December 2005
In this month, JT-60U experimental operation in FY2005 has started in a good wall condition after the boron coating of the first wall in November. The major objective in this month was to evaluate the effects of toroidal ripple reduction owing to the ferritic steel inserts. Attempts to achieve high beta exceeding the no-wall stability limit has been started as one of the major JT-60U objectives in 2005-2006. Results including other experiments are shown below.
(1) Evaluation of ferritic steel effects on magnetic field measurements
The data to evaluate effects of ferritic steel on the diamagnetic measurement was collected by energizing poloidal field coils under various toroidal fields with the wall temperature of 150°C.
(2) MSE calibration
The calibration of MSE diagnostics was performed by injecting neutral beams into a gas-filled torus with a toroidal field and a vertical field. The data for low toroidal fields (1.6 T and 2.1 T) are acquired for the first time in JT-60U, to confirm the nonlinearity of polarization angles on the toroidal field in some channels.
(3) Fast ion confinement with ferritic steel inserts
The decay time of a neutron emission rate after a short pulse NB injection (beam blip) was measured to evaluate the confinement of fast ions, which is expected to be improved due to the ripple reduction with the ferritic steel inserts under the toroidal field coils. Though the plasma parameters were attempted to be made similar to those of a reference discharge before the installation of the ferritic steel, the electron density was higher by about 20%, which results in a shorter slowing down time of fast ions. The measured decay time of the neutron emission rate, however, was similar to that in the reference discharge. This suggests the improvement of fast ion confinement.
(4) Plasma rotation with ferritic steel inserts
The plasma toroidal rotation profiles after the installation of the ferritic steel was compared with those in reference discharges before the installation of the ferritic steel, to evaluate effects of ripple reduction on the toroidal rotation. The change in the toroidal rotation profile toward the co direction (the same direction to the plasma current) was observed in the peripheral region of L-mode plasmas with the same injection power. Though the peripheral toroidal rotation varied toward the counter direction with an increase in the perpendicular NB power, similarly to the tendency observed before the installation of the ferritic steel, the change rate was smaller. These observations indicate the improved confinement of fast ions.
(5) H-mode confinement with ferritic steel inserts
H-mode plasmas were produced with the same configurations (small and large), plasma currents and toroidal fields as those in the reference discharges before the installation of the ferritic steel, in order to evaluate how the H-mode confinement and pedestal parameters were affected by fast ion loss and plasma toroidal rotation. The peripheral toroidal rotation was changed to the co direction as expected. The pedestal pressure increased, particularly for the case of co-toroidal momentum input, in both configurations. The H-factor also increased significantly, particularly for the large volume configuration with co-toroidal momentum input.
(6) High beta exceeding the no-wall stability limit (resistive wall mode study)
Achievement of high beta exceeding the no-wall stability limit was attempted in a large volume configuration, in which the wall stabilization effect and increased heating power by the reduction of ripple loss were expected. High βp H-mode plasmas with weak positive shear were employed. The wall conditioning to reduce the particle recycling was performed in order to improve the performance. As a result, a high βN of 3.6 was obtained at Bt = 1.58 T and Ip = 0.9 MA in a configuration with an outer gap of ~20 cm. The attainable βN was limited by mini collapses, which accompanied magnetic fluctuations with a growth time of 10 ms as precursors, suggesting appearance of the resistive wall mode.
(7) Neoclassical tearing modes (NTMs)
(a) Influence of sawtooth oscillation on the onset and evolution of NTMs
The influence of sawtooth oscillation on NTMs was studied in high βp H-mode plasmas by generating sawtooth oscillation with on-axis ECCD before NB heating. Though the influence on the onset of the NTM (the threshold beta value) was not clear, the initial growth of the NTM was enhanced in sawtoothing plasmas. The saturation level of the NTM was smaller in a plasma with 4 units (~2.6 MW) EC injection where larger sawteeth appeared. In this discharge, the NTM growth was interrupted by each sawtooth crash. This result suggests the possibility of NTM control with sawtooth oscillations.
(b) Stabilization of a 2/1 NTM with ECCD
Stabilization of a 2/1 (m=2, n=1) NTM with ECCD was attempted, where m and n are poloidal and toroidal mode numbers, respectively. The discharge scenario was optimized to obtain a stationary 2/1 NTM in a low beta (βN~0.8) phase after triggering it in a high beta phase. The 2/1 NTM was successfully stabilized with 4 units EC injection, where the EC-driven current density was estimated to be comparable to the local bootstrap current density.
(8) Impact of electron heating on ITBs
The impact of electron heating on an ion temperature (Ti) ITB was studied by injecting an ECRF wave into a high βp H-mode plasma. The toroidal rotation of the target plasmas (before the ECRF injection) was changed by using on-axis co or counter tangential NB, and it was found that the toroidal rotation shear was reduced (the toroidal rotation profile became flatter) and the Ti ITB was degraded with 2 units on-axis ECRF injection in both cases. The off-axis 3 units ECRF injection at r/a~0.4 (in the ITB region) also degraded the ITB in both cases, though the impact was weaker than 2 units on-axis ECRF injection.
(9) Off-axis NBCD
Controllability of the current profile by using off-axis NBCD was investigated. The MSE polarization angles showed an increase in the current density in a region of r/a = 0.55-0.75 of a plasma where a co-off-axis NB was injected, while they showed a decrease in the current density in the same region of a discharge where a counter-off-axis NB was injected. These results suggest the existence of off-axis NBCD.
(10) Penetration of inductive current
Penetration of an inductive current was measured with MSE during Ip modulation. The plasma current was modulated with a frequency of 2 Hz and an amplitude of +-30 kA (2% of the total plasma current) in an NB-heated 1.5 MA plasma with NTMs. Phase delay in MSE angles was observed across the q = 2 surface. A decrease in the coherence between MSE polarization angles and the plasma current was also observed near the same location.
(11) Emission rates of CH/CD and C2 spectral bands
Intensity of CH/CD and C2 spectral bands was measured when a given amount of hydrocarbon gas was injected into the divertor region, in order to determine calibration factors to evaluate the amount of hydrocarbon molecules generated on the carbon tiles. The data was obtained for C2H6, CH4 and CD4 gases as a function of the electron temperature in the divertor plasma on the target plates. A larger emission rate was observed for CD4 than CH4, suggesting isotope effects.
(12) 2 dimensional structure of recombination
Intensity of CIV and DI recombining lines was measured along vertical and horizontal chords (the total number of chords was 92) to obtain a 2 dimensional structure in the divertor region. The CIV emission was localized on the inner divertor leg just above the X-point, while the DI emission had a broad profile, for a detached divertor plasma with MARFE.