Speaker
Description
Thermal quench (TQ) marks the point of no return in a tokamak
disruption. It not only brings a thermal load management issue at the
divertor plates and first wall, but also determines the runaway
seeding for the subsequent current quench (CQ). There are two ways to
trigger a TQ, one is the globally stochastic magnetic field lines that
connect the hot core plasma to the cold boundary, while the other is
high-Z impurity injection. In both situations, a nearly collisionless
plasma is made to intercept a radiative cooling mass (RCM), being that
an ablated pellet or a vapor-shielded wall. Previous JET experiments
and more recent DIII-D data have demonstrated a wide range of TQ
durations with and without high-Z pellet injection, which is
concerning for future tokamak reactor operations.
With fully kinetic VPIC simulations and analytical theory, we have
uncovered three underlying parallel transport mechanisms that govern
the thermal collapse of a fusion-grade and hence nearly collisionless
plasma. They are: (1) thermal collapse of surrounding plasmas due to a
localized RCM is dominated by convective energy transport as opposed
to conductive energy transport, and as the result, TQ comes in the
form of four propagating fronts with distinct characteristic speeds,
all originated from the RCM, and core thermal collapse is a lot slower
than one would expect based on electron thermal conduction of
Braginskii or free-streaming; (2) cooling of perpendicular electron
temperature closely follows that of parallel electron temperature, and
in a nearly collisionless plasma, is mostly driven by fast
electromagnetic kinetic instabilities of two kinds, sequentially in
time; (3) the overall TQ inevitably has a transition from the
collisionless phase to the collisional phase, the duration of which
have distinct physics scalings, and the two of which are sandwiched by
a transition period of its own unique physics scaling. Altogether, we
can now predict the TQ history of a plasma at given density and
temperature as a function of the magnetic connection length.
These physics advances inform the strategies for avoiding and
mitigating the deleterious effects of TQ on both thermal load
management and the subsequent Ohmic-to-runaway current conversion. In
the ITER scenario of high-Z pellet injection for spreading plasma heat
load via radiation, pellet assimilation and spatial homogenization are
both tied to the TQ physics. The staged pellet injection suffers
particularly strong constraints that result in severe performance
degradation. In situations where runaway avoidance is a priority, one
can no longer rely on impurity radiation for thermal load management.
The drastically different TQ durations in the collisionless and
collisional regimes point to the alternative mitigation approach that
relies on dilutional cooling via massive hydrogen injection to place
the entire TQ in the collisional phase, in which case the CQ and TQ
span the same period. If there is not enough lead time for
predisruption pellet injection, the physics insights place stringent
constraints on what an optimal passive mitigation method would entail,
especially when runaway avoidance is also a consideration.
Work supported by U.S. DOE OFES and OASCR.
| Speaker's title | Mr |
|---|---|
| Speaker's email address | xtang@lanl.gov |
| Speaker's Affiliation | Los Alamos National Laboratory, Los Alamos, New Mexico |
| Member State or IGO | United States of America |
