Density effects on the detectability of axisymmetric Bernstein modes in a finite-length non-neutral plasma.

We use a 2-D (r-z) PIC code to model high-frequency axisymmetric oscillations in a finite-length pure-ion plasma. These modes are not detectable in the wall surface charge of infinite-length plasmas because of axisymmetry and lack of z-dependence. This is not true in a finite-length plasma, because the perturbed density has to have nodes a short distance beyond the ends of the plasma. This gives the modes a sinusoidal dependence, with a k_z such that an integral number (approximately) of half-wavelengths fit into the plasma. This z-dependence makes the mode detectable in the wall surface charge. Near a density of 6/7 of the Brillioun limit modes with different k_z are well separated in frequency. The higher the k_z in these modes, the lower the frequency. When the density is lowered to values which are more easily accessed experimentally, however, the modes are much more closely spaced and therefore overlap and are no longer distinguishable. Because the modes with higher k_z tend to be damped, this causes moderately heavy damping of all the modes at these lower densities, making them very difficult to observe. We will quantify what size of density perturbation is necessary to produce experimentally measurable signals as a function of plasma density.