Annual Report (January to December 1996)

Annual Report (MS-Word)

Progress of Three Large Tokamak Cooperation

January to December 1996
Executive Committee

1. Cooperation activities

Effectiveness and productivity through the coordination of the collaborative program have been further enhanced. The cooperation among the three large tokamaks was remarkably successful in 1996, with the Implementing Agreement contributing to the development of a profound understanding of the tokamak physics and to the development of improved operational regimes of integrated fusion performance, which are expected to be applicable to ITER as well.
The progress of the large tokamak research in 1996 was significant, as represented at JT-60 by the new records for the fusion triple product of 1.5x10²¹ m^-3 s keV, the ion temperature of 45 keV and the achievement of plasma conditions which if repeated in a 50:50 DT plasma would give Q_DT

1, the exploration of the enhanced reverse shear (ERS) regime and the observation of toroidal Alfv

n eigen (TAE) modes driven by fusion alpha particles on TFTR, and the performance developments including a plasma energy of 15 MJ in the improved divertor conditions at JET. Furthermore, ITER physics R & D tasks were also robustly implemented in individual programs.
As to the task-assignment programs, which were strongly emphasized in the past years as the most effective means of coordinating the collaborative work, one proposal "Impurity Content of Radiative Discharges" was newly approved by the Executive Committee. Data from JT-60 and JET relevant to the scaling of line averaged Z_eff with radiated power and machine size has been combined in a multi-machine database. The five other task assignments also continue to produce the significant results. (1) TFTR and JT-60 have continued the high-

works, extending the regime to higher currents and with reversed shear cores. A new record of the equivalent Q_DT (Q_DT^equiv) of over 1.0 was achieved in the JT-60 high-

experiments with the reversed shear configuration with remote participation by TFTR. (2) As for disruption studies, fast plasma shutdown has been demonstrated in JT-60 by impurity pellet injection. It has been determined that runaway electron generation at the disruptive termination can be avoided when magnetic perturbations are enhanced spontaneously or by the external helical field. (3) Thermal fatigue tests on Vapatron were conducted at JAERI. The Vapatron made of CuCrZr was prepared by JET, and tested in the JAERI Electron Beam Irradiation System facility. There was a major emphasis on the thermal fatigue lifetime of the Vapatron under a severe heat load which gives the surface temperature of about 700

C. It was demonstrated that the Vapatron has a lifetime of more than 4000 cycles even at high temperature. (4) The first demonstration of heating and current drive by a negative-ion based NBI with up to 400 keV, 2.5 MW in JT-60 has indicated an effectiveness of N-NBI as a core heating and current drive method in reactor-relevant plasmas. Shear optimization experiments are also planned in JET high poloidal beta plasmas with a large fraction of bootstrap current and co-injected NB current as the seed current. (5) JAERI installed an ISDN video conferencing system in the JT-60 control room, in cooperation with PPPL and LANL. JAERI also completed work on a Data Link System (DLS) for exchanging experimental data from JT-60 through the Data Link for tasks agreed to under the IEA Large Tokamak Agreement. Data from approximately 700 typical shots was transferred to this system for testing under the authorized procedure for remote data access. Scientists from PPPL and JAERI also participated in an IEA sponsored workshop on remote collaboration in fusion at the IAEA meeting in Montreal.

2. Meeting Activities and Personnel Assignments

Three workshops were held in 1996 under the Implementing Agreement. Two of them, "Key Physics and Related Operational Issues for High-Reactivity Discharges" and "Non-Inductive Steady State Research", were jointly held at JT-60 on February 19-21. The third one is "Effects of Plasma Behaviour on Tokamak Structural Components" held at JET on September 10-12. The number of personnel exchanges of which exceeded four weeks was only 2, while 38 scientists participated in the review tours. These numbers of the personnel exchanges and the participants in the review tours were nearly identical those in the previous year.

3. Meeting of the Executive Committee

The eleventh Executive Committee meeting took place at TFTR on June 10 and 11, 1996. The coordinated assignments in the previous year as well as the annual strategic work program were reviewed. Additionally, proposed workshops and personnel assignments for the coming year were discussed and authorized.

4. Status of the Three Large Tokamaks

I. JET

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.

II. JT-60

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, Q_DT^equiv > 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 T_i(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 Q_DT^equiv = 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.

III. TFTR

The high current high-l_i 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.

5. Contribution to the ITER project

As documented in the previous sections, the three large tokamaks coordinated their research programs to make significant contributions to the ITER EDA. The achievements produced by the cooperative work under the Implementing Agreement have actually made and will continue to make remarkable contributions to understand the physics of and to define the operational regimes for ITER.