Experimental studies of plasma-antenna coupling with the JET Alfvén Eigenmode Active Diagnostic

Abstract

This paper presents a dedicated study of plasma-antenna (PA) coupling with the Alfvén Eigenmode Active Diagnostic (AEAD) in JET. Stable AEs and their resonant frequencies f , damping rates γ < 0, and toroidal mode numbers n are measured for various PA separations and limiter versus X-point magnetic configurations. Two stable AEs are observed to be resonantly excited at distinct low and high frequencies in limiter plasmas. The values of f and n do not vary with PA separation. However, |γ| increases with PA separation for the lowf , but not high-f , mode, yet this may be due to slightly different edge conditions. The high-f AE is detected throughout the transition from limiter to X-point configuration, though its damping rate increases; the low-f mode, on the other hand, becomes unidentifiable. The linear resistive MHD code CASTOR is used to simulate the frequency scan of an AEAD-like external antenna. For the limiter pulses, the high-f mode is determined to be an n = 0 GAE, while the low-f mode is likely an n = 2 TAE. During the transition from limiter to X-point configuration, CASTOR indicates that n = 1 and 2 EAEs are excited in the edge gap. These results extend previous experimental studies in JET and Alcator C-Mod; validate the computational work performed by Dvornova et al 2020 Phys. Plasmas 27 012507; and provide guidance for the optimization of PA coupling in upcoming JET energetic particle experiments, for which the AEAD will aim to identify the contribution of alpha particles to AE drive during the DT campaign.

A novel measurement of marginal Alfvén Eigenmode stability during high power auxiliary heating in JET

“…The damping rate was also noted to increase rapidly and nonlinearly with the edge magnetic shear s 95 (see Fig. 6a in [7]), likely due to both continuum and radiative damping. This result was in agreement with past works [10,14,17,18,23,33].…”

“…The observation of this stable AE was actually surprising in several ways: First, significant ion Landau damping was expected from the Neutral Beam Injection (NBI) heating power P NBI ≈ 20 MW during the discharge since significant NBI damping of AEs has been observed before in many devices, including JT-60U [38], TFTR [13], and JET [4,10,20]. The plasma was also in X-point configuration, which makes stable AE detection even less likely [7,39]. Yet we were able to track a stable AE in real time, during X-point configuration, and at the highest external heating power to-date (∼25 MW).…”

“…The AEAD was operated on almost 800 plasma discharges, from which a database of almost 7500 stable AE measurements was assembled. Note that this database is actually an expansion of that reported earlier in [6] and [7] because new data were acquired after their publications.…”

“…As the scanning antenna frequency passes through that of a stable AE, the plasma resonates like a driven, weakly damped harmonic oscillator [1], and the frequency-filtered magnetic response -obtained from a toroidal array of fast magnetic probes -determines f 0 , γ, and n [6]. The external antennas are more likely to excite AEs near the plasma edge than in the core; these often include Global, Toroidicity-induced, and Ellipticity-induced AEs (GAEs, TAEs, and EAEs, respectively) [7].…”

Results from the Alfvén Eigenmode Active Diagnostic during the 2019-2020 JET deuterium campaign

“…Unless otherwise noted, data in this paper include both limiter and X-point configurations. The effects of plasma shaping and plasma-antenna coupling on AE measurements have been investigated in past works [14,20,22,25,26,28,30,31,33] and will be explored for our database in an upcoming study [50]. In this section, we highlight a few salient observations and trends, but note that extracting physics from the database will require careful data filtering, proper statistical analysis, and physics-based guidance.…”