Annual Report (MS-Word)
I. Task Assignment Programs
Collaborative program in 1997 was coordinated in practice with emphasis placed on the task assignment programs for the data and information exchange. The six task assignments were continued to produce the significant results.
(1) In the task of the high research, TFTR and JT-60 continued the enhancement of the plasma performance by avoiding the MHD activities. All of three large tokamaks extended their operation regime to the reversed shear (or optimized shear) plasmas and reported the achievement of internal transport barrier.
(2) In the task of the disruption studies, plasma behaviours in the phase of high performance plasma termination were investigated in details in all tokamaks and it was clarified that the further characterization of disruption phenomena is necessary for the advanced tokamak scenario. Researches on haro current asymmetry and the avoidance of electron runaway generation progressed.
(3) In the task of the divertor plate technology, valuable technical information of the results on heating test of CVD-W coated on copper mock-ups and on high heat flux Beryllium samples and the Deuterium implantation in these samples was exchanged between JT-60 and JET.
(4) In the task of the neutral beam current drive research, a basic study of NNB current drive was carried out, which demonstrated that the total driven current and current density profiles were consistent with the theoretical predictions. The TAE mode driven by NNB was also confirmed in consistent with the theory.
(5) In the task of the remote participation experiments, after the last year's demonstration among JT-60, TFTR and LANL, the guideline in JT-60 experiments were discussed at the executive committee meeting and according to this guideline the 9 subjects of the remote collaboration has started from November between JT-60 and TFTR. TFTR is continuing the Internet-based joint code development projects, working off a sheared source repository on a PPPL file server. JET and TEXTOR groups in Europe are participating in these projects.
(6) In the task of the impurity content of radiation discharges, the experimental scaling law of the effective charge number of a plasma is now in construction mainly based on the JET data. TFTR supershot data have been already added to this database and the new data of JT-60U pumped divertor discharges will be a valuable addition to this database soon.
II. Tripartite Workshops
Four workshops were held in 1997 under the Implementing Agreement.
(1) "TAE Modes and Energetic Particle Physics" was held at JAERI Naka on February 25-27. A lot of new and interesting results on experimental and theoretical studies on the Alfven eigenmodes and other important issues on energetic particle physics in large tokamaks were reported and detailed discussions were made.
(2) "Nonlinear MHD and Extended MHD" was held at the University of Wisconsin on April 30 - May 2. Nonlinear MHD simulations which provide valuable insights and understandings of the various MHD phenomena in tokamak plasmas were presented. Extended-MHD models such as a fluid moments formulation and/or a particle/fluid hybrid model were also discussed.
(3) "Performance Optimization with Current Profile Control" and (4) "Termination of High Performance Regimes and Disruptions" were jointly held as "High Performance Regimes" at JET Joint Undertaking on June 16-19. Presentations and discussions were given on termination events such as disruptions and limitations and sustainment of the high performance regimes including optimised shear and high beta plasmas.
III. Personnel Assignments
The number of personnel exchanges of which exceeded four weeks was 3, while 30 scientists participated in the review tours. The long term visiting research scientists participated in experiments and data analysis and contributed to the key experimental campaigns of each tokamak. The short term exchange of scientists and engineers were very effective for the exchange of detailed information and the reflection to their own projects.
IV. Meeting of the Executive Committee
The twelfth Executive Committee meeting took place at JET on July 22 and 23, 1997. The coordinated assignments in the previous year as well as the annual strategic work program were reviewed. Proposed workshops and personnel assignments for the coming year were also discussed and authorized.
The major activity at JET in 1997 has been the planning, preparation and execution of the DTE1 series of Deuterium-Tritium experiments.
The DTE1 experiments required extensive technical and procedural preparations and used the AGHS(Active Gas Handling System), a close coupled tritium processing plant, to repeatedly reprocess and recycle the 20 g of tritium on site to supply about 100 g of tritium to the tokamak. The preparation and execution of DTE1 was aided by visiting scientists and engineers under the IEA agreement, there were 5 visits from PPPL (USA) and 5 visits from JT-60 (Japan).
The DTE1 series of experiments has led to new world records for fusion power production (16.1 MW) ; fusion energy (22 MJ) ; fusion duration (4 MW for ~4 s) and fusion Q (~0.8 for 0.4 s). Highlights of the experiments are summarised below.
A) Important Results for ITER: (1) Direct energy confinement scaling from JET suggests only a small effect on predicted for ITER and significant (20 to 30%) reductions in threshold power for ITER which should increase operational flexibility. (2) Three important ICRF heating schemes have been shown to be effective in D-T plasmas: CD; 2CT and CT (He3). (3) There are clear evidences of alpha particle heating and TAE modes driven by high energy RF tails, but no alpha particle driven modes in D-T plasmas.
B) Fusion Performance: (1) 8.5 MW of fusion power has been obtained in optimised shear operation. (2) Steady state ELMy H-mode has now been demonstrated in ITER geometry in D-T. A particular JET pulse has, Pfuse=4 MW steady for 3.5 s with Wfuse=22 MJ and Q=Wfuse/Win=0.2 for 3.5 s or eight energy replacement times. (3) Hot-Ion ELM Free H-mode is the highest performance mode in JET and produces : Pfuse=16 MW(> 10 MW for 0.7 s) and Wfuse=14 MJ with Pfuse/Pabs=0.66 for ~0.15 s.
Major efforts of JT-60 are focused on the physics of the new divertor, improvement of confinement and exploration of steady state operation.
The experiment with the new W-shaped pumped divertor started in June 1997. A quasi- steady state ( ~ 5 sec ) helium exhaust was demonstrated by using helium NB with 60 keV of beam energy and 1.4 MW of power. The ratio of effective helium confinement time to the energy confinement time, He*/, was ~4, which satisfies the ITER helium exhaust requirement of He/ < ~10. A high enrichment factor of helium in the divertor of ~1 was obtained. A long sustainment of ELMy H-mode discharge for 9 seconds with H-factor of ~1.7 and normalized beta of 1.4 ~ 1.8 was demonstrated with a large total NB heating energy of 203 MJ, without any serious increase in the production of carbon impurity. This is probably due to the effects of W-shaped divertor geometry and the particle pumping. Before the divertor modification, the total NB heating energy was limited for ~3 seconds and ~70 MJ.
In the reverse shear (RS) experiment, quasi-steady state ELMy H-mode was achieved by step-down of NB power after the formation of internal transport barrier (ITB) at the plasma current flat-top phase to avoid of reaching the beta limit. The RS ELMy H-mode with H-factor of ~2.4 and ~ 1.7-1.9 was maintained for ~2 seconds.
With 350 keV and 3 MW of N-NB injection, TAE like mode was investigated. The frequency of 50 ~ 100 kHz with toroidal mode number of 1 ~ 2 was obtained with a relatively small fast ion beta value of 0.2%.
Eight research scientists and engineers joined in the JT-60 experiments for negative ion beam physics, TAE mode physics and transport studies, and made the integrated researches in comparison with TFTR and JET results. During August and September, 1997, PPPL scientists participated in collaborative experiments on reversed-shear plasmas, with two scientists on site at JT-60 and others off site at PPPL.
Experiments were conducted on TFTR from January to April 4, 1997 when TFTR was shut down. The tokamak is now being decommissioned in preparation for eventual removal and disposal.
Measurements were made of the plasma flow characteristics accompanying the formation of transport barriers during neutral beam heating of plasmas with reversed magnetic shear. Detailed spatial and temporal data were obtained in plasmas which underwent transitions to the Enhanced Reversed Shear (ERS) mode, showing the occurrence of localized, transient precursors in the poloidal flow velocity in the region where steep gradients in the density evolve.
The high-li technique was integrated with two other elements of improved tokamak operation. First, the confinement was improved by reducing limiter recycling using new methods of coating the limiter surface with lithium. Second, the peak power flux to the limiter surface was reduced by creating a radiating boundary layer, or mantle, around the plasma through controlled injection of impurities.
The new system for limiter coating (DOLLOP) supplied a quasi-continuous stream of lithium droplets to the edge plasma by directing a repetitively pulsed laser onto a molten lithium target. Its use contributed to suppressing the particle influx from the limiter below the level produced by standard lithium pellet conditioning alone. Use of DOLLOP reduced the Zeff, to as low as 1.3, and improved the confinement.
The radiative mantle was formed in the edge region by injecting krypton or diluted xenon (5% in deuterium) gas into the plasma during neutral beam heating. With appropriate control of the radiated power level, the plasmas did not suffer deleterious thermally induced influxes from the limiter for the pulse duration of the NBI (1s). In a plasma combining the high-li startup with DOLLOP conditioning and the radiative mantle at the highest D-T NBI power, a peak fusion power of 7.7 MW was produced.