Identifying the microtearing modes in the pedestal of DIII-D H-modes using gyrokinetic simulations

Abstract

Recent evidence points toward the microtearing mode (MTM) as an important fluctuation in the H-mode pedestal for anomalous electron heat transport. A study of the instabilities in the pedestal region carried out using gyrokinetic simulations to model an ELMy H-mode DIII-D discharge (USN configuration, 1.4 MA plasma current, and 3 MW heating power) is presented. The simulations produce MTMs, identified by predominantly electromagnetic heat flux, small particle flux, and a substantial degree of tearing parity. The magnetic spectrogram from Mirnov coils exhibits three distinct frequency bands---two narrow bands at lower frequency ($\sim$35-55 kHz and $\sim$70-105 kHz) and a broader band at higher frequency ($\sim$300-500 kHz). Global linear GENE simulations produce MTMs that are centered at the peak of the $\omega_*$ profile and correspond closely with the bands in the spectrogram. The three distinctive frequency bands can be understood from the basic physical mechanisms underlying the instabilities. For example (i) instability of certain toroidal mode numbers (n) is controlled by the alignment of their rational surfaces with the peak in the $\omega^*$ profile, and (ii) MTM instabilities in the lower n bands are the conventional collisional slab MTM, whereas the higher n band depends on curvature drive. While many features of the modes can be captured with the local approximation, a global treatment is necessary to quantitatively reproduce the detailed band gaps of the low-n fluctuations. Notably, the transport signatures of the MTM are consistent with careful edge modeling by SOLPS.

Microtearing modes as the source of magnetic fluctuations in the JET pedestal

“…Such variations in MTM physics appear to be responsible for the multiple bands that are observed in some discharges. For example, DIII-D discharge 162940 exhibits three distinct frequency bands: narrow bands at 40 kHz, 80 kHz, and a broader band centered at 400 kHz, which are in quantitative agreement with simulations as described in reference [41].…”

Time-dependent experimental identification of inter-ELM microtearing modes in the tokamak edge on DIII-D

“…On JET, detailed comparisons with gyrokinetic simulations showed that experimentally-observed frequency bands in magnetic fluctuations were consistent with expectations from MTMs and that MTM fluctuations produce experimentally-relevant transport [25]. Additional ongoing work at DIII-D [27,28] and elsewhere is continuing to support the notion that MTMs are present in the H-mode pedestal, perhaps alongside additional instabilities.…”

“…In particular, models based off of KBM constraints have been extensively developed and are able to reproduce observed pedestal heights and widths to within ∼ 20% on many machines [6,7]. However, ongoing gyrokinetic work has suggested that other modes, especially the MTM [9][10][11][12], may also contribute to transport mechanisms in the pedestal region, potentially having large effects on energy regulation [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28].…”

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…On the longer timescale of the electron temperature gradient (ETG) recovery, the TEM turbulence (figure 7(b)) exhibits a threshold in ETG (figure 7(c)) and then saturates later in the ELM recovery. MTM scale electro-magnetic modes (figure 7(d)) driven by grad-T e can also contribute to the anomalous Q e through to the end of the ELM cycle as suggested by non-linear simulations [32][33][34], although their experimental identification is not yet conclusive in this set of experiments. Finally, simulations predict that ETG modes also contribute to Q e between ELMs [36], although the spatial scales are so short that no direct measurements of ETG scale fluctuations in the pedestal are available.…”