Speaker
Dr
Andrey Kabantsev
(University of California, San Diego)
Description
Experiments and theory on magnetized electron columns have characterized a novel *algebraic* damping of drift "diocotron" modes, caused by a weak flux of electrons through the resonance (critical) layer [1]. This flux-driven damping also eliminates the unwanted ion-induced exponential instability of diocotron modes. Here we suggest that a similar flux-driven damping may mitigate the classical flute MHD instability of (*quasi*)neutral plasmas confined in non-uniform magnetic fields.
Our electron plasmas rotate at rate $\omega_{E\times B}$, and the (nominally stable) diocotron modes are characterized by amplitude $A_{d}$, $k_{z}=0$, $m_{\theta}=1,2,3,..,$ frequency $\omega_{d}(m_\theta)$, with a wave/fluid critical radius $r_{c}(m_\theta)$, where $\omega_{E \times B}(r_{c})=\omega_{d} /m_{\theta}$. Application of weak external field asymmetries produces a low $(\sim 1/100)$ density halo of electrons moving radially outward from the plasma core, with flux rate $F\equiv (-1/N_{e}) dN_{e}/dt$. We find that *algebraic* damping of the diocotron modes begins when the halo reaches the critical radius $r_{c}(m_\theta)$, proceeding as $A_{d}(\Delta t)=A_{d}(0)-\gamma\ast\Delta t$ , with $\gamma=\beta (m_\theta)\ast F$.
We have also investigated the diocotron mode instability which occurs when a small number of ions are transiting the electron plasma [2]. In outline, the differential bounce-averaged azimuthal drifts of electrons and ions polarize the diocotron mode density perturbations, developing instability analogous to the classical flute MHD instability. The exponential growth rate $\Gamma$ is proportional to the fractional neutralization $(N_{i}/N_{e})$ and to the amount of charge separation between electrons and ions in the periodic wave perturbations.
We find that the flux-induced *algebraic* damping can stabilize the exponential ion-induced instability up to the driven amplitudes limited by $A_{d}\leq\beta F/\Gamma$. Controlling the cross-field flux of electrons by (changing) applied to the cylindrical wall electrostatic asymmetries we have prevented the ion-induced diocotron instability from growth in a broad range of fractional neutralization. By its very nature the *algebraic* damping of exponential instabilities is most effective at low wave amplitudes $A_{d}$, so this new mitigation mechanism is extremely sensitive even for seemingly small fluxes $F$ through the Landau-resonant layers.
1. A.A. Kabantsev, C.Y. Chim, T.M. O'Neil, and C.F. Driscoll, Phys. Rev. Lett. 112, 115003 (2014)
2. A.A. Kabantsev and C.F. Driscoll, Trans. Fus. Sci. and Tech. 51, 96 (2007)
Primary author
Dr
Andrey Kabantsev
(University of California, San Diego)
Co-author
Prof.
Fred Driscoll
(University of California, San Diego)
