Christopher Douglas Cothran

On the Mechanism of Pulsed Electron Beam Production From an Uninterrupted Plasma Cathode

Electron extraction from a hollow cathode plasma discharge through a primary anode, biased with a dc accelerating potential, is modulated to produce pulsed or continuous electron beams without the interruption of the plasma cathode discharge. This is achieved with the addition of a secondary anode, within the hollow cathode. When this secondary anode is electrically connected to the primary anode, beam production ceases; when the secondary anode is electrically isolated, beam production resumes. Previous works demonstrated the utility and operation of this device; however, the precise physical principles allowing beam modulation were not well characterized. In this paper, we show that beam modulation can be understood in terms of changes in plasma potential within the hollow cathode that either promote or prevent the formation of an electron sheath at the exit of the hollow cathode, which is mediated by the electrical state of the secondary anode. Maintaining a plasma within the hollow cathode during a state change of the secondary anode depends on the secondary anode meeting the criteria for global nonambipolar flow. These effects are explored with both the Langmuir probe measurements within the hollow cathode during the steady-state operation, and the high-speed current and voltage monitoring during beam pulses.

Advances in Impedance Probe Applications and Design in the NRL Space Physics Simulation Chamber

Impedance probe measurements are made at ionospheric (F-region) plasma conditions (n_e ≈ 10⁴-10⁶ cm^-3 and λ_D ≈ 1-10 cm) created in the Space Physics Simulation Chamber at the Naval Research Laboratory. Measurements of probe-plasma impedance are used to provide comparative data and identify possible problems, with the goal of developing flight-capable diagnostics for sounding rocket experiments. The ability of the diagnostic technique to infer electron density and plasma potential in an ionospheric plasma is demonstrated with laboratory measurements. In addition, preliminary data from prototype instruments built in our laboratory are presented.

Electron Beam Propagation in Magnetic Fields

Cold cathode plasma electron beam sources are robust alternatives to hot filaments and field emission sources. In applications requiring pressures above 1 mtorr or reactive gas backgrounds, cold cathode plasma sources can produce high-power electron beams while also exhibiting long lifetimes. The images presented here are produced by electron beams extracted from cold cathode plasmas.

Experiments and simulations of antenna coupling in space plasmas

Summary form only given. An ongoing problem in the plasma physics community is the understanding of wave excitation and near field antenna coupling in a magnetized plasma. The anisotropy of the plasma index of refraction, the nonuiniform current distribution on the antenna, and the presence of the plasma sheath surrounding the antenna makes it difficult to determine a framework for building efficient exciters, as it is not immediately apparent which parts of the physics are most important. This is evident in looking at the body of experimental work in the literature, where one can find a broad range of antenna designs in use, but very rarely is a rigorous quantitative theory combined with the experimental results. With an eye toward better understanding antenna plasma coupling, we present new experimental and theoretical results related to excitation of waves in a magnetized plasma across a large plasma parameter space. The antenna is a simple monopole which can be oriented parallel or perpendicular to the magnetic field, and can also be biased to control the width of the sheath interface with the plasma. The amplitude and phase of the transmitted and reflected power are measured to compare the antenna radiation resistance with the actual waves being detected at a distance. The experimental results will be augmented by rigorous numerical modeling to form a more complete picture of the physical process by which waves are excited. The experiments were performed in the Space Physics Simulation Chamber at NRL. The chamber is 2-m diameter, 5-m long vacuum vessel surrounded by five 3-m diameter water-cooled magnet coils capable of producing a wide variety of magnetic field profiles of up to 250 Gauss. The plasma is created in a pressure of p ≈ 10-4 Torr Argon with a 1-square-meter array of glowing, biased tungsten filaments. The electron density can be set over a very broad range (106 <; n <; 1012cm-3) while electron and ion temperatures are typically Te ≈ 0.5 eV and Ti ≈0.05 eV.

Whistler and lower hybrid wave propagation experiments at the NRL SPSC

We present results of recent whistler and lower hybrid wave propagation experiments in the Space Physics Simulation Chamber facility at the Naval Research Lab. The waves are driven and detected using balanced dipole and loop antennas connected to a vector signal analyzer which measures the amplitude and phase of the wave in two dimensions (r and z). In addition the frequency of the signals is also swept over a range of several hundred megahertz, providing a very comprehensive picture of the near and far field antenna radiation patterns over a variety of plasma conditions. The magnetic field is varied from a few Gauss to 200 Gauss, with the density variable over at least three decades from 107−1010cm−3. Observations to be presented are the efficacy of resonant vs non-resonant antenna coupling in driving large amplitude waves, ducting of waves with density channels, and wave propagation in linear and non-linear regimes.

Continous multi-keV electron beam source using a hollow cathode

A multi-kilovolt, compact electron beam source is being developed for use in a laboratory plasma physics experiment to simulate the energetic electron population present in the magnetosphere and the associated space plasma physics phenomena. In parallel, a similar source is intended for welding and metal forming applications. The source operates by extracting electrons from a hollow cathode plasma that has been biased to the required beam voltage. The hollow cathode operates in the mTorr pressure range (Argon) at approximately 400V and current up to about 150mA (power supply limited) in a uniform 100G field. A variety of electron extraction geometries have been implemented, including grids and plates of differing thicknesses and hole diameters, yielding a largest extraction efficiency of approximately 30%. The thermal management problem of continuous operation is solved by making the hollow cathode wall also serve as a vacuum wall so that it may be cooled externally.