Description
COMPASS Upgrade (COMPASS-U) is a compact tokamak which is being built at the Institute of Plasma Physics in Prague [1]; COMPASS-U will explore various plasma scenarios, characterized by different magnetic fields, plasma currents and plasma shapes [2]. Systematic stability calculations with MARS-F code [3] were performed on the main representative scenarios for COMPASS-U operation and the most unstable modes were identified [4]. The main scenarios, characterized by 3 < q95 < 4, were found to be stable with respect to tearing modes for nominal β because of the stabilizing Glasser-Greene-Johnson effect [5]. Two alternative scenarios were examined, one with negative triangularity and one with higher-current, both characterized by q95 < 3. They were found to be ustable for nominal β: the destabilization was attributed to coupling to pressure-driven external modes [6]. Stability with respect to neoclassical tearing modes (NTMs) was assessed by using generalized Rutherford equation. The saturation width of the (2,1) magnetic islands was found to be large, which make them susceptible to locking to error fields. Electron cyclotron heating (ECH) and current drive (ECCD) are the most largely adopted strategies to control and suppress NTMs [7, 8]. COMPASS-U will be equipped with a triple-frequency ECRH system, which will be employed mainly to centrally heat the plasma and prevent impurity accumulation [9]. Stabilization of NTMs by means of electron cyclotron waves was evaluated and the ECH was found to be more efficient than ECCD in the examined scenarios. This will have consequences on the design of the launcher.
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[9] M. Farnik et al., Plasma Physics and Controlled Fusion 67, 025030 (2025)