November 2004

  In this month, tokamak operation was carried out for taking (1) reference data before installing ferritic steel to reduce the field ripple and (2) tritium degassing data. The data for (3) long pulse operation of LH and EC systems, (4) resistive wall mode, (5) mass-dependence of H-mode pedestal width, (6) fast CXRS measurement, (7) beam particle self-interaction, (8) SOL (scrape-off layer) plasma fluctuations, (9) divertor spectroscopy, (10) carbon-13 injection, (11) oxygen glow, and (12) others were also obtained. The results are shown below.

(1) Reference data before installing ferritic steel: Insertion of ferritic steel tiles into the JT-60U vacuum vessel is planned to reduce the toroidal field ripple and the resultant loss of beam ions. Reference data before the ferritic steel insertion were taken to investigate the effect of ripple reduction on (a) transport of fast ions and (b) toroidal rotation generated by fast ion loss.
  For (a), data on confinement time of fast ions measured by short pulse injection of NBI, IRTV images for hot spots by fast ion losses, and radial profiles of neutron emission were obtained.
  For (b), data on toroidal rotation, confinement and pedestal parameters in H-mode plasmas were obtained in three plasma configurations with 0.4%, 1% and 2% of ripple on the plasma surface. The injected toroidal momentum and power were scanned in each configuration. The density was scanned in one configuration (1% ripple). In L-mode plasmas, data on toroidal rotation was obtained for various kinds of NB (co, counter and perpendicular, on- and off-axis) and different values of q95 (3.4, 4.1 and 5.2), including response of the toroidal rotation to modulated injection of perpendicular NBs.
(2) Tritium degassing: Tokamak operation with hydrogen gas puff and hydrogen neutral beams was carried out for two weeks to release the tritium from the carbon tiles on the first wall. The I(Dα)/I(Hα) (Dα line intensity to Hα line intensity) ratio just after the plasma breakdown decreased from ~1 to ~0.1, indicating that the tritium degassing progressed successfully.
(3) Long pulse operation of LH and EC systems: The pulse length of LH was extended up to 30 s in a power modulation mode with a duty cycle of 75%. The peak power was 0.63 MW and the width of each pulse was 30 ms. The injected energy was 13.3 MJ. The EC pulse length was also extended up to 44.6 s by using 4 gyrotrons in series. The injected energy reached 15 MJ with an average power of 0.35 MW.
(4) Resistive wall mode: In low beta plasmas with a low surface safety factor, the growth rate of resistive wall mode (RWM) was found to decrease with decreasing the distance between the plasma surface and the vessel wall. The data for the RWM at a higher beta (βN~2.1-2.4) was also obtained in reversed shear plasmas.
(5) Mass-dependence of H-mode pedestal width: The H-mode pedestal width in hydrogen plasmas was compared with that in deuterium plasmas, keeping the plasma current, the toroidal field, the average electron density and the plasma shape constant. It was found that almost identical pedestal profiles (width, height, gradient) were obtained in hydrogen and deuterium discharges by adjusting the heating power. This suggests that the pedestal width is not determined by the poloidal Larmor radius at the pedestal.
(6) Fast CXRS measurement: The ion temperature and the toroidal plasma rotation in ELMy H-mode plasmas were measured with charge exchange recombination spectroscopy with fast sampling. A reduction of the ion temperature and a variation in the toroidal rotation in the direction of the plasma current were observed to take place within ~1 ms after Dα bursts.
(7) Beam particle self-interaction: The ionization cross-section of the neutral beam injected into a tokamak plasma will be affected by the interaction with circulating fast ions generated by neutral beam itself; this is called the "beam-particle self-interaction (BPSI)" effect. N-NB with 350 keV and 1.5 MW was injected into a low-density (~1x1019 m-3) plasma. The shine-through power evaluated by the temperature rise of facing tile on the N-NB line suggested the existence of the BPSI effect.
(8) SOL (scrape-off layer) plasma fluctuations: The fluctuation level of the ion saturation current (δIs/Is) was measured with Langmuir probes. The δIs/Is was ~40% on the midplane and 4-6 times as large as δIs/Is near the X-point (7-10%). Density dependence of δIs/Is was observed at different poloidal and radial locations. On the midplane, δIs/Is was enhanced at the outer flux surface with an increase in the electron density while it was nearly constant near the separatrix. On the other hand, δIs/Is near the X-point decreased slightly at the high density.
(9) Divertor spectroscopy: In order to evaluate the molecular flux from the divertor plate using spectroscopy, the relation between the number of molecules and the intensity of the spectral bands is required. To determine the relation, H2, D2, CH4 and C2H6 gases were injected into the divertor region of L-mode plasmas and the spectral bands were observed at the injection region.
(10) Carbon-13 injection: In order to study the carbon transport in the SOL/divertor region, 13CH4 was injected as a tracer in the outer divertor region. The distribution of 13C will be investigated by analyzing graphite tiles after opening the vessel.
(11) Oxygen glow: An oxygen discharge is considered to be one of leading candidates for methods of removing the tritium in carbon deposition laygers in ITER. In order to address effects of the oxygen discharge, a glow discharge with He including 1000 ppm O2 (O2/He) was performed for 20 minutes for the first time in JT-60U. Most of injected oxygen was absorbed in carbon tiles during the O2/He discharge. Desorption of hydrocarbon was enhanced by oxygen. Absorbed oxygen was slowly desorbed in the form of CO2 during a pure He glow discharge after the O2/He glow discharge. No increase in oxygen impurity contents was observed in tokamak discharges on the following day.
(12) Others: Other experimental studies, in connection with response of the toroidal rotation to LH and EC waves, Alfven ion cyclotron wave, coupling and current drive properties of the LH wave, real-time current profile control, detachment physics and tungsten generation from the tungsten-coated divertor tiles, have been carried out.