Collisional damping of plasma waves by H^- in room-temperature electron plasmas

Collisional effects play an important role in the dynamics of plasma waves, by setting a minimal damping rate and by disrupting the wave-particle (Landau) resonant damping. For room-temperature electron plasmas accumulating H^- ions, this damping results from the long-range e/H^- frictional drag, and it is relatively independent of the spatial structure (m_r, m_θ, m_z) of the wave. Here, the collisional damping rate γ_cl(H^-) is one-half of the electron-H^- scattering rate ν_ei(n_H,T_e), i.e., γ_cl(H^-) = ν_ei /2, but with an unusual Coulomb logarithm [1] for the repulsive (like-sign) particle collisions.

Operating with room-temperature electron plasmas (n_e ≈ 10⁷/cm³) accumulating a 10% fraction of H^- ions, the observed damping rates γ_cl(H^-) show three-fold increase over their background level (γ_bg ≈ 10³/sec) for a variety of plasma waves with low wave numbers (m_r, m_θ, m_z). This increase is somewhat less than the theory estimate ν_ei /2 ≈ 4.6×10³/sec, probably due to ongoing centrifugal mass separation of electrons and heavy ions. The “background” damping rate is sensitive to the plasma injection geometry, so it may be caused by an unnoticed few percent fraction of negative ions produced during the electron injection.

[1] D.H.E. Dubin, Phys. Plasmas 21, 052108 (2014)