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Experiments have been performed in testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high-power microwave performance. Magnetic priming consists of N/2 azimuthal magnetic perturbations applied to an N-cavity magnetron for rapid generation of the desired number of electron spokes for the [pi]-mode. Magnetic perturbations were imposed by utilizing three high-permeability nickel-iron wires embedded beneath the emission region of the cathode, spaced 120[degrees] apart. Magnetic priming was demonstrated to increase the percentage of [pi]-mode shots by 15% over the baseline case. Mean peak power for [pi]-mode shots was found to be higher in the magnetically primed case by almost a factor of two. Increases in mean microwave pulsewidth were also observed in the magnetically primed case when compared to the unprimed case (66-ns primed versus 50-ns unprimed). Magnetron starting current for the magnetically primed [pi]-mode exhibited a reduction to 69% of the unprimed baseline starting current. Index Terms--High power microwaves, magnetron, mode competition, priming, startup.

The effects of small random fabrication errors on the small-signal characteristics of a traveling wave tube are examined by using the classical 1-dimensional small-signal theory of Pierce. The results have shown that the effects of perturbation in the circuit phase velocity have dominated the effects of perturbations in the gain parameter and the cold-tube circuit loss.

Plasma breakdown at dielectric windows in the output of high power microwave systems has become an important challenge with the advent of GW power level sources. The modeling and simulations have played a crucial role in understanding the physics of plasma breakdown. In this work, we report a number of recent advances which have significantly improved understanding of the physics of vacuum multipactors, gaseous discharges driven by microwaves, and the transition from one regime to the other by varying the gas pressure. Index Terms--Microwave windows, dielectric breakdown, vacuum breakdown, multipactor, microwave discharges.

The simple first-principles approach to modeling tumor size to account for spatial effects is extended including the development of the necrotic radius, by making four primary assumptions. Excellent agreement is obtained with experimental data and the final steady-state viable rim thickness.

A phase-locking model is developed from circuit theory to qualitatively explain the various regimes observed in magnetron injection-locking experiments. The model recovers quantitatively many of the phase-locking frequency characteristics observed in the experiments.

A demonstration that magnetic priming hastens the startup of relativistic magnetrons is conducted using 2-D MAGIC simulations. Three electron spokes are formed almost three times faster when magnetic priming is used (36 down to 13 ns.) and it was found that magnetic priming can provide rapid startup and mode-locking in relativistic magnetrons.

Single particle orbit considerations show that the cycloidal orbits of electrons in a gap with crossed electric and magnetic fields lead to rapid spoke formation if the external magnetic field has a periodic variation. This spoke formation is primarily a result of kinematic bunching, which is independent of the radio frequency electric field and of the space charge field. A parametric instability in the orbits, which brings a fraction of the electrons from the cathode to the anode region, is discovered. These results are examined in light of the recent rapid startup, low noise magnetron experiments and simulations that employed periodic, azimuthal perturbations in the axial magnetic field. Index Terms--Low-noise, magnetron, microwave, parametric instability, vacuum electronics.

Azimuthally varying axial magnetic fields have been utilized to perform 'magnetic priming' of magnetrons for rapid startup, low noise, and mode control. An overview of the latest magnetic priming experimental and simulation results are presented in this paper. Magnetic priming experiments in dc-operating microwave oven magnetrons show sideband elimination, even with the cathode heater turned off. Simulations using three three-dimensional (3-D) improved concurrent electromagnetic particle-in-cell (ICEPIC) codes with two different computational algorithms recover the oven magnetron experimental results obtained with magnetic priming including fast mode growth, rapid spoke formation, and the tendency toward lower noise operation. A new, axially symmetric, azimuthally varying magnetic field geometry for oven magnetrons is explored and preliminary results are reported. Simulations using two-dimensional (2-D) MAGIC code for the University of Michigan/Titan relativistic magnetron show that the oscillation startup time can be dramatically decreased (almost by a factor of 3) and mode competition can be suppressed with magnetic priming. Index Terms--High-power microwaves, magnetron, microwave noise, mode competition.

The fundamental Child-Langmuir limit on the maximum current density in a vacuum between two infinite parallel electrodes is one of the most well known and often applied rules of plasma physics. We develop a simple model using vacuum capacitance, conservation of energy, and conservation of charge to derive the Child-Langmuir space-charge-limited emission. This capacitive model provides physical insight into the origins of the well known [(voltage).sup.3/2]/[(gap distance).sup.2] scaling of the classical current density and does not require the solution of the nonlinear differential equation normally associated with the Child-Langmuir formulation. In addition, the full space-charge-limited solution is reproduced without imposing the condition that the electric field be driven to zero at the cathode surface.

This article reviews the advances in the science and applications of energy beam ablation of materials at the University of Michigan. Types of energy beams applied to materials ablation are: excimer lasers, laser-ablation-assisted plasma discharges, and channelspark electron beams. The characteristics of the ablation plasma plumes generated by these energy deposition sources are discussed and compared. Plume expansion velocities are in the range of cm/[[micro]seconds] for all three types of ablation plumes. Electron beam ablation plumes show higher ionization states than laser ablation plumes. Index Terms - Ablation, e-beam, laser-ablation.

Results are summarized of experiments on a gyrotron utilizing a rectangular-cross-section (RCS) cavity region. The major issue under investigation is polarization control of microwave emission as a function of magnetic field. The electron beam driver is the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V = 0.8 MV, [I.sub.diode] = 1-10 kA, [I.sub.tube] = 0.1-0.5 kA, and [t.sub.e-beam] = 0.4-1.0 [micro]s. The annular e-beam is spun up into an axis-encircling beam by passing it through a magnetic cusp prior to entering the RCS interaction cavity. Experimental results show a high degree of polarization in either of two orthogonal modes as a function of cavity fields. The RCS gyrotron produced peak powers of 14 MW in one polarization ([TE.sub.10]) and 6 MW in the cross-polarized mode ([TE.sub.01]). Electronic efficiencies for this device reached as high as 8% with transverse efficiency of 16%. Experimental results on the beam alpha ([Alpha] = [V.sub.[perpendicular]]/[V.sub.//]) diagnostics, where alpha is the ratio of the e-beam's transverse velocity to its parallel velocity, agree well with the single electron trajectory code. MAGIC code results are in qualitative agreement with microwave measurements. Microwave emission shifts from the dominant fundamental mode polarization ??, to the next higher order mode polarization ?? as the solenoid magnetic field is raised from 1.4-1.9 kGauss. Frequency measurements using heterodyne mixers support mode identification as well as MAGIC code simulations.

We use a simple transmission line model to evaluate the rf power deposited on a dielectric window by multipactor discharge. The calculation employs Monte Carlo simulation, using realistic secondary electron yield curves as input, and taking into account the distributions in the emission velocities and emission angles of the secondary electrons. Beam loading on the external rf, as well as the evolution of the dc electric field due to dielectric charging, are also accounted for. It is found that the buildup of the multipactor space charge, rather than beam loading, causes saturation. Over a wide range of operating conditions and materials, it is found quite generally that the multipactor delivers on the order of 1 percent, or less, of the rf power to the dielectric. A simple estimate is given in support of this ratio, using the susceptibility diagram that was constructed from kinematic considerations. Comparison with experimental results is given.

Microwave pulse shortening experiments have been performed on a rectangular-cross-section (RCS) gyrotron driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters V = -800 kV, [I.sub.tube] = 0.3 kA, and pulselengths of 0.5-1 [micro]s. Pulse shortening typically limits the highest (10 MW level) microwave power pulselength to 100-200 ns. Potential explanations of pulse shortening are being investigated, particularly plasma production inside the cavity and at the e-beam-collector. We report the first optical spectroscopy diagnostic measurements inside an operating gyrotron as a means of exploring plasma effects on pulse shortening. Plasma hydrogen H-[Alpha] line radiation has been characterized in both time-integrated and temporally-resolved measurements and correlated with microwave power/cutoff. Hydrogen is believed to originate from water absorbed on internal tube surfaces in the gyrotron.

A study was conducted to examine the surface instability of multipulse laser ablation on metallic targets. The laser ablation plume was generated using a KrF excimer laser on aluminum targets while the stainless-steel vacuum system was evacuated utilizing a rotary pump-backed turbomolecular pump. Calculated cumulative growth was represented as a function of the number of pulses. Results indicated that the plume's density profile was time dependent.

Computer modeling has been utilized to guide gyrotron backward-wave oscillator (gyro-BWO) experiments at the University of Michigan over a wide range of tapered interaction regions and tapered magnetic fields. E-GUN code is used to examine beam and diode characteristics, while MAGIC is used to analyze the dynamics of the problem, such as particle kinematics and microwave power production. Several innovative techniques are used to create matching boundary conditions for a backward propagating wave. MAGIC simulations predict optimum performance of the gyro-BWO operating in a [TE.sub.01] mode within a combination of uniform interaction region and a tapered axial magnetic field which increases 7.5% in the direction of beam propagation. Experiments have been performed to investigate the effects of tapering magnetic fields and tapered interaction region radii on the high-power microwave emission from the gyro-BWO over the frequency range from 4.0 to 6.0 GHz. These experiments were performed on the Michigan Electron Long-Pulse Accelerator (MELBA) with parameters: V = -0.7 to -0.9 MV, [I.sub.diode] = 1-10 kA, [I.sub.tube] = 1-4 kA, [[Tau].sub.e-beam] = 0.4-1.0 [[micro]second]. Tapered interaction regions of 37%, 23%, 9.4%, and 6.4% were built and tested to determine their effect on microwave power, pulselength, and inferred energy compared to the uniform interaction region. Magnetic tapering trim coils with a range of -10.6% < [Delta]B/[B.sub.o] < + 15.0% were constructed which allow the orientation of the field taper to be changed without breaking the vacuum. The peak microwave power from individual shots was from 30 to 55 MW. Experiments on magnetic field tapering indicate that positive tapered fields improve microwave power and energy output.

An experimental program is developed to study the cumulative beam breakup instability (BBU) of electron beam accelerators. The program yields more information on the physics of BBU instability. The development of BBU in a low density cavity system is predicted by the use of continuum model. A discrete cavity model yields more accurate predictions. The BBU instability is caused by beam loading at high electron beam densities.

A sophisticated model for the characterization of cavity coupling for the control of beam breakup growth was presented. The model took time delay between coupled cavities into account and covered the possibility of wave cutoff and resonance in the coupling path. Dominant beam breakup mode peak spatial exponentation was decreased by a factorof two. Although other modes were destabilized, they exhibited lower growth rates.

A simple derivation of the new scaling of Child--Langmuir law in the quantum regime is presented. Based on a dimensional argument of the Schrodinger equation and the Poisson equation, the limiting current in the deeply quantum regime is found to be proportional to the square root of the gap voltage and to the inverse fourth power of gap spacing. The importance of electron exchange-correlation interactions in the quantum regime is discussed. Index Terms--Beams, cathodes, nanotechnology, space charge.

By the use of a simple model, we explicitly incorporate the coupling between the driver cavity and the booster cavity in a relativistic klystron amplifier (RKA). We show that this RKA configuration may turn into an injection-locked oscillator only when the beam current is sufficiently high. Other features revealed by this model include: the downshifted frequency mode ('0' mode) is unstable whereas the upshifted frequency mode ('[Pi]' mode) is stable; the growth rate of the '0' mode is relatively mild so that the oscillation can start only in an injection-locked mode; the oscillation does not require the presence of reflected electrons; and the separation of the cavities must be sufficiently short. These, and other features, are found to be in qualitative agreement with the recent experiments on the injection-locked relativistic klystron oscillator (RKO) that were conducted at Phillips Laboratory.

Beam breakup (BBU) is analyzed theoretically and experimentally for annular electron beams, and compared to BBU in a pencil beam. Annular beams carry six times more current than an axis-focussed pencil beam due to their increased cross-sectional area and limiting currents. BBU stimulation due to rf action of the magnetic field is more important than that due to rf action of the electrical field of defecting mediums. An annular beam close to a metallic drift tube wall with a small axial electric field suffers BBU due to reflected magnetic field action rather than due to the electric field.

Several studies are conducted to explain the different scaling laws employed for the electrical contact resistance with dissimilar materials. The parametric dependence of the contact resistance of the materials on the geometry of different regions is also analyzed.

The conformal mapping technique is used to study the field improvement of several rectilinear geometries, including a single rectangular ridge, a trapezoidal ridge, and their superposition. The results revealed that the composite field improvement factor of the double ridge with a microprotrusion on top of a macroprotrusion is dominated by the product of the individual protrusions' field enhancement factors over a very wide range of geometric aspect ratios, as assumped by Schottky product rule.