2024 – Characterization of runaway electron plateau spatial profiles and final loss structure

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.

Experimental Approach

The target shot (#191357) is an ECH-heated, inner wall-limited (IWL), L-mode discharge, which uses deuterium as the working gas. The REs are generated by first shutting down the plasma discharge with an Argon pellet injection (ArPI) at t = 350 ms to create a RE plateau, followed by an injection of 100 Torr-L of D2 starting at t = 450 ms and lasting 150 ms, using the fueling valves gasB+gasC+gasD, to recombine the RE plateau background plasma and to purge the Argon. The plasma current is ramped up, starting at 450 ms, to reach the target value at 600 ms. The plasma is then compressed towards the center post (CP) starting at t = 650 ms. As the plasma touches the wall at about t = 800 ms, a final loss MHD instability is triggered which results in a rapid loss of the RE beam. The target RE current just before the final loss event is Ip~ 550 kA. The same target shot is desired for the entire run in order to establish a consistent frame of reference for the fastcam and to make good shot-to-shot comparisons when the RMP is introduced. The fastcam diagnostic will be prepared with a dedicated setup to allow for a reduced region of interest (ROI) selection viewing the CP, allowing for acquisition rates up to 500 kHz. This and a large passband filter (100 nm passband) will ensure a detailed measurement of the instability and mode structure. The BGO array will be use to reconstruct the toroidal phase of the RE beam interacting with the CP, that will be compared with the RMP phase in the second phase of the experiment.

Interested in a behind-the-scenes look at DIII-D? Join us for a virtual OR in-person tour during Fusion Energy Week (May 5-9)! Sign up for a tour here.

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