Characterization of runaway electron plateau spatial profiles and final loss structure
2024 Research Campaign, Disruption Mitigation
Purpose of Experiment
The main purpose of this experiment is twofold. The first is to obtain, for the first time, data on the poloidal structure of the runaway electron (RE) final loss instability, which can then be compared to past simulations of the RE final loss structure. Such experimental data has not been obtained before and it is therefore crucial for identifying which simulations are physically correct and to understand the dynamics of the RE plateau current distribution. The second purpose is to determine if resonant magnetic perturbations (RMPs) can affect the toroidal phase of the RE final loss strike. This will shed light on the viability of using RMPs to control the RE plateau wall strikes, which may be used in ITER if proved successful here.
Background: RE beams that form after a disruption carry most of the current, which is usually comparable in magnitude to the pre-disruption plasma current. The RE beams seem to be very stable, lasting for seconds in current devices, but eventually are lost to the wall. This loss happens on a very quick timescale on the order of 10s of microseconds. Such an event is commonly referred to as a final loss, as all the RE of the beams are lost and the beam is terminated. This is a very dangerous phenomena as all the energy carried by the beam is rapidly deposited in a localized region and may cause heavy damage to the wall and the plasma-facing components (PFC). The instability causing this loss has been studied theoretically by different groups, but there is no consensus on the correct mechanism that causes it. For example, Boozer et al. [1] suggested that the instability is due to the stochastic region surrounded by a small annulus of good magnetic surface. When the good magnetic surfaces are broken, for example due to contact with the wall, all the RE in the stochastic region are lost. Meanwhile, Bandaru et al. [2] suggested that a combination of low plasma density and a hollow current profile causes a fast growth of a double-tearing mode, which in turn leads to stochastization of the magnetic field and a prompt loss of REs. Alternatively, Paz-Soldan et al. [3] suggested that the culprit is a 2/1 kink instability, occurring when the edge safety factor qa reaches 2. The goal of the experiment is to determine the final loss mode structure by observing a final loss event during a center post compression of a RE beam with the DIII-D fastcam and BGO diagnostics.