The negative triangularity tokamak: stability limits and prospects as a fusion energy system

Abstract

The paper discusses edge stability, beta limits and power handling issues for negative triangularity tokamaks. The edge MHD stability is the most crucial item for the power handling. For the case of negative triangularity the edge stability picture is quite different from that for conventional positive triangularity tokamaks: the 2nd stability access is closed for localized Mercier/ballooning modes due to the absence of magnetic well, and nearly internal kink modes set the pedestal height limit weakly sensitive to diamagnetic stabilization just above the margin of localized mode Mercier criterion violation. While negative triangularity tokamak is thought to have low beta limit with its magnetic hill property, it is found that plasmas with reactor relevant values of normalized beta β N > 3 can be stable to global kink modes without wall stabilization with appropriate core pressure profile optimization against localized mode stability and also with increased magnetic shear in the outer half radius. The beta limit is set by n=1 mode for the resulting flat pressure profile. The wall stabilization is very inefficient due to strong coupling between external and internal modes. The n>1 modes are increasingly internal when approaching the localized mode limit and set a lower beta in case of peaked pressure profile leading to Mercier unstable core. With the theoretical predictions supported by experiments, a negative triangularity tokamak would become a perspective fusion energy system with other advantages including larger separatrix wetted area, more flexible divertor configuration design, wider trapped particle free SOL, lower background magnetic field for internal poloidal field coils and larger pumping conductance from the divertor room.

Dependence of the L-Mode scrape-off layer power fall-off length on the upper triangularity in TCV

“…This paper reports on experimental studies aiming at (i) understanding the effect of plasma shaping and reversing toroidal magnetic field polarity to support further theoretical work, e.g. the importance of drifts and turbulence on scrape-off layer heat flux and (ii) scaling the effect of negative triangularity for accessing the reactor design with negative triangularity [5,6,7]. The Tokamakà Configuration Variable (TCV) [8,9] is able to achieve a large variety of plasma shapes due to 16 independently powered poloidal field coils and an open vessel geometry.…”

“…A triangularity dependence of the power fall-off length λ q in combination with reversed toroidal field has been observed in ASDEX Upgrade [16]. Negative triangularity has been investigated in TCV [17,18] and reactor studies [5,6,7]. Such investigations showed increased energy confinement with negative triangularity in low [19] and medium [20] density plasmas, explained by a reduction of turbulent transport [21].…”

Achievement of Reactor-Relevant Performance in Negative Triangularity Shape in the DIII-D Tokamak

“…Historically the −δ shape has been considered to have low β stability limits to ballooning modes due to the fraction of plasma volume with bad field line curvature. However, a recent study of the −δ shape found a limit β N > 3.0 is possible [13]. Moreover, a study of growth rates of n = 1 kink modes using the GATO code [14], employing scaled DIII-D experimental profiles, found a beta limit β N = 3.1, so the notion of low β limits in this shape is not borne out in modeling or experiment.…”

DEMO physics challenges beyond ITER

“…This risk poses a serious question mark on the suitability of H-mode as a reactor scenario, since a reliability of 100% would be required to any chosen ELM mitigation or suppression method, a challenging engineering target to meet (even disregarding other drawbacks all active ELM mitigation or suppression techniques might have concerning the plasma performance). For this reason, a plasma scenario which is naturally ELM-free, as for example the QH-mode [37], the I-mode [38], or even negative triangularity [39,40,41] would be extremely beneficial for a machine like EU-DEMO, whose mission includes stringent availability requirements -to a much higher extent than ITER, which is still an experiment.…”