M Mori

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.

Onset condition for ELMs in JT-60U

For the standard shape of JT-60U at low triangularity , the onset condition for giant ELMs is clearly correlated with the high-n ballooning limit in the first stability regime over wide ranges of plasma parameters (, ...). The limit of the normalized edge pressure gradient (-parameter) increases with elongation (1.5 - 1.8) and internal inductance . Recently, a new connection of the poloidal field coils enabled us to scan triangularity from the original value of up to (currently limited at MA). The -scan showed that the limit of edge density, edge pressure and the -parameter increase with . When both and are high (, ), minute-grassy ELMs appear. The edge -parameter during the minute-grassy ELMs can be higher than that for the onset of giant ELMs.

Formation of internal and edge transport barriers in JT-60U

Spontaneous formation of internal transport barrier was observed in JT-60U high- beta p 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 transport barrier.

ELMy H-mode with high- beta N and high- beta p in JT-60U

By modification of current and heat deposition profiles, the integrated core plasma performance was improved in ELMy H-mode. In a quasi steady state, beta p approximately 2.5-3, beta N approximately 2.5-3.1 and H-factor approximately 1.8-2.1 were sustained simultaneously under the full current drive condition (bootstrap current 55%, beam driven current 45% at Ip=0.5MA). The achievable beta p value and the energy confinement performance increases systematically with safety factor at the edge qeff and a high qeff operation (qeff>6) is required to obtain favorable integrated performance. To obtain these improved high beta ELMy plasmas, it is essential to control MHD activities; suppression of beta p-collapse and control of ELM activity.

Study of internal transport barriers by comparison of reversed shear and high- discharges in JT-60U

Reduction of electron thermal diffusivity was investigated by comparing the internal transport barrier (ITB) with that of high- discharges. In reversed shear discharges heated by neutral beams, reduction was significant when negative magnetic shear was strong. Radial location of ITB propagated outwards and then localized near the region of q-minimum for both the ion and the electron channels. Reduction in was also observed in the electron heating regime by ion cyclotron heating

Comparison of edge neutral effect on the condition of H-mode transition before and after the modified divertor in JT-60U

The JT-60U divertor geometry was recently modified to a W-shaped structure in order to reduce the back flow of neutrals from the divertor. The H-mode threshold power decreased subtly compared with the open divertor case. As regards density dependence, the exponent of density was 0.5 to 0.75. In addition, we found that the outflux from the outer baffle plate should be considered for the evaluation of neutrals.

Active control of H-mode

The present status of investigations concerning active control of the H-mode is discussed, including density control for steady H-mode operation, control of heat flux to lower the divertor heat load, and control of the transport barrier for further improvement of confinement and MHD stability. Control of ELM activity is extremely important for density control in the H-mode. Some examples of controlling ELMs are discussed. The remote radiative cooling of the main plasma and the divertor plasma is necessary to lower the divertor heat load. Since the heat flux across the separatrix to keep the H-mode has to be higher than the threshold power for the HL transition, the feasibility of cooling the H-mode plasma with the main radiation loss depends on the ratio of the threshold power to the total heating power. Control of the edge transport barrier with plasma shaping has been demonstrated. The toroidal field dependence of the threshold power for an internal transport barrier (ITB) formation of the high- mode is significantly different from that for an edge transport barrier formation in the H-mode, indicating the transition physics might be different. Formation of an ITB was successfully demonstrated by ion Bernstein wave resonant heating and by negative magnetic shear.

Threshold power for the H-mode transition in JT-60U plasmas

The following scaling of threshold power for the H-mode transition is obtained for ; where is around unity for and for and . The threshold power increases with decreasing density in the range . For there is no dependence of . However, for the threshold power increases with increasing .

Time evolution of transport properties in JT-60U H-mode plasmas with improved core confinement

The time evolution of confinement and transport properties of ICC H-mode plasmas in JT-60U has been studied. We found the change of confinement character on a timescale much smaller than the thermal energy confinement time, , at the L - H and H - L transitions. A slow change on a timescale of is also found during the H-mode phase.

Non-dimensional threshold scaling of edge plasma quantities in JT-60U

Exploratory work on scaling the H-mode threshold power with edge non-dimensional quantities was first undertaken based on the JT-60U H-mode database. Predominant edge dependence as well as the strong negative contribution of edge were found. Obtained scaling is consistent with the global parameter dependences and also satisfies the non-dimensional constraint.