David N. Walker

Whistler wave propagation and whistler wave antenna radiation resistance measurements

Whistler waves are a common feature of ionospheric and magnetospheric plasmas. While the linear behavior of these waves is generally well understood, a number of interesting observations indicate that much remains to be learned about the nonlinear characteristics of the mode. For example, in space, very low frequency (VLF) emissions triggered by whistler modes launched from ground-based transmitters have been observed. Emission is assumed to come from transverse currents formed by counterstreaming electrons that are phase bunched by the triggering signal. In the laboratory, it has been shown that with increasing amplitude of the driving signal applied to an antenna, the whistler mode radiation pattern forms a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. These observations have prompted an Naval Research Laboratorys (NRL) Space Physics Simulation Chamber study of whistler wave dynamics. The goals are to investigate whistler wave ducting, self-focusing, and amplification, and to study nonlinear whistler-plasma interactions.

Characterization of Joule heating in structured electric field environments

We have recently performed a detailed characterization of ion Joule heating perpendicular to an axial magnetic field in the laboratory in a simulated ionospheric plasma environment which contains localized electric field structuring. Since Joule heating is often regarded as an important mechanism contributing to energization of outflowing heavy ions observed by higher-altitude auroral satellites, this work has particular relevance to space physics issues, and, to our knowledge, has not been investigated systematically in a controlled environment. Since transverse (to B) ionospheric electric fields are often spatially and temporally structured, with scale lengths often as small as an ion gyroradius, the ability to systematically vary the spatial extent and magnitude of an electric field region and to observe the effect on ion energy is important. The experiment makes use of a concentric set of separately biasable ring anodes which generate a radial electric field with controllable scale length perpendicular to an ambient axial magnetic field. Joule heating results from ion-neutral collisions occurring within this transverse, dc electric field. Until there is sufficient neutral pressure to raise the ion-neutral collision frequency (ν in ) to an observable Joule heating threshold, ion cyclotron wave heating, which is induced by shear in E x B rotation, can be the primary channel for ion energization. We have discussed in earlier papers the conditions under which this occurs, and we have treated the transition between the two forms of ion heating. We concentrate primarily in this work on constructing the fields themselves and on the relationship between the subsequent collisional heating and the Pedersen conductivity as an initial indication of the validity of the measurement results. We are able to demonstrate that measurable heating is produced by even relatively small scale structures of the order of the ion gyroradius. In addition, we show that measured heating is consistent with predictions of Joule heating as a function of ion-neutral collisions. Finally, this work can have major implications for ionospheric studies where large-scale electric fields are often assumed in the calculation of Joule heating.

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.

Determining the electron distribution function from RF measurements using an impedance probe

Using a network analyzer which returns Re(Zac) and Im(Zac) for a spherical probe in a plasma, we have demonstrated the existence of collisionless resistance in the sheath, shown that this leads to a method of finding the electron sheath density profile, and proposed a method of measuring electron temperature using the rf results1. The magnitude of the applied signal from the network analyzer is much smaller in magnitude than typical applied dc potentials and it is therefore transparent to the existing plasma/probe interface. Recently, from determination of plasma potential, we are able to construct the electron distribution function, f(E), from the rf measurements. This method requires only a first derivative of the inverse Re(Zac) with respect to bias. We will present the method and results for f(E) for three spherical probes of varying sizes.

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.

Theory and observations of ion energization in the presence of large amplitude ion cyclotron waves and a DC electric field in a weakly ionized plasma

Summary form only given, as follows. Recently, a series of experiments conducted in the Naval Research Laboratorys Space Physics Simulation Chamber (SPSC) have shown ion energization in a weakly ionized argon plasma in the presence of large amplitude ion cyclotron waves and a DC electric field. The SPSC is a 1.8 m diameter by 5 m long cylinder equipped with a microwave discharge plasma source, an external magnetic field, and a variable background pressure. Wave and bulk parameters are measured with heatable Langmuir probes, emissive probes, and a perpendicular ion energy analyzer. DC electric fields are created within the SPSC plasma column by modifying the plasma potential tranverse to the external magnetic field. The experimental observations indicate that the argon ion perpendicular temperature can be increased at least by a factor of 2 in the presence of 1-10% plasma density fluctuations, a inhomogeneous DC electric field, and large amplitude ion cyclotron fluctuations. The ion collision frequency is composed of both ion-neutral and ion-electron contributions and can be varied to be much less than or greater than the ambient ion cyclotron frequency. We have computed the expected ion energization in the SPSC using several approaches, i.e., particle-in-cell simulations, quasilinear heating, large amplitude trapping, and ion collisional heating. We will present the results of these analyses and compare with the experimental observations.