Electron diffusion in the annular Penning trap

Transport by cross-field diffusion has been studied in the annular Penning trap in which a nonneutral plasma of electrons is contained between concentric cylinders. At densities sufficiently low (<105 cm−3) to suppress mobility transport arising from the space charge electric field, the dominant sources of transport are diffusion from collisions of electrons with added helium gas and asymmetry transport from stray fields. The collisional diffusivity is shown to scale linearly with collision frequency and inversely with the square of the axial magnetic field. The measured mean energy is initially 0.3 eV and the least energetic electrons are lost more slowly as a consequence of the energy dependence of the diffusivity. Decay constants are about a factor of four higher than calculated from the electron–helium momentum transfer collision frequency. Both the asymmetry transport and the collisional transport are shown to depend upon the cleanliness of the trap surfaces.