Robert A. Meger

Theoretical modeling of the plasma erosion opening switch for inductive storage applications

A theoretical model for the plasma erosion opening switch (PEOS) is presented which predicts its voltage, current and impedance history as a function of the input waveforms, geometry, and switch parameters. Scaling relations for the switch operation are developed from this model. System requirements for pulse compression and power multiplication using inductive storage are derived from a simple lumped circuit analysis and a transmission line analysis. These requirements are shown to be satisfied using the PEOS as a fast opening, vacuum switch in a configuration relevant for existing high‐power accelerators. The switch model is incorporated into a transmission line code for comparison with recent inductive storage experiments. Code results agree well with the data showing conduction times of ∼60 ns and switching times of ∼10 ns with peak currents of ∼600 kA.

Vacuum Inductive Store/Pulse Compression Experiments on a High Power Accelerator Using Plasma Opening Switches.

A new type of an opening switch which operates in vacuum and opens on a nanosecond timescale at voltages > 1 MV is described. This Plasma Opening Switch (POS) is able to conduct several hundred kiloamperes for approx. 50 nsec while a vacuum inductor is charged, then to open in < 10 nsec, delivering a large fraction of the stored energy to an electron-beam load. Preliminary experiments are described and a simple model of the POS operations is presented.

Current distribution in a plasma erosion opening switch

The current distribution in a plasma erosion opening switch is determined from magnetic field probe data. During the closed state of the switch the current channel broadens rapidly. The width of the current channel is consistent with a bipolar current density limit imposed by the ion flux to the cathode. The effective resistivity of the current channel is anomalously large. Current is diverted to the load when a gap opens near the cathode side of the switch. The observed gap opening can be explained by erosion of the plasma. Magnetic pressure is insufficient to open the gap.

Theoretical overview of the large-area plasma processing system (LAPPS)

A large-area plasma processing system (LAPPS) is under development at the Naval Research Laboratory. In the LAPPS, the plasma is generated by a sheet electron beam with voltages and current densities of the order of kilovolts and tens of milliamps per cm2. The plasma dimensions are a metre square by a few centimetres thick. The beam is guided by a magnetic field of 50-300?G. Since an electron beam of this type efficiently ionizes any gas, high electron densities of n~1012-1013?cm-3 are easily generated at 30-100?mTorr background pressure. In addition to large area and high electron density, the LAPPS has advantages for plasma processing. These include independent control of ion and free radical fluxes to the surface, very high uniformity, very low electron temperature (Te, {<}1?eV, but can be controllably increased to a desired value) and a geometry that is well suited for many applications. This paper sketches an initial theoretical overview of issues in the LAPPS and compares aspects of the theory to a preliminary experiment.

Production of large-area plasmas by electron beams

An analysis is presented for the production of weakly ionized plasmas by electron beams, with an emphasis on the production of broad, planar plasmas capable of reflecting X-band microwaves. Considered first in the analysis is the ability of weakly ionized plasmas to absorb, emit and reflect electromagnetic radiation. Following that is a determination of the electron beam parameters needed to produce plasmas, based on considerations of beam ionization, range, and stability. The results of the analysis are then compared with a series of experiments performed using a sheet electron beam to produce plasmas up to 0.6 m square by 2 cm thick. The electron beam in the experiments was generated using a long hollow-cathode discharge operating in an enhanced-glow mode. That mode has only recently been recognized, and a brief analysis of it is given for completeness. The conclusion of the study is that electron beams can produce large-area, planar plasmas with high efficiency, minimal gas heating, low electron temper...

Effect of pulse sharpening on imploding neon Z‐pinch plasmas

The radial implosion of hollow, cylindrical neon gas columns, driven by currents of up to 1.45 MA, produces a linear Z pinch with over 70% of the radiation in neon K lines. A plasma erosion opening switch (PEOS) is used to eliminate prepulse and to reduce the current rise time from ∼60 to ∼20 ns. Incorporation of the PEOS improves the uniformity of the Z pinch and increases the radiation yield.

Demonstration of a plasma mirror for microwaves

A plane sheet of partially ionized plasma produced by a pulsed, hollow-cathode glow discharge in an axial magnetic field has been shown to reflect X-band microwaves as well as an equivalent sheet of metal. The plasma mirror can be established and extinguished on a time scale of about 20 mu s and could form the basis of an extremely agile radar system. >

Experimental investigations of the formation of a plasma mirror for high‐frequency microwave beam steering

The Naval Research Laboratory (NRL) has been studying the use of a magnetically confined plasma sheet as a reflector for high‐frequency (X‐band) microwaves for broadband radar applications [IEEE Trans. Plasma Sci. PS‐19, 1228 (1991)]. A planar sheet plasma (50 cm×60 cm×1 cm) is produced using a 2–10 kV fast rise time square wave voltage source and a linear hollow cathode. Reproducible plasma distributions with density ≥1.2×1012 cm−3 have been formed in a low‐pressure (100–500 mTorr of air) chamber located inside of a 100–300 G uniform magnetic field. One to ten pulse bursts of 20–1000 μs duration plasma sheets have been produced with pulse repetition frequencies of up to 10 kHz. Turn on and off times of the plasma are less than 10 μs each. The far‐field antenna pattern of microwaves reflected off the plasma sheet is similar to that from a metal plate at the same location [IEEE Trans. Plasma Sci PS‐20, 1036 (1992)]. Interferometer measurements show the critical surface to remain nearly stationary during th...

Long conduction time plasma erosion opening switch experiment

A plasma erosion opening switch, coupled to a small capacitor bank, conducts 120 kA for 400 ns before opening in 40 ns. Voltages above 170 kV are produced through the use of an electron beam diode. These voltages exceed the initial capacitor bank voltage by a factor of 4. Current and magnetic field measurements indicate that the same current conduction and opening processes observed in earlier erosion switch experiments are involved here at tenfold greater conduction times, verifying the current controlled nature of plasma erosion switch operation.

Beam-generated plasmas for processing applications

The use of moderate energy electron beams (e-beams) to generate plasma can provide greater control and larger area than existing techniques for processing applications. Kilovolt energy electrons have the ability to efficiently ionize low pressure neutral gas nearly independent of composition. This results in a low-temperature, high-density plasma of nearly controllable composition generated in the beam channel. By confining the electron beam magnetically the plasma generation region can be designated independent of surrounding structures. Particle fluxes to surfaces can then be controlled by the beam and gas parameters, system geometry, and the externally applied rf bias. The Large Area Plasma Processing System (LAPPS) utilizes a 1–5 kV, 2–10 mA/cm2 sheet beam of electrons to generate a 1011–1012 cm−3 density, 1 eV electron temperature plasma. Plasma sheets of up to 60×60 cm2 area have been generated in a variety of molecular and atomic gases using both pulsed and cw e-beam sources. The theoretical basis ...