Pressure Measurement Using a Pure Electron Plasma

A pure electron plasma can be employed as a pressure sensing medium. The plasma is confined in a Malmberg trap composed of a set of electrically isolated colinear cylinders embedded in a uniform axial magnetic field. Axial confinement of the plasma is produced by negative voltages applied to the end cylinders relative to the middle cylinder(s). Radial confinement is produced by the magnetic field, and under the correct conditions is inversely proportional to the background neutral density. It is shown that a pure electron plasma could potentially serve as a primary vacuum standard for the 10^{-8} to 10 ^{-5} Pa pressure regime. In this regime, the plasma can approach a quasi-thermal equilibrium state during its expansion toward the trap wall. While near thermal equilibrium, the rate of expansion of the plasma as a function of helium pressure can be predicted from fundamental physical constants if the total charge, mean-square-radius, and temperature of the plasma are known. The minimum pressure is determined by pressure -independent causes of plasma expansion. Collisions between electrons do not cause expansion because they conserve the mean-square-radius of the plasma. Previous experiments, and the present results, indicate that azimuthal asymmetries in the confining fields are the predominant cause in Malmberg traps. However, resonant particle models used previously to explain the coupling of the asymmetries to the plasma are inconsistent with new data on the empirical relation between plasma properties and the pressure-independent expansion rate. The maximum pressure is determined by the requirement that the plasma remain near thermal equilibrium during its expansion. A uniform temperature and a specific density profile characterize equilibrium, and they are produced and maintained by electron-electron collisions. The transport due to collisions with the neutral gas can change both the density and temperature profiles. However, a new calculation shows that as long as the plasma temperature remains uniform, collisions with neutrals will not perturb the equilibrium density profile. With its lower limit of 10^{ -8} Pa, this new type of pressure sensor would operate beyond the range of existing vacuum standards.