No abstract

No abstract

High-density disruptions and low-q disruptions have been investigated by soft-X-ray imaging on the JIPP T-II tokamak. Although an m = 3/n = 2 mode is identified together with a 2/1 mode before partial disruptions in high-density discharges, the resulting disturbance is limited and the central region is not affected. The growth of the 3/2 mode detected in low-q discharges leads to a rearrangement of the soft-X-ray profile by a 1/1 mode. The essential feature of major disruptions is the abrupt central thermal quench. Before major disruptions the predominant 2/1 mode is modulated by a central 1/1 component. The 1/1 mode is found to play an important role in the final stage of major disruptions, both in high-density discharges and in low-q discharges.

The neoclassical bootstrap current effect is investigated in the JT-60 tokamak. The experimental resistive loop voltages are compared with the calculations, using the neoclassical resistivity, with and without the bootstrap current, and the Spitzer resistivity for a wide range of plasma current (Ip = 0.5-2 MA) and poloidal beta (βp = 0.1-3.2). The neoclassical bootstrap current is calculated directly with the force balance equations for viscous and friction forces according to the Hirshman–Sigmar theory. The bootstrap current driven by the fast ion component is also included. The calculated resistive loop voltage is consistent with the neoclassical prediction including the bootstrap current. It is shown that up to 80% of total plasma current is driven by the bootstrap current in the regime with an extremely high poloidal beta value (βp = 3.2) while the beam driven current is negligibly small.

No abstract

A nondisruptive /3-limiting phenomenon in a large tokamak under a large bootstrap current fraction, up to ^80% of the plasma current, is described; )3 = (plasma pressure)/(magnetic pressure). During long-pulse neutral-beam-heated discharges in the JT-60 tokamak, it occurs at Pp--3, leading to a limit of the normalized p lower than the Troyon limit. The MHD feature is characterized by a largeamplitude partial relaxation with a fast growth time. A hollow current profile evolution in the high-^,, regime plays an essential role in the MHD stability, analysis of which shows that the ideal « = 1 kinkballooning modes can be unstable just before the collapse.PACS numbers: 52.35.Py, 52.55.Fa In nuclear fusion research, the high-pp regime in tokamak operation is potentially of much interest as it may offer an alternative route to the development of an ignition device in which the requirement for external sources of noninductive current drive is very much reduced [1-3]. Here, the poloidal beta is defined as Pp =2^o{p)/Bp, where {p) is the volume-averaged plasma pressure and Bp is the averaged poloidal field at the plasma circumference. While the discharges can be sustained in principle with much lower driven currents than are generally considered necessary, the stability of highpressure discharges in a large tokamak under a large bootstrap current fraction is crucially important to determine whether such a reactor scenario is feasible. Actually, a long-pulse heating experiment is indispensable to investigate the current profile evolution and its effects on the stability. However, tokamak experiments have so far poorly addressed the subject. This Letter describes a plimiting phenomenon associated with the stability of hxgh'Pp plasmas carrying a large bootstrap current fraction in a large tokamak.In JT-60, \\\g\\-Pp experiments have been carried out in a sawtooth-free regime utilizing high-power neutral-beam heating for a long pulse duration up to -5 s, in which a fast internal disruption, unlike a sawtooth collapse, has been observed. A significant reduction of the stored energy by (20-30)% is followed by a collapse. It limited the attainable normalized p in the discharges to much lower values than the Troyon limit. It has been dubbed the ''Pp collapse" since the events occur for high-j3^ discharges at Pp-^^ and the mechanisms appear to be closely related to the Pp in terms of the bootstrap current fraction and the MHD stability. Note that the long heating duration as compared to the field diffusion time scale allows the minimal value in the q profile (^min) to be sufficiently higher than unity in the core plasma. Various attempts to extend the p values have been carried out in tokamaks and have encountered a variety of p-\\n\\i\ng phenomena: for instance, soft p collapses for high toroidal-beta (Pt) experiments in PBX-M [4], DIII-D [5], and JET [6]; Pp saturation for high-Pp experiments in ASDEX [7] and PBX-M [4]; and a disruptive plimit at Pp-^2 for supershot experiments in TFTR [8]. Pressure-driven high-Az ballooning mode...

Parameter dependence of the radial structure of edge poloidal rotation is studied with spectroscopic measurements for L-and H-mode plasmas in the JPT-2M tokamak [Y. Miura et al., in Plasma Physics and Controiied Nuclear Fusion Research, Proceedings of the 13th International Conference, Washington 1990 (IAEA, Vienna, 199 I), Vol. 1, p. 3251. The poloidal flow is in the electron diamagnetic direction and appears suddenly near the plasma edge at the H-mode transition. The poloidal rotation velocity profile in the H mode has a peak at the separatrix. No critical normalized ion collisionality, Yap, for the transition of the L to the H mode is observed. The size of the poloidal flow in the H mode has no dependence on the poloidal gyroradius.

Spontaneous formation of intemal transport barrier was observed in JT-60U high-& discharges, where q=3 surface is the most likely explanation for its radial location. Further improved confinement was brought about by the subsequent transition to the edge nansport barrier.