[en] The expansion of pure electron plasmas due to collisions with background neutral gas atoms in the Electron Diffusion Gauge (EDG) experiment device is observed. Measurements of plasma expansion with the new, phosphor-screen density diagnostic suggest that the expansion rates measured previously were observed during the plasma's relaxation to quasi-thermal-equilibrium, making it even more remarkable that they scale classically with pressure. Measurements of the on-axis, parallel plasma temperature evolution support the conclusion

[en] Measurements of the expansion rate of pure-electron plasmas have been performed on the Electron Diffusion Gauge (EDG) device at background helium gas pressures in the 5 x 10(superscript -8) Torr to 1 x 10(superscript -5) Torr range, where plasma expansion due to electron-neutral collisions dominates over plasma expansion due to trap asymmetries. It is found that the expansion rate, defined as the time rate of change of the particles' mean-square radius, scales approximately linearly with pressure and inversely as the square of the magnetic field strength in this regime, in agreement with classical predictions

[en] The expansion of pure electron plasmas due to collisions with background neutral gas atoms in the Electron Diffusion Gauge experiment device is observed. Measurements of plasma expansion with the new, phosphor-screen density diagnostic suggest that the expansion rates measured previously were observed during the plasma's relaxation to thermal quasi-equilibrium, making it even more remarkable that they scale classically with pressure. Measurements of the on-axis, parallel plasma temperature evolution support the conclusion

[en] The expansion of pure electron plasmas due to collisions with background neutral gas atoms in the Electron Diffusion Gauge experimental device is observed to be in good agreement with the predictions of a macroscopic fluid model with uniform electron temperature. Measurements of the expansion with a two-dimensional (2-D), phosphor-screen density diagnostic suggest that expansion rates measured with the 1-D diagnostic were observed concurrently with substantial changes in the plasma that are not due to electron-neutral collisions. Measurements of the on-axis, parallel plasma temperature evolution support this conclusion and further indicate that the plasmas are continuously losing energy during the expansion, presumably through inelastic collisions with trace background gases

[en] The expansion of the Electron Diffusion Gauge (EDG) pure electron plasma due to collisions with background neutral gas atoms is characterized by the pressure and magnetic field scaling of the profile expansion rate (d/dt)< r²>. Data obtained at higher background gas pressures than previously studied is presented. The measured expansion rate in the higher pressure regime is found to be in good agreement with the classical estimate of the expansion rate

[en] Smaller-diameter pure electron plasmas are generated in the Electron Diffusion Gauge (EDG) using a thoriated tungsten filament wound into a spiral shape with an outer diameter which is 1/4 of the trap wall diameter. The m=1 diocotron mode is excited in the plasma by means of the resistive-wall instability, using a resistor-relay circuit which allows the mode to be induced at various initial amplitudes. The dynamics of this mode may be predicted using linear theory when the amplitude is small. However, it has been observed [e.g., Fine et al., Phys. Rev. Lett. 63, 2232 (1989)] that at larger amplitudes the frequency of this mode (relative to the small-amplitude frequency) exhibits a quadratic dependence on the mode amplitude. In this paper, the frequency shift and nonlinear dynamics of the m=1 diocotron mode in the EDG device are investigated

[en] A pure electron plasma is confined in a Malmberg-Penning trap and its confinement and stability properties are studied. Of particular interest are the effects that collisions between plasma electrons and background neutral gas atoms have on the plasma expansion and on the evolution of the m=1 diocotron mode. Essential features of the m=1 diocotron mode dynamics in the absence of electron-neutral collisions have been verified to behave as expected. The mode frequency, the resistive growth rates, and the frequency shift at nonlinearly large amplitudes are all in good agreement with predictions. When background neutral gas is injected, the evolution of the mode amplitude is found to be sensitive to the gas pressure down to pressures of 5x10^-10 Torr, the lowest base pressure achieved in the EDG device. The evolution of the plasma density profile has also been monitored in order to examine the shape of the evolving density profile, and to measure the expansion rate. The density profile has been observed to expand radially while maintaining a thermal equilibrium profile shape, as has been predicted theoretically. The plasma expansion rate is affected by the background neutral gas pressure, but the measured expansion rate is generally faster than the expansion rate predicted by considering only electron-neutral collisions

[en] The evolution of the amplitude of the m=1 diocotron mode is used to measure the background neutral pressure in the Electron Diffusion Gauge (EDG), a Malmberg-Penning trap. Below 5x10^-8 Torr, the dependence on pressure scales as P^1/4, and is sensitive to pressure changes as small as ΔP=5x10^-11 Torr. Previous studies on the EDG showed that the diocotron mode is more strongly damped at higher neutral pressures. Both the diocotron mode damping rate and the plasma expansion rate depend similarly on experimental parameters, i.e., conditions which favor expansion also favor suppression of the diocotron mode. The sensitivity of the mode evolution is examined as a function of the resistive growth driving conditions, which are controlled by the amount of wall resistance connected to the trap

[en] Measurements of the expansion rate of pure-electron plasmas have been performed on the Electron Diffusion Gauge (EDG) device at background helium gas pressures in the 5x10^-8 Torr to 2x10^-5 Torr range, where plasma expansion due to electron-neutral collisions dominates over plasma expansion due to trap asymmetries. It is found that the expansion rate, defined as the time rate of change of the particles' mean-square radius, scales approximately linearly with pressure and inversely as the square of the magnetic field strength in this regime, in agreement with classical predictions

[en] 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 L_p≅15 cm, plasma radius R_p≅1.3 cm, and characteristic electron density 5x10⁶ cm^-3< n<3x10⁷ 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 5x10^-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. (c) 2000 American Institute of Physics