The resistance in the walls of the tokamak vessel can cause disruptive energy loss

Under certain conditions, fusion devices known as tokamaks can experience a sudden loss of energy in the vessel walls. Researchers call this process of energy loss stalling. One cause is a magnetohydrodynamic (ie, conducting plasma in a magnetic field) instability or coupling mode with the vacuum vessel.

New research demonstrates that the rate of thermal energy loss is consistent with the development of a specific instability, the tear wall resistance mode (RWTM). Experimental measurements show that the plasma temperature decreases on a time scale consistent with mode development. The simulations show that the RWTM would be stable in the presence of a perfectly conducting wall and also that the unstable mode grows to sufficient amplitude to cause the rapid loss of plasma energy. This rapid loss of energy is called thermal decay. The simulated amplitude and onset condition agree with the experimental results.

The goal of developing fusion power leads researchers to develop experiments for the ITER tokamak. Currently under construction, ITER will be the largest and most powerful tokamak in the world once completed. This research informs how quickly thermal quenches can occur in ITER. This will affect how operators mitigate these outages. Uncontrolled shutdown events in a large machine like ITER can cause significant damage to the craft and must be avoided.

Simulations of a typical ITER reference scenario predict that the plasma will be unstable in RWTM. If the thermal damping is driven by RWTM as observed and modeled in existing devices, then the thermal damping at ITER will be much larger than originally expected. This information can help operators modify ITER’s disturbance mitigation system, thereby reducing the associated risks.

In tokamak disturbances, plasma energy is rapidly transferred to the walls of the device. The time duration of this thermal damping process sets the requirements for any mitigation techniques that may be applied. In recently published research in Plasma Physics from HRS Fusion and the DIII-D National Fusion Facility, a Department of Energy user facility, scientists have described a detailed physics-based understanding of this process by combining experiments, simulations, and theory to study the evolution of plasma instabilities during an interruption.

The simulations demonstrate both that the scaling of the instability growth rate is consistent with expectations based on the container conductivity and that the thermal decay time is proportional to the linear growth time. The simulated growth rate and amplitude of the RWTM is consistent with the time scale of the thermal quenching in the experiment. Extrapolating this result—from the DIII-D tokamak, where the thermal quench is typically a few milliseconds, to ITER—suggests that the thermal quench duration in ITER may be on the order of 70–100 milliseconds. Importantly, this work provides a physical basis for determining the relevant time scale in ITER, and a longer thermal quench reduces mechanical constraints on various perturbation mitigation techniques.