Thermal Quench in DIII-D locked mode disruptions

The cause of tokamak disruptions has not been well understood.

It has not been known what instability causes disruptions or how to avoid them.

Recent work identified the thermal quench (TQ) in JET locked mode disruptions with

a resistive wall tearing mode (RWTM) [1].

New research [2] finds a similar instability in a DIII-D locked mode shot [3].

The instability is studied with simulations, theory, and comparison to experimental data.

Linear theory and simulations show the mode is stable for an ideal wall, and unstable

with a resistive wall.

Its growth rate scales as the resistive wall time to a negative fractional power.

The scaling depends on the tearing stability parameter with and without an ideal wall.

Nonlinear simulations show that the mode grows to large amplitude, causing a thermal quench.

The mode growth time in experiments and simulations is comparable to the thermal quench time.

The onset condition for the RWTM is that the plasma is tearing stable with an ideal

wall, and unstable with a resistive wall.

This can be the case when the q = 2 rational surface is sufficiently

close to the edge of the plasma.

These results are important for ITER [4],

greatly mitigating the effects of disruptions. The ITER thermal quench time will be

much slower, because the wall resistive penetration time is

50 times slower than in JET and DIII-D.

[1] H. Strauss and JET Contributors,

Effect of Resistive Wall on Thermal Quench in JET Disruptions,

Phys. Plasmas 28, 032501 (2021)

[2] H. Strauss, B.C. Lyons, M. Knolker,

Locked mode disruptions in DIII-D and application to ITER,

arXiv:2206.06773 (14 Jun 2022)

[3] R. Sweeney, W. Choi, M. Austin, et al.,

Relationship between locked modes and thermal quenches in DIII-D, Nucl. Fusion 58, 056022 (2018)

[4] H. Strauss, Thermal quench in ITER disruptions,

Phys. Plasmas 28 072507 (2021)