Studies of electron-ion streaming instabilities will be performed on the LAPD device by driving strong field-aligned currents. For these experiments, current densities exceeding the threshold for ion acoustic (j > n e c_s) and Buneman instabilities (j > n e v_th,e) will be necessary. The experiments will use pulsed grids as well as Barium-oxide coated nickel cathodes which are brazed directly to ceramic-encapsulated coaxial heating filaments (Thermocoax) for heating the cathode to thermionic emission temperature (840-920C). For a 10 centimeter cathode in an LAPD discharge (T_e ~ 10 eV, n ~ 10**12 cm^-3), a current of 10A is required to surpass the ion acoustic threshold and 1kA is required to surpass the Buneman threshold; both currents are accessible using standard discharge power supplies and transistor switches. These thresholds may also be lowered by operating in the cold (T_e ~ 1 eV) afterglow plasma. During the experiments, the nature of fluctuations driven by the field-aligned currents will be investigated, as well as the generation of anomalous resistivity. When the threshold for the Buneman instability is exceeded, a search for and study of electron solitary holes will be performed. In the Earth's ionosphere and magnetosphere electron solitary holes have Debye length scale sizes. Velocities of electron solitary holes range to nearly the electron thermal (or drift) speed.

Gekelman and his group are engaged in experiments with narrow, pulsed currents and the plasma flows associated with their production. An example of one of these structures is shown in the figure above). The intense, pulsed, currents (30A/cm² ~ v_drift) have turbulent fluctuations within them and are a likely source of electron phase space holes. These currents can be enhanced by using narrow emitting cathodes and collection grids, which we will manufacture for these experiments.

In the LAPD the Debye length is 16-30 microns. Constructing probes to measure Debye length structures is a great challenge. In the preliminary set of experiments, we will use a test chamber associated with the Large Plasma Device (LAPD) at UCLA. As we develop microprobe technology the experiments will be moved to the LAPD itself. The test chamber is 4 meters long and a meter in diameter. It has an inductively coupled RF plasma source which produced plasmas with densities below 10¹¹ cm^-3. The RF source can be pulsed at hundreds of Hertz, and the experiments will be done in the quiescent afterglow plasma when the rf noise from the source is absent. Apart from this proposal we plan to build a 30 centimeter diameter cathode in this chamber as a second source. It will produce a plasma with density similar to the LAPD. The maximum magnetic filed attainable in the test chamber is 500G. The field will determine the properties of the currents but, to first order, is not important for electron hole generation. Since the electron holes are of order of the Debye length we will first do these experiments in a plasma of density 10¹⁰ cm^-3 and T_e ~ 0.1 eV in which case the Debye length is 52 microns. The initial probes we will use were provided by Prof. Yogesh Gianchandani, and Dr. Jamille Hetke at the University of Michigan. We propose to design suitable electronics in collaboration with the UCLA school of engineering.

There are several geometries of the sensor but all contain detectors which are much smaller than the Debye length, and are spaced 50-100 microns apart. Since the signals will be small (~ 100 microvolts), amplifiers will be designed at UCLA if necessary. They will be mounted several centimeters from the probe tips and will be used to drive miniature coaxial cables in the probe shaft. Additional amplifiers will be used outside of the device, and the signals will then be digitized. The probe survived the plasma environment of LAPD for two days without damage. The demands on the amplifiers for these probes are non trivial. This proposal contains funds for fabrication of these microamplifiers. With twelve fast channels we can study the propagation of structures as small as the Debye length as well as their statistical properties. In a controlled laboratory experiment we can change the plasma density and temperature as well as the velocity and density of the currents that will be used to produce the electron holes.

The electron hole research will be led by Gekelman, the director of the Basic Plasma Science Facility at UCLA. This work will be done in collaboration with Troy Carter and Jack Judy (UCLA Dept of Electrical Engineering) as well as Paul Kintner (Cornell University). Kintner has participated in many rocket and satellite studies of the auroral ionosphere and is keenly interested in studying these structures. He has agreed to participate in analysis of the data and help us relate the results to measurements in the Earth's ionosphere. We will make use of tools at the UCLA MEMs center as well as our relationship with the University of Michigan.

Research Plan

• Year 1: Fabricate small heated nickel cathode and capacitor bank driver for field-aligned current studies. Perform initial testing in LAPD test chamber. Design and fabrication of initial miniature Langmuir probe array and electronics for small scale fluctuation studies.
• Year 2: Studies of fluctuations and resistivity in field-aligned current experiments (using standard diagnostic probes). Initial experiments in LAPD for wider regime. Initial testing of microprobes and electronics in search for electron hole generation.
• Year 3: Continued studies of field-aligned current driven instabilities and anomalous resistivity in test chamber and in LAPD. Iteration on microprobe and associated electronics for measurement of small scale structures. Design of microwave scattering system to complement probe studies of Buneman turbulence and electron solitary holes.
• Years 4-5: Continued studies of field-aligned current driven instabilities and anomalous resistivity. Fabrication and implementation of microwave scattering system for studies of Buneman turbulence and electron solitary holes. Iteration on microprobe and electronics design for electron-holes studies.