In this month, evaluation of ferritic steel effects on magnetic field measurements and conditioning operation have been carried out as an initial operation stage after installation of the ferritic steel in the tokamak vessel. Boron coating of the first wall and Rayleigh scattering calibration of the Thomson scattering systems have also been done.
(1) Evaluation of ferritic steel effects on magnetic field measurements
The ferritic steel installed in the vessel to reduce toroidal field ripple generates poloidal fields and poloidal and toroidal flux, which should be evaluated correctly to determine the plasma position and shape, to determine the plasma stored energy by diamagnetic measurement, and to reconstruct the plasma equilibrium. The poloidal field generated by ferritic steel depends on the toroidal magnetic field as well as the poloidal magnetic field since the magnetization of ferritic steel depends mostly on the toroidal magnetic field.
Effects of ferritic steel on the diamagnetic measurement was evaluated by energizing poloidal field coils under various toroidal field with the wall temperature of 300C. The effects of saturated ferritic steel were found to be small.
Effects of ferritic steel on poloidal field pick-up coils and on poloidal flux loops were evaluated by numerical calculations. The function to evaluate the effects of ferritic steel on magnetic sensors is added to the code to calculate the plasma shape by using the Cauchy condition surface (CCS). The position of the last closed flux surface (LCFS) obtained by the CCS code has been compared with that by other diagnostics including the charge exchange recombination spectroscopy (CXRS), the motional Stark effect diagnostics (MSE) and divertor Langmuir probes. The CXRS has its observation points on the upper low-field side of the plasma, where the effects of ferritic steel are expected to be largest. The position of LCFS in H-mode plasmas was determined as the foot of steep ion temperature gradient (pedestal) in the CXRS measurement. It was found that the position of LCFS by the CCS code and that by the CXRS measurement agree with each other within a few cm.
(2) Conditioning operation
Wall conditioning was carried out by the combination of the Taylor discharge cleaning (TDC) and the glow discharge cleaning (GDC) before the beginning of tokamak operation.
NBI port conditioning, EC conditioning and wall conditioning were carried out after the tokamak operation became possible. The wall temperature was kept 300C in the first two weeks and was 150C in the third week. The pulse length of tangential NBs, #7 to #10, has been extended up to 14-19 s. The pulse length of #14 and other perpendicular NBs (#2, 3, 4, 6, 12, 13) have been extended up to 14 s and 10 s, respectively. The pulse length of EC (4 units) has been extended up to 3.0 to 4.0 s. The H/D ratio, evaluated by the ratio of Hα to Dα at t = 0.3 s, decreased smoothly to 0.1. The ratio of OVIII to CVI also decreased.
The measurements of the toroidal rotation and the pedestal temperature were made in conditioning discharges, and the results suggested the change of toroidal rotation toward the direction of the plasma current and improvement of the pedestal pressure through the reduction of toroidal field ripple. These will be studied in detail in experiments in December after the boron coating of the first wall.
(3) Boron coating of the first wall
After three weeks of conditioning operation, boron coating of the first wall was performed by GDC with the He gas and the decaborane (B10D14) vapor. A total amount of 50 g decaborane was used during ~32 hours GDC.
(4) Rayleigh scattering calibration
The Rayleigh scattering calibration was performed for Thomson scattering diagnostics with the ruby laser and that with the YAG laser.