Annual Report (January to December 1987)

(Annual Report to IEA)

IEA Three Large Tokamak Cooperation

(January to December 1987)

Executive Committee

1. Cooperation Activities

The fruitful cooperation among the three large tokamaks is well under way. Four workshops were held as shown in Attachment 1. There were 33 personnel exchanges which started in l987 as shown in Attachment 2. Reports on these activities (Forms A, B and C) are filed in Attachment 3. The personnel exchanges have proved to be good occasions to gain scientific and technical experience and to build up friendships among staffs for future communication.
The Personnel Assignment Agreement between JT-60 and JET was signed in March, completing the trilateral Personnel Assignment Agreements linking the three projects.

2. Meeting of the Executives Committee

The second Executive Committee meeting took place at Princeton on February 26 and 27, 1987. The members were Dr J Poffe from JET, Dr D Grove and Dr D Meade from TFTR, Dr J Willis from USDOE. Dr M Yoshikawa and Dr Y Shimomura from JT-6O. The Committee elected Dr Grove as chairman. Dr Meade succeeded him after the meeting until the next meeting. Dr J Sinnis replaced Dr Meade as alternate member. JAERI acted as the Secretariat, represented by Dr A Kitsunezaki.
Discussions were made on the following items.
The plan of personnel exchanges and workshops for one and a half years was agreed upon. It was decided to use numbers for each workshop and personnel exchange to keep track of them.
The list of plans of workshops and personnel exchanges should be made for every one and a half years starting April 1 and should be reviewed at the Executive Committee meeting.
There were two comments on workshops that the timing should not be the same as other important meetings and that the number of workshops should not be too many.
It was confirmed that there is no difficulty in adding personnel exchange after the Executive Committee meeting with agreement made by letter.
Procedures of workshops, personnel exchanges and information exchanges which should be done by key persons, assigned participants or task coordinators were proposed by the Secretariat and agreed upon.
The minutes of the second meeting of the Executive Committee is attached to this report. (Attachment 4)

A General Meeting was held at Princeton at the occasion of the Performance Limitation workshop, where the progress to date was reviewed and modification of the workshop plan in 1988 was discussed. The usefulness of these meeting was queried given that the large tokamak projects are now exploring different paths. In general it was felt that there were still topics of sufficient common interest to warrant the continuation of these meeting.
The next meeting of the Executive Committee will be held at the JET Joint Undertaking on February 29 and March 1, 1988.

3. Status of the Three Large Tokamaks

All the three tokamaks are making continued progress towards providing scientific feasibility of nuclear fusion with improved device capability and plasma performance.
The status and major results of the three large tokamaks are summarized below.

JET

The 1987 operation period began after a major shutdown in which the following parts were replaced or newly installed; the central OH coil, vessel restraints, toroidal belt limiters, 8 ICRF antennae, the second NBI box, and an ORNL multipellet injector. ICRF now couples up to 14 MW from 8 antennae and monster sawteeth lasting up to 2.7 s have been produced. The central electron temperature rises to 1 0 keV in these discharges. H-mode has not been obtained with ICRF heating alone. Neutral beam operation was delayed by technical problems but in the last weeks of 1987 injection power in the range 4-6 MW of 70 kV deuterium was used. This enabled H-modes to be recovered similar to those seen in l986. A study of the threshold beam power for L to H transitions shows a high sensitivity to the toroidal field strength (P_th B_t^2.5). With the new poloidal field system it has been possible to operate at 6 MA in the limiter mode and 3.5 MA in the X-point configuration. A fusion product of n_i(0)T_i(0)= 2x10²⁰ m^-3s keV equal to the best 1986 value has been achieved. Peaked density profiles have been obtained using the ORNL pellet injector.

TFTR

The neutral beam performance has been extended to 22 MW lasting for 2 s. This makes it possible to study a long-term evolution of supershots. The best results, central ion temperature T_i(0) > 25 keV and n_e(0)T_i(0) value of 3x10²⁰ m^-3s keV, were obtained with nearly-balanced injection. Non-inductively driven currents have been investigated with the long-pulse neutral beams. Modeling of the plasma surface voltage requires inclusion of the neoclassical bootstrap current. An interesting regime with some properties similar to the H-mode has been observed for limiter plasmas when qa is close to certain values, 3.2 or 2.7.
TFTR has been shut down since July to reorient the NB injectors for maximum balanced performance and to install an ICRF system for profile peaking.

JT-60

During a shutdown in April and May, limiters and divertor plates were changed to graphite. With enhancement of the poloidal coil and power supply systems, the plasma current (3.2 MA limiter, 2.7 MA divertor) now exceeds the original design values (2.7 and 2.1 MA, respectively). The total combined heating power up to 30 MW with NB and RF has been injected into hydrogen plasmas. The plasma stored energy increased up to 3.1 MJ with I_p, and n_e(0)= 1.8x10¹⁹ m^-3s at T_i(0)=3.7 keV was obtained in limiter plasmas. This data point enters well into the JT-60 target area as laid down by the Atomic Energy Commission in July l975 in equivalent, deuterium plasma parameters taking into account of the square root dependence of on ion mass. Short H-mode periods were observed in the outside X-point divertor discharges with heating power higher than 16 MW. Another type of improved energy confinement was obtained with the combined NB and ICRF heating in low density discharges associated with strong acceleration of fast ions.
A new divertor coil for lower X-point divertor operation and a 4-pellet injector are being installed for confinement study which will start in March 1988.