2025 – Can magnetic islands in an FPP be automatically stabilized by RF condensation?

Can magnetic islands in an FPP be automatically stabilized by RF condensation?

2025 Research Campaign, Transient Control

Purpose of Experiment

In ITER or any future FPP, disruptions triggered by large magnetic islands pose a significant threat to performance and stability. Current driven at the island O-point by electron cyclotron (EC) waves has been shown to provide a stabilizing effect on these islands. Recently, a nonlinear effect (called RF condensation) has been proposed, which would concentrate RF power deposition and current drive at the island O-point and enhance the temperature perturbation in the island. This may allow for automatic stabilization of magnetic islands even with initially broad power deposition profiles. A code called OCCAMI has been developed to self-consistently calculate the nonlinear enhancement of the island temperature perturbation for a given plasma with some input RF power. This experiment on DIII-D aims to quantitatively validate the OCCAMI code and RF condensation theory in general. Our calculations indicate that RF condensation effects will be strongest for large, locked islands. For both locked and rotating islands, we will use as many gyrotrons as possible to launch EC waves into the magnetic island at various power levels, scanning over a range of poloidal launch angles. We expect nonlinear effects to be strongest for very high toroidal launch angles and high input power. For each scan, the temperature at the island O-point will be measured using electron cyclotron emission (ECE) diagnostics and compared to predictions from the OCCAMI code. We will compare the temperature perturbations at the island O-point at various RF power levels to measure the nonlinearity of the island heating.

Experimental Approach

We will use a resonant magnetic perturbation (RMP) imposed by an n = 1 I-coil current to reliably obtain an n = 1 island, which we will optimize to be (i) as large as possible and (ii) stable enough to run our required scans with the gyrotrons. To do this, we will scan over neutral beam (NB) torque, I-coil current, and I-coil rotation rate. We will also study an island locked at some optimal RMP phase which maximizes the island temperature perturbation and the RF current drive. For our experimental scans over both rotating and locked islands, we will set each gyrotron to its maximum possible toroidal launch angle (205, 207, or 208 degrees depending on the gyrotron) and maximum power. We will scan the gyrotron poloidal angle from 95 degrees to 108 degrees, and back down, taking one second each way. Then, the gyrotrons will be reset to ½ and then ¼ power, and the same scans over poloidal angle will be repeated. After these scans, we will use the ECE and ECEI diagnostics to determine the temperature along radial lines through the O-point and X-point of a locked island, thereby measuring the “island temperature perturbation” (T(O) – T(X))/T(X) as a function of poloidal launch angle. Finally, we will divide the measured island temperature perturbations at full power by the measurements at ½ and ¼ power. This will allow us to quantitatively determine the strength of any nonlinear effects.

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