7th Asia-Pacific Conference on Plasma Physics, 12-17, Nov. 2023, Nagoya, Japan.

Recent progress on the equilibrium and stability properties ofNT tokamaksLinjin Zheng, M. T. Kotschenreuther, F. L. Waelbroeck, M. E. Austin, B. Agnew
Institute for Fusion Studies, University of Texas at Austin
Y. NishimuraInstitute of Space and Plasma Sciences, National Cheng Kung University, Taiwan∗ E-mail: lzheng@austin.utexas.edu
There is continuing interest in investigating
negative triangularity (NT) tokamaks. Here, we report the results of two investigations: first, we examine the effects of rotation on the MHD stability
in NT; Second, we examine the effects of X point
position on the equilibrium, stability, and transportin NT scenario in comparison with the positive triangularity (PT) case.
For the MHD stability analyses, we use theAEGIS and DCON stability codes to investigate theDIIID-like equilibria constructed by EFIT, VMEC,etc.We found that, although NT configurations may
be more unstable for low-n modes than the PT case,for intermediate-n modes the situation is reversed.
The NT configuration becomes more stable for intermediate n modes (n = 3-10) [1]. This is consistent with the experimental observation that shows
the NT configuration usually has a lower turbulencetransport level [2].
We then extend the studies to include the effects of rotation to see how the resistive wall modesin the NT configuration are affected as comparedwith the PT configuration. This is particularly motivated by noting that the wall interface with theplasma is quite different between the NT and PT
configurations. The geometrical differences condition the effects of plasma rotation on the low nmodes. We find that, although low-n ideal MHD
modes are more unstable in the NT case, the rotation stabilization of the resistive wall modes is moreeffective in the NT case than in the PT case [3].
We also study the X point effects on equilibrium, stability, and transport in the NT scenario
as compared with the PT one. We note that thepositions of X points are quite different between
the NT and PT cases. The X points locate in thehigh field side in the PT case, while in the low fieldside for the NT case. With the analytical Solovev
solutions we are able to show that the NT configuration always gives lower safety factor value than
the PT case [4] even without taking into accountthe bootstrap current, which has been pointed out
to drive the q value even lower in the NT case due
to larger trapped particle population [5]. We also
examine the local q distribution along the magnetic
field line. We found that the high local q region
accumulates near the X point in the NT case, while
being broader in the PT case. This leads us to
examine further how these features affect the equilibrium, stability, and transport.
In the equilibrium, we previously obtained theasymptotic solution of vacuum field at the X point
tip with a conformal transformation [6]. We findthat, while there is no solution for an equilibriumwith an X point on the plasma-vacuum interface,
the hyperbolic plasma-vacuum boundary does givethe solution with the new X point in the vacuum.
This indicates that one has to chip off a thin layer
in order for an MHD equilibrium with X points toexist. Due to the difference in the accumulation ofhigh q value along the magnetic field between theNT and PT cases, chipping off a thin layer can causea large difference of edge q value between the NTand PT cases. The difference consequently affects
the edge stability and transport.
In the stability study, we developed the coordinate system with dual-regional q values. This isshown to be more suitable to study the system withX points. It extracts the extreme high local q region
at the vicinity of the X points from the global MHD
stability studies. In view of the non-existence of an
equilibria with X points on plasma edge, we then
are able to investigate the edge chipping off effects
on the peeling/ballooning modes in both NT andPT scenario.
In the transport studies, we have shown thatwith the X points sitting in the low field side, theNT configuration turns out to be less effective in
confining the trapped particles at plasma edge ascompared to the PT case [7]. We will further study
the edge chipping-off effects on the transport to see
how deep the confinement of trapped particles is affected. The aspect ratio and elongation effects will
also be discussed.
References[1] Linjin Zheng, M. T. Kotschenreuther, and F. L.
Waelbroeck, Nucl. Fusion 61, 116014 (2021).
[2] Y. Camenen et al, Nucl. Fusion 47 510 (2017).
M. E. Austin et al, Phys. Rev. Lett. 122, 11500(2019).
[3] Linjin Zheng, M. T. Kotschenreuther, F. L.
Waelbroeck, and M. E. Austin, IAEA-FEC, London (2023).
[4] Linjin Zheng, M. T. Kotschenreuther, F. L.
Waelbroeck, and M. E. Austin, Sherwood fusion
theory conf. (2023).
[5] M. Kikuchi, et al. 1st International e-Conference
on Energies (2014).
[6] Linjin Zheng, M. T. Kotschenreuther, F. L.
Waelbroeck, “Asymptotic vacuum solution at tokamak X-point tip” APS-DPP 2023, submitted toPhys. Plasmas.
[7] Y. Nishimura, F. L. Waelbroeck, and L. J.
Zheng, Phys. Plasmas 27, 012505 (2020).