Speaker
Description
Fast ions generated by neutral beam injection (NBI) or radio-frequency (RF) heating can resonantly interact with background plasma and excite Alfvénic eigenmodes (AEs), in turn leading to enhanced fast-ion transport and potential damage to reactor components [1]. The combined use of RF and NBI heating exhibits mixed effects on AE activity—mitigating or enhancing mode excitation in NSTX(-U) [2–4], and reducing AE-induced fast-ion losses in ASDEX Upgrade [5]. Understanding the RF and AE interplay with fast ions, e.g. from NBI or alpha particles in burning plasmas, is essential for the operation of future burning plasma devices.
In low-B field spherical tokamaks (STs), and particularly for high-energy RF-accelerated ions, the guiding-center (GC) approximation breaks down, necessitating a full-orbit (FO) treatment [6]. We benchmark a full-orbit RF implementation [7] against the GC quasilinear Stix operator [8] implemented in the ASCOT code [9], and validate the results against ASDEX Upgrade INPA diagnostic [10] experimental data.
The validated tool was employed to analyze NSTX(-U) cases where the High-Harmonic Fast Wave (HHFW) influences the AE stability [3]. Significant differences between FO and GC approaches are highlighted and applied to understand the heating/current drive patterns that will be possible with the new NSTX-U neutral beam system. A major difference between the approaches is RF-induced anomalous transport captured by the FO treatment. Sub- and super-diffusive regimes are identified as a function of the ratio of the resonance-layer distance to the Larmor radius, where for sufficiently small ratios the behavior returns to the diffusive limit, implied by the Stix operator. We will discuss the effects on AE stability, especially for poorly confined, highly energetic ions in low-B-field NSTX scenarios. To investigate the effects of RF waves on the AE saturation regime, a perturbative method for AE mode amplitude and phase evolution [11, 12] has been implemented in ASCOT5. We will use this tool to characterize AE properties, with emphasis on AE chirping as they are routinely observed at the NSTX(-U) tokamak, and to develop strategies for employing RF waves as an AE control mechanism in tokamaks.
[1] A. Loarte et al., Plasma Physics and Controlled Fusion 45 (2003) pp. 1549-1569
[2] W. W. Heidbrink et al., Plasma Physics and Controlled Fusion 48 (2006) pp. 1347-1372
[3] E. D. Fredrickson et al., Nuclear Fusion 55 (2015) pp. 013012
[4] M. Podestà et al., Nuclear Fusion 52 (2012) pp. 094001
[5] J. Galdon-Quiroga et al., in preparation
[6] G. Kramer et al., Plasma Phys. Control. Fusion 55 (2013) 025013
[7] P. Oyola et al., in preparation
[8] T. H. Stix et al., Nuclear Fusion 15 (1975)
[9] E. Hirvijoki, PhD Thesis, 2014
[10] J. Rueda-Rueda et al., Plasma Physics and Controlled Fusion 66 (2024) 035008
[11] R. B. White, The theory of toroidally confined plasmas.
[12] A. Bierwage et al., Plasma and Fusion Research 16 (2021) 1403087