Effects of background gas pressure on the dynamics of a nonneutral electron plasma confined in a Malmberg–Penning trap

Abstract

The effects of electron–neutral collisions on plasma expansion properties and the evolution of the m=1 diocotron mode are investigated in the Electron Diffusion Gauge (EDG) experiment, a Malmberg–Penning trap with plasma length Lp≃15 cm, plasma radius Rp≃1.3 cm, and characteristic electron density 5×106 cm−3<n<3×107 cm−3. Essential features of the m=1 diocotron mode dynamics in the absence of electron–neutral collisions are verified to behave as expected. The mode frequency, the growth rate of the resistive-wall instability, and the frequency shift at nonlinearly large amplitudes are all in good agreement with theoretical predictions. When helium gas is injected into the trap, the evolution of the mode amplitude is found to be very sensitive to the background gas pressure down to pressures of 5×10−10 Torr, the lowest base pressure achieved in the EDG device. The characteristic time scale τ for nonlinear damping of the m=1 diocotron mode is observed to scale as P−1/2 over two orders-of-magnitude variation in the background gas pressure P. The evolution of the plasma density profile has also been monitored in order to examine the shape of the evolving density profile n(r,t) and to measure the expansion rate. The density profile is observed to expand radially while maintaining a thermal equilibrium profile shape, as predicted theoretically. While the expansion rate is sensitive to background gas pressure at pressures exceeding 10−8 Torr, at lower pressures the cross-field transport appears to be dominated by other processes, e.g., asymmetry-induced transport. Finally, the expansion rate is observed to scale approximately as B−3/2 for confining fields ranging from 100 to 600 G.