Overview of JT-60U progress towards steady-state advanced tokamak

Abstract

Recent experimental results from steady-state advanced tokamak (AT) research on JT-60U are presented with emphasis on time scales longer in comparison with the characteristic time scales in plasmas. To achieve this, modification of the controls for the operation, heating and diagnostics systems have been carried out. As a result, ∼60 s current flat top and a ∼30 s H-mode are obtained. The long pulse modification has opened a door into a new domain for JT-60U. High normalized beta (βN) of 2.3 is maintained for 22.3 s and 2.5 for 16.5 s in a high-βp H-mode plasma. A standard ELMy H-mode plasma has also been extended and changes in the wall recycling on the longer time scale have been unveiled. The development and investigation of plasmas relevant to AT operation have been continued in long discharges as well as in discharges where higher NB power is available (⩽10 s). Higher βN (∼3) is maintained for 6.2 s in a high-βp H-mode plasma. High bootstrap current fraction (fBS) of ∼75% is sustained for 7.4 s in a reversed shear plasma. Neo-classical tearing mode (NTM) suppression by localized ECCD is found to be more effective with ECRF injection preceding the mode saturation. The mode is suppressed with less power compared to the injection after the mode saturates. The domain of the NTM suppression experiments is extended to the high-βN regime, and the effectiveness of m/n = 3/2 mode suppression by ECCD is demonstrated at βN ∼ 2.5–3. Genuine tokamak plasma start up without a central solenoid is demonstrated. In a current hole region, it is shown that no scheme drives current in any direction. Detailed measurement of energetic ions in both space and energy showed dynamic change in the energetic ion profile due to collective instabilities. The impact of toroidal plasma rotation on ELM behaviour is clarified in the grassy ELM and QH domains. Retention of hydrogen isotopes in the divertor tiles is analysed.