Speaker
William Heidbrink
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I5.J602.pdf
First direct observation of whistler waves driven by relativistic electrons in
a toroidally-confined laboratory plasma
D.A. Spong1, W.W. Heidbrink2, C. Paz-Soldan3, X.D. Du2, K.E. Thome3, M.A. Van Zeeland3
1
Oak Ridge National Laboratory, Oak Ridge, USA
2
University of California, Irvine, USA
3
General Atomics, San Diego, USA
Whistlers are dispersive electromagnetic waves that can be driven unstable by energetic
electrons in both space and laboratory plasmas. They are unstable in the outer radiation belts
of planetary atmospheres, where they are known as chorus waves. Perpendicular scattering of
energetic electrons by whistlers contributes to the aurora. The first detection (in 1894!) was
generated by lightning. In the present experiment, a confined population of ~ 10 MeV
“runaway” electrons drives whistlers unstable in the DIII-D tokamak (Fig. 1). The detected
100-200 MHz waves are in the band between the ion cyclotron and lower hybrid frequencies
and satisfy the cold-plasma dispersion relation, with the expected dependencies on magnetic
field and density. Whistler activity is correlated with the intensity of hard x-rays produced by
the runaways. Fluctuations occur in discrete frequency bands, and not a continuum as would
be expected from plane wave analysis, suggesting the important role of toroidicity. An MHD
model including the bounded/periodic nature of the plasma identifies multiple eigenmode
branches. For a toroidal mode number n = 10, the predicted frequencies and spacing are similar
to observations. The instabilities are stabilized with increasing magnetic field, as expected from
the anomalous Doppler resonance. The whistler amplitudes show intermittent time variations.
Predator-prey cycles with electron cyclotron emission (ECE) signals are observed, which can
be interpreted as wave-induced pitch angle scattering of moderate energy electrons. Such
nonlinear dynamics are supported by quasi-linear simulations indicating that electrons are
scattered both by whistlers and high frequency magnetized plasma waves. The whistler wave
predominantly scatters the high energy electrons, while the magnetized plasma wave scatters
the low energy electrons, abruptly enhancing the ECE signal. If whistlers that pitch-angle
scatter runaways are excited in future devices, the enhanced runaway dissipation could reduce
the likelihood of runaway-electron induced damage.
Work supported by the US DOE under DE-FC02-04ER54698, DE-AC52-07NA27344, DE-
FG02-07ER54917, DE-SC00-16268, and DE-AC05-00OR22725.
Figure 1. Whistler waves detected by a
magnetic probe. The time evolution of the
dominant toroidal magnetic field is indicated
by the dashed line; the density is nearly
constant in this discharge. The frequency
exhibits the expected linear dependence on
magnetic field. The modes are more easily
destabilized at lower magnetic field,
probably because the electron energy is
higher. The banded, discrete nature of the
observed modes is evident.
