Hydrogen discharges have been performed for four weeks for degassing tritium from the vacuum vessel. Even though the operation was performed at a low vacuum vessel temperature of 150C compared with the temperature in previous degassing campaigns of 300C, there was almost no difference in the decay of the D/H ratio. Using these hydrogen discharges, experimental data have been taken as described in the following sections. High-performance operation of the ECRF and NNB systems has also been demonstrated.
Impact of electron heating on ITB plasmas has been investigated. Electron heating by ECRF and NNB were applied in RS plasmas. The ITBs in Te, Ti and ne could be maintained with PPNB = 1.3MW, PNNB = 4.6 MW and PECRF = 2.9 MW. Preliminary analysis shows that 74% of the total absorbed power was fed to the electrons. It was also demonstrated that ITBs in Te, Ti and ne could be formed under the electron heating dominant situation with PECRF = 2.9 MW and PPNB = 2.5 MW. In LHCD-prepared RS plasmas, ECE radiation temperature over 25 keV was obtained by core ECRF heating. On the contrary, degradation of Ti was observed, when ECRF was applied in high-βp like ITB and L-mode plasmas.
ITBs with large foot radii (~ 0.9a) were also formed in hydrogen discharges as in deuterium discharges.
Measurement of SOL flow and plasma profiles has been performed in order to investigate puff&pump effect on SOL plasmas. During strong gas puffing into the main chamber, the electron density in the SOL was ~20 % higher than that during strong gas puffing into the divertor at the same main-plasma density. On the other hand, the Mach number profiles were comparable. Reduction in Zeff in the main plasma with the puff&pump method may be attributed to better shielding of carbon ions in high-collisionality SOL/divertor plasmas rather than increment of the flow velocity.
Characteristics of Alfvén eigenmodes have been investigated by NNBI in reversed shear plasmas. In order to measure the q-profile accurately, we performed the experiment with relatively high magnetic field (3.7 T) and plasma current (1.3 MA). Plasmas with various q-profiles were explored. We observed GAE in the region of 2.5 < qmin < 3.0. The observed frequency of CAE (Cylindrical Alfvén Eigenmodes) increased consistently with a calculation.
Using the four gyrotrons, high-power ECRF injection has been demonstrated (2.8 MW x 3.6 s). The LHRF and ECRF systems were successfully applied to obtain high-electron-temperature RS plasmas; the ECE radiation temperature reached ~26 keV.
Investigation of plasma-current ramp up without external magnetic flux injection was started in collaboration with Profs. Takase of Tokyo Univ. and Mitarai of Kyushu Tokai Univ. Simultaneous injection of LHRF, ECRF and NB succeeded to raise the plasma current from 120 to 400 kA without external magnetic flux injection. Effect of trapped electrons on ECCD was also investigated in collaboration with Dr. Petty of D III-D.
By optimization of operational parameters of the ion source, high-power injection (381 keV, 6.2 MW, 1.7 s) of hydrogen beam has been succeeded. Furthermore, by correcting deflection of the beamlet, long pulse injection (Eacc = 355 keV, PNNB = 2.6 MW, Δt = 10 s) was also demonstrated.
In collaboration with Dr. Okano of Central Research Institute of Electric Power Industry, enhancement of the effective ionization cross section for high-energy NB by beam-particle self interaction was investigated using NNBI.