MSE measurements of the high field side lower hybrid current drive profile
2025 Research Campaign, Thrust: High Field Side LHCD
Purpose of Experiment
The purpose of this experiment is to measure the lower hybrid (LH) non-inductive current profile using the Motional Stark Effect (MSE) diagnostic. This will be compared against predictions from the ray-tracing/Fokker-Planck codes GENRAY/CQL3D to assess the accuracy of simulation for the high field side lower hybrid current drive experiment. The plasma density and wave N|| will be varied to begin understanding the regions of parameter space in which good simulation accuracy should be expected. Previous MSE measurements in LHCD experiments on other devices have found significant differences between simulation and experiment. However, in DIII-D’s case, a clear means of closing the spectral gap has been identified in simulation without relying on scrape-off layer interactions. Simulation accuracy is thus expected to be improved relative to previous experiments. This would bode well for the future of LH as the spectral gap in reactor-class devices is also readily bridged in simulation.
Experimental Approach
The reference shot for this experiment is 199749, which is ideal due to its low collisionality, allowing for larger driven currents. To investigate simulation accuracy, a density and N|| scan will be completed. The two N|| selected for this experiment are 2.3 and 2.7. Throughout the density scan, N|| = 2.7 is predicted to remain in the single pass regime, whereas N|| = 2.3 falls deeper into the multi-pass regime as density increases (see Figures 1 and 2). The first set of shots will restore 199749 and establish conditions ideal for LHCD. RMP coils will be used for density pumpout to obtain a line average density of ~2.5e19 m-3 for a stationary flattop. 199749 does requires some minor adjustments, detailed in the miniproposal. Beyond restoring the target shot, these first shots will also serve as the reference shots for the low density case. The second set of shots will introduce the LH waves and will form the lower end of the density scan. To drive substantial current, at least 300 kW of LH power will be requested. Peak current densities are predicted to be a factor of 2-5 larger than those successfully measured in previous top launch experiments [11]. Additionally, the current is expected to be driven closer to the axis in this experiment, further improving its measurability. Should application of LH produce no effect on the MSE signals, we may try dropping the density and/or increasing the temperature before moving to the higher density cases. The third and fourth set of shots correspond to these higher density cases, with the highest density points being completed first so as to bookend the scan. For each density point, a reference case without LH is completed first. That shot is then repeated twice more, once for each N||. If time allows, either the inner gap or DRSEP would be varied.