A novel path to runaway electron mitigation via deuterium injection and current-driven MHD instability

Abstract

Relativistic electron (RE) beams at high current density (low safety factor, q a ) yet very low free-electron density accessed with D2 secondary injection in the DIII-D and JET tokamak are found to exhibit large-scale MHD instabilities that benignly terminate the RE beam. In JET, this technique has enabled termination of MA-level RE currents without measurable first-wall heating. This scenario thus offers an unexpected alternate pathway to achieve RE mitigation without collisional dissipation. Benign termination is explained by two synergistic effects. First, during the MHD-driven RE loss events both experiment and MHD orbit-loss modeling supports a significant increase in the wetted area of the RE loss. Second, as previously identified at JET and DIII-D, the fast kink loss timescale precludes RE beam regeneration and the resulting dangerous conversion of magnetic to RE kinetic energy. During the termination, the RE kinetic energy is lost to the wall, but the current fully transfers to the cold bulk thus enabling benign Ohmic dissipation of the magnetic energy on longer timescales via a conventional current quench. Hydrogenic (D2) secondary injection is found to be the only injected species that enables access to the benign termination. D2 injection: (1) facilitates access to low q a in existing devices (via reduced collisionality & resistivity), (2) minimizes the RE avalanche by ‘purging’ the high-Z atoms from the RE beam, (3) drives recombination of the background plasma, reducing the density and Alfven time, thus accelerating the MHD growth. This phenomenon is found to be accessible when crossing the low q a stability boundary with rising current, falling toroidal field, or contracting minor radius—the latter being the expected scenario for vertically unstable RE beams in ITER. While unexpected, this path scales favorably to fusion-grade tokamaks and offers a novel RE mitigation scenario in principle accessible with the day-one disruption mitigation system of ITER.

Bayesian optimization of massive material injection for disruption mitigation in tokamaks

“…Indeed, RE beams with the same RE current may cause serious damage, or no detectable effect at all, depending on how the beam is lost to the wall. Recent results indicate that a combination of a low impurity concentration bulk plasma and large-scale magnetohydrodynamic instabilities may enable termination of megaampere-level RE currents without damage to the wall (Paz-Soldan et al 2021; Reux, Cédric et al 2021).…”

Runaway electron deconfinement in SPARC and DIII-D by a passive 3D coil

“…Numerous experiments with post-disruption RE current plateaus (the largest carrying almost 1MA of current) have been performed on DIII-D [7,8,10,58] without causing significant damage to the device, making it a valuable test-bed for RE mitigation concepts. The DIII-D modeling of the IWL plasmas showed a larger loss fraction (∼ 90%) than the linear plasma response modeling from Ref.…”

“…The edge safety factor increases monotonically as the simulation progresses and the plasma current decays. Experimentally, this monotonic increase is not required [10,50] as the plasma can also shrink as it moves into the wall, and a reduction of minor radius is used as an experimental knob to reduce the edge q according to q a ∝ aB T /I P [10] in order to trigger kink instabilities, but the ideal-wall boundary condition used in the NIMROD model precludes this effect. Each simulation begins with 10530 RE test-particles, which are distributed uniformly in normalized poloidal (with random poloidal angles) and uniformly in energy ranging from 1-50 MeV, with pitch (p ⊥ /p) ranging from 0-0.5.…”

“…Experiments aimed at mitigating tokamak disruptions with massive material injection (gas or pellets) have long sought unsuccessfully to exceed the so-called Rosenbluth density to maintain E < E crit even at postthermal quench temperatures [3][4][5]. Injection of either high-Z (Ne, Ar, Kr, Xe) [6,7] or low-Z (He, D 2 ) [7,8] material into an existing runaway electron beam has also been pursued, with D 2 injection showing strong promise experimentally for benign termination of a runaway electron beam by a kink instability, supported by extended-MHD modelling [9][10][11], which has been extended to ITER scenarios [12]. Strategies to prevent the formation of a mature RE beam involve enhancing transport of the seed REs in physical or momentum space, e.g., by stochastic magnetic fields [13][14][15][16][17][18] or wave-particle interactions [19,20], to produce a loss rate that exceeds the avalanche growth rate.…”

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…Studies of high current runaway electron (RE) beams [8,9] reveal excitation of current-driven (low edge safety factor, q a ) m/n = 2/1 kink instabilities (figure 4) that promptly terminate the RE beam on an Alfvenic time-scale, with minimal heating of plasma facing component surfaces [10,11], offering a new alternate pathway to RE beam mitigation without collisional dissipation. MARS-F modeling [9] predicts that the absence of wall heating is due to both an increase of the wetted area during the MHD-driven RE loss (figures 4(e) and ( f )) and an inhibition of the conversion of magnetic energy into kinetic energy normally observed during RE loss events (when the RE beam regenerates during CQ). Observations of IR emission from the centerpost during RE beam loss (figure 4( f )) confirm the MARS-F predictions that the wetted area would include the full toroidal and a substantial poloidal range after the MHD event disperses the RE beam.…”