Interactions of waves and particles in plasmas with reference to an experimental study of current-driven turbulence

Thesis

This thesis is an experimental study of a plasma which is made strongly turbulent by conducting an electric current. The experiments are performed on the cylindrical potassium plasma column which exists between the hot and cold end-plates of a single-ended Q-machine. The plasma is continuously produced at the hotplate by contact ionization and confined radially by a strong axial magnetic field such that ω_ce » ω_pe. The electrons and ions are initially close to thermal equilibrium with the hotplate, with a common temperature ≃ 0.2eV. The plasma density (~ 10⁹ cm^-3), is such that the time for collisions between electrons and ions is very long compared to the time taken for an electron to travel the length of the column.

Pulsed electron currents of duration ~ 2 μs are drawn through the plasma by applying positive voltage pulses (≲ 100V) to the cold plate. Simple electrical measurements indicate that, above a critical current, the plasma has an anomalously low conductivity accompanied by the onset of a high level of potential fluctuations, which are detected in the plasma by electrostatic probes.

A diagnostic technique has been developed to study the temporal evolution of the distribution function of axial electron velocities. This involves time-resolved measurements of the dispersion of electron plasma waves during the current pulse. Analysis of this dispersion gives unambiguous measurements of the electron density n, drift velocity v_d, and root mean square (or thermal) velocity v_e, during the pulse. The results indicate that, after an. initial transient, n and v_d are axially uniform, n remains equal to the initial value and the current drawn from the plasma gives a good measure of their product. The critical current corresponds to v_d = 1.3 v_c, which linear theory predicts, is the threshold for the electron-ion two-stream instability. Above this threshold, v_e quickly exceeds v_d and, in contrast to the predictions of linear theory and computer simulation, continues to rise for constant v_d to high values, (indicating electron temperatures ≲ 80 eV), limited only by the energy containment time of the column. The values of conductivity deduced from the effective scattering rate and from the electrical measurements are in good agreement, and are consistent with an empirical scaling law found in a pulsed toroidal discharge by other workers.

Measurements on the potential fluctuations indicate a broad frequency spectrum: in the low frequencies the correlation lengths were too short to be measured; in the high frequencies the fluctuations were shown to consist of relatively coherent, long wavelength electron plasma waves.

Authors: Clark, W

• Peer review status: Peer reviewed
• DOI: 10.5287/ora-mzkgqbg4o
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford