Speaker
Description
Benign termination of mega-ampere level runaway current has been
convincingly demonstrated in JET [1] and DIII-D [2],
establishing it as a leading candidate for runaway mitigation on
ITER. This comes in the form of a runaway flush by parallel streaming
loss along stochastic magnetic field lines formed by global
magnetohydrodynamic (MHD) instabilities, which are found to correlate with a
low-Z (deuterium) injection that purges the high-Z impurities from a
post-thermal-quench plasma. After the runaway flush, there are two
scenarios determining whether the runaway current reforms.
The scenario of no-runaway-reconstitution is enabled by (1) high
degree of high-Z impurity purge by massive deuterium injection and (2)
limited assimilation of deuterium in the post-purge plasma. Both
conditions are conducive to reduce radiative losses that would
have otherwise prevented electron reheating by Ohmic dissipation of
the plasma current, which is critical to establish and sustain a
parallel electric field below the runaway avalanche threshold.
Although electron reheating is most efficient at vanishing impurity
density and low deuterium density, the current ITER target for current
quench duration demands effective electron cooling to cap the electron
temperature to a few tens of eV. This translates into a limit on how
high the deuterium density must be, the exact magnitude of which is
controlled by the residue impurity content.
In the likely scenario of insufficient electron reheating, the parallel
electric field stays above the runaway avalanche threshold and runaway
current can be reconstituted. Here the quantity of interest is the
plasma current drop before the Ohmic-to-runaway conversion is
completed. This critically depends on runaway seeding, which is
dramatically different for a post-flush plasma. Specifically, the
trapped ``runaways'' dominate the seeding for runaway reconstitution,
in sharp contrast to that in a post-thermal-quench plasma where much
lower energy hot tail, Dreicer mechanism, and tritium decay/Compton
scattering are at play. The high-Z impurities previously injected to
mitigate the thermal quench can greatly enhance such a trapped "runaway"
population, which remains well confined in the presence of stochastic
magnetic fields, and thus survives the 3D MHD flush. Furthermore,
their relatively high energy, around 1 MeV, ensures a low collisional
detrapping rate while the magnetic surfaces reheal at low electron
temperature. Most interestingly, the incomplete purge of high-Z
impurities helps drain the seed but produces a more efficient
avalanche, two of which compete to produce a 2-3~MA step in current
drop before runaway reconstitution of the plasma current.
The different post-flush scenarios and the corresponding plasma
parameter regimes demarcated here, help place the existing experiments
in perspective in relation to ITER requirements. The present work will
also elucidate strategies through which the phase space distribution
of runaways during the current plateau can be tailored to minimize
this trapped population of electrons, along with determining critical
parameters for expediting the decay of the remnant electron population
before the magnetic flux surfaces are able to reform. This work was
supported by DOE OFES and OASCR.
[1] Reux et al., Phys. Rev. Lett. (2021), [2] Paz-Soldan et al. Nucl. Fusion (2021)
| Speaker's title | Mr |
|---|---|
| Speaker's email address | cmcdevitt@ufl.edu |
| Speaker's Affiliation | University of Florida |
| Member State or IGO | United States of America |
