Review of JT-60U experimental results in 1998

Based on the high confinement regimes such as reversed shear mode, high-βp mode and H-mode, the JT-60U experiment in 1998 was devoted to expand the discharge regimes in terms of 1) achievement of high fusion gain, 2) concept optimization for long sustainment of the advanced modes for >>τE and >τp* with the current profile close to the steady-state solution, 3) high confinement by electron heating (Te>Ti), 4) high confinement at high electron density and/or at high divertor radiation and 5) active control of multiple parameters including both core and divertor plasmas. As for the reversed shear mode, high performance discharges satisfying QDTeq (DT equivalent fusion gain ) >1 were obtained reproducibly and the record value of QDTeq=1.25 was achieved in a reactor-relevant thermonuclear dominant regime due to the optimized discharge scenario using feedback control of the neutron production rate where β-values were kept in the MHD stable region during the Ip ramping phase with a large radius of the internal transport barrier (ITB). The reduction of Zeff obtained after installation of W-shaped pumped divertor increased fusion reaction rate. Concerning long sustainment, the reversed shear ELMy H-mode with H89PL-factor∼1.5-2 and βN=1.0-1.4 was kept for 5.5 s with NB heating. By off axis LH current drive, the reversed shear current profile with the ITB was kept constant for 4.7 s under full non- inductive current drive condition (LHCD=77%, bootstrap=23%) at Te-1.2Ti. In the high-βp ELMy H-mode regime, benefits of the high triangularity shape were demonstrated. At a high triangularity δX∼0.46, βN=2.5-2.7 was sustained for 3.5 s even at the low value of q95=2.9-3.3. The product of βNxH-factor sustainable for >5τE (>τp*) increases with δx and reaches ∼6 at δX∼0.46. In addition to extension of the discharge regimes, the key phenomena determining transport and stability around ITBs were studied intensively. For the study on high confinement by electron heating, the negative ion source NB (NNB) with 350-400 keV, LH and IC second harmonic minority heating have been used. By NNB heating, an ELMy H-mode plasma was sustained with H-factor∼1.6 at Te=1.4Ti. High Te(0)∼12 keV was also sustained by LHCD of ∼2 MW. The Alfven eigenmodes were studied with NNB injection in weak or reversed magnetic shear plasmas. The Toroidicity-induced Alfven eigenmodes were observed in weak shear plasmas with <βh> ≥0.1% and 0.4≤Vb// /vA≤1. In order to maximize safety at disruption, we have also focused on softening of the disruption effects on the vacuum vessel structures. Reduction of halo current was demonstrated by impurity gas puffing. The divertor study in 1998 focused on investigating the geometry effect of the W-shaped divertor and the pumping effects. The W-shaped divertor structure with the private dome separating the inner and outer divertor channels realized the effective inner leg-pumping both in L-mode and H-mode plasmas. The pumping efficiency is higher at narrower gap of the inner hit point and the pumping slot. The main gas puff and divertor pumping was effective for reduction of impurity level and the enhancement of impurity shielding. The dome structure also prevented the upstream transport of hydrocarbons generated by chemical sputtering, and reduced carbon influx into the main plasma. Particle flow from the main plasma to the divertor region through the SOL layer was studied intensively. (author)