The spatial profile of density in electron beam generated plasmas

In electron beam generated plasmas, plasma generation is confined to the beam volume and is largely independent of reactor geometry. When the beam is magnetically confined, these plasmas exhibit excellent uniformity along the axis of beam propagation but have highly non-uniform charged particle density profiles normal to the beam axis. The structure of this spatial profile is driven by plasma chemistry and diffusion. This article explores the spatial variation of plasma density in different gas backgrounds as a means to separate the role of diffusion from electron/ion recombination in these systems. Small relative concentrations of molecular gas (H2/CH4) added to noble gas backgrounds are found to dramatically affect the spatial plasma density profile due to the effect of electron/ion recombination. Diffusion in different noble gases is treated in detail, with attention paid to the role of ion mass and electron temperature. Changes in ion mass affect the spatial plasma density profile by altering the ion mass term of the ambipolar diffusion coefficient. In addition spatial variation in electron temperature also strongly influences the ambipolar diffusion coefficient by changing the electron/neutral collision frequency.