Jim Browning

ICRF heating with mode control provided by a rotating field antenna

A rotating field antenna set — a pair of two closely spaced dual half-turn antennas — is used to tailor the azimuthal spectrum of the antenna field. It is demonstrated that the azimuthal mode of ICRF waves can be controlled by proper phasing of the antenna set. Ion heating is shown to be strongly dependent on the azimuthal mode number. When the antenna set is operated in the m = −1 (left rotating) mode, the m = −1 modified ion cyclotron wave is selectively excited, resulting in efficient ion heating.

The response of a microwave multipolar bucket plasma to a high voltage pulse

A collisional model that describes the response of a microwave multipolar bucket plasma to a high voltage pulse is developed for plasma source ion implantation (PSII). The primary purpose is to develop a theoretical model of PSII plasma physics to be used in conjunction with a model of ion-target interaction to optimize PSII processing. Measurements of the sheath position and target current in a 100 mtorr helium plasma are found to be consistent with the model. Sheath thicknesses predicted by the collisional model are significantly less than those predicted by similar noncollisional models. >

Gate field emitter failures: experiment and theory

Intrinsic failures of gated field emitters have been studied. The gate-emitter voltage drops from typical values of 140 V to 10-70 V in less than 10 ns at the onset of a failure. Measurements with an electrostatic probe indicate that plumes of ions and electrons are ejected into vacuum. The measured ion current to the probe is typically 10% of the electron current. The voltage during the event and the ion-to-electron current ratio measured at the probe are characteristic of a cathodic vacuum arc plasma. For series resistors less than 1 k Omega , the arc is continuous, while the series resistors greater than 10 k Omega , the arc is intermittent. Initiation of the failure based on ion-space charge enhancement of the emitter electric field is modeled with the plasma simulation code PDS1. These structures provide a controlled geometry for studying arcs of micron size dimension. >

A low-frequency crossed-field amplifier for experimental investigations of electron-radio frequency wave interactions

A low-frequency (100-200 MHz) crossed-field amplifier has been designed and constructed for the purpose of studying electron-radio frequency wave interactions. The device has been designed to allow in situ diagnostic probe measurements of the interaction region during the amplification process. Details of the design and operating parameters in the beam-injected mode are described. A maximum gain of 7 dB for a circuit of short electrical length (six slow wave wavelengths) has been obtained. Measurements of the local radio-frequency field during amplification, of the electron density profile, and of the electron plasma current-voltage characteristics in the interaction region demonstrate the use of probes to diagnose the electron plasma in a crossed-field amplifier configuration. These experiments will provide a basis for investigations of noise generation and nonlinear interactions in the more complex emitting-sole, reentrant crossed-field amplifier. >

Measurements of electromagnetic waves in Phaedrus-B: bench-mark test of antena wave field calculations

It is shown that the predictions of a numerical code (ANTENA) and the data of wave field measurements in the Phaedrus-B tandem mirror are consistent (±25%) for right-handed (−) wave fields and less so (±40%) for left-handed (+) wave fields in the plasma core, and that they disagree for + fields near the column edge. Shorting out or reduction of the wave azimuthal electric fields by limiters is the probable cause of this discrepancy. The ICRF fluctuating wave fields are shown as || contour maps in the r-z plane, where the + data peak at a smaller radius than predicted. The waves are characterized by different dominant axial wave numbers for the left- and right-handed circularly polarized fields.

Ion‐space‐charge initiation of gated field emitter failure

Failures of individual micron‐scale gated field emitters are observed to be similar in many respects to cathodic arcs. The initiation of a failure event at elevated pressures by ion‐space‐charge enhancement of the electric field at the emitter tip is simulated and compared with experimental results. The experimental results show a significantly lower pressure failure threshold than that predicted by the simulation. This discrepancy may indicate the presence of additional processes.

Experimental observations of gated field emitter failures

Intrinsic failure events in gated field emitters have been studied. The gate-emitter voltage, typically 140 V during operation, drops to 10-70 V at the onset of the failure. Measurements with a diagnostic probe indicate that plumes of ions and electrons are ejected into vacuum with the ion current typically 10% of the electron current. The arc voltage and the ion-to-electron current ratio are characteristic of a cathodic vacuum arc. For series resistors less than 1 k Omega , the arc is continuous, whereas for series resistors greater than 10 k Omega , the arc is intermittent.<<ETX>>

A Miniature Inductively Coupled Plasma Source for Ion Thrusters

A small inductively coupled plasma source has been developed for use in an ion thruster. A 10.1-mm-diameter thick film silver spiral antenna is fabricated on a low-temperature cofired ceramic with a 35-μm dielectric over the antenna. The antenna has been able to sustain an argon plasma over a frequency range from 448 to 1020 MHz with pressures ranging from 50 mtorr to 1.75 torr. Plasma start powers ranged from 3 to 50 W with minimum sustain powers down to 0.1 W. Antenna electric field measurements have been made in air and compared with simulation of the antenna field using COMSOL. These results show that the antenna pattern is dominated by the standing wave pattern of the spiral antenna. Simulations of the RF power density versus frequency compare well with the plasma start power variation except for a large start power peak between 600 and 700 MHz.

Radio-frequency wave interchange stability experiments below the ion cyclotron frequency

It has been demonstrated in the Phaedrus‐B tandem mirror [Phys. Fluids 31, 714 (1988)] that radio‐frequency (rf) waves applied below the ion cyclotron frequency (ω/Ωi ≊0.75) can stabilize a mirror plasma against the interchange instability. Results presented here demonstrate stabilization over a range of frequencies (0.65≤ω/Ωi≤0.8) with the mMHD =−1 instability stabilized and higher‐order (‖mMHD‖≥2) low‐amplitude modes present. Radial profiles of the rf magnetic fields at a number of axial locations as well as parallel wavenumber measurements indicate mrf =−1 slow shear Alfven wave excitation over this frequency range. Measurements at frequencies below 0.75Ωi indicate attenuation of rf waves by gas baffles and a gas box.

Faceted magnetron concept using field emission cathodes

A magnetron concept using field emission cathodes has been modeled with the Air Force Research Laboratory particle-in-cell code ICEPIC and the two-dimensional particle trajectory simulation Lorentz2E. In this approach, field emitters are used to provide a distributed cathode in place of a traditional thermionic cathode. The emitters are placed below the interaction space in a shielded structure. The cathode is comprised of facet plates with slits to protect the emitters. Simulation of an L-band rising sun magnetron shows that the faceted magnetron will oscillate using both five and ten facet cathodes. The startup times are very similar to that of a cylindrical cathode magnetron. The electron trajectories of the shielded slit structure have been modeled, and the results indicate that electrons can be injected through the slits and into the interaction space using lateral edge emitters and a pusher electrode design.