitude. In particular, stability properties are analyzed of a relativistic electron beam gyrating in a uniform magnetic field with a strong spatiotemporal correlation in electron gyrophases. Both field amplitude equations and dispersion relations have been obtained for small-amplitude electromagnetic and electrostatic waves on a spatiotemporally gyrating relativistic electron beam. Detailed properties of such a system are studied numerically. It is shown that, if the phase velocity of the gyration pattern is close to the axial velocity, then stimulated radiation emission occurs at a frequency that is must greater than the Doppler-shifted electron frequency in both relativistic and mildly relativistic regimes. Numerical examples are presented for cyclotron maser interactions at 35 GHz and 94 GHz for electron beam voltages ranging from 100 kV to 500 kV confined by an axial magnetic field less than 6 kG.15

Electron beam halo formation is studied as a potential mechanism for electron beam losses in high-power periodic permanent magnet focusing klystron amplifiers. In particular, a 2D self-consistent electrostatic model is used to analyze equilibrium beam transport in a periodic magnetic focusing field in the absence of radio-frequency signal, and the behavior of a high-intensity electron beam under a current-oscillation-induced mismatch between the beam and the periodic magnetic focusing field. Detailed simulated results are presented for choices of system parameters corresponding to the 50 MW, 11.4 GHz periodic permanent magnet focusing klystron experiment performed at the Stanford Linear Accelerator Center. It is found from the self-consistent simulations that sizable halos appear after the beam envelope undergoes several oscillations, and that the residual magnetic field at the cathode plays an important role in delaying the halo formation process. Finally, a confinement criterion is obtained for a highly bunched beam propagating through a perfectly conducting drift tube in a uniform magnetic field.

Crossed-field electron vacuum devices are resonant devices. When properly tuned, they operate at a single frequency and have an average background distribution. Thus one can use the cold-fluid equations and a Fourier decomposition to separate the physical quantities into a background (DC) mode and a pump (RF) mode. We have improved our previous calculations on these devices and can now understand how the background plasma density varies and evolves as the RF wave travels down the slow-wave structure. We study the evolution of an RF pump wave through the device and find that in general, chaotic (period-2) instabilities can occur if the device is too long. We also present results for the high magnetic field case, (typical CFA/magnetron regime), for the moderate magnetic field case (ultra-low noise regime), and discuss how these solutions correspond to device operation. Lastly, we discuss our results and point out future work in need of study.

This paper analyzes the effects of space charge shielding on the steady state of multipactor discharge on a dielectric. Analytic methods are used to obtain an exact function for the potential in the discharge, assuming a Maxwellian distribution of emitted electrons. An equation for the amount of power deposited on the dielectric by the multipactoring electrons, for a given saturation level is given. A simple method for obtaining the saturation level, for a given material, is obtained.

The general form is derived, from first principles, for the collective bremsstrahlung recoil force on a test particle participating in a bremsstrahlung process in a relativistic nonequilibrium beam-plasma system. The force is expressed in terms of the particle and photon distribution functions and the bremsstrahlung transition rate. This relationship is useful in calculations of collective radiation processes and the conditions for the occurrence of bremsstrahlung radiative instability in relativistic beam-plasma systems.

Microwave window breakdown is investigated in vacuum and atmospheric conditions. An S-band resonant ring with a frequency of 2.85 GHz and a power of 80 MW with a 4 MW magnetron as a source is used. Window breakdown on the vacuum side is simulated using a dielectric slab partially filling an evacuated waveguide. Various high-speed diagnostic methods yield a complete picture on the breakdown phenomenology, with far reaching similarities to dc surface flashover. During the initiation phase, free electrons are presented, which can be influenced by magnetic fields, followed by a saturated secondary electron avalanche with electron-induced outgassing. Final breakdown occurs in the desorbed gas layer above the surface. In order to simulated window breakdown on the gas-side, a segment of the resonant ring separated by two windows was filled with gas at variable pressure, and breakdown was initiated by field- enhancement tips on one of the gas-side surfaces. Threshold power densities for breakdown are measured, and first results on the phenomenology of this gas breakdown are compared with the processes of flashover in vacuum.

Our group first reported the operation of a relativistic backward wave oscillator (BWO) in the so-called `cross- excitation' regime in 1998. This instability, whose general properties were predicted earlier through numerical studies, was a consequence of using a particularly shallow rippled- wall waveguide (slow wave structure--SWS) that was installed in the experiment to diagnose pulse shortening in a long pulse electron beam-driven high power microwave (HPM) source. This particular SWS was required to accommodate laser interferometry measurements during the course of microwave generation. Since those early experiments we have further studied this regime in greater detail using two different SWS lengths. We have invoked time-frequency analysis, the smoothed-pseudo Wigner-Ville distribution in particular, to interpret the heterodyned signals of the radiated power measurements. These recent results are consistent with earlier theoretical predictions for the onset, voltage scaling, and general behavior for this instability. This paper presents data for a relativistic BWO operating in the single frequency regime for two axial modes, operating in the cross-excitation regime, and discusses the interpretation of the data, as well as the methodology used for its analysis. Although operation in the cross-excitation regime is typically avoided due to its poorer efficiency, we discuss how it may be exploited in HPM effects studies.

A series of experiments have been conducted to investigate the critical mechanisms involved in pulsed rf breakdown. This research has examined fundamental issues such as microparticle contamination, grain boundaries, residual gas, pulse duration, field emission, and the spatial distribution of plasma during a breakdown event. The motivation of this research is to gain a clearer understanding of the processes involved in breakdown and to determine methods to increase the breakdown threshold thereby increasing the available power in high power microwave sources and accelerator components.

X-band klystrons capable of 75 MW and utilizing either solenoidal or Periodic Permanent Magnet (PPM) focusing are undergoing design, fabrication and testing at the Stanford Linear Accelerator Center (SLAC). The klystron development is part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). SLAC has completed a solenoidal-focused X-band klystron development effort to study the design and operation of tubes with beam microperveances of 1.2. As of early 2000, nine 1.2 (mu) K klystrons have been tested to 50 MW at 1.5 microsecond(s) . The first 50 MW PPM klystron, constructed in 1996, was designed with a 0.6 (mu) K beam at 465 kV and uses a 5-cell traveling-wave output structure. Recent testing of this tube at wider pulsewidths has reached 50 MW at 55% efficiency, 2.4 microsecond(s) and 60 Hz. A 75 MW PPM klystron prototype was constructed in 1998 and has reached the NLC design target of 75 MW at 1.5 microsecond(s) . A new 75 MW PPM klystron design, which is aimed at reducing the cost and increasing the reliability of multi- megawatt PPM klystrons, is under investigation. The tube is scheduled for testing during early 2001.

An integral part of any vacuum rf device is the cathode. Many rf and microwave tubes utilize thermionic cathodes. However, these cathodes are generally limited to current densities less than 100 A/cm², a limitation too great for the majority of High Power Microwave tubes. At the Air Force Research Laboratory, Directed Energy Directorate, we have to study a variety of explosive emission cathodes. This paper presents results on studies of several types of cathodes tested in a simple circular geometry. We also present results of research on different types of anode material. The data includes measurements of current, voltage, cathode lifetime, and cathode/anode out-gassing.15

Low jitter, triggered spark gap switches are highly desirable components for a wide variety of High Power Microwave applications. In particular, this switch enables a wide range of new applications ranging from protection circuitry to high power transient arrays that are currently technically infeasible with the considerable jitter associated with high power switches. The device presently under advanced development is a novel triggering scheme using a ferroelectric ceramic as the electron source coupled with a high gas flow rate which allows reliable triggering at low trigger voltages, even at high repetition rates. The switch chamber is tailored to reduce field stresses and provide a low inductance current path with a very compact geometry. The high gas flow rate allows the replacement of the gas in the discharge region within the time scale necessary to sustain the required repetition rates of up to 1000 Hz.

Recent advances in the understanding of dynamical systems and chaotic behavior have resulted in the investigation of HPM source design issues. Modern dynamical systems theory can improve our understanding of the dynamics of space charge dominated beams and the RF waveforms generated by them. This paper will review the work done to date using time series analysis techniques to study the state space dynamics of high power microwave sources using simulation (particle-in-cell) code results. Low-dimensional chaos has been observed in simulation results from a variety of HPM sources, including the MILO (Magnetically Insulated Line Oscillator). Additionally, the particle behavior within the diode portion of HPM tubes can have chaotic characteristics. Knowing when these features occur and how they develop are important first steps in our ability to control and/or eliminate them. Central to understanding source behavior is the initial use of joint time frequency analysis to assess whether the dynamics are stationary or not. Subsequently we use delay coordinate embedding techniques to reconstruct an effective state space global dynamics. From this, Poincare sections are examined. Lyapunov exponents are then calculated to determine whether the behavior of the source is noise or deterministic chaos.

Fast rising, high-voltage, low jitter trigger pulses have been in high demand for a variety of applications. A recent application required triggering a compact Marx generator with very low trigger jitter. The lateral gallium arsenide photoconductive semiconductor switch (PCSS) being developed at the Air Force Research Laboratory (AFRL) has been implemented in this application. PCSS technology is an attractive solution because of the very low trigger jitter, high voltage switching capability and inherent trigger isolation properties. The PCSS has been packaged into an integrated unit for triggering a Marx generator. The Photoconductive Trigger Generator (PTG-30) requires external 24 VDC, TTL trigger and pressurized insulating gas for operation. Performance characteristics of the PTG-30 are variable output voltage from 5 to 13 kV, in four discrete steps. At maximum output voltage, the pulse risetime is approximately 350 pS. The high-voltage output pulse has an average of 22 ps rms temporal switching jitter with respect to a fast-rising trigger signal. The PTG-30 incorporates all required support components internal to the unit, including: laser diode and driver assembly, solid state FET-based pulse modulator, high-voltage DC to DC converter, and a pulse forming line. The versatility of this unit also allows for direct connection to a variety of antennas and other loads. This paper will discuss the manufacture and performance characteristics of the PTG-30 as well as experimental results from triggering a coaxial Marx generator.

In support of cathode development at the Air Force Research Laboratory, a new ultra-high vacuum cathode test facility is being assembled to complement the existing repetition-rate test pulser. The existing test bed is a 500 kV, 100 Ohm, 1 microsecond(s) duration pulser capable of firing at up to 1 Hz. The new facility is designed to operate at lower voltages (20 - 200 kv), lower impedance (50 - 75 Ohm), and variable pulse lengths (200 - 800 ns) in a single-shot mode. This Threshold Cathode Test Facility (TCTF) will be used to generate data regarding emission turn-on field strengths, outgassing volumes and constituents, vacuum level effects, and anode effects for a variety of field-emitting and explosive- emitting cathode materials. Presented herein are the design parameters of TCTF including diagnostic capabilities and electrostatic simulations of the diode region both with and without beam current.

We have designed, built, and tested two 18-inch diameter antennas, a reflector Impulse Radiating Antenna, and a Pulse Radiating Antenna Element. We provide extensive measurements of the two antennas, made both in the time domain range of Farr Research, and at the frequency domain range of Mission Research. We then evaluate these antennas for possible use as a lightweight component in both a Synthetic Aperture Radar system and in a Remote Target Identification system.

The results of experimental tests of the wide-bandwidth relativistic microwave amplifier,--dielectric Cherenkov maser,--are reported that revealed plasma presence in its interaction space, and the analysis of electromagnetic properties of a hybrid system,--dielectric-lined waveguide loaded with a plasma layer,--is presented. Experiments with the high-current electron beam showed the effect of strong enhancement of output microwave power by means of an external driver signal at different X-band frequencies, however, the said enhancement was poorly reproducible. Circumstantial evidences were obtained indicating that plasma could be produced at the dielectric surface from electron bombardment of the liner as well as from an rf breakdown caused by a microwave drive signal. The dispersion relation of the system with magnetized near-surface hollow plasma have been derived, and the spectra and electromagnetic field radial distributions for both waveguide and plasma modes have been analyzed depending on the plasma density and layer thickness. It has been found that for dense plasma, even a very thin layer gives a strong frequency upshift of a waveguide mode and reduces its coupling impedance at a given frequency whereas plasma of modest density improves the coupling impedance. Features of usual and hybrid waveguide and plasma modes are discussed in detail. It has been shown that for the hollow plasma configuration, a hybrid plasma mode is characterized with the longitudinal electric field component sign change across the layer.

In this paper, we have researched modulation of the space charge in MILO and obtained that the first order component of the harmonic current will enhance with the increasing of the voltage on the diode and reduce with the increasing of the operating frequency and the length of the slow-wave structure.

Microwave pulse compressors are of interest for plasma experiments, particle accelerators and, hopefully, radar. At the shortest microwaves (at frequencies higher than 10 GHz), high power compressors must be based on use of oversized, quasi-optical, mode selective structures.

A means for removing the parasitic feedback in microwave amplifiers, the main obstacle to achieving a high gain, is described here. The method is based on utilizing the resonant interaction between fast cyclotron waves on an electron beam and the electromagnetic waves that are propagating in the opposite direction. This is an effective method to prevent detrimental self-excitation in amplifiers whose operation is based upon the stimulated Cherenkov radiation of a forward-propagating electron beam in a guiding magnetic field. Conditions for the resonant interaction are provided by proper choice of the guiding magnetic field. At such resonances the counter-propagating waves are in stop-bands and, therefore, cannot propagate. Results of theoretical and experimental investigations of cyclotron absorption of counter-propagating waves in amplifiers are given in the present work. It is shown that the resonant cyclotron interaction leads to a complete suppression of the feedback and that the threshold of self- excitation becomes unachievable even for large reflections. Only a minor decrease in the amplification results in comparison with an ideal amplifier without reflections. It follows from these results that a spatially varying magnetic field can be applied along the axis of the amplifier to expand the zone of the cyclotron absorption and thereby exclude a re-tuning of the self-excitation frequency.

Evolution of initial perturbation in the case of electron beam instability development in various systems is investigated generally. The approach is based on equation for slowly varying amplitude of induced wave packet. It is shown that the space-time dynamics of fields in the case of beam instability development is described by partial differential equation of third order independently on system geometry, presence of external fields, specific parameters etc. This equation is solved and analytical expression for fields' space-time structure is obtained and analyzed. The shape of induced wave packet is presented. The velocities of unstable perturbations vary between the group velocity of resonant wave (without beam) and beam velocity. The dependence of the growth rates of the perturbations from their velocities is obtained.

The Microwave Facilitated Atmospheric Energy Projection System (MFAEPS) uses microwave energy at resonant frequencies of oxygen that occur around 60 GHz or the lone absorption peak at 118 GHz to reduce the atmospheric concentration of O₄⁺ a thermally unstable cluster ion that is vital to the most significant reaction for squelching electron transport via recombination in cold dry air: O₄⁺ + e^- equals O₂ + O₂. A voltage pulse generator discharges creating an ionizing wave that follows the microwave pulse. The plasma created by the voltage pulse generator's discharge propagates in the path created by the microwave pulse because the heated oxygen can form less O₄⁺ and thus a lower impedance pathway is created for the avalanche progression of the plasma. The microwave pulse heats O₂ in its path from 20 K to 700 K above ambient temperature. Multiple ionizing waves can be launched in rapid succession down the same path to deliver more current to the target. The neutral but paramagnetic oxygen molecule absorbs energy at the 60 GHz and 118 GHz frequencies because the magnetic moment of the oxygen molecule interacts with the rho-type triplet rotational ground states of oxygen. MFAEPS can be scaled up from environmentally friendly non-lethal crowed control to electronic warfare counter measures and anti-cruise missile defense systems.

An experimental investigation of the generation in a 8-mm wavelength range in a one-sectional overmoded slow-wave structure (SWS) near the high-frequency boundary of the TM₀₁-mode passband versus the SWS length and electron beam geometry has been made. An optimization of a two-sectional multiwave Cerenkov generator has been carried out on the basis of the obtained results. The generation of the radiation with a 8.84-mm wavelength has been realized. The power radiated into the atmosphere was 500 - 600 MW at the efficiency of 6 - 7%, 20-ns pulse length at a half-maximum, and spectrum width being not over 0.5%. The possibility to obtain the linearly polarized radiation with the power to 150 MW in a one-sectional SWS under condition of using a hollow electron beam with an ellipsoidal profile has been shown.

Numerical simulations of the eigenwave properties of the experimental corrugated slow-wave structure in the lowest and two higher its passbands for the TM-type waves have been carried out by means of the electrodynamical methods of linear theory of the Cerenkov type devices. A detailed comparative analysis of the frequency dependencies obtained at these passbands for the transmission coefficients as well as the mode analysis of the passed and reflected power flows have been made. Longitudinal distributions of the electric field axial component at resonance frequencies of the used structure have been considered. The electrodynamical properties of the experimental corrugated slow-wave structure in its two higher passbands are shown to be similar to each other but appreciably differing from the ones in the lowest passband of the E₀₁ mode. This is explained by a hybrid character of the waves in the indicated higher passbands of the used structure.

Resonance properties of a backward-wave oscillator on the basis of an overmoded open resonator have been investigated by the electrodynamical methods of linear theory of the Cerenkov type microwave devices. The dispersion diagrams of the eigen electron waves of a corrugated slow-wave structure loaded by a high-current electron beam with different azimuthal indices have been calculated. Starting conditions of generation of microwaves in the microwave devices based on them have been determined depending on the number of corrugations and the diode voltage value. The obtained results are compared in detail with the well-known experimental data.

Gallium arsenide photoconductive semiconductor switches (PCSS) are being studied as enabling technologies for a variety of applications. High grain PCSS can be triggered with small laser diodes or laser diode arrays. Some of the applications require low temporal jitter of the switches relative to the trigger laser. The purpose of this study was to compare the temporal switch jitter times for different systems: we varied the type of trigger laser and its risetime, the type of pulse charger and transmission line that was discharged through the PCSS, and the geometry of PCSS used. One of the PCSS was an opposed contact PCSS geometry used by the Air Force Research Laboratory. The other was a coplanar geometry switch made by Sandia National Laboratories. It is found that the optical trigger laser characteristics are dominant in determining the PCSS jitter while the nature of the contact geometry (opposed or coplanar) is not as important.

In this paper we suggest a high power CW mm-Wave source driven by a compact electron storage ring with modest size and energy. The source is working as a `power transformer' of a standard RF power into mm-waves via coherent radiation of fully bunched electron beam. The mm-wave power in a form of TEM Gaussian beam is generated in a high-field long period (high-K) helical wiggler. The wiggler provides for an effective energy exchange between the electron beam and the TEM mm-wave. The storage ring is designed to preserve electron beam with long (multi-hours) life-time. An optical resonator is used for feed-back to make the mm-wave FEL system an oscillator. The storage ring and mm-wave FEL provide for the bunching of the e-beam into mm-size buckets and for amplification of mm-wave inside the cavity.