Annual Report (MS-Word)

Progress of Cooperation among
The Three Large Tokamak Facilities

Annual Report (January to December 2000)
Executive Committee

I. Cooperative Activities

The Three Large Tokamak Collaboration has continued successfully in 2000, with active personnel and task assignment programs. The Implementing Agreement functioned efficiently to promote improved understanding of tokamak physics and development of fusion technologies.

1. Executive Committe Members

Dr. J. Pamla : EFDA Associate Leader for JET
Dr. S. Clement-Lorenzo : DG Research, CEE
Dr. M. Watkins : Head of Program, EFDA-JET CSU
Dr. D. Robinson : Director, EURATOM-UKAEA Association

Dr. K. M. Young : Head of International Collaborations Division, PPPL
Dr. E. Oktay : Office of Fusion Energy Sciences, DOE
Dr. N. Sauthoff : Head of Off-site Research Department, PPPL
Dr. J. Willis : Director, Research Division, 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. R. Yoshino : Senior Staff, Office of Planning
Dr. K. Ushigusa : General Manager, Large Tokamak Experiment Division I Department of Fusion Plasma Research

2. Personnel Assignments, Workshops and Executive Committee meeting

The total number of personnel assignments in 2000 was fifty (TFTR<-->JET (21), JET<-->JT-60 (13) and TFTR<-->JT-60 (16)), six of which were long-term exchanges over four weeks. The major activities are as follows: collaborations on transport and MHD studies (46%); technology development (plasma control, diagnostics, material and tritium handling) (22%); heating, current drive and fueling (18%); radiative divertor (14%).
Three workshops were held: "Transport barriers at edge and core" (59 participants), "Fueling: Core and edge density control" (19 participants), and Diagnostics for burning plasma experiments" (40 participants).
The Fifteenth Executive Committee meeting was held at JET in May 2000. The coordinated assignments in the year were reviewed, and the annual strategic work program and extension of implementing agreement were discussed in this Meeting.

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.
Since the last report on this task, JT-60 has achieved a highest beam driven current efficiency, IpneR/PNBI of 1.55 x 1019 A/W/m2 using N-NBI in a high- H-mode plasma with a high electron temperature (13 keV) produced by ECH, and a fully non-inductive current drive in a high-confinement and high-density regime (HH 1.4, n/nG 0.8) in a reversed shear plasma using N-NBI and ECH. The ExB shearing rate analysis for several JT-60U RS plasmas was done by scientists of PPPL, who made important contributions to a JT-60U paper presented at the 18th IAEA conference in October. To improve understanding of X-mode reflectometer data on the JT-60U RS plasmas, simulations were developed to elucidate the roles of reflected and scattered waves. These simulations, which were reported in the 42nd APS meeting in October, were carried out with on-site participation by scientists of PPPL. Tokamak transport model testing, in which the 'stiffness' of temperature profile is carefully studied, has been carried out by using JT-60U ELMy H-mode plasmas, showing that the predictions of both the RLWB and IFS/PPPL models generally agree with the measured temperatures.
An extensive programme of JET experiments has been performed to study the properties of plasmas with negative central magnetic shear. The q-profile shape, heating power and applied torque have been varied in an investigation of the relative roles of magnetic shear and plasma flow shear in the internal transport barrier generation process. High fusion yields have been obtained (transiently up to 4 x 1016 DD neutrons per second) in discharges with 'deep' shear reversal, similar to previous JT-60U and TFTR experiments, using 21MW of additional heating power. In these conditions, the figure of merit H reached about 7 and 1.2 thereby providing a substantial neo-classical bootstrap current fraction. Investigations of the impurity transport properties and long pulse potential of this regime are continuing.

(2) Disruption studies
Disruptions observed for reversed or optimized magnetic shear plasmas with ITB has been investigated further from a view point of "Disruption Avoidance" and "Steady state operation". In JT-60U, large pressure gradient near ITB and finite edge plasma current reduces ideal stability limit significantly when the minimum safety factor, qmin, and/or the safety factor near the plasma edge, q*, is close to integer values. Resistive instabilities such as resistive interchange modes and tearing modes are also observed as instabilities which can lead to disruption or hard collapse.
Study of external kink and RWM near the beta collapse has been initiated as a joint effort by JAERI and PPPL.
Spontaneous termination of runaway current which is generated at disruption is observed in JT-60U during the vertical plasma displacement event when the safety factor at the plasma surface, qs, decreases to be qs ~2. It is confirmed that runaway current termination starts with appearance of spike-like magnetic fluctuations. Growth rates of the spike-like perturbed magnetic field decrease by an order of magnitude during the termination of runaway current. When magnetic perturbations with a slow growth rate appears, runaway current decays, and heat flux pulses are generated.
Halo current is generated after the runaway termination and reaches the maximum level at qs ~1 while it is small during the runaway termination. Study of the vertical plasma displacement event and halo current has been also proceeded in JET, as well as the analysis of the possibility of improving the sensor for vertical control in order to reduce the effects of ELMs.

(3) Divertor Plate Technology
As a part of continuing co-operation between JET and JAERI, tests were carried out to investigate the suitability of unidirectional Carbon-Fibre-Composite (CFC) material for the measurement of the neutral beam power density distribution.
This technique (which has been used at JET in the last six years) is based on the unique thermal properties of the unidirectional CFC material, which allows accurate measurement of the beam pulse power density distribution from two infrared images recorded before and a few seconds after the beam pulse.
The tests of the new unidirectional material manufactured in Japan (Toyo-Tanso) were performed using neutral beams of the JET Neutral Beam Test Bed in September 2000. The objectives of the tests were a) to check the adequacy of the new material for power density distribution measurement, and b) to check the influence of various experimental parameters on the accuracy of the measurement.
Test samples were exposed to high power beams with peak power densities in the range 5 - 70 MW/m2. Measured power densities agreed well with the data obtained using conventional calorimetry - the ratio between CFC and conventionally measured data is 0.990.03 over the entire range of power densities. The test confirmed that the new material is perfectly suitable for the measurement of 2D power density distribution with an accuracy better than 5%. This technique is being used for the diagnostic of neutral beams but can be also implemented in other areas, e.g. for measuring the power density distribution in the divertor region.

(4) Neutral Beam Current Drive Research
During this period, the work between PPPL and JAERI on the JT-60U negative ion sources was concentrated upon the task of improving the spatial uniformity of the plasma illuminating the extraction grid. Among many techniques tried, the most successful involved using exterior resistors of different values to offset the differences in the internal arc impedance among the eight filament groups. The experiment with operating the filaments emission-limited rather than space-charge-limited was also began to improve control over the primary electron energy in order to optimize negative ion production, and also to improve electron emission uniformity. Coupled with improvements from previous years under this task, the best deuterium acceleration efficiency has increased from 55% to 74%. Because this results in a reduction in the fraction of the power striking the grids, the reliability of the sources has increased, with many more 1.5 - 2 second shots at power sufficient for current drive experiments. These source results were presented in a poster and in a rapporteured talk at the 18th IAEA Fusion Energy Conference in Sorrento in October, 2000.
The JET and JT-60U neutral beams provide a significant non-inductive current drive, together with LHCD and the neo-classical bootstrap current. The large bootstrap current was combined with the neutral beam driven current to provide most of the plasma current for a few energy confinement times. The improved core confinement was also obtained.
The experiment of Toroidal Alfven Eigenmodes using the negative-ion-based neutral beam injection was carried out based on the experimental proposal made by collaborative discussions between PPPL and JAERI. Two PPPL scientists stayed in Naka for analyzing and discussing newly obtained experimental results. The result was analyzed in details with the HINST code by the data exchange through the Data Link System. Two collaborative papers were presented in the 18th IAEA Fusion Energy Conference.

(5) Impurity Content of Radiative Discharges
ELMy H-modes with argon seeding have been studied on JET in various plasma configurations: (i) with the X-point lying on the dome of the divertor in normal and high triangularity and (ii) with the vertical target configuration. For all these cases quasi-stationary phases of 5 to 8 energy confinement times have been obtained with an averaged value of n/nGR=0.9 (0.8 for the vertical target case), H97=1 and ~1.9 in the phase following the deuterium and argon puff (after-puff regime). The deuterium puff combined with argon seeding allows the density to be raised to close to the Greenwald level. During puffing a heavy puff leads to a deterioration of the energy confinement towards L-mode levels (H89<1.5) but after the switch-off of the gas puff the confinement returns to H-mode levels (H89=1.9) while the density remains near the value reached during the puff (e.g. n/nGR=0.95) and Zeff below 2. Without argon seeding the maximum density which could be reached was limited to 0.8 nGR. In the after-puff phase a small deuterium puff was added to refuel the discharge without confinement degradation in addition to a small amount of argon to maintain the radiated power fraction. The addition of argon in the deuterium puff changes the character of the ELMs from Type I to Type III. Type I ELMs are recovered in the after-puff phase but with a reduction in frequency and amplitude.

(6) Remote Participation in Experiments
During the Year 2000 the Remote Participation activities in the European Fusion Programme concentrated on the exploitation of the JET Facilities by the European Associated Laboratories. In this framework, remote data access to JET data for elaboration at the Associated Laboratories has steadily increased. In parallel the secure remote login to JET's computers has increased even more rapidly. This is used to run data analysis tasks remotely on JET's computer systems, for controlled access to the JET Facilities' internal web pages, and, most importantly, for following the experiments (e.g. via web broadcast of live control room computer screens). Internet-based teleconferencing has started to be used, in particular for distributed Task Force meetings, and, more recently, for joint Pulse Schedule preparation. Network monitoring tools are being deployed to monitor the necessary Internet connectivity. Most tools and infrastructure measures for Remote Participation of the European Associations in the exploitation of the JET Facilities are technically directly applicable also to collaboration between other fusion laboratories.
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. By using the Data Link System and the video conference capabilities, participants from both JAERI and PPPL were able to discuss experimental results and analyzed results of JT-60 improved confinement plasma in September.
The following research topics were carried out between JT-60U and PPPL through 'Remote Participation":

As a results of "Remote participation in Experiments", six joint papers were presented at the IAEA Fusion Energy Conference.

(7) Scaling of Access to ITB Plasmas
Substantial progress has been made on the analysis of access to ITBs and on new techniques to produced ITBs: negative shear targets on JET with LHCD pre-heat, ITBs with pure ECRH heating on JT60-U.
In JET, ITB access has been investigated for various conditions, including variation of magnetic shear, of magnetic field and of injected momentum. When strong shear reversal is obtained with LHCD-preheat, there is either no threshold or a very low one, even for ITBs with strongly improved confinement. This confirms results already observed on DIIID and TFTR. For low shear targets without LHCD preheat, ITBs are obtained with a clear power threshold scaling proportional to the magnetic field. Experiments with different momentum input for the same neutral beam power indicate a dependence on the toroidal rotation of threshold and performance which is being analysed.
In JT-60U, the threshold power for ITB formation in reversed shear plasmas was investigated aiming at studying (a) the dependence on plasma current (for a fixed toroidal field) and (b) the dependence on toroidal momentum input using co/counter injected NBI. The results are under analysis. In ECCD experiments, an electron ITB was found to be formed with off-axis co ECCD that caused a reversed shear profile. In this case, the absorbed power was quite low (1-1.5 MW) and Te/Ti was high. This is also in line with the observation that low power is needed to generate ITBs with negative shear target plasmas.
Continuing analysis of TFTR data reveals that access conditions to an ITB regime is highly dependent on configuration and not only on power and magnetic field. With balanced neutral beam injection with reverse shear, power requirements for entry into the Enhanced Reverse Shear regime vary in proportion to the toroidal field (B or B2) for a given edge q. As seen in JET, deepening of the shear reversal lowers the power threshold. Low power transitions occur in RS plasmas with qmin = 2 appearing in the discharge. Also, strong co-rotation yields an enhanced confinement state with reverse shear whose transport levels depend on the local ExB shear, not on the power itself. In these plasmas, no real power threshold is observed.

II. Planned activities in 2001

The next Executive Committee meeting will be held in June 2001 at JAERI. Exchanges for the first half of the year will be proposed and agreed electronically by January 2001. 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

This Agreement is of great value to all the Parties involved, and has therefore been amended as needed, and extended for another five years. Following the termination of the JET Joint Undertaking in December 1999, the Committee acted according to Article 10 of the Implementing Agreement upon the change of status of JET and re-iterates the statements in the Minutes of the extraordinary Executive Committee meeting of Dec. 6 and 7 1999 that "the European Contracting party is solely the European Atomic Energy Community (EURATOM); and that the countries referred to in the Implementing Agreement be, with respect to EURATOM, the countries of the member states of EURATOM and Switzerland".
At the extraordinary Executive Committee meeting in December 1999, the Committee, acting according to article 11 c) of the Agreement, proposed some amendments to the Contracting Parties. The extension of the Implementing Agreement for another 5 years from 15th January 2001 was agreed unanimously, pending the resolution of two issues with the respective authorities of the Contracting Parties. The representatives of the Contracting Parties resolved these issues at a meeting in Paris in January 2000, and through subsequent e-mail and telephone communications. Some textual amendments to the Agreement were approved at the Executive Committee meeting of May 2000, which are based on the recommendations of the representatives of the Contracting Parties.
The Committee also discussed the Multi-year Work Plan for the IEA Large Tokamak Agreement (see Appendix), where the new term of the agreement is January 15, 2001 to January 14, 2006. It was agreed that this should comprise the five Tasks (Transport/Confinement studies, Tokamak macroscopic stability, Divertor and plasma boundary studies, Fast particle and Current drive studies, and Tritium and remote-handling technologies), Personnel Assignments, Remote Participation and Workshops relevant to the Large Tokamak Programmes.


Plan of Activities to be carried out under the IEA Large Tokamak Agreement: 2001 - 2005

1. Introduction:
Three principal activities will be carried under the IEA Large Tokamak Agreement in the period 15 January 2001 to 14 January 2006. These activities are:

1) Carrying out research on the JET and JT-60U tokamaks for the further development of tokamak scenarios in preparation for the next step device, including coordination of experimental work.

2) Arranging for personnel exchanges and for remote data analysis and participation among the two facilities and the U.S.

3) Organizing workshops on key programmatic issues in collaboration with the other IEA Fusion Executive Committees.

The coordination of experimental work is the main purpose of the Agreement, the other two activities serving to facilitate this work, to disseminate results and to plan further studies. These activities are summarized briefly.

2. Description of Activities:
2.1 Tasks to be carried out under the IEA Large Tokamak Agreement (2001 - 2005)

The general aim of the first four Tasks is to participate in the further development of tokamak scenarios in preparation for the next step, in particular by studying ELMy H-mode plasmas near operational boundaries and by progressing toward steady state in several scenarios. Progress requires several physics issues to be addressed in an integrated manner. Design and planning of experiments on the Large Tokamaks will contribute to the data base for the next step. Development of related diagnostics should be also involved in their respective task areas, 1 to 4. Theory and modeling will support experimental activity. Furthermore, remote participation will enhance the effectiveness and efficiency of the collaboration through advances in communication.

The fifth Task will address issues of a more technological nature, such as tritium technologies and remote handling.

All these activities are key to the success of next step operation and exploitation.

2.1.1. Transport/Confinement Studies
This task includes transport/confinement physics related to transport barriers, and basic physics of modes such as reversed/optimized shear, H-mode and high-bp plasmas. ELMy H-mode plasmas near operational boundaries, particularly at high density, will be studied. The physics and technologies for steady-state operation, and scaling of access to these modes are also involved in this task.

2.1.2 Tokamak Macroscopic Stability
This task treats MHD instabilities such as bp-collapse, NTM and RWM, avoidance of which is key to obtaining and sustaining high performance discharges. Various macroscopic instabilities, which are related to ITB and ETB, are also studied. Physics and technologies for controlling these modes should be included. Disruption characterization, avoidance and mitigation are also important; suppression of runaway electrons due to disruptive phenomena should be included.

2.1.3. Divertor and Plasma Boundary Studies
Physics and technologies to understand and control heat and particle flow in the divertor/edge region, including reduction of the ELM amplitude and consistent with high performance core plasmas, should be studied. Study of enhanced radiation at the divertor/edge in the high density and improved confinement regimes is one of the most urgent issues. Particle fuelling and exhaust techniques should also be discussed. Plasma-wall interactions such as co-deposition and erosion will be important. This task should be directed towards the optimization of divertor design including plasma-facing material for the next step.

2.1.4. Fast Particle and Current Drive Studies
Confinement and transport of fast particles in high performance tokamak operation will be addressed. Alpha-particle physics should be studied, including simulation experiments with other sources of fast ions. Collective phenomena such as TAE modes and MHD instabilities linked with fast particles and the effect on current drive and heating efficiency are also important issues. Various heating and current drive physics and technologies including core current drive and local off-axis current profile control are also included in this task. Wave-particle coupling and optimization of different current-drive techniques should be carried out. Hardware developments will be a component of this task.

2.1.5. Tritium and Remote-Handling Technologies
Measurement of tritium on first-wall surfaces and studies of removal techniques will be considered. Decontamination, impurity processing and other tritium-handling activities will be included. Innovative techniques for remote handling of in-vessel components should be considered.

2.2. Personnel exchanges and remote data analysis and participation among JET and JT-60U and the U.S.

Assignments of staff to participate on-site at JET and JT-60U and within the U.S. will be arranged through this Agreement. Also data analysis off-site (at the home institutions) in support of the Tasks is also provided under the Agreement. Any such off-site analysis must be approved by the appropriate program management and by the Executive Committee before it can take place.

2.3. Workshops on key programmatic issues

Workshops (~ two per year) will be arranged by the Executive Committee in support of the Task areas listed above. Examples of such workshops might be a) Tokamak Plasma Shaping, b) Improvement of Confinement Performance in Large Tokamaks, c) Tritium Retention Issues.

These Workshops will be coordinated with the other Executive Committees under the auspices of the Fusion Power Co-ordinating Committee of the IEA and with conferences organized by other organizations.