#
56th Annual Meeting of the APS Division of Plasma Physics

## Volume 59, Number 15

##
Monday–Friday, October 27–31, 2014;
New Orleans, Louisiana

### Session QI2: Particle Beams and Waves

3:00 PM–5:00 PM,
Wednesday, October 29, 2014

Room: Bissonet

Chair: Chuang Ren, University of Rochester

Abstract ID: BAPS.2014.DPP.QI2.4

### Abstract: QI2.00004 : Diocotron Mode Damping from a Flux through the Critical Layer*

4:30 PM–5:00 PM

Preview Abstract
Abstract

####
Author:

C. Fred Driscoll

(University of California, San Diego)

Experiments and theory characterize a novel type of spatial
Landau damping of diocotron modes which is {\it algebraic}
rather than {\it exponential} in time; this damping is caused by
a flux of particles through the wave/rotation critical
layer.\footnote{A.A. Kabantsev, et al., Phys.~Rev.~Lett. 112, 115003 (2014).}
These $k_z = 0$ diocotron (drift) modes with azimuthal mode
numbers $m_\theta = 1,2...$ are dominant features in the
dynamics of non-neutral plasmas in cylindrical and toroidal
traps; and they are directly analogous to Kelvin waves on 2D
fluid vortices.
Spatial Landau damping is the resonant
interaction between a mode at frequency $f_m$ and the plasma
rotation $f_E (r)$, at the critical radius $R_c$
where $f_m = m_\theta f_E(R_c)$. This is mathematically analogous to
velocity-space Landau damping with $f_k = k v / 2 \pi$.
\textbullet Experimentally, diocotron modes on pure electron plasmas
exhibit exponential Landau damping when the {\it initial} plasma
density is non-zero at $R_c$. Here, we demonstrate that a
steady outward {\it flux} of particles through $R_c$ causes
diocotron modes to damp algebraically to zero amplitude, as
$D(t) = D_0 - \gamma_m t$ . The outward flux is controlled and
measured experimentally, and the damping rates $\gamma_m$ are
proportional to the flux. In general, any weak non-ideal
process which causes outward flux may cause this damping.
\textbullet Analytics and simulations have developed a simple model of
this damping, treating the transfer of canonical angular
momentum from the mode to particles transiting
the nonlinear trapping region at $R_c$. The
model qualitatively agrees with experiments for $m_\theta = 1$,
but nominally predicts a discrepant algebraic exponent for
$m_\theta = 2$, perhaps due to the amplitude dependence of the
trapping structure. Overall, this novel flux-driven damping is
determined by the {\it present} magnitudes of the wave and
outward flux, in contrast to the Landau analysis of phase
mixing of the {\it initial} density.

*Supported by NSF/DOE Partnership grants PHY-0903877 and DE-SC0002451.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2014.DPP.QI2.4