Annual Report (MS-Word)

Progress of Three Large Tokamak Cooperation

January to December 1993
Executive Committee

The cooperation activities and achievements accomplished in 1993 under the IEA three large tokamak cooperation programme are hereby described in this document.

1. Cooperation activities

The cooperation among three large tokamaks in 1993 was remarkably successful, and it did function very well. This year, Executive Committee is proud of the way they showed how the collaboration work has to be. It has to be deplored that previous collaboration activity was literally equivalent to the exchanges of personnel and arranging workshops. Fusion research activities had been so far machine-oriented, and it was difficult to define the common underlying physics. However in 1993, rudimentary task-assignment oriented programme was introduced in three major areas of the tokamak fusion research, of which collaborative work as a result, enhanced the plasma performance substantially and contributed much for deeper understanding of tokamak physics.

One is the high beta poloidal plasma collaboration mainly between JT-60 and TFTR. The primary objective of this joint experiment was to find common physics principles between high beta poloidal discharges of JT-60U and supershots of TFTR. Some of the discharging conditions of JT-60 were actually preprogrammed by the TFTR group in order to investigate the size scaling for high beta poloidal regime of JT-60U by varying the size of plasmas and by comparing the data with that from TFTR. The assumption was that if the beam-target interaction dominates the present fusion reaction, a well focused NBI on a small target plasma should be the best for maximum neutron production as well as for optimized reactor condition. The scaling of the stored energy, confinement properties neutron yield, and MHD properties were surveyed. It has been concluded that one could optimize the high beta plasma in the small volume discharges of 40-50 m3, and the result of collaborative work will be presented as a joint paper at the IAEA meeting in Madrid, 1994.

The second issue was the disruption studies, mainly at JET and JT-60. The principal activity is to merge the disruption data base of each device, which contain the same variables accumulated under the same criteria. Established data base was applied to compare the disruption dynamics between the two machines aimed at singling out crucial parameters such as the effect of ion core, vertical stabilization, amount of halo currents and creating stability diagrams. The active control of imminent disruptive instabilities by means of NB injection as well as the application of Disruption Control Windings at JT-60 and Stabilization Coils at JET will also be comparatively investigated in 1994.



The last issue is the development of divertor plate technology, involving JET and JT-60. Power handling in the next device, such as ITER, is proclaimed to be one of the crucial issues. However, adequately feasible divertor technology has not been established. In this programme, present brazing technology of Oxygen Free High Conductivity Copper on the Carbon Fiber Composite will be extended to the application of CuCrZr Vaportron base plates. Investigation on the optimum Brazing procedure as well as the measurement of burnout critical heat flux will be the principal area of work which each collaborating laboratories are involved. This year, a sample tile of Oxygen Free High Conductivity Copper on the Carbon Fiber Composite was produced at JT-60, and the beam bombardment test was carried out at JET.

As TFTR started the D-T experiment in late 1993, many prominent scientists from JET and JT-60 joined the tritium preparation work and the tokamak experiment.

Two workshops were held in 1993 as shown in Attachment 1: one at TFTR on Alpha Physics and Tritium Issues in Large Tokamaks on February 17-19 and the other at JET on Termination of High Performance Regimes on August 2-4. The number of personnel exchanges of which period exceeded four weeks was 12 as shown in Table 1, while 36 scientists participated in the review tours. Detailed information on the personnel exchanges are depicted in Attachment 2. Reports on these activities, written in Forms A, B and C, are also listed in Attachment 3.


Fig. 1 Yearly variations of the number of participants in the personnel exchanges and review tours since 1986. Solid and open squares represent the participants sent from JET and JT-60, respectively. Slanted open square and cross symbols represent numbers from TFTR and the total numbers.

As can be seen in figure 1, number of personnel exchanges are slightly increasing every year. However in 1993, participants in the review tours remarkably increased. This is mainly due to the task assignment arrangement and the D-T experiment at TFTR. Previously, the executive committee arranged two workshops on average. Therefore, workshops were held as usually seen in previous years.

2. Meeting of the Executive Committee

The eighth Executive Committee meeting took place at TFTR on May 25 and 26, 1992. The names of attendee are as follows: Drs. A. Gibson and J-P Poff* from JET, Drs. H. Kishimoto and M. Azumi from JT-60, Drs. R. J. Hawryluk and K. M. McGuire, H. Furth, P. Rutherford, D. M. Meade, K. M. Young, M. Williams and Mr. M. T. Browning from TFTR and Drs. S. A. Eckstrand, M. Johnson from U.S.D.O.E.

The Committee elected Dr. R. Hawryluk as the chairman until the next meeting. JT-60 also acted as the secretariat, represented by Dr. T. Fukuda. The plans for workshops and personnel exchanges for coming one and a half year were agreed upon. The recent activities of the FPCC and CRD were reported by Dr. A. Gibson on behalf of Mr. Yamada of IEA, based on the material: IEA / CERT / FP / CC / A ( 93 ) 1 / REV1. The Committee agreed to continue to seek benefits deriving from Non-Member country participation in its activities and to keep the FPCC informed. It was also reported that the FPCC will in turn discuss general priorities and options of the participation of Non-Member country, considering the Governing Board approval.

In order to make the mutual scientific and technical benefit that has been derived from the personnel assignments clear, the revision of FORM B was proposed. The minutes of the eighth Executive Committee meeting is attached to this report as Attachment 4, and the next meeting of the Executive Committee will be held at JET on May 9 and 10, 1994.

3. Reports of FPCC and CRD activities

The preparation of the manuscript for the review package, which will be pubulished as Collaboration in Energy Technology in 1990 - 93 is being undertaken. Announcement about the industrial sponsorship was distributed from the IEA secretariat. This issue will be discussed at the next executive Committee Meeting at JET.

4. Membership of the Executive Committee

The members of the Executive Committee are as shown in Appendix 1.

5. Status of the Three Large Tokamaks

As is described in detail in the succeeding section, TFTR proceeded with the full D-T experiment in late 1993. TFTR thereby set a world record of about 6.1 million watts of controlled D-T fusion power. The neutron production rate was also the world record of 2.3 x 1018 / s. As Secretary of Energy Hazel R. O' Leary remarked, this world record is indeed a great step in the development of fusion energy, and it is the most significant achievement in fusion energy in the past two decades. Significance of the tritium experiment is not only establishing the world records, but also the fact that it enables unprecedented studies of the plasma behavior expected in a reactor.

An important figure of merit for a fusion reactor is the fusion power density. Although TFTR is much smaller than a commercial fusion reactor, the power density in the core of the TFTR is comparable to that which will be required for such a power reactor. The TFTR results extend by a factor of almost two those attained in 1991 by JET which produced 1.7 million watts of fusion power from plasmas containing about 10% tritium. The new record is also more than 10 million times greater than was possible at the time TFTR was proposed in 1974. The result will certainly lead to the study of ignited plasmas in ITER.

The major issues of present research work at TFTR are the tritium transport and alpha particle heating. The first full D-T experiment in the world shall It is noteworthy that TFTR undertook the tritium operation after the long dedicated effort of delibrately difficult counteractions against the safety regulations in the U. S. Therefore, the first few weeks were spent sole for the tritium tracing experiment.

JET is pushing the modification work strongly. On the other hand, JET has sent several scientist to TFTR for the future full D-T experiment at JET. Also, in the field of power handling technology, which is intimately relevant to the ITER design studies, bombardment examination of the divertor tile manufacured at JT-60 was undertaken.

Upgraded JT-60 has also made significant progress in the fusion product and the establishment of high poloidal beta plasmas after the deliberate wall conditioning including the boronization. By the central heating of small bore plasmas, JT-60 has achieved the world record of D-D neutron reaction rate and the highest fusion product. It is also remarkable that some of the dicsharges at JT-60 during the high performance campaign were actually conducted by the TFTR group.

The status and major results of the three large tokamaks are summarized below:

I. JET

JET has been in the process of modification during the whole year of 1993. Failures of toroidal field coils deterred the modification work twice. However, the delay in the work plan was delibrately made minimum, and totally new control system, saddle coils and pellet injectors are being completed for the anticipated plasma operation in February 1994. The interior of the torus will have been completely rebuilt. The most significant change will be that JET will be equipped with a pumped divertor. The internal divertor coils will be capable of producing a wide range of bottom, single null X-point configurations with the scrape-off layer interacting in the divertor with either bottom, or side, inertially - cooled CFC (Carbon) dump plate tiles. The key programme area for 1994 will be to investigate, understand and optimize the performance of this pumped divertor, with a reactor relevant plasma.

Within the range of pumped divertor configurations there are many options to be explored: grad B drift direction; co and counter injection; bottom or side target plates; fat and slim plasmas; fueling techniques; gas target regimes etc. In addition, other configurations are possible (e.g. double null X-point) and two major-new systems are to be exploited: the high power (10 MW) Lower Hybrid system and the Saddle Coils for instability studies and disruption control. The ICRH system, which is essential to the programme, has undergone a complete change of configuration and will need to be recommissioned as will the Neutral Beam systems and any available pellet systems. If a second tritium experiment is to be carried out the Active Gas Handling System (AGHS) will have to be commissioned on load to JET.

  1. Status of 1993 Shutdown.
    Removal of all components was successfully completed on time. The manufacture and assembly of the four divertor coils and casings inside the vessel is completed. The installation of new target plates, cryopump, RF antenna, poloidal limiters and the LHCD system is in progress.

  2. Status of the Active Gas Handling System.
    The JET Active Gas Handling System (AGHS) is designed for the supply of tritium to JET and the processing of JET exhaust gasses. This system is unique in the world, it has facilities for gas chromatograph separation, for cryodistillation and for exhaust detritiation of a gas steam which might be generated by a major vacuum failure. It has a design daily separation capability of: 3.4 m3 of H2 (300g), 0.34 m3 of D2 (60g) and 0.11 m3 T2 (30g) with up to 10% impurity content (treated by cryoseparation and catalytic oxidation). In addition 100 litres /day of helium can be separated onto cryogenic activated charcoal, monitored and discharged.

    The capacity of the plant makes the system a relevant prototype for ITER. Evaluating its performance with actual tokamak exhaust gasses will be a very important task for JET.

    The system is now complete and commissioning trials are well advanced. Both the gas chromatograph and cryodistillation unit have demonstrated effective separation of H2 and D2, in the case of cyrodistillation the H2 produced was pure to 8 parts in 106 and the D2 to 3 parts in 104. The exhaust detritiation and impurity cracking (CH4) systems have also been successfully tested and a number of improvements are being implemented as a result of these tests. Good progress is being made towards obtaining the necessary regulatory consents and the first operation with tritium using ~ 0.1 g tritium residues from PTE1 is scheduled for 1994.

  3. Divertor Tile Technology
    A Base Programme aimed at assessing the Pumped Divertor and exploiting the installed equipment in a co-ordinated programme will be carried out using Carbon dump plates. Examination of the heat capacity on to the carbon plate was undertaken in collaboration with JT-60. This result will be extended to the divertor tile assessment experiment in 1994, in which important aspects of controlling the maximum dump plate power loading; impurity production; impurity retention for target & introduced (e.g. He, Ne) impurities; model validation; divertor geometry effects (to give data for a MKII divertor); effect of cryopump; gas target investigations are surveyed. Gas Target Regimes will also be investigated.

  4. Preparation of the experiments in 1994.
    High Current H-Mode discharges at 5 or possibly 6 MA will also be investigated, as a basis for scaling to ITER. These may well be the highest current H-modes produced before ITER comes into operation. The development of a Long Pulse, high performance H mode, of the type only sustainable in JET, would be the clearest evidence that the divertor physics issues had been understood and controlled. A challenging aim for this work would be to develop an H-mode at 3 MA with a duration of 30 s, temperatures of 10 keV and with a confinement enhancement of two over Goldston scaling.

    JET will also join the High , High bootstrap current H-Mode collaboration programme, which at the moment JT-60 and TFTR work togrther. JET offers unique proven facilities, within the European programme, to investigate this mode of operation which has been proposed as a basis for advanced Tokamak reactor concepts such as SSTR. On the basis of the present resultsm, JET will accommodate generation of 2 MA plasmas with bootstrap fraction of 70 % and sustain them until energy and particle confinement has reached equilibrium. The plasmas could also be extended to moderate q (at toroidal fields of ~ 1.4 T) to overlap the regime to be studied by the TPX project giving valuable information while that project is still in construction or even still in design. The further aim would be to maintain the confinement enhancement seen in the present ~ 1.5 MA, q ~ 10 discharges into the 3MA / 2.8T operation range, whilst maintaining the bootstrap fraction of over 50 %.

  5. ITER Related Issues
    Most of the programme areas proposed for 1994 are directly relevant to ITER. In this section a few specific areas with explicit ITER relevance are listed.

    Main Areas:
    (a) Controlled toroidal field ripple experiment.
    (b) Excitation and study of TAE modes using the Saddle Coils.
    (c) Fuelling Experiments using High Speed and Centrifuge Pellet Launchers (if available).
    (d) A possible burn control experiment using feedback from e.g. neutrons to ICRH power.
    (e) ICRH with high minority concentration.

    Physics Issues:
    A number of areas are identified where JET has the capability to make important contributions to physics issues which impact the ITER design.
    (a) Dimensionally similar scaling (* scaling) experiments with the following discharge types: (i) X-point L-mode; (ii) Elmy H-mode; (iii) enhanced H-mode.
    (b) Isotope Scaling for Elmy H-mode.
    (c) H-mode threshold studies for forward and reverse BT with the aim of producing a prediction for ITER.
    (d) MHD(2,1) mode stabilisation using ICRF.
    (e) It should be possible to use the saddle coils to study tearing modes, control island behaviour and to vary error fields in order to give an opportunity to understand and control instability behaviour.

  6. PTE 2 programme
    Objective of this programme is to produce a single D - T discharge with high fusion power ( ~ 10 MW ) and high Q ( ~ 0.7 - 1 ) about 1 s, supported by similar but longer duration high performance deuterium pulses. The experiment will demonstrate high Q large Fusion power production and will allow observation of the effect of introducing significant amounts of (2 MW) of a power, which will be centrally peaked, into the plasma. In addition carrying out a PTE 2 will give valuable experience for the final tritium phase of JET. It will greatly reduce the experimental time needed to establish full operation of the AGHS for this phase. It will increase confidence in proceeding to handle the larger (50 to 100 gram) quantities of tritium needed in that phase and it will ease and support the case to be made to the Regulatory Authorities.

II. JT-60

High power heating and current drive experiments with NB, IC and LH have progressed in JT-60U with successive wall conditionings such as boronization. Significantly improved plasma performances have been achieved in high discharges associated with H-mode. NB and LH current drive experiments have been made up to 3.5 MA. Construction of a high energy negative-ion NB system has been initiated to investigate beam-driven steady-state operations. Divertor physics and disruption control are other important issues of JT-60U.

(a) High- H-mode confinement
The high- enhanced confinement in high power NB heated plasmas has been improved further by the formation of multi-stage transport barriers. First improvement with a factor of h = 2.5 appeared with formation of the transport barrier in the core region of the plasma with discharges of BT = 4.4 T, Ip = 2.1 MA, R / a = 3.1 m / 0.7 m and = 1.7. Subsequent improvement was associated with H-mode, characterized by the edge barrier formation. The h-factor reached about 3.6 and the fusion triple product was increased up to nD (0)Ti (0) = 1.1 x 1021 keVsm- 3; nD(0) = 4.3 x 1019 m- 3, Ti(0) = 37 keV, = 0.68 s. The D-D neutron rate was 5.6 x 1016 / s. The occurrence of ELMs terminated this improved confinement phase.

(b) High-, high bootstrap current discharges
Current profile control with quasi-perpendicular and tangential beam injections allowed high normalized beta ( = 2.5 - 3.5 ) discharges with Ip = 0.4 MA and BT = 1.5 T. In the case of co-injection of tangential beams, the plasma current was driven fully non-inductively; approximately two-third by the beam driven current and the rest by the bootstrap current. Then the h-factor was around 1.7. High- discharges with longer duration were obtained with the co-tangential injection rather than the counter-tangential one.

(c) LH current drive
Operation of a new 48 x 4 multi-junction grill for LHCD has been initiated recently. Total injected power up to 8.3 MW was achieved by using both this grill and a 24 x 8 grill without hot spot phenomena on the divertor plates. In the preliminary experiments, the full current drive has been demonstrated up to 3.5 MA discharges with the current drive efficiency of 2.5 x 1019 A / m2 W. The current profile driven by LH waves becomes flatter at higher plasma current discharges.

(d) IC heating
Efficient sawtooth stabilization by the minority 2 wCI heating has been demonstrated in low q ( qeff = 3.5 ) and high density ( ~ 5 x 1019 m-3) discharges. The coupled power was 5 MW in the out-of-phase mode with a wide gap of 13 cm between the separatrix and the Faraday shield. The sawteeth-free period reached 1.5 sec with the IC heating alone and 1.7 sec with the combined IC and NB heating.

(e) Wall conditioning and divertor physics
A multiple feeding system of decaborone (B10H14) has been introduced for uniform boronization of the first wall. The recycling rate was reduced by a factor of 2 - 3 compared with that in the previous non-uniform boronization, and the low recycling rate was essential to achieve the above mentioned plasma performance.

It has been confirmed that the product of the averaged electron density and the safety factor ( x qeff) is the key parameter for divertor heat load, which decreases with increase in qeff.

(f) Disruption studies
Divertor discharges at qeff > 6 ( q95 > 5 ) are almost free from disruptions. The current decay time at disruption was typically longer than about 5 ms and could be prolonged with the increased peripheral plasma temperature.

III. TFTR

Until November 1993, TFTR group was devoted to the physics oriented work and was making final preparations for the D-T campaign. Pre D-T research on TFTR has emphasized optimization of performance in deuterium plasmas, transport studies and studies of energetic ion and fusion products physics.

Recent D-T experiment is hereby described below in the chronological order. The initial experiments in early December were with 2% tritium and 98% deuterium gas. Aout 60 shots with trace tritium, some with beams and some with gas puffing were performed.

On Thursday, December 9, high power D-T experiments on TFTR was initiated. Five experiments were performed. The first four experiments were with one tritium neutral beam source and varying numbers of deuterium sources. The total power was varied from 5 MW to 28 MW to evaluate the performance of the neutron detector and other diagnostics. The fifth shot was with 24 MW of power and a nearly "50-50" D-T mix. This experiments was conducted about 11:08pm. The reslt of this discharge is described in TABLE 2.

On Friday, December 10, the first set of high power D-T experiments was completed. Five additional high power experiments were conducted with different numbers of tritium and deuterium neutral beam sources. The programmatic milestone of 5 MW of fusion power this year was achived. The maximum fusion power was about 6 MW. Detailed information of this very discharge is shown in TABLE 3.

In addition, valuable data was obtained on the confinement of a D-T plasma, recycling of Hydrogenic species from the walls, and checkout diagnostics in a D-T environment. Preliminary indications are that: the confinement in a D-T plasma is somewhat better than in a deuterium comparison shot; deuterium wall recycling is important in determining the ratio of deuterium to tritium in the plasma core; enhanced alpha particle loss has not been observed as a function of fusion power in these experiments and nearly all diagnostics are functioning well in high power D-T experiments and are providing high quality data.

On Monday, December 13, a current scan was performed with one tritium neutral beam source to establish a baseline for the loss of alpha-particles to the escaping detectors.

On Tuesday, December 14th we performed an experimental proposal, which is concerned with tritium accounting and cleanup after full tritium experiments. The experiment ran very well and was completed after about 1 shift of running. TFTR is now waiting for the final regenerations of the beamlines to complete the tritium accounting and retention studies after this first phase of tritium experiments.

On Thursday, December 16th an experimental proposal to study alpha heating in D-T experiments was examined. The goal of this experimental proposal is to test the alpha-heating predictions by measuring the central Te. This is a difficult experiment because of the competing effects of isotope scaling. The result from Thursday was that the T-T discharges had 2 / 3 of the neutrons observed in a DT discharge. This would suggest that there is a considerable loss of deuterium recycling at the walls and that there is 30% deuterium in the core of a nominally T-T plasma.

The number of D-T shots to date is:

Trace tritium experiments (< 2 % mixtures) 59
Tritium gas puffing experiments 13
Tritium neutral beam experiments 27