Annual Report (MS-Word)

Progress of Cooperation among
The Three Large Tokamak Facilities

Annual Report (January to December 1999)
Executive Committee

I. Cooperative Activities

The Three Large Tokamak Collaboration has continued successfully in 1999, with active personnel and task assignment programs. The Implementing Agreement functioned efficiently to promote improved understanding of tokamak physics and enhanced tokamak plasma performance.

1. Executive Committe Members

Old membership (Until Dec. 6, 1999)
Dr. J. Jacquinot : JET Director
Dr. M. Watkins : Scientific Assistant to the JET Director Acting Head of Publications
Dr. P. Thomas : Head of Experimental Division Acting Head of Torus and Measurements Department

During EECM (From Dec. 6, 1999) transferred membership to
Dr. J. Pamla : EFDA Associate Leader for JET
Dr. S. Clement-Lorenzo : DG Research, CEE
Dr. M. Watkins : EFDA-JET CSU An additional alternate member will be nominated by UKAEA

Dr. K. M. Young : Head of International Collaborations Division, PPPL
Dr. J. Willis : Director, Research Division, Office of Fusion Energy Sciences, DOE
Dr. N. Sauthoff : Head of Off-site Research Department, PPPL
Dr. E. Oktay : Office of Fusion Energy Sciences, DOE

Dr. H. Ninomiya : Deputy Director, Department of Fusion Plasma Research
Dr. M. Kikuchi : General Manager, Tokamak Program Division Department of Fusion Plasma Research
Dr. M. Nagami : Senior Staff, Office of Planning
Dr. R. Yoshino : General Manager, Large Tokamak Experiment Division II Department of Fusion Plasma Research

2. Personnel Assignments, Workshops and Executive Committee meeting

The total number of personnel assignments in 1999 was forty-eight (TFTR<-->JET (19), JET<-->JT-60 (7) and TFTR<-->JT-60 (22)), seven of which were long-term exchanges over one month. The major activities are as follows: collaborations on transport and MHD studies (52%); heating and divertor plate technology (12.5%); steady-state operation including plasma control technology (12.5%); diagnostics for current profile and scrape-off-layer measurements (12.5%); and plasma radiation (10.5%).
Two workshops were held: "Advances towards Steady-state of Regimes with Internal Transport Barriers"(32 participants) and "Collaborative exploitation of fusion facilities" (30 participants).
The Fourteenth Executive Committee (EC) meeting was held at PPPL in June 1999. The coordinated assignments in the year as well as the annual strategic work program were reviewed in this Meeting. In December 1999, an Extraordinary Executive Committee meeting was held at JET in order to discuss and implement IEA collaborative issues emanating from the transition from the JET Joint Undertaking to EFDA and to finalize the committee's recommendation on the extension of the Agreement.

3. Collaborative activities and achievements

Seven Task assignment Programs have been conducted intensively as follows.

(1) Research on High- and related modes of operation
Research on high- modes, including reversed-shear or optimized-shear plasmas with internal transport barriers (ITBs), has continued in JT-60U and JET.
In JT-60U, the plasma "jog" technique was used to measure detailed temperature profiles at the ITB in experiments with on-site participation by scientists of PPPL. Collaboration continues with PPPL on MHD topics including the barrier-localized, edge-localized and resistive wall modes and thermal energy loss at mini-collapses. The time dependent analysis code TRANSP was introduced to evaluate the ExB shearing rate and a study of the correlation length with respect to the ITB formation was continued. A TOPICS/ TRANSP benchmark test was carried out and the JT-60 data were tested against the IFS-PPPL, Multi-Mode and RLWB models.
An extensive investigation of optimized shear plasmas has been conducted in JET with the MkII-Gas Box divertor. The effects of heating method and timing on the q profile and ITB formation have been studied. At low field, 2.5T, substitution experiments demonstrated that centrally deposited ICRF and NB heating were equivalent in their ability to produce ITBs. By optimizing the heating waveforms and "seeding" the plasma with argon to produce edge radiation, wide ITBs were produced with H89P = 3, = 2.5 at 2.5MA/2.5T and H89P = 2.3, = 1.95 at 3.4MA/3.4T yielding up to 4x1016 DD neutrons/s. Scientists of PPPL participated on-site in some of these experiments.

(2) Disruption studies
Disruptions observed for reversed or optimized magnetic shear plasmas with ITB should be investigated further from a view point ofˇˇ"Disruption Avoidance" as discussed in Workshop W41. In JT-60U the maximum ion temperature gradient just before the major disruption increases with the decrease in qmin. At qmin ~2, hard disruptions occur even at low . The same phenomenon has been also observed at qmin ~3.
Generation of a halo current spike when the safety factor at the plasma surface, qs, decreases to around 1 has been clarified in JT-60U from the detailed equilibrium analysis including the halo currents. The same halo current spike has been also observed in JET.
Avoidance of runaway electrons in macro-scale magnetic turbulence has been demonstrated in the experiments in JT-60U and analyzed by numerical simulation. Spontaneous and intrinsic termination of runaway current has been demonstrated in JT-60U when qs decreases to ~3 or ~2 by the controlled horizontal plasma shift, the VDE, and the plasma current ramp-up. The low-n kink mode has a possibility to cause this termination. Toroidally asymmetric deformation of the magnetic fluxes under the low-n kink mode may degrade the confinement of high-energy electrons (several tens of MeV). This degradation is now under investigation in JT-60 and PPPL.

(3) Divertor Plate Technology
As part of the continuing cooperation between JET and JAERI, tests were set up to investigate further the 'hypervapotron' cooling technique which shows promise for future Divertor Plate designs.
Fatigue cycling tests were performed on two JET hypervapotron elements using electron beams at the JAERI Electron Beam Irradiation System (JEBIS) in April 1999. The objective of the tests was:
a) to check the survivability of the existing aged components (standard JET hypervapotron design), and
b) to check the adequacy of the new hypervapotron design for higher power.
Two test samples were fabricated from existing elements, which already experienced > 15000 full stress cycles. They were exposed to electron beams with peak power density 30% above the design value (i.e. up to 15 MW/m2), and survived 10000 (standard design) and 7000 (new design) stress cycles without any signs of plastic deformation or surface cracks.
The results of the fatigue tests confirmed that the beam stopping elements presently used at JET could be used with confidence for several more years. The hypervapotron technique can now be considered to be validated on production samples at the 15 MW/m2 level.

(4) Neutral Beam Current Drive Research
During 1999 the PPPL/JAERI negative ion neutral beam (NNB) collaboration continued its course of study to understand the physical processes limiting the performance of this new technology and to find workable ways to alleviate these problems. We have reduced the fraction of power hitting the accelerator and ground grids by more than a third, which is gradually allowing longer beam pulses at higher powers. We have achieved good success in finding techniques to infer the source plasma non-uniformity. These techniques involved using the relative values of the arc currents flowing through the eight filament groups as reverse probes, measuring the beam itself with a scanning calorimeter etc. We have had partial success at understanding the non-uniformity as probably arising from the net longitudinal magnetic field arising from the arc current flowing through the filaments. We have reduced the non-uniformity by installing series resistors in the arc circuit so that the values of these resistors can be used to alter the impedance of each arc group. We have also reduced the non-uniformity by altering the relative connections of the filaments to make the connection length of the longitudinal field bumpier.
With participation in the experiment from PPPL, excitation conditions for Toroidal Alfvn Eigenmodes, fishbone-like burst modes and chirping modes were investigated in JT-60U with NNB injection. The PPPL participants analyzed chirping modes in detail with the HINST code and showed qualitative understandings of the frequency chirping. These new results were presented in the 6th IAEA Technical Committee Meeting on Energetic Particle in Fusion held at JAERI Naka on October 12-14, 1999.

(5) Impurity Content of Radiative Discharges
Noble gas (Ne, Ar and Kr) seeding experiments have been performed in ELMy H-mode and reversed shear plasmas in order to maintain the high enhancementˇˇfactor of the energy confinement with the detached divertor.
Pumping efficiency of the novel gas was improved in the W-shaped divertor with pumping, and Ar gas was mostly used.
Feedback technique of the gas puffing rate using bolometer signals as an actuator was demonstrated, in particular in the ELMy H-mode plasma, with the large radiation fraction (70-80% of net input power).
Impurity transport in the main plasma is analyzed both by IMPACT code (JAERI) and MIST code (PPPL), and the results are compared.
Data of Zeff and radiation loss from JT-60U will be added into the multi-machine database.

(6) Remote Participation in Experiments
The present data link between the U.S. and Japan includes a frame-relay line with 768 kbps capacity and an ISDN line with 128 kbps. Collaboration research between JT-60 and TFTR involves 5 continued and 2 new topics under the Task Proposal on the Exchange of Data and Information entitled "Remote participation in Experiments.
By using the Data Link System and the video conference capabilities, participants from both JAERI and PPPL were able to discuss experimental and analyzed results of JT-60 improved confinement plasma in July, August and October.

(7) Scaling of Access to ITB Plasmas
Scaling of access to ITBs issues were discussed at a Three Large Tokamak Workshop on: Advances towards steady-state regimes with Internal Transport Barriers (W41) held at JET from 21 to 24 June. JET results were reported on scaling of the dependence of access power versus magnetic field, on a scan of target safety factor at constant power, constant magnetic field and similar current profile and on the dependence with the power deposition profile. From TFTR, it has been shown in a comparative study of core and edge transport barrier dynamics of DIII-D and TFTR plasmas, that ExB shear effects on turbulence-induced transport play a dominant role in governing barrier dynamics. On JT-60U, the dependence of access power for ITBs in reversed shear scenarios has started focusing on magnetic field and plasma current dependence. Substantial analysis, in particular concerning the absorbed power (large orbit losses at low plasma current, large shine through at low density) is needed.
The discussions at the Tripartite Workshop were aimed at having some rationale on how to proceed in order to establish the required scaling laws for ITBs access power. It was agreed after long arguments to proceed in parallel along two main routes: The Physics Oriented Route based on an ExB shearing rate model and assuming that transport is ultimately of neo-classical nature and the Experimental Oriented Route based on strong reversed shear or weak shear configuration. The criteria can consist of an upper limit on the shear within a given radius (r/a < 0.3 for instance). Then these two scenarios can be tested on JET, JT-60U and other medium-size devices.
It was felt that the similarity experiments could be proposed in a year's time scale and that the best way to co-ordinate the efforts were likely through the IEA co-operation among the Three Large Tokamaks.

These Tasks will be reviewed in 2000 and the Committee will modify them, as necessary, to conform these Tasks to the current emphasis in the technical program.

II. Planned activities in 2000

The next ExC meeting will be held in June 2000 at JET. The coordinated assignments in the year as well as the annual strategic work program will be reviewed in this Meeting.

III. Issues related to the collaboration

1. Extension of the Agreement

The IEA Three Large Tokamak Agreement expires in January 2001. The Committee recognizes the significance of this Agreement in the fusion research and therefore recommends that the Agreement should be extended for another 5 years. Therefore the Committee recommends the Contracting Parties to work together to take the necessary extension procedure, noting the closure of TFTR and the change of the JET organization.