The effects of neutral gas pressure and electron temperature on the dynamics of the electron diffusion gauge experiment electron plasma

The dynamics of pure, electron plasmas confined in cylindrically symmetric, Malmberg-Penning traps are strongly affected by imperfections in the trap fields and collisions with background gas molecules present in the vacuum. These imperfections in the trap torque the azimuthally rotating plasma, causing it to expand radially.; The Electron Diffusion Gauge (EDG) device is used to determine whether the effects of background gas on an electron plasma can be quantitatively predicted and used to calibrate ionization gauges, the standard equipment for measuring gas pressure in the ultra-high-vacuum (UHV) regime. Earlier studies of EDG plasmas in this regime suggested that the plasma expansion is primarily due to field imperfections rather than interaction with the background gas (primarily helium), and that an observed damping of the m = 1 diocotron mode is more sensitive to the gas pressure. Recent measurements indicate that m = 1 diocotron mode growth observed at high electron-source heating voltages can be much more sensitive to other components of the background gas than the mode damping is to helium. It appears likely that ions unintentionally produced in the electron source are traversing the plasma and causing the destabilization, and that quantitative prediction of this effect will be difficult.; At higher (HV) background gas pressures, the plasma expansion rate has been observed to agree with the theoretical expansion rate predicted with a fluid description of the plasma that includes elastic collisions with background gas molecules. This agreement is impressive because the model describes uniform-temperature plasmas, and the plasmas in EDG do not necessarily have uniform temperatures at HV pressures. Greatly improved measurements of the plasmas' radial density profiles and new measurements of the on-axis plasma temperature indicate that the majority of these plasmas do not have thermal, quasi-equilibrium density profiles initially. Measurements later in the UHV pressure evolution of thermal quasi-equilibrium plasmas reveal that expansion due to field imperfections is a factor of four lower than that estimated previously. Non-increasing temperatures measured during this later part of the evolution further suggest that the plasma is indeed losing the electrostatic potential energy liberated by the expansion, presumably through inelastic collisions with impurities in the background gas.