7thAsia-Pacific Conference on Plasma Physics, 12-17 Nov,2023at Port Messe Nagoya

Infernal mode stability in negative-triangularity plasmas

X. M. Zhang1, L. Li1, Y. Q. Liu2, Z. Y. Dong1, J. W. Li1 and F. C. Zhong11College of Science, Donghua University, China, 2 General Atomics, USAe-mail (speaker):2222438@mail.dhu.edu.cn
Tokamak plasma with negative-triangularity (NT) shape is
an attractive operational configuration because this type ofplasma has been found to be capable of achieving H-mode
like confinement while operating in L-mode regime. Thelatter helps solve the plasma exhaust problems in future
fusion reactors by avoiding the edge localized modes(ELMs). In recent experiments in DIII-D [1] and TCV [2],high-performance plasmas, with the normalized beta
values of �� = 2.2~2.8 have been achieved without
ELMs in the NT configuration. This, combined with othertechnical advantages, renders the NT plasma a promising
operational choice for future reactor design.
This work aims at numerical investigation of magnetohydrodynamic (MHD) instabilities in NT plasmas. In
particular, we consider the infernal mode stability in NT
versus its positive-triangularity (PT) counterpart.
Compared to other MHD modes such as the external kinkand the resistive wall mode [3] and the tearing mode [4]which have recently been studied for NT plasmas, theinfernal model is less exploited. This instability typicallyoccurs in tokamak plasmas with reversed central magnetic
shear, where the minimum safety factor (���� ) is close to arational value [5]. In such a case, sufficiently high plasmapressure (which is achievable in NT experiments asmentioned earlier) can drive the infernal mode instability.
Utilizing the toroidal code MARS-F [6], this work
systematically scans the plasma triangularity from negative
to positive values while fixing the (non-monotonic) safety
factor profile (Fig. 1(a-b)). Linear stability of the n=1 (n isthe toroidal mode number) ideal infernal mode is thennumerically computed, with examples of the results
reported in Fig. 1(c).
For the q-profile considered here, we find that theinfernal mode is more unstable in the NT plasma than that(a)(b)in the PT counterpart. Sufficiently high-beta drives
instability within the whole range of the triangularity scan.
However, the least unstable case occurs for the PT plasma.
Moreover, at fixed ���� =2.01 and �� =2, a stable window
opens, again towards the positive triangularity value. The
physical reason for the NT-destabilization is the lessfavorable curvature, when the mode is located towards theplasma edge where the triangularity effect is strong.
Other families of equilibria have also been considered,with varying ���� (near q=2) but fixed �� and thetriangularity value of 0.5. MARS-F modelling finds
instability windows near q=2 as shown in Fig. 1(d). This,
combined with the radially localized eigenmode structure(not shown here), confirms the infernal mode nature inthese NT plasmas. The slight shift of the peak growth rate,with respect to the q=2 value, is due to the coupling of theinfernal mode to the other (high-pressure driven) kinkinstability. This is different from the case of a pure infernalmode, with growth rate peaking at the rational-q location
as e.g. reported in Ref. [7].
This work is supported by National Natural Science
Foundation of China (NSFC) [grant numbers 12075053]
and by National Key R&D Program of China undercontract No. 2022YFE03060002.
References[1] Austin M E et al 2019 Phys. Rev. Lett. 122 115001[2] Marinoni A et al 2021 Nucl. Fusion 61 116010[3] Ren J et al 2016 Plasma Phys. Control. Fusion 58115009[4] Wang R Y et al 2023 Nucl. Fusion 63 016023[5] Hastie R and Hender T 1988 Nucl. Fusion 28 585[6] Liu Y Q et al 2000 Phys. Plasmas 7 3681
[7] Dong G Q et al 2017 Phys. Plasmas 24 11251(c)(d)

Figure 1. With scanning the plasma triangularity including both the NT plasmas and PT plasmas, (a) the plasma boundary
shape, (b) the radial profiles of safety factor, and (c) the growth rate with different plasma pressure, �� = 2.29, �� = 2 and�� = 1.8. (d) reports the growth rate versus ���� (near q=2) for the relative equilibrium with different plasma pressure,
where the solid line denotes the analytic results from Ref. 7, and the value of the triangularity is 0.5.