Squeeze Effects on Plasma Wave Damping

We present a theory for the damping of cylindrically symmetric plasma modes due to a cylindrically symmetric squeeze potential of magnitude $\phi_s$ applied to the center of a non-neutral plasma column. Squeeze divides the plasma into passing and trapped particles; the latter cannot pass over the squeeze. Damping of the mode in collisionless theory is caused by Landau resonances at energies $E_n$ for which the bounce frequency $\omega_b(E_n)$ and the wave frequency $\omega$ satisfy $\omega=n\omega_n(E_n)$. Particles experience a non-sinusoidal wave potential along their bounce orbits due to the squeeze potential. As a result, squeeze induces bounce harmonics with $n\gg 1$ in the perturbed distribution. The harmonics allow resonances at energies $E_n\le T$ and cause a substantial damping at phase velocities much larger than thermal velocity, which is not expected for unsqueezed plasma. In the regime $\omega/k\gg\sqrt{T/m}$ ($k$ is the wave number) and $T\gg\phi_s$, the resonance damping rate has a $\phi^2_s$ dependence. This behavior is consistent with the observed experimental results.