First Experiments with ${e^{-}} \mathord{\left/ {\vphantom {{e^{-}} {H^{-}}}} \right. \kern-\nulldelimiterspace} {H^{-}}$ Plasmas: Enhanced Mode Damping and Transport

Negative Hydrogen ions are produced and confined in a room-temperature electron plasma, causing enhanced mode damping and particle transport effects. We accumulate an $H^{-}$ charge fraction ${n_{H^{-}} } \mathord{\left/ {\vphantom {{n_{H^{-}} } {n_{e} \sim 20\% }}} \right. \kern-\nulldelimiterspace} {n_{e} \sim 20\% }$ in about 200 seconds, as externally excited $H_{2} $ molecules undergo dissociative electron attachment in the plasma. The accumulated $H^{-}$ fraction causes a novel \textit{algebraic} damping of diocotron mode amplitude $A(t)$, and the damping is coincident with an \textit{enhanced} outward drift $\upsilon_{r} $ of the $H^{-}$ ions. That is, ${dA} \mathord{\left/ {\vphantom {{dA} {dt=-\alpha }}} \right. \kern-\nulldelimiterspace} {dt=-\alpha }$, with $\alpha \propto n_{H^{-}} \ast \upsilon_{r} $. We observe that heating the ${e^{-}} \mathord{\left/ {\vphantom {{e^{-}} {H^{-}}}} \right. \kern-\nulldelimiterspace} {H^{-}}$ plasma terminates the enhanced damping and enhanced centrifugal separation, both of which resume when plasma re-cools by cyclotron radiation at $B=$1.2T. Other interesting observations include: (1) enhanced $e^{\mathrm{-\thinspace }}$cooling from collisions with $H^{-}$ cooled by neutrals; (2) enhanced damping of plasma waves due to ${e^{-}} \mathord{\left/ {\vphantom {{e^{-}} {H^{-}}}} \right. \kern-\nulldelimiterspace} {H^{-}}$ collisional drag; (3) strong exponential damping of diocotron modes in a ``floppy'' nearly-pure $H^{-}$ plasma, created by rapid axial ejection of the electrons. Additional novel drift modes and instabilities are predicted theoretically in such a plasma [1]. [1] D.H.E. Dubin, Phys. Plasmas 17, 112115 (2010).