Annual Report (MS-Word)
Two pumped divertors have been installed and tested in JET under ITER relevant conditions. The excellent power handling of the MkIIA divertor has been demonstrated and has allowed new performance developments including a record plasma energy of 15 MJ and a high fusion triple product in high current quasi-steady state discharges.
Long pulse discharges with type I ELMs have been established which when extrapolated to ITER, using dimensionless scaling techniques, attain the energy confinement time required in ITER. The ITERH93-P confinement scaling is confirmed over a broad range of parameters but the scaling with B is found to be more favourable. If this is confirmed on other experiments, the confinement time predicted for ITER should increase by about 10 to 15%.
In addition to the hot ion H-mode, a second high performance regime has been developed at JET. It is based on optimising the plasma current profile. With low central magnetic shear and q > 1 everywhere in the plasma, internal confinement barriers are observed above a power threshold which is presently about 17 MW. This regime appears similar to that obtained with similar geometry in DIII-D. It is significant that the confinement barrier appears in JET despite a lower fuelling and toroidal momentum input in JET than in DIII-D.
The work on JET has benefited from exchange of information and developments under a number of Task Agreements and from collaborations established and expertise transferred under a number of personnel assignments within the auspices of the IEA Agreement. These exchanges continue to broaden the research which is carried out in the JET programme.
A substantial portion of the current JT-60 program is devoted to the contribution to the broad range of ITER Physics R&D. Major efforts are focused on the improvement of confinement and exploration of steady state operation.
As for the confinement improvement, QDTequiv > 1 has been achieved with reversed shear discharges with the plasma current of 2.8 MA, the energy confinement time of 0.97 sec and the center ion temperature of 16.5 keV. In high- H-mode discharge, the highest fusion triple product of 1.5x1021 keV s m-3 has been recorded with Ti(0) = 45 keV. In those discharges, H/q95 reached ~ 1 at q95 of 3, this value exceeds that required in ITER ignition operation at ~ 0.7. It was found that the triangularity on the plasma shape has an important role on sustainment of high confinement. The edge stability of high- H-mode plasmas was significantly improved, and giant ELMs were suppressed with increasing triangularity.
As for the steady state operation research, 7.5 sec sustainment of the reversed shear configuration were successfully performed by LHCD. Quasi-steady high performance discharge of ~ 2.5, H-factor ~ 2.3 and QDTequiv = 0.27 was sustained for 2.5 sec in high triangularity (=0.35) operation. The first demonstration of the heating and current drive by a negative-ion based NBI with up to 400 keV, 2.5 MW has indicated an effectiveness of N-NBI as a core heating and current drive method in reactor-relevant plasmas. Results observed have indicated that the reversed shear is one of the attractive candidate for an advanced steady state operation scenario. Stabilization of TAE mode was also demonstrated with reversed shear configuration.
The high current high-li regime has demonstrated good energy confinement and favorable MHD stability up to 2.31 enabling the achievement of fusion power production (8.7 MW) comparable to that achieved in supershots at similar heating powers. The proven ability to achieve high values of offers the potential of still higher values of fusion power in TFTR. The formation of an internal transport barrier in the enhanced reverse shear regime has dramatically reduced the ion heat and particle flux from the core. This is accompanied by a substantial reduction in core plasma fluctuations and a steeping of the plasma pressure gradients controlling the evolution of the barrier and the current profile to maintain stability.
ICRF heating schemes of importance to ITER have been validated and a new scheme for RF current drive through mode-conversion in a mixed-species plasma has been demonstrated. Parasitic absorption by impurities is found to decrease mode conversion current drive efficiency in D-T plasmas. ICRF heating has been shown to sustain the ERS mode without particle fueling, which is important to the development of this operational regime. The interactions of energetic ions, including fusion alphas, with ICRF waves have been investigated and support the development of the alpha-channeling concept.
Fusion alpha particles have behaved classically in quiescent MHD plasmas. Sawteeth have been shown to redistribute fusion alphas near the plasma core. In discharges with weak magnetic shear, TAE modes driven by fusion alpha-particles have now been observed and found to be in accord with theoretical predictions.
The sharing of theoretical and experimental data with JET and JT-60 enabled very good progress in 1996 with the exchange of research physicists on visits and assignments. The additional capability with remote participation in experiments and the implementation of enhanced communications via direct network capabilities, the internet, video-conferencing, use of the JAERI-developed X-Windows control programs and the prototype developed by JAERI of the Data Link System for exchanging experimental data has fostered improved coordination of efforts on a real-time basis.