Annual Report (January to December 1994)

Annual Report (MS-Word)

Progress of Three Large Tokamak Cooperation

January to December 1994
Executive Committee

1. Cooperation activities

Further enhancement of the effectiveness and productivity through the coordination of the collaborative program having been intensively pursued, which has been intrinsically prerequisite to the recent fusion research, the co-operation among the three large tokamaks was remarkably successful also in 1994, and hence the Implementing Agreement functioned efficiently to contribute much to the profound understanding of the tokamak physics and to suggest the operational regime of integrated fusion performance relevant to ITER as well.

Progress of the large tokamak research in 1994 was significant, as represented by the achievement of ITER relevant plasma parameters i.e., D-T fusion power production of 10.7 MW at TFTR and the fusion product of 1.2 x 10²¹ m^-3 keV

m^-3 at JT-60. By the dynamic profile control and suppression of MHD instability of high beta-poloidal plasmas, notably large fusion product of 4 x 10²⁰ m^-3

keV ( equivalent Q_DT = 0.25 ) was sustained for 1.5 s in the quasi-steady-state ELMy H-mode discharge also at JT-60. In addition, JET has demonstrated the 20s steady-state H-mode as well as 7s high beta-poloidal discharge. The results from JT-60 and JET both indicate clearly the advantage of high beta-poloidal regime for the steady-state tokamak operation, which is accompanied by a large fraction of the bootstrap current. A number of scientists from JET and JT-60 participated in the TFTR D-T campaign, while TFTR proposed and conducted a part of high beta-poloidal runs at JT-60. Tritium transport was also intensively studied at TFTR, and the energetic ion loss in TFTR plasmas was investigated in collaboration with JT-60. Extensive joint work between JET and JT-60 was beneficial for ITER EDA.

Task-assignment program started in 1993, was continued with success in three main areas of recent tokamak research, which integrates the main thrust of research activities of contracting parties. One of the issues was the collaboration on high beta-poloidal plasma research started with JT-60 and TFTR. JET joined this program in 1994 to contribute much for the demonstration and scaling of high beta-poloidal plasmas, having undertaken the small-bore high beta-poloidal experiment based on the JT-60 proposal. TFTR scientists focused on the significance of beam deposition profile in high beta-poloidal discharges, and the joint experiment was carried out at JT-60. The results of collaborative work described above were presented as a joint paper at international meetings. The second issue was to seek for the amelioration method of major disruptions. Experimental results from JET and JT-60 have documented that the reduced stored energy right before the energy quench is beneficial for the reduction of impurity influx, which moderates the current quench activity. In addition, it was also shown that beryllium tile was remarkably advantageous to relax the major disruption. The growth rate of vertical displacement instability is significantly reduced by the relaxation of current quench, which can reduce the halo current and resulting stress on the vacuum vessel as well as the runaway electrons. The result of this collaborative work was also published and presented as a joint paper at international meetings. The third is the development of divertor plate technology, mainly involving JET and JT-60. Divertor power handling is inevitably one of the crucial issues for ITER, and it is what JET copes with in the advanced divertor program. Based on the encouraging preliminary result in 1993, a divertor test section was produced at JT-60 in 1994, which has unidirectional high conductivity carbon fiber composite tiles brazed on the Cr Zr Cu Hypervapotron heat sink supplied by JET, and its burnout critical heat flux will be examined at JET in early '95.

2. Meeting activities and personnel assignments

One workshop was held in 1994 under the Implementing Agreement at TFTR on "Review of D-T Plasmas" on March 2-4. The number of personnel exchanges of which period exceeded four weeks was 4, while 31 scientists participated in the review tours. The number of personnel exchanges was decreased in 1994, however, participants in the review tours was as many as 1993, which is indicative of the increasing demand of co-operative work in spite of the various limitations.

3. Meeting of the Executive Committee

The ninth Executive Committee meeting took place at JET on May 19 and 20, 1994. The coordinated assignments in the previous year as well as the annual strategic work program were reviewed, and proposed workshops and personnel assignments for the coming year were discussed and authorized. Introduction of the mutual data transfer method among the contracting parties by means of the computer communication was also discussed. The recent activities of the FPCC and CRD were reported from IEA. The Committee also discussed the industrial participation in the Implementing Agreement, and the extension and amendment of the Agreement after the completion of the TFTR D-T experiment.

4. Status of the Three Large Tokamaks

I. JET

After a commissioning procedure which started from the beginning of '94, JET resumed operation in mid-February with the new divertor configuration. Experimental programme started in early May, and the discharge optimization has been intensively carried out at I_p / B_T = 2 MA / 2.8 T with ELMy H-mode phases reaching a duration of 20s. The additional heating system has also been successfully commissioned, and the non-inductive current drive of 2 MA was achieved. The combined heating power has thereby reached 26 MW.
The heat load capability was improved by an order of magnitude, and 140 MJ has been injected into the plasma whereas in the previous configuration a carbon bloom occurred after 15 MJ. However, the ELM-free periods are shorter and the ELM frequency is higher due to the increased recycling and reduced triangularity, even so ELM-free durations of 2s have been achieved. In the transient peak performance regimes, the fusion product has reached 85% and Q_DD 75% of previous best values despite the now smaller plasma volume.
Operating regimes have been established at 4 and 5 MA. The divertor cryopump is operating successfully and is being used for divertor pumping and helium exhaust studies. Detached plasmas with radiative transfer in the divertor have been obtained using nitrogen puffing. This work is directly relevant to ITER.
High values of ~ 2% have been maintained for 7s ( = 3, = 1.5 at I_p / B_T = 1.5 MA / 1.4 T). Collaboration with JT-60 and with TFTR have suggested important operating regimes and experimental investigations. Important experiments for the near future are experiments with variable toroidal ripple and a comparison of the present operation using CFC divertor target plates with operation on beryllium target plates.

II. JT-60

During the 1994 campaign, major experimental efforts have been devoted to further enhancement of the integrated fusion performance at JT-60, focusing on the attainment of steady-state improved confinement regimes. As a result of the dynamic control of heat deposition and current density profile as well as the suppression of -collapse, a large fusion product of 4 x 10²⁰ m^-3skeV ( equivalent Q_DT = 0.25 ) was sustained for 1.5 s in the ELMy H-mode discharge, while the highest performance achieved was 1.2 x 10²¹ m^-3skeV ( equivalent Q_DT = 0.46 ). Termination of the steady-state high performance was caused by the carbon influx from the divertor plates. It was also documented that Helium accumulation was subtle in the ELMy H-mode, in spite of the deteriorated Helium exhausting capability relative to the L-mode.
Non-inductive current drive experiment with substantial bootstrap and NB driven current fraction have also been intensively carried out. Production of the reversed shear was found to have influences on the achieved values of normalized and poloidal beta both around 3 and the H-factor of 2.2 for 1 s. The full current drive condition was sustained for 0.65 s. Production of the internal transport barrier is the key factor for the attainment of high- mode. The influence of the heat deposition and momentum input profile on the transport barrier formation was investigated in collaboration with TFTR group.
In addition, recent turbulent transport studies prevailed that the long-wavelength mode can possibly be responsible for the deteriorated confinement in the L-mode discharges. This is consistent with the TFTR results.

III. TFTR

The TFTR device continued to optimize the D-T plasma performance with the increased toroidal field capability and beam power, and number of scientists from JET and JT-60 participated. A maximum fusion power of 10.7 MW was obtained, and it was sustained above 10 MW for approximately 0.1 s. The highest fusion product obtained hereby reached 5 x 10²⁰ m^-3skeV. Lithium pellet injection allowed the supershot conditions to be obtained at the high plasma current of 2.7 MA.
The observed neutron flux was in good agreement with calculated value based on the electron and ion temperature and density profiles. The loss of alpha particles can be attributed to the classical first orbit loss. In these high power D-T experiments, collective instabilities induced by the presence of alpha particles have not been documented. The stored plasma energy, electron and ion temperature increased in deuterium-tritium plasmas compared with similar deuterium plasmas, corresponding to an increase in from 160 ms to 210 ms.
Tritium transport has also been intensively carried out with multi-channel neutron collimator, and it has been documented that tritium diffusivity is similar to that of He and the thermal diffusivity of deuterium, which is consistent with the E x B drift theory. For r / a < 0.5, the deduced ion thermal diffusivity is a factor of 1.5 lower in the D-T plasma compared to the D-D plasma. This suggests a strong sensitivity of ion heat conduction to isotopic composition in supershots plasmas. in these supershot discharges increases with the mass of the Hydrogenic species.
H-mode features also differs from the D-D discharges i.e., larger Da drop and ELM amplitude, smaller ELM frequency, and / ^ITER-89P > 4.3 was achieved in D-T limiter H-mode discharges. The amount of reduction in _i is also significant in D-T discharges.
Second harmonic tritium heating with 5.5 MW of ICRF power superimposed on the 23 MW of NB power has resulted in increasing the ion temperature from 26 to 36 keV. T_e increased from 8 to 10.5 keV due to direct electron damping as well as ³He minority heating.

5. Contribution to the ITER project

As documented in the previous sections, the three large tokamaks have made best efforts to co-ordinate their research programs and to carry them out intensively to make significant contributions to ITER EDA. The achievements hereby produced by the co-operative 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 regime of ITER.

6. Extension of the Implementing Agreement

Based on the consent that the continuation of the co-ordinated collaborative work under the Implementing Agreement is indispensable for the progress of the world fusion program, Executive Committee members unanimously agreed to propose the extension of the agreement to IEA for its approval.