Observation of a fast beta collapse during high poloidal-beta discharges in JT-60

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...

Profile and Shape Control of the Plasmas with Transport Barriers towards the Sustainment of High Integrated Performance in JT-60U.

“…This section treats the dependence of the ITB characteristics on the magnetic shear profile [3,7,8,12,17,[21][22][23]. In JT-60U, ITBs have been observed over a wide range of the magnetic shear profile.…”

“…In addition to providing physics basis for each individual element, recent JT-60U research has been directed toward the optimization of the operational concepts with the goal of achieving the high integrated performance, its sustainment in long pulses, and extension to the reactor relevant parameter regimes [7]. Most of these experiments have been based on the reversed shear (RS) mode [8][9][10][11], the high-β p mode [12][13][14][15][16][17], and their combination with the H-mode ( Fig. 1(b)).…”

A Review of Fusion and Tokamak Research Towards Steady-State Operation: A JAEA Contribution

“…The low li boundary is limited by surface kink modes or locked mode while the upper li boundary is limited by tearing mode activities for low  regime. Advanced tokamak operation modes such as weak positive shear regime (or high  p regime) and reversed shear regimes are below the sawtooth boundary [87,88]. [86].…”

“…In this regime, improved core confinement is observed in high power neutral-beam heated large tokamaks such as JT-60U [87][88][89] and TFTR [90,91]. Ideal MHD stability of a weak shear regime was studied by Ozeki [78], showing that a SSTR relevant regime can be stable for ideal MHD modes with q(0) > 2 and wall stabilization.…”

Highβpplasma formation using off-axis ECCD in Ohmic heated plasma in the spherical tokamak QUEST

“…It is also proposed to operate in a shaped configuration to increase β p [5]. High β p plasma has been produced by injecting multi-megawatt NBI power to Ohmic plasma in different machines [6][7][8]. The maximum achievable β p is limited by a so called equilibrium limit, where an Inboard Poloidal field Null (IPN) appears at the high field side of the vacuum vessel.…”