High performance reversed shear plasmas with a large radius transport barrier in JT-60U

Abstract

The operation of reversed shear plasmas in JT-60U has been extended to the low-q, high-Ip region keeping a large radius transport barrier, and a high fusion performance has been achieved. Record values of deuterium-tritium (DT)-equivalent power gain in JT-60U have been obtained: QDTeq = 1.05, τE = 0.97 s, nD(0) = 4.9 × 1019 m-3 and Ti(0) = 16.5 keV. A large improvement in confinement resulted from the formation of an internal transport barrier (ITB) with a large radius, which was characterized by steep gradients in electron density, electron temperature and ion temperature just inside the position of qmin. Large negative shear regions, up to 80% of the plasma minor radius in the low-qmin regime (qmin∼2), were obtained by plasma current ramp-up after the formation of the ITB with the pressure and current profiles being controlled by adjustment of plasma volume and beam power. The ITB was established by on-axis beam heating into a low density target plasma with reversed shear that was formed by current ramp-up without beam heating. The confinement time increased with the radius of the ITB and the decrease of qmin at a fixed toroidal field. High H factors, up to 3.3, were achieved with an L mode edge. The effective one fluid thermal diffusivity χeff had its minimum in the ITB. The values of H/q95 and βt increased with the decrease of q95, and the highest performance was achieved at q95 ∼3.1 (2.8 MA). The performance was limited by disruptive beta collapses with βN∼2 at qmin∼2.

Parameter Regime of Ion Internal Transport Barrier Formation in the Large Helical Device

“…author's e-mail: nagaoka@nifs.ac.jp was first used to refer to tokamak plasmas [4], and several types of ITB occur in tokamak plasmas depending on the discharge, such as a box-type ITB in reversed shear configurations and a weak ITB in positive shear configurations [5,6]. There are two types of ITB definition.…”

Observation of Stable Superdense Core Plasmas in the Large Helical Device

“…Various confinement improvement modes have been discovered including edge transport barriers (ETBs, or H mode) [2] and internal transport barriers (ITBs) [3][4][5]. In this Letter, we describe improved confinement in super dense core (SDC) plasmas, in diverted discharges in the Large Helical Device (LHD), a heliotron configuration in which the rotational transform is provided by external magnetic coils.…”

Evolution of the ITER program and prospect for the next-step fusion DEMO reactors: status of the fusion energy R&D as ultimate source of energy

“…of Q ∼ 0.65 [1] in deuterium-tritium (DT) plasmas by the JET (EU), and equivalent Q ∼ 1 in deuteriumdeuterium(DD) plasmas by the JET and by the JT-60 (Japan) [2]. The plasma temperature achieved is relevant to fusion power reactors (up to 40 keV), as can be seen from Figure 2.…”