Characterization of the SOL in various scenarios using the Helium Beam at the DIII-D Helicon Antenna

2025 Research Campaign, Divertor Science and Innovation

Purpose of Experiment

The goal of this experiment is to characterize and better understand the changes to an ELM suppressed scrape off layer (SOL) in the presence of resonant magnetic perturbations (RMPs), focusing on density pumpout, the 2-point model power relationship, and impurity exhaust. Many diagnostics suffer in the edge, meaning new diagnostics will prove helpful in this area. The He beam diagnostic at DIII-D works by puffing a small amount of neutral helium into the device, and uses line ratio spectroscopy to yield electron temperature and density profiles over the lifetime of a shot. It is located at 195 R0 with a 9 cm maximum span, and has a diagnostic range covering typical SOL parameters, 1×1018 m-3<ne< 4×1019 m-3 and 5 eV <Te< 250 eV. The experiment will be based on an ITER Similar Shape (ISS) reference shot 200635 with n=3 RMPs, and is broken into two major parts: First, a power scan from 6 to 9 MW of NBI power, with and without RMPs. Next, a series of high density (2.5×1019 m-3 pedestal) impurity exhaust studies with He, Ne, and N will be completed. Expected outcomes of this experiment are radial profiles of ne and Te in the SOL, along with their change in behavior over time. Combined with several diagnostics and EMC3-EIRENE modeling, this experiment will give us a better understanding of plasma behavior from the edge extending into the core.

Experimental Approach

This experiment is scheduled for the afternoon after Kole Rakers (MP 2025-13-02), with whom I share a reference shot. This means I will not need any patch panel changes or to re-establish the reference. The plan is as follows: Step 1: Two-Point Model for a 3D SOL (2-6 Shots) We will start with a shot with no RMPs based on 200635 and scan from 6-9 MW of NBI power, with a step roughly every 800 ms. This will be done while keeping torque as constant as possible. We will then repeat with n=3 RMPs. If these shots go well, we will repeat them both, this time with a small injection of Ne to understand its dissipation. We will define the success of these shots based on ELM suppression via Dalpha signal, decreased density(density pumpout), near-constant plasma torque, and strong signals on essential diagnostics. Step 2: RMP and Impurity Edge Impacts in Higher Density Discharges (6-8 Shots) We will recreate our reference shot, but have a higher target density of 2.5e19/m3. After we have successfully seen ELM suppression in both RMP and non-RMP scenarios, we will move forward with Ne and He pulse trains (allowing us to measure impurity confinement times). If time allows, we will repeat these shots with N, and repeat the power scan at this higher density as well.

See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).