S. Takeji

High performance experiments in JT-60U reversed shear discharges

The operation of JT-60U reversed shear discharges has been extended to a high plasma current, low q regime keeping a large radius of the internal transport barrier (ITB), and a record value of equivalent fusion multiplication factor in JT-60U, QDTeq = 1.25, has been achieved at 2.6 MA. Operational schemes to reach the low q regime with good reproducibility have been developed. The reduction of Zeff was obtained in the newly installed W shaped pumped divertor. The β limit in the low qmin regime, which limited the performance of L mode edge discharges, has been improved in H mode edge discharges with a broader pressure profile, which was obtained by power flow control with ITB degradation. Sustainment of the ITB and improved confinement for 5.5 s has been demonstrated in an ELMy H mode reversed shear discharge.

Characteristics of Alfvén eigenmodes, burst modes and chirping modes in the Alfvén frequency range driven by negative ion based neutral beam injection in JT-60U

The excitation and stabilization of Alfv?n eigenmodes and their impact on energetic ion confinement were investigated with negative ion based neutral beam injection at 330-360?keV into weak or reversed magnetic shear plasmas on JT-60U. Toroidicity induced Alfv?n eigenmodes (TAEs) were observed in weak shear plasmas with ?h ? 0.1% and 0.4 ? vb||/vA ? 1. The stability of TAEs is consistent with predictions by the NOVA-K code. New burst modes and chirping modes were observed in the higher ? regime of ?h ? 0.2%. The effect of TAEs, burst modes and chirping modes on fast ion confinement has been found to be small so far. It was found that a strongly reversed shear plasma with internal transport barrier suppresses AEs.

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

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.

Internal transport barrier with improved confinement in the JT-60U tokamak

Characteristics of the internal transport barrier (ITB) were studied. The region of steep and , i.e. the ITB front, propagated from the core outwards. The thickness of the ITB front was about 3 cm. The ITB worked as a particle transport barrier as well as a thermal transport barrier for ions. The threshold heating power for ITB formation strongly increased with electron density and was independent of the toroidal magnetic field. ITB with was sustained for twice the global energy confinement time . A repetitive relaxation phenomenon at ITB was observed, which induced spikes like ELMs but had a different poloidal distribution.

Long sustainment of JT-60U plasmas with high integrated performance

The article treats the recent development of quasi-steady ELMy high ?p H?mode discharges with enhanced confinement and high ? stability, where long sustainment time, an increase in absolute fusion performance and extension of the discharge regime towards low q95 (~3) are emphasized. After modification to the new W shaped pumped divertor, a long heating time (9?s) with a high total heating energy input of 203?MJ became possible without a harmful increase in impurity and particle recycling. In addition, optimization of the pressure profile characterized by the double transport barriers, optimum electron density and/or high triangularity ? made it possible to extend the performance in long pulses. The DT equivalent fusion gain QeqDT ? 0.1 (? = 0.16) was sustained for ~9?s (~50?E, ~10?*p) and QDTeq ? 0.16 (? = 0.3) for 4.5?s at Ip = 1.5?MA. In the latter case with higher ?, an H?factor ( = ?E/?ITER89PLE) of ~2.2, ?N ? 1.9 and ?p ? 1.6 were sustained with 60-70% of the non-inductively driven current. In the low q95 (~3) region, the ? limit was improved by the high ? (~0.46) shape, where ?N ? 2.5-2.7 was sustained for ~3.5?s with the collisionality close to that of ITER-FDR plasmas. The limit of the edge ? parameter in the ELMy phase increases with ?, which is the main reason behind the improved ? limit in a long pulse at high ?. The sustainable value of ?NH also increases with ?. Sustainable ?N is limited by the onset of low n resistive modes. Direct measurement of island width shows agreement with the neoclassical tearing mode theory.

Alfvén eigenmodes in reversed shear plasmas in JT-60U negative-ion-based neutral beam injection discharges

Rapid frequency sweeping modes observed in reversed magnetic shear (RS) plasmas on the Japan Atomic Energy Research Institute Tokamak 60 Upgrade (JT-60U) [H. Ninomiya and the JT-60 Team, Phys. Fluids B 4, 2070 (1992)] have been identified as reversed-shear-induced Alfven eigenmodes (RSAEs), which are ideal magnetohydrodynamic Alfven eigenmodes (AEs) localized to the region of minimum safety factor, qmin, and are excited by negative-ion-based neutral beam injection. The chirping and subsequent saturation of the mode frequency are consistent with theoretical predictions for the transition from RSAEs to toroidal Alfven eigenmodes (TAEs). The previously observed rapid frequency sweeping modes in ion cyclotron wave heated plasmas in JT-60U can also be similarly explained. The observed AE amplitude is largest during the transition from RSAEs to TAEs, and fast ion loss is observed when the AE amplitude is largest at this transition. It is preferable to operate outside the transition range of qmin, e.g., 2.4

Magnetic safety factor profile before and after sawtooth crashes investigated with toroidicity and ellipticity induced Alfvén eigenmodes

A study of toroidicity and ellipticity induced Alfven eigenmodes (TAEs and EAEs) that are excited before and after sawtooth crashes during ion cyclotron range of frequency (ICRF) heating in JT-60U is presented. From the TAEs that are observed before sawteeth and that reside inside the q = 1 surface an upper limit has been set for q in the plasma centre at the time of the crash. After the sawtooth crash, EAEs that reside at the q = 1 surface are often observed. In a number of discharges the start of the EAE activity is delayed to up to 150 ms after the crash. In some cases TAE activity was observed between the sawtooth crash and the onset of the EAE activity. These TAEs could be modelled successfully only when it was assumed that the central safety factor (q0) rises above unity after the giant sawtooth crash. The appearance of the TAE activity immediately after the giant sawtooth crash is a strong indication that the fast particle drive remains in the plasma centre. This is consistent with theoretical estimates for the confinement of deeply trapped ICRF ions. The delayed appearance of the EAEs is also consistent with the disappearance of the q = 1 surface from the plasma at the giant sawtooth crash. The only way to obtain agreement between the experimentally measured EAE frequencies and the NOVA-K simulations is to assume that the q = 1 surface reappears in the plasma at the start of the EAE activity. The delayed appearance of the EAE activity seems to be correlated with the electron temperature just before the crash.

Analysis of error field due to ferritic steel in the advanced material testing program of JFT-2M

The reduction of ripple due to the use of a ferritic steel is studied computationally for two types of vacuum vessel (VV): one is made of nonmagnetic material with a ferritic section, and the other is made of ferritic steel. The appropriate setting of the ferritic section in the nonmagnetic material VV results in a ripple reduction in the whole plasma region of the low field side and the ripple amplitude can be reduced by a factor of 3: the ripple amplitude is reduced from 1.8% to 0.6% on the plasma boundary. The ripple amplitude in the case of the ferritic VV with the realistic horizontal port is comparable with that in the case of the nonmagnetic VV with the ferritic section.

Ferromagnetic and resistive wall effects on the beta limit in a tokamak

The ferromagnetic and resistive wall effects on the beta limit in a high aspect ratio tokamak are investigated. It is shown that the beta limit is reduced to 90% of that without ferromagnetic effect for a high aspect ratio tokamak, when the relative permeability of the ferromagnetic wall is 2. In these analyses, parabolic profiles for both plasma current and pressure are employed and the radius and thickness of the resistive wall are rw = 1.43a and d = 0.07a (a is plasma minor radius), respectively. The stability window with respect to the external kink modes is shown to be reduced by a high permeability effect in the case of a uniform current tokamak plasma, which is different from the case where only the finite resistivity effect is considered. The effect of toroidal plasma flow is also investigated, and it is shown that the toroidal background flow velocity of 0.3vpa, vpa is poloidal Alfven velocity, is sufficient for the resistive wall to have the stability effect of an ideal wall. The ferromagnetic effect of the wall destabilizes both resistive wall and ideal kink modes.

Resistive instabilities in reversed shear discharges and wall stabilization on JT-60U

Resistive instabilities and wall stabilization of ideal low toroidal mode number, n, kink modes are investigated in JT-60U reversed shear discharges. Resistive interchange modes with n = 1 are found to appear in reversed shear discharges with large pressure gradient at the normalized beta, βN, of about unity or even lower. The resistive interchange modes appear as intermittent burst-like magnetohydrodynamic (MHD) activities and higher n ≤ 3 modes are observed occasionally in higher βN regime. No clear degradation of the plasma stored energy is observed by the resistive interchange modes themselves. It is also found that resistive interchange modes can lead to major collapse owing to a coupling with tearing modes at the outer mode rational surface over the minimum safety factor. Stability analysis revealed that stability parameter of tearing modes, ∆′, at the outer mode rational surface is affected by the free-boundary condition. The result is consistent with the experimental evidence that major collapse tends to occur when plasma edge safety factor, q∗, is near integer values. Stabilization of ideal low n kink modes by the JT-60U wall is demonstrated. Magnetohydrodynamic perturbations that are attributed to resistive wall modes are observed followed by major collapse in wall-stabilized discharges.