JT-60U MONTHLY SUMMARY

from August to November, 1998


Papers from JT-60U in the 17th IAEA Fusion Energy Conference

Including the new results shown later, 15 papers were presented from JT-60U in the 17th IAEA Fusion Energy Conference, 19 - 24 October 1998, Yokohama, Japan:

  1. 'JT-60U High Performance Regimes', OV1/1, S.Ishida et al.,
  2. 'Characteristics of Halo Current in JT-60U', EXP3/11, Y.Neyatani, et al.,
  3. 'Long Sustainment of JT-60U Plasmas with High Integrated Performance', CD2/EX9/2, Y. Kamada et al.,
  4. 'High Performance Experiments in JT-60U Reversed Shear Discharges', EX1/2, T. Fujita, et al.,
  5. 'Reduced Transport and Er shearing in Improved Confinement Regimes in JT-60U', EX5/4, H. Shirai, et al.,
  6. 'Formation Condition of Internal Transport Barrier in JT-60U', EX5/2, Y. Koide et al.,
  7. 'Particle Confinement and Transport in JT-60U', EXP1/17, H. Takenaga, et al.,
  8. 'Localized MHD Activity near Transport Barriers in JT-60U and TFTR', EX8/4, J. Manickam et al.,
  9. 'Core Density Fluctuations in Reversed Magnetic Shear Plasmas with Internal Transport Barrier on JT-60U', PDP/03, R.Nazikian, et al.,
  10. 'LHCD Current Profile Control Experiments towards Steady-State Improved Confinement on JT-60U', CD1/4, S. Ide, et al.,
  11. 'Heating and Non-Inductive Current Drive by Negative-Ion Based NBI in JT-60U', CD1/1, T. Oikawa, et al.,
  12. 'Alfven Eigenmodes and Their Impact on Plasma Characteristics in JT-60U', EX8/6, Y. Kusama, et al.,
  13. 'Steady-State Exhaust of Helium Ahs in the W-shaped Divertor on JT-60U', EX6/5, A. Sakasai, et al.,
  14. 'Divertor Characteristics & Control on the W-shaped Divertor with pump of JT-60U', EXP4/05, N. Hosogane, et al.,
  15. 'Heat and Particle Transport of SOL/Divertor Plasma in the W-shaped Divertor on JT-60U', EXP4/06, N. Asakura, et al.,

The papers 4 - 10 and 13 treat the reversed shear plasmas.

In addition, the following related papers are useful to understand the key physics for understanding the JT-60U plasmas:

  1. 'Simulation Study on Avoiding Runaway Electron Generation by Magnetic Perturbations', THP2/26, S. Tokuda et al.,
  2. 'Characterization of Disruption Phenomenology in ITER', ITERP1/14, R. Yoshino et al.,
  3. 'Discontinuity Model for Internal Transport Barrier Formation in Reversed Shear Plasmas', TH1/2, Y. Kishimoto et al.,
  4. 'Improvement of MHD Stability in Negative / Weak Shear Configurations for a Steady State Tokamak', THP2/31, T. Ozeki, et al.,
  5. 'Edge Database Analysis for Extrapolation to ITER', ITERP1/13, M. Shimada et al.,

OPERATION AND CONFINEMENT PHYSICS

The database of the halo current at major disruption was expanded to the high plasma current region up to 2MA. We measured electron temperature in the halo region outside the separatrix for the first time in JT-60U by the YAG Thomson scattering system. The value was 10 eV in the current quench phase. By neon gas puffing, it was demonstrated that the local peak value of the halo current was decreased by 60%.

For establishment of the advanced feedback control required for integration of favorable core and divertor performances, we started a new scheme to control multiple plasma parameters with multiple actuators. Using NB heating power, gas puffing from the main chamber and gas puffing from behind the divertor baffle as actuators, we could sustain DD neutron production rate, main plasma density and divertor radiation power almost steadily for 6 sec (~50 x energy confinement time) with a newly developed feedback control matrix.

Concerning sustainment of the reversed shear plasmas with the internal transport barrier (ITB), we could sustain H-facror=1.5 - 2.0, normalized beta (betaN)= 1.0-1.4 for 5.5 sec in the reversed shear ELMy H-mode with NB heating. In these discharges, the minimum value of q (qmin) decreases in time by current diffusion. By application of off-axis current drive with Lower Hybrid Current Drive (LHCD), we could sustain qmin constantly (qmin ~ 2) in the reversed shear mode with the ITB under full non-inductive current drive condition (H-factor ~1.4, betaN ~1). In this discharge, electron temperature was higher than ion temperature (Te ~ 1.2 Ti). As for the formation of ITBs, we have found that the pressure gradient at the ITB can be modified by change of the toroidal momentum input profile and the location of the ITB can be shifted inward by step down of heating power. A clear ITB for electron temperature can be formed in the positive magnetic shear region. Such drop in electron thermal diffusivity at the positive shear region was also observed in the high beta-p mode plasmas at high triangularity. A large drop in particle diffusivity by factors of 5 - 6 was evaluated by modulating helium gas puffing rate. In order to clarify the reason of reduction of transport at the ITB, we started the measurement of density fluctuation using the reflectometer under collaboration with Princeton Plasma Physics Laboratory (PPPL). We have clarified that the correlation length of the micro turbulence clearly decreases in the ITB layer. As for the MHD stability near the ITB layer was also systematically studied. The radial profile of the eigen function of the low-n kink ballooning mode thus the effect on confinement was found to be strongly affected by magnetic shear, location of ITB and q-min through comparison between JT-60U and TFTR with detailed analyses of the ideal MHD stability analyses with the PEST code (under PPPL-JT60U collaboration).

The progress of development of the high beta-p ELMy H-mode is also favorable toward steady state high integrated performance. For sustainment of high beta-N value, the benefit of the high triangularity shape was demonstrated. In high beta-p ELMy H-mode characterized by ITB and the edge transport barrier (ETB), beta-N=2.5 - 2.7 and the product of beta-NxH-factor ~5.5 were sustained for 3.5s even at the low value of q95=2.9-3.3 at triangularity ~0.46 (Bt=1.9-2.1T, Ip=1-1.1MA).This high value of beta-N at the low safety factor can expand possibility of the reduced size ITER which requires a higher beta-N then that in ITER-FDR (beta-N=2.2). The quasi-steady value of H-factor x beta-N sustainable for >5x energy confinement time (> ~ effective particle confinement time) increases with triangularity. This is mainly because the pedestal beta-N increases with triangularity. The sustainable beta is limited by the neoclassical tearing modes. As for high confinement at high electron density, the density range with H-factor >2 was extended from ~50% to ~60% of the Greenwald density limit by increasing triangularity from 0.2 to 0.4. However, for a further increase in density, strong gas puffing is required and the H-factor decreases with density. For the simultaneous improvement of both core and SOL/divertor plasmas, behavior of the plasma edge plays the key role. Based on the detailed measurement of the edge pedestal structure in H-mode, the edge pedestal width in the ELMy phase reaches 10-15cm which is 2-3 times wider than that in the ELM-free phase. The width scales with the poloidal gyro radius of thermal ion at the pedestal shoulder. The edge pressure gradient increases with triangularity. In particular, at high triangularity (>0.3), high-q95(>5-6) and high-beta-p(>1.5-2) the type I ELMs disappear and the pedestal width increases further which suggests the access to the second stability regime.

CURRENT DRIVE AND HIGH ENERGY PARTICLE PHYSICS

The plasma current driven with the negative-ion-based neutral beam (NNB) injection was increased up to 600 kA by injecting the NNB of 3.7 MW into the plasma with the central electron temperature of 4.2 keV. The dependence of current drive efficiency on the central electron temperature and the beam energy and the effect of beam ion deposition on the current drive efficiency have been clarified.

The H-mode transition was triggered by the NNB injection of 4 MW and the ELMy H-mode with a H-factor of 1.6-1.8 was sustained in the electron heating dominant regime. The central electron temperature was higher than the central ion temperature (Te(0) ~ 1.4 Ti(0)). About 77% of the total neutral beam power was transferred to electrons and the central electron temperature was ~40% higher than that of ions.

By applying lower hybrid (LH) waves for current drive and heating of revered magnetic shear plasmas, the reversed shear configuration with the internal transport barrier (ITB) was sustained for 6 s without shrinking the location of the ITB. The LH driven current was increased up to ~77% and fully non-inductive current drive was realized for the first time in the reversed-shear plasma. A large amount of the plasma current was driven inside the ITB by LH waves and it contributed to keep hollow current density profile necessary for sustaining the reversed shear configuration.

Alfvén eigenmodes (AEs) were investigated by injecting NNB of 330-360 keV into plasmas with low toroidal magnetic field of ~1.2-2.1 T. New chirping and burst

DIVERTOR AND BOUNDARY PHYSICS

Helium exhaust characteristics in the reversed shear plasma (plasma current = 1.7 MA, toroidal field = 3.7 T and line averaged electron density = 3x1019 m-3) were investigated by using a short pulsed He gas puff (edge fueling). The central ion temperature of 8-10 keV was kept and H-factor of 1.6 - 2.0 were achieved during the NB heating with the constant NB power. The He residence time inside the ITB was 1.9 times as long as that outside the ITB. If the decay time of the He density inside the ITB is assumed as the local tau-*He, the ratio of tau-*He / tau-E = 8 - 10 were achieved, within the range of future fusion reactors.

The direction of the SOL plasma flow was measured with Bt reversed (Ip direction was normal). The SOL plasma flow direction was from the midplane to the divertor. It was confirmed that the SOL parallel flow at the outer midplane directs against the ion grad B direction. The SOL flow may be the parallel flow to compensate the difference of plasma pressure along the field line due to the toroidal effect on the poloidal ion drift. SOL plasma flow was also measured during the ELMy H-mode (ion grad-B direction towards the divertor) with the fast sampling of the ion saturation currents. At ELM pulses (a few ms) the flow reversal was reduced, and recovered rapidly between ELMs.

The detail dependence of the divertor pumping rate on the Gap-in (the distance from the inner strike point to the pumping slot) was investigated. It was found that the divertor pumping rate linearly decreases with increasing Gap-in.

Doppler broadening of a C IV line in the divertor plasmas has been measured in order to estimate profiles of ion temperature in the divertor plasmas of L-mode discharges. Ion temperature around the null point decreased from 32 eV to 20 eV, as the electron density increased up to MARFE onset.