August-October 2006

In order to resume experiment after air ventilation, wall conditioning was carried out.

From August 21st to 25th, wall conditioning after the air ventilation was carried out by combination of Taylor discharges, glow discharges, and tokamak discharges at a vacuum vessel temperature of 300C. The first tokamak discharge with the pulse length of 15 s was obtained on August 24th. Neutral beam injection started on August 25th. From August 29th to September 1st, wall conditioning was carried out using tokamak discharges with NB heating at a vacuum vessel temperature of 150C. The oxygen contents and the H/D ratio decreased successfully.

JT-60 experiment was resumed and following experiments were carried out.

(1) Study of particle control under saturated wall conditions
It has been observed that wall pumping continued for longer than 20 s in high-density H-mode discharges with an X-point MARFE and detached divertor plasma. Co-deposition of deuterium with carbon impurity did not account for the continuous wall-pumping rate.

(2) Current profile control using lower-hybrid current drive (LHCD)
Using the power control of off-axis LHCD, the minimum value of the safety factor (qmin) was controlled in real-time in a high βp mode plasma (Ip = 0.8 MA, Bt = 2.5T, q95=5.8, βN = 1.3-1.6, fBS = 46%, fCD = 0.87). The qmin followed the reference value rising in time from 1.3 to 1.7. During the control, electron and ion internal transport barriers were formed, leading to change in the central bootstrap current density and the electric conductivity. The change in central current density, and hence the qmin, was successfully controlled through the LH power.

(3) Burning plasma simulation experiments
Burning plasma simulation scheme has been developed using 2 groups of NB, where one simulated α particle heating and the other simulated external heating. The NB heating power for the simulation of α particle heating (PNBα) was determined using real time measurements of density and ion temperature (Ti) with consideration of temperature dependence of DT fusion reaction rate. The strong loop of increases or decreases in Ti and PNBα was observed with the temperature dependence of ~Ti2 corresponding to the DT fusion reaction rate in the range of Ti = 10-20 keV. The increasing loop was associated with the profile change at the constant confinement improvement factor in the early NB heating phase. On the other hand, the decreasing loop was induced by gas-puffing with confinement degradation.

(4) Toroidal rotation and momentum transport
Beam perturbation techniques with perpendicular NBs were applied in L-mode plasmas in order to investigate the characteristics of momentum transport and plasma rotation in the toroidal direction. The inverse momentum diffusivity at r/a=0.6 increased linearly with the plasma current. Toroidal rotation velocity profiles in the cases with and without external torque input can be almost reproduced by transport coefficients estimated from the transient momentum transport analysis at low beta (βN < 0.4).

(5) Resistive wall mode study
The plasma rotation threshold for stabilization of Resistive Wall Mode (RWM) was investigated by controlling the toroidal rotation with external momentum input by means of tangential NB injection. The weak reversed shear plasmas with Bt = 1.58 T and Ip = 0.9 MA was placed close to the outer vacuum vessel for wall stabilizing. The plasma rotation was successfully controlled from -100 to 0 km/s with keeping constant beta by feedback control of perpendicular NBs power. The observed threshold was 0.3% of the Alfvén velocity and much smaller than the previous experimental results obtained with magnetic braking. This low critical rotation was found to change little even when the critical beta became close to the ideal wall limit.

After JT-60 experiments were closed for this campaign, degassing tritium from the materials in the vacuum vessel was carried out. Degassing tritium has successfully been completed using H2 GDC. The GDC was performed for ~13 hours at a vacuum vessel temperature of 300C and for ~7 hours at a vacuum vessel temperature of 150C.

21st Fusion Energy Conference, Chengdu 16-21 October 2006
As a summary of the JT-60 project in 2005-2006, nineteen papers (9 orals and 10 posters) were presented as follows:
Overview of JT-60U Results for Development of Steady-State Advanced Tokamak Scenario, H. Takenaga and the JT-60 Team, (OV/1-2).
Improved Performance in Long-pulse ELMy H-mode Plasmas with Internal Transport Barrier in JT-60U, N. Oyama, et al., (EX/1-3).
Evolution of Bootstrap-Sustained Discharge in JT-60U, Y. Takase, et al., (EX/1-4).
Active Control of Neoclassical Tearing Modes toward Stationary High-Beta Plasmas in JT-60U, A. Isayama, et al., (EX/4-1Ra).
Enhanced H-mode pedestal and energy confinement by reduction of toroidal field ripple in JT-60U, H. Urano, et al., (EX/5-1).
Confinement Degradation of Energetic Ions due to Alfvén Eigenmodes in JT-60U Negative-Ion-Based Neutral Beam Injection Plasmas, M. Ishikawa, et al., (EX/6-2).
Off-axis Current Drive and Current Profile Control in JT-60U, T. Suzuki, et al., (EX/6-4).
Plasma Rotation and Wall effects on Resistive Wall Mode in JT-60U, M. Takechi, et al., (EX/7-1Rb).
ELM Propagation and Fluctuations Characteristics in H- and L-mode SOL Plasmas on JT-60U, N. Asakura, et al., (EX/9-2).
Studies on impact of electron cyclotron wave injection on the internal transport barriers on JT-60U, S. Ide, et al., (EX/P1-5).
Controllability of large bootstrap current fraction plasmas in JT-60U, Y. Sakamoto, et al., (EX/P1-10).
Driving Mechanism of Toroidal Rotation and Momentum Transport in JT-60U, M. Yoshida, et al., (EX/P3-22).
Particle Control under Wall Saturation in Long-pulse High-density H-mode Plasmas of JT-60U, H. Kubo, et al., (EX/P4-11).
Hydrogen Retention and Carbon Deposition in Plasma Facing Wall and Shadowed Area of JT-60U, K. Masaki, et al., (EX/P4-14).
Radiation processes of impurities and hydrogen in detached divertor plasmas of JT-60U, T. Nakano, et al., (EX/P4-19).
Dynamic Transport study of the plasmas with transport improvement in LHD and JT-60U, K. Ida, et al., (EX/P4-39).
Ferritic Insertion for Reduction of Toroidal Magnetic Field Ripple on JT-60U, K. Shinohara, et al., (FT/P5-32).
Observation of spontaneously excited waves near the ion cyclotron range of frequency on JT-60U, M. Ichimura, et al., (EX/P6-7).
Density Limit in Discharges with High Internal Inductance on JT-60U, H. Yamada, et al., (EX/P8-8).