Speaker
Description
Runaway electrons of MeV and higher energies can dominate the plasma
current during ITER startup and the current quench phase of a major
disruption. The plasma regime spans from reasonably low-density and
high-temperature (startup) to high-density and low-temperature
(disruption mitigated by high-Z impurities), and somewhere in between
(disruption mitigation by low-Z injection). Here we describe the
recent physics findings informing how runaways can be managed in these
diverse situations. The first result is on how runaway-induced wave
instabilities, particularly the slow-X modes as opposed to the
whistler waves, can efficiently transfer the plasma current from
high-energy runaways to suprathermal electrons, during which
relativistic runaways can even reverse their direction with respect to
the magnetic field. These findings come from fully-kinetic simulations
that remove the shackle of quasilinear formulation previously reported
in the literature, and the physics is of interest to warm plasmas
during ITER startup or in reheated plasmas during a current quench.
The second result is on the standard ITER scenario of high-Z impurity
mitigated disruption in which runaway dissipation and transport loss
are greatly enhanced by high-Z impurities while plasma column scrapes
off against the first wall during a vertical displacement event
(VDE). The runaway loss pattern on the first wall, as the result of
both collisional transport and VDE scrape-off, is of particular
interest in assessing the wall tolerance for the runaway impact. A
hybrid model that couples quasi-static MHD with drift-kinetic runaway
electrons has been simulated to account for the ITER VDE dynamics with
the full kinetic physics of runaway dissipation and transport loss, as
well as the scrape-off process. The third result is on the use of
solid tungsten particulates for standoff termination of the
relativistic runaway electrons. The idea is that instead of having the
runaways scrape-off against the wall, one can place a cloud of
tungsten particulates in front of the first wall to be impacted by the
VDE, so runaways can terminate on these solid particulates. This is
similar to the previous dust shield concept for divertors, but the new
twist is that the tungsten particulates can facilitate safe
termination by both runaway energy attenuation and effective pitch
angle scattering, which can alter the runaway orbits (e.g. from
passing to trapped) for broader deposition pattern on the first
wall. Here we will show both effects by the tungsten particulates via
MCNP calculations. The fourth result is on the feasibility of doing
away with thermal quench mitigation by radiative cooling. The idea is
to inject enough amount of hydrogen that the plasma would be
dilutionally cooled to be collisional for open field line transport,
but still warm enough that the inductive electric field from
$E_\parallel = \eta j_\parallel$ stays below the avalanche threshold
electric field if not the Connor-Hastie critical field. Interestingly,
MHD simulations with Braginskii transport coefficients are supposed to
be theoretically sound for such a mitigated collisional plasma. Here
we will show the most up-to-date PIXIE3D simulations that establish
comparable TQ and CQ time scales in such mitigation scenario.
| Speaker's title | Mr |
|---|---|
| Speaker's email address | xtang@lanl.gov |
| Speaker's Affiliation | Los Alamos National Laboratory, Los Alamos, NM |
| Member State or IGO | United States of America |
