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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.

Abstract. Edge impurity transport has been investigated in the stochastic layer of Large Helical Device (LHD) and the scrape-off layer (SOL) of Huan Liuqi-2A (HL-2A) tokamak, as a comparative analysis based on the three-dimensional (3D) edge transport code EMC3-EIRENE and on the carbon emission profile measurement. The 3D simulation predicts impurity screening effect in the both devices, but also predicts different impurity behavior against collisionality and impurity source location between the two devices. The difference is caused by geometrical structures of the magnetic field lines in the stochastic layer and X-point poloidal divertor SOL, i.e., number of poloidal turns of flux tubes affecting poloidal distribution of plasma parameters and impact of perpendicular transport on parallel pressure conservation and energy transport. These processes have an influence on the impurity screening efficiency at upstream and downstream positions of field lines. The carbon emission measured in the stochastic layer of LHD clearly indicates the screening effect in high density region. The result can be qualitatively interpreted by the present modeling, although the modeling shows a slight difference in the quantitative behavior of carbon ions in the stochastic layer of LHD. On the other hand, comparison of the carbon emission profile from HL-2A with the modeling is not straightforward. It is found that the impurity distribution in the HL-2A SOL is very sensitive to the impurity source location. In order to interpret the experimental observation a further study is necessary, in particular, on the impurity source distribution in the divertor plate and the first wall.

The features of ion and electron fishbone instabilities have been investigated during neutral beam injection (NBI) and electron cyclotron resonance heating (ECRH) on HL-2A. Some new phenomena, such as frequency jumps and V-font-style sweeping, have been presented in the paper. Three kinds of i-fishbones, including hybrid sawtoothfishbone (sawbone), run-on fishbone and classical fishbone, have been identified during NBI. During high power (P ECRH > 0.7 MW) ECRH, the experimental results indicate that the e-fishbone frequencies are higher than those during low power ECRH, and are provided with up-and down-chirping behaviours, and sometimes also with V-font-style sweeping. The periodic mode frequency jumps have also been detected by a soft x-ray array. It is possible to correlate these phenomena with the redistribution of energetic electrons.

Strong burst of the internal kink mode has been observed during ECRH on the HL-2A. It has been experimentally identified that the energetic electrons, which deviate from Maxwell velocity distribution, excite the mode, which was so called electron fishbone (e-fishbone). The energy distribution of the electrons is indirectly measured by a hard X-ray detector (CdTe) with the pulse height analysis (PHA).When the counts of the energetic electrons with 35-70keV increase to a higher level, the mode can be observed obviously. The e-fishbone can be excited during off -axis ECRH deposited both the high field side and low field side. The modes propagate toroidally parallel to the precession velocity of deeply trapped ions which is in the same direction as the plasma current (co-current) and poloidally parallel to the electron diamagnetic drift velocity. In order to further identify with e-fishbone mode, the resonance condition of wave-particle has been investigated. Comparing with experimental results, the calculation analyses show that the mode frequency is close to the precession frequency of the barely trapped electrons (BTEs) or the barely circulating electrons (BCEs) when the magnetic shear is very weak or negative.

Measurements with a toroidally and poloidally displaced three-dimensional set of Langmuir probe arrays have revealed details of turbulence, geodesic acoustic modes (GAMs), zonal flows and their interactions in the edge region of HL-2A tokamak plasmas. The coexistence of intensive low frequency zonal flows (LFZFs) of f < 4 kHz and the GAMs of 7 kHz < f GAM < 20 kHz has been unambiguously demonstrated. The poloidal and toroidal symmetries of the flows, including the GAMs, are verified experimentally. In particular, the coherency of the flows over a large toroidal scale of 2100 mm at a magnetic flux surface is emphasized. The LFZF packets are shown to propagate outward and inward as equally likely events, whereas the predominantly outward propagation of the GAM packets is analyzed. The nonlinear three-wave coupling of the flows with ambient turbulence is shown with a bicoherency analysis and an envelope modulation of the latter by the former. The intensity of the LFZFs is observed to increase and decrease with increases in ECRH power (∼300-700 kW) and safety factor q ∼ (3.5-6.2), respectively, whereas the intensity of the GAMs increases with increases in both ECRH power and q.