2024 – Plasma shaping, rotation, and m, n>2, 1 tearing mode origins

Plasma shaping, rotation, and m, n>2, 1 tearing mode origins

2024 Research Campaign, Transient Control

Purpose of Experiment

Large-scale magnetic instabilities are a major cause of disruptions in tokamaks. The high projected stored energy in ITER and other future devices makes this rapid particle and energy loss very damaging to plasma-facing components; plans for future reactors and must be optimized to avoid dangerous instabilities. We are also interested in interactions between instabilities, which have been shown to transfer energy between them or trigger disruptions. The objective of the proposed experiment is to use simultaneous internal magnetic and density fluctuation measurement with the Radial Interferometer-Polarimeter diagnostic to identify the effects of plasma shaping and rotation on magnetic instability behavior. The work will also probe the instabilities’ radial energy distribution and its effects on interactions between fluctuations. It has already been shown that elongated and triangular plasmas are more prone to disruptive instabilities. This work emphasizes higher-helicity instabilities, which are less immediately dangerous but can allow some particles and/or energy to escape, reducing self-heating, or interact to trigger other instabilities. This experiment intends to quantify the effects of plasma elongation, triangularity, and rotation on higher-helicity instability growth and structure. The experiment will be used to validate and expand on previous findings, using internal measurements, of mode interactions. The researchers will measure instability growth and triggering mechanisms as early as possible.

Experimental Approach

This experiment will vary neutral beam fueling power and plasma torque to identify plasma rotation profiles with maximum magnetic fluctuation activity. It will be very important for the plasma rotation in each shot to be constant in time, since changing rotation also changes the plasma susceptibility to fluctuations and the likelihood that they will interact. Two rotation settings will be selected for the next step: one with strong fluctuations or multiple fluctuations of different frequencies, and one with the lowest possible rotation in which fluctuations still occur. It is predicted that, as has been seen in other cases, high rotation may cause the plasma to be more unstable, but low rotation will promote fluctuation interaction. Ellipticity will then be varied to measure its effects on magnetic instability strength. The plasma will be vertically moved during each shot to measure the strength of instabilities at vertical positions outside the plasma center. If time permits, the plasma will be made more triangular to further promote instability and/or plasma current will be varied in time to move instabilities around within the plasma.