May - Jul 2002
Four weeks of JT-60U operation has been performed in May and July 2002, aiming at further improvement in the integrated fusion performance and extension of the physics basis for contribution to ITPA and fusion science. In particular, the latter was carried out largely in collaboration with universities and research institutions in Japan and abroad. The outstanding results produced in the 2002 experimental campaign is highlighted by (1) the RF plasma initiation and NB current ramp up to 0.7 MA without the use of OH field coil, which is the largest in the world, (2) the "search and suppress" feedback control of the NTM island for the full recovery of βN and (3) an achievement of the fusion product of 3.3 - 3.5 X 1020 m-3 s keV under the nearly full-current drive condition at 1.8 MA.
Particularly, in the plasma start up experiment (1), which was performed in collaboration with University of Tokyo, Kyushu, Kyushu-Tokai and Kyoto Universities, an initial plasma current of 0.2 MA was produced by the EC preionization and inductive flux input from the vertical field and shaping coils. The plasma current was then raised by LHCD to a level of 0.4 MA, which is adequate for confining the 85 kV beam ions. A hollow current profile with "current hole" was maintained during the current ramp. An increase of βp caused by NB heating results in the increased vertical field and shaping coil current, and further increase in the plasma current to 0.6 to 0.7 MA has been accomplished by the extremely large bootstrap fraction. The produced plasma has a significantly high βp of 3.6, in other words εβp=1.0, and βN (=βt aBt/Ip) =1.6, seemingly close the stability limit in low li negative shear plasmas in JT-60U. The confinement property was also significant with both ITB and H mode, as represented by the enhancement factor of 1.6 over the ITER98 (y, 2) scaling. The q profile is deeply reversed with qmin = 5.6 at a/r = 0.7, and the bootstrap fraction exceeded 90%. This result sheds lights on the possibility of reducing the eventually eliminating the central OH solenoid in the tokamak and ST fusion reactor, which can dramatically reduce the burdens related to the engineering design complexities.
(2) In the newly developed NTM feedback stabilization system, the location of 3/2 island is detected in real time, by the combination of Te fluctuation profile measured by the high-resolution heterodyne radiometer and the CCS equilibrium reconstruction system. The EC reflection mirror is steered to continuously auto-track the position of the island center. Therefore, the local ECCD is effective even after the partial recovery of βN, which results in the Shafranov shift of the plasma column. As a consequence, the complete stabilization of NTM and full recovery of βN have been demonstrated for the first time in the world. It was also found that early ECH could reduce the necessary power to suppress NTM in collaboration with Kyoto University.
(3) In comparison with the achievements made last year, in which nD(0) τE Ti(0) of 3 X 1020 keV ・ s ・ m-3 was achieved, the heating profile has been modified to increase the central plasma pressure. As a result, the record value above was further extended by 15 % and the largest stored energy of 8 MJ at 1.8 MA was obtained, although it was not possible to attain the complete full current drive condition due to the limitations in the NNB pulse length.
Although the period of run weeks was extremely costraining in the 2002 campaign, plentiful accomplishments have been made, such as the confinement enhancement factor of 0.95 near n/nGW (nGW = Ip/π>a2) =1 in Ar seeded plasmas, and numerous results which is directly relevant to the physics basis of steady-state burning plasmas and contributes much to the ITPA activities. Namely, they are the impact of electron heating and impurity transport in ITB plasmas, scaling of the ELM heat load on the divertor plates, current drive efficiency in high Te plasmas and off-axis or counter ECCD efficiency, current drive in strongly negative shear plasmas with current hole, dynamics and scaling of the pedestal characteristics, Te ITB formation conditions and active control of the ITB quality by the perturbative schemes. The detailed analysis on above topics is in progress, and the results are going to be presented at the coming IAEA meeting in Lyon.