The High-Density Fixed Point for Nonneutral Plasma Compression.

A new ``strong-drive'' regime was recently discovered for the radial compression of single component plasmas in Penning-Malmberg traps using a rotating electric field [the so-called rotating wall (RW) technique]\footnote{J. R. Danielson and C. M. Surko, {\it Phys. Rev. Lett.} {\bf 95}, 035001 (2005); and {\it Phys. Plasmas} {\bf 13}, 055706 (2006).}. The transition to this regime occurs via a bifurcation, and the steady-state density exhibits hysteresis as a function of the applied RW voltage. Plasmas can be compressed until the E x B rotation frequency, $\omega_{E}$ ($\omega_{E} \propto$ n, the plasma density) approaches the applied frequency, $\omega_{RW}$. Here, we discuss a simple nonlinear model that explains these observations as convergence to an attracting, high-density fixed point - a torque-balanced steady state. Measurements of the RW torque magnitude and dependence on drive voltage are presented. Quantitative agreement is found with a newly developed theory\footnote{M. W. Anderson and T. M. O'Neil, adjacent poster.} that calculates the torque near the fixed point produced by the Debye-shielded RW electric field. Applications of the RW technique in this high-density, strong-drive regime and factors limiting its utility will be discussed.