Demonstration of Tokamak Discharge Shutdown with Shell Pellet Payload Impurity Dispersal

Abstract

The first rapid tokamak discharge shutdown using dispersive core payload deposition with shell pellets has been achieved in the DIII-D tokamak. Shell pellets are being investigated as a possible new path toward achieving tokamak disruption mitigation with both low conducted wall heat loads and slow current quench. Conventional disruption mitigation injects radiating impurities into the outer edge of the tokamak plasma, which tends to result in poor impurity assimilation and creates a strong edge cooling and outward heat flow, thus requiring undesirable high-Z impurities to achieve low conducted heat loads. The shell pellet technique aims to produce a hollow temperature profile by using a thin, low-ablation shell surrounding a dispersive payload, giving greatly increased impurity ablation (and radiation) rate when the payload is released in the plasma core. This principle was demonstrated successfully using outer diameter = 3.6 mm, thickness = 40 μm diamond shells holding boron powder. The pellets caused rapid (< 10 ms) discharge shutdown with low conducted divertor heat fluence (~0.1 MJ/m 2 ). Confirmation of massive release of the boron powder payload into the plasma core was obtained spectroscopically. Some evidence for the formation of a hollow temperature profile during the shutdown was observed. These first results open a new avenue for disruption mitigation research, hopefully enabling development of highly effective methods of avoiding disruption wall damage in future reactor-scale tokamaks.

Runaway dynamics in tokamak disruptions with current relaxation

“…In addition, it can also be relevant for scenarios exhibiting ‘inside-out thermal quench’, which can occur in case of a shell pellet injection (Hollmann et al. 2019), but may also happen in shattered pellet injection cases, depending on the detailed dynamics of impurity deposition and radiative collapse.…”

“…This situation may be representative of an internal plasma instability, which can also arise even in non-disruptive plasmas, such as during sawteeth activity. In addition, it can also be relevant for scenarios exhibiting 'inside-out thermal quench', which can occur in case of a shell pellet injection (Hollmann et al 2019), but may also happen in shattered pellet injection cases, depending on the detailed dynamics of impurity deposition and radiative collapse.…”

Runaway Electron Seed Formation at Reactor-Relevant Temperature

“…Previously, small variations (in T e , or in the pellet integrity) were sufficient to prevent RE formation [65,34], making RE avoidance demonstrations meaningless in such marginal conditions. In contrast, a demonstration of RE avoidance via advanced pellet injectors [66,67] or other techniques [68,69,70] would be more meaningful in the robustly RE producing high T e ITB scenario, though the strong avalanche multiplication would still be absent. T e cases highlighted.…”

Modeling of carbon pellets disruption mitigation in an NSTX-U plasma

“…This is the basic idea of a shell pellet, which protects the payload until it reaches the core and deposits its payload there in order to induce an inside-out thermal quench. Experiments [14] at DIII-D have demonstrated this technique and simulations were also conducted [15] using the NIMROD code [16]. As a test case to this approach, we performed a simulation which tries to resemble the injection of a hollow carbon pellet filled inside with carbon dust.…”