S.A. Anderson

Cathode effects on a relativistic magnetron driven by a microsecond e-beam accelerator

Experiments have been performed on a relativistic magnetron driven at e-beam accelerator peak parameters: voltage = -0.4 MV, current = 16 kA, and pulselength = 0.5 /spl mu/s. The magnetron is a six-vane device operating at about 1 GHz with extraction from two cavities. For equal power in both extraction waveguides, the peak microwave power of this device is between 200 and 300 MW. Microwave pulse-shortening limits pulselengths to the range of 10-100 ns. Time-frequency analysis of microwave emission indicates operation at about 1.03 GHz, close to the pi mode frequency identified from cold tests and the three-dimensional MAGIC code. Two cold cathodes were tested: 1) an emitting aluminum knob in the vane region with no endcap and 2) an extended cathode with a graphite fiber emission region in the vanes and endcap outside the vanes. Electron endloss current has been measured for the two cathodes. With no endcap, the cathode exhibited endloss current fraction up to 50% of the total; with one endcap, the cathode reduced the endloss current fraction to as little as 12%. Both cathodes produced peak total-electronic efficiency in the range of 14%-21%.

Radio-frequency plasma cleaning for mitigation of high-power microwave-pulse shortening in a coaxial gyrotron

Results are reported demonstrating that radio-frequency (rf) plasma cleaning is an effective technique for mitigating microwave-pulse shortening (i.e., lengthening the pulse) in a multimegawatt, large-orbit, coaxial gyrotron. Cleaning plasmas were generated by 50 W of rf power at 13.56 MHz in nitrogen fill gas in the pressure range 15–25 mTorr. Improvements in the averaged microwave energy output of this high-power-microwave device ranged from 15% to 245% for different initial conditions and cleaning protocols. The mechanism for this improvement is believed to be rf plasma sputtering of excess water vapor from the cavity/waveguide and subsequent removal of the contaminant by cryogenic vacuum pumps.

Relativistic magnetron experiments and novel theory on the limiting current in a relativistic, magnetically-insulated diode

This paper is a progress report on the relativistic magnetron experiment. It also presents, for the first time, the maximum emission current density for time-independent, relativistic, cycloidal electron flows in a magnetically insulated diode.

Two-dimensional optical emission imaging of a XeCl discharge in a microwave resonant cavity

Optical emission of a XeCl excimer discharge, within a 1.2-cm diameter quartz discharge tube, excited by 2.45-GHz microwaves in an Asmussen resonant cavity, operating in the TM/sub 012/ cavity mode, has been imaged onto a two-dimensional (2-D) charge-coupled device (CCD) camera. Spatial imaging of the discharge provides information on the heating regions of the cylindrical plasma as well as the contraction of the plasma diameter with increasing pressure. This technique also provides a real-time diagnostic for production of ultra-violet (UV) emission using a UV bandpass filter.

Relativistic L-band magnetron and nonrelativistic oven magnetron experiments

Summary form only given, as follows. Investigation is underway of the effects of space charge densities and impedances of the relativistic L-band magnetron and nonrelativistic oven magnetron. A Maxwell Physics International L-band magnetron is being driven by the MELBA accelerator. Parameters attained include: voltage=-400 kV, current=1-10 kA, and pulselengths=0.1-0.5 microseconds. Time frequency analysis of the heterodyned microwave signals finds generation at two frequencies 1.04 GHz and 0.95 GHz in the relativistic magnetron. Measured microwave power has been generated of at least 150 MW out of one cavity (with 2 output cavities). Microwave power versus beam current and beam pulselength will be presented. Stark broadening of H lines is being explored as an electric field diagnostic. Commercial kilowatt oven magnetrons operating in S-band (2.45 GHz) are also being investigated. Oven magnetron efficiency and time-resolved frequency spectra are studied under varying parameters.

Pressure dependence of XeCl excimer plasmas excited by microwaves

Summary form only given, as follows. High-pressure excimer lamp discharges are of interest in the manufacturing industry as a source of intense ultraviolet light for such applications as the curing of inks, coatings, and adhesives. Microwave pumped excimer discharges play an important role in the development of such lamps by eliminating the use of electrodes and thus increasing lifetimes, providing intense ultraviolet emission in desired wavelength bands, and offering high overall coupling efficiency. In this work, an Asmussen microwave resonant cavity is used to excite a XeCl excimer discharge within a range of 50W-150W of absorbed power at pressures up to several hundred Torr. A continuous wave power supply at a fixed frequency of 2.45 GHz is used to produce a XeCl excimer discharge in a resonant cavity operating in the TM/sub 012/ cavity mode. Coupling efficiencies from the microwave supply to the discharge are typically greater than 80% and as high as 94%. The discharge is contained within a quartz tube that runs along the center of the resonant cavity with Xe:Cl/sub 2/ ratios ranging from as low as 1:2 to as high as 10: 1. Forced airflow is used to cool the surface of the discharge,tube and improve molecular formation at higher pressures. As pressure is increased, the diffuse volume of the plasma shrinks to form very intense filaments within the discharge. Higher power levels are needed to sustain a stable discharge at higher pressures.

Multipactor experiment on a dielectric surface

Summary form only given, as follows. A novel experiment to investigate single-surface multipactor on a dielectric surface was developed. The experiment consists of a small brass microwave cavity in a high vacuum system. The cavity is /spl sim/15 cm in length with an outer diameter of /spl sim/10 cm. A pulsed, variable frequency microwave source at /spl sim/21.4 GHz, 100 W-4500 W peak excites the TE/sub 111/ mode with a strong electric field parallel to a dielectric slab (/spl sim/.2 cm thickness) that is inserted at the mid-length of the cavity. The microwave pulses are monitored by calibrated microwave diodes. An electron probe measures electron current and provides temporal measurements of the multipactor electron current with respect to the microwave pulses. Multipactor is observed to occur simultaneous to microwave power initiation. Phosphor is used to detect multipactor electrons by photoemission. E/sub RF/ measurements are made using bead pull perturbation analysis. The motivation of this experiment is to verify recent theoretical calculations of single surface multipactor on a dielectric.

Relativistic L-band magnetron experiments driven by a microsecond e-beam accelerator

This research program investigates high power microwave generation utilizing a microsecond electron beam accelerator to drive a relativistic magnetron. Peak microwave power levels have been achieved exceeding 200 MW total (100 MW per-cavity for two-cavity extraction) from a six-vane structure. Time-frequency analysis shows that microwave emission is primarily single-mode with a total pulse duration in the range of 50 - 100 ns. Relativistic magnetron end-loss current measurements have been performed. Preliminary total efficiency estimates for the relativistic magnetron are in the range of 13%, including endloss current. If endloss current is subtracted, the magnetron electronic efficiency nearly doubles to 25%. The goal of future research is to explore techniques for increasing the microwave power, efficiency and pulselength of relativistic magnetrons.

XeCl excimer fluorescence in a microwave discharge

Summary form only given. Ultraviolet light sources are becoming increasingly useful to manufacturing applications such as the curing of inks, coatings, and adhesives. Intensive ultraviolet light can provide efficient curing without intense heat. Microwave driven lamps have many advantages including long lifetime, high overall efficiencies and high intensity emission; XeCl is one of many rare-gas halide excimers used in these systems. The experiment described here is used to understand the production mechanisms of XeCl as well as the optimal conditions for ultraviolet emission. A continuous wave discharge produced by 2.45 GHz microwaves is used to produce 308 nm fluorescence (B/spl rarr/X) and 236 nm fluorescence (D/spl rarr/X). Varying ratios of Xe and Cl/sub 2/, are controlled by mass flow controllers and mixed in a mixing cell before flowing into a quartz tube, which lies along the centerline axis of an Asmussen microwave cavity. The emission of 236 nm tends to be much dimmer than the more prominent 308 nm emission. Emission from Xe atoms are also observed. Pressure ranges from a few torr to over 100 torr are used with microwave powers ranging from 100 W to 300 W. A comparison of the XeCl emission using different ratios of Xe and Cl/sub 2/, and trends in power and pressure will be presented.

Magnetron simulations and experiments

Summary form only given, as follows. Simulations and experiments are underway at UM to investigate the characteristics of GW-level, cold cathode, relativistic magnetrons versus kW-level, thermionic cathode magnetrons. The goal of the research is to understand why kW magnetrons exhibit 50-80% efficiencies, but relativistic (>100 MW) magnetrons are limited to /spl sim/30% efficiency. We present simulations relevant to relativistic magnetron experiments beginning at the University of Michigan. Planned MELBA experimental relativistic e-beam parameters are: voltage=-500 kV, current=1-10 kA, and pulselength from 100-500 ns. The main relativistic magnetron under study is the Maxwell Physics International L-band magnetron. Commercial oven magnetrons are also being studied. Mode competition and mode hopping are being addressed by utilizing time-frequency-analysis of the heterodyned microwave signals, we are planning to explore the effects of the vastly different impedances and space charge densities that characterize each device.