JT-60U MONTHLY SUMMARY

September and October 2007

  The "theme group" for JT-60 research was modified. The "theme leaders" were changed to N. Oyama and A. Isayama. The "MHD sub-theme leaders" were changed to G. Matsunaga (JAEA) and S. Sakakibara (NIFS). The new "theme group" is listed below.

  Theme leaders : N. Oyama and A. Isayama

  A manhole of the tokamak vessel was closed on September 21. Leak tests of the vessel were completed in October, both before and after the vessel baking at 150C. The temperature of the tokamak vessel was stated to increase 260C on October 25 for wall conditioning before the start of tokamak operation in November.

  The followings were done, in addition to usual maintenance, during the maintenance period from November 2006 to October 2007; (1) Extension of injection duration for 3 perpendicular NBs, (2) installation of guard tiles for ferritic steel tiles faced to a large port edge, (3) insertion of carbon sheets between carbon tiles and outer dome wing base for improvement of contact, (4) restoration of an LH antenna, (5) improvement of supersonic molecular beam injection system, (6) installation of peculiar tiles for analysis and (7) improvement of diagnostics systems.

(1) Extension of injection duration for 3 perpendicular NBs: Power supply was modified for 3 perpendicular NBs (#2, #3 and #6) to extend the operation duration to 30 s, in addition to 4 tangential NBs (#7, #8, #9 and #10) and 1 perpendicular NB (#14). It is expected that the beam injection duration will be limited less than 30 s by temperature rise at armor tile or beam limiter. These NBs are injected slightly co-direction and high confinement was attained with these NBs. The extension of the injection duration for these NBs allows us to choice NB combination flexibly for long sustainment of high confinement.

(2) Installation of guard tiles for ferritic steel tiles: Damage was observed in the ferritic steel tiles faced to a large port edge after the last campaign (2005 and 2006) due to localized heat load caused by fast ion loss. The carbon guard tiles were installed on the large port edge to suppress the damage.

(3) Insertion of carbon sheet between carbon tiles and outer dome wing base: A carbon tile came off the divertor dome during the last experimental campaign (2006) due to small thermal conduction caused by less contact between the carbon tile and the outer dome wing base. In order to improve the contact, carbon sheets were inserted between carbon tiles and outer dome wing base.

(4) Restoration of an LH antenna: In the last campaign (2005 and 2006), only 6 rows out of the total 8 rows of the LH antenna were used, because 2 rows have sever damage, melting in the stainless base. The stainless base for 7th row was restored during the maintenance period in addition to those for 6 rows used in the last campaign. In the forthcoming operation, 7 rows of the LH antenna can be used, which will result in 17% increase of available power.

(5) Improvement of supersonic molecular beam injection system: Seal materials installed in the injector heads were damaged in the last campaign due to long baking at high temperature of 300C. The seal materials were changed to high heat-resistant material, which were selected based on the material test performed in CEA-Caradache and JAEA. Also, the material test results indicated that the feeding gas pressure in the injector head, which tends to reduce the stresses on the seal during the baking, plays a role. Therefore, reduction of baking temperature and high feeding gas pressure (5 atm) during the baking was planned.

(6) Installation of peculiar tiles for analysis: A carbon tile with stripe shaped tungsten coating was installed at the outer divertor, in order to investigate prompt re-deposition of tungsten. A gap width scan tile was also installed, in order to investigate hydrogen isotope re-deposition at the gap. These tiles will be removed and analyzed after next ventilation.

(7) Improvement of diagnostics systems:
(7-1) Li beam probe (LiBP): A Li beam was installed for measurements of density (intensity measurement), temperature and rotation (CXRS) and current (polarization) in the edge region. Beam energy is 10-30 keV and beam current is 10 mA. The beam is injected vertically and signals are detected from outer midplane.

(7-2) Fast CXRS: Time resolution of the CXRS system was improved from 16.6 ms to 2.5 ms by using fast CCD. This system will be used for real-time control.

(7-3) RWM sensing coil: Two one-turn-loop coils were installed for active diagnostics of MHD stability. Magnetic perturbation will be externally induced and response of plasma will be measured. A maximum coil current is 100 A and a frequency of 10-150 kHz is available.

(7-4) Imaging bolometer: Imaging bolometer was modified in collaboration with NIFS. In order to suppress neutron damage, metalic foil was changed from Au to Ta and periscope was installed. Also, spatial resolution was improved from 15 cm to 3 cm.

(7-5) Doppler reflectometer: Doppler reflectometer was installed with O-mode mm wave, which can scan the frequency of injected wave. Thus, it is possible to measure correlation length by combined with existing O-mode reflectometer.