First Test of Long-Range Collisional Drag via Plasma Wave Damping

In magnetized plasmas, the rate of particle collisions is enhanced over classical predictions when the cyclotron radius $r_{c}$ is less than the Debye length $\lambda_{D}$. Classical theories describe local velocity scattering collisions with impact parameters $\rho < r_{c}$. However, when $r_{c} < \lambda_{D}$, long-range collisions exchange energy and momentum over the range $r_{c} < \rho < \lambda_{D}$ with negligible parallel-perpendicular velocity scattering. Previous experiments and theory have shown that these long-range collisions enhance cross-field diffusion, heat transport, and viscosity by orders of magnitude over classical predictions\footnote{C. F. Driscoll et al., Phys. Plasmas \textbf{9}, 1905 (2002)}. Here, we present the first experimental confirmation of a new theory\footnote{D.H.E. Dubin, Phys. Plasmas \textbf{21}, 052108 (2014)}, which predicts enhanced parallel velocity slowing due to these long-range collisions. These experiments measure the damping of Trivelpiece-Gould waves in a multispecies pure ion plasma. The damping is dominated by interspecies collisional drag when Landau damping is weak. In this ``drag damping'' regime, the measured damping rates exceed classical predictions of collisional drag damping by as much as an order of magnitude, but agree with the new long-range enhanced collision theory. The enhanced slowing is most significant for strong magnetization and low temperatures. For example, the slowing of anti-protons at a density of $10^{7}$ cm$^{-3}$ and a temperature of 10 K in a 6 T trap is enhanced by a factor of 30.