Speaker
Description
Wave-particle interaction (WPI) can
produce effective pitch-angle scattering for electrons under
runaway acceleration by the parallel inductive electric field.
Enhanced pitch-angle scattering can impact the runaway energy gain in
two ways. The first is entirely in momentum space, in which the
resonant pitch-angle scattering sets up an energy barrier for
electrons that follows the resonant condition in electron energy and
pitch as a function of wave frequency and parallel wave-number. The
underlying physics is a competition between electric field
acceleration and pitch angle scattering. Acceleration by parallel
electric field dominates at small pitch, but becomes subdominant
compared with synchrotron radiation damping for large enough pitch.
Rapid increase in pitch through resonant WPI can thus turn
accelerating electrons at low pitch to a slowing-down population at
high pitch, which effectively reshapes the runaway vortex to much
lower energy that is set by the resonance condition in momentum and
pitch space. This is a robust process as long as a strong magnetic
field is present so synchrotron damping is appreciable at high pitch
or a finite aspect ratio of the flux surface allows a sizable trapped
region in momentum space.
While a beam-like runaway distribution is known to excite fast plasma
waves through the anomalous Doppler-shifted cyclotron resonance, and
the saturation of this velocity space instability modifies the runaway
energy distribution, external injection of specially designed fast
electromagnetic waves (~500 MHz) has the advantage of targeting the
runaways at energies of 1 MeV or below. This is because the damping
of the wave, as opposed to excitation of a wave instability, is
through the normal Doppler-shifted cyclotron resonance.
The second way resonant and non-resonant WPI can limit the runaway
electron energy is through enhanced spatial transport even if the
magnetic surfaces are intact. Only passing electrons can experience
runaway acceleration, and the maximum energy gain after each toroidal
transit is simply the loop voltage so the runaway energy is bounded
from above by the number of toroidal turns of a passing runaway
electron before it hits the wall. The dwell time of a passing
electron is thus directly tied to the maximum energy it can reach
under runaway acceleration. This picture is complicated by the fact
that passing electrons can become trapped as the result of enhanced
pitch angle scattering. Such trapped energetic electrons no longer
experience runaway acceleration while suffering much faster radial
loss.
Experimental observation on DIII-D suggests that compressional Alfven
waves (CAE) in the MHz range are correlated with the runaway plateau,
which motivated the question if and how external CAE injection
can provide runaway control.
Three key issues in these approaches are (1) collisional damping of
the externally injected wave; (2) flux surface averaged wave-spectrum
that enters the quasilinear pitch angle scattering coefficient, which
helps set the power efficiency of the scheme, and (3) how non-resonant
WPI in CAE range modifies runaway transport. With the helicon and CAE
wave systems coming online on DIII-D, we will investigate how these
injected waves can provide energy control for the runaways.
| Member State or International Organization | United States of America |
|---|---|
| Affiliation | Los Alamos National Laboratory |
