Annual Report (MS-Word)
1. Executive Committee Members
Dr. J. Pamla : EFDA Associate Leader for JET
Dr. S. Clement-Lorenzo : DG Research, CEC
Dr. M. Watkins : Head of Program, EFDA-JET CSU
Dr. D. Robinson : Director, EURATOM-UKAEA Association
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. R. Stambaugh : Key Advisor of International Collaboration in U.S., GA
Dr. H. Ninomiya : Deputy Director, Department of Fusion Facilities, JAERI
Dr. M. Kikuchi : Deputy Director, Department of Fusion Plasma Research, JAERI
Dr. K. Ushigusa : Senior Staff, Office of Planning, JAERI
Dr. Y. Miura : General Manager, Large Tokamak Experiment and Diagnostics Division, Department of Fusion Plasma Research, JAERI
2. Personnel Assignments, Workshops and Executive Committee Meeting
The total number of personnel assignments carried out in 2002 was thirty-five (US.<-->JET (13), JET<-->JT-60 (10) and U.S.<-->JT-60 (12)), one of which was the long-term exchange over four weeks. The major activities are as follows: collaborations on Transport/Confinement (11 assignments, 31%); Macroscopic Stability (3 assignments, 9%); Divertor and Plasma Boundary (4 assignments, 11%); Fast Particle and Current Drive (12 assignments, 34%); Tritium and Remote-Handling Technologies (5 assignments, 14%).
Four workshops were held: "Real Time Control of ITB Discharges Approaching Steady State", "Electron Transport", "ELMs", and "Implementation of the ITPA Coordinated Research Recommendations".
The Seventeenth Executive Committee meeting was held at PPPL for 6-7 June 2002. The coordinated assignments in the year were reviewed, and the annual strategic work program was discussed in this Meeting.
3. Present Status of Each Parties
JT-60 has made 4 weeks of extensive research operations in 2002. As a large-sized tokamak equipped with a variety of devices for heating, current drive and profile control, JT-60 has high ability to address Advanced Tokamak scenarios and to approach the conditions required in reactors (ITER or demo): low values of normalized Larmor radius and collisionality, high toroidal field, high temperature with Te>~Ti, small central fueling, small ELM activities, etc. JT-60 experiments were also advanced toward high integrated performance.
N-NB and EC power increased up to 6.2 MW and 3 MW, respectively. A high H-mode plasma with full non-inductive current drive has been obtained at 1.8 MA and the fusion triple product reached 3.1x1020 m-3keV. High beta with =2.7 was maintained for 7.4 s. NTM suppression with EC was accomplished using a real-time feedback control system and improvement in was obtained. High confinement reversed shear plasmas with Te>Ti were obtained. Impurity accumulation related to strong ITBs in a reversed shear plasma and degradation of ITB by ECH in a weak positive shear plasma have been found. N-NB heating in an Ar-seed plasma extended the density region to 95% of Greenwald density with HHy2=0.9.
A stable existence of current hole was observed. The current profile control in high bootstrap current reversed shear plasmas was demonstrated using N-NB and LH. A new operation scenario has been established in which a plasma with high bootstrap current fraction and ITBs is produced without the use of OH coil. A new type of AE mode has been proposed and found to explain the observed frequency chirp quite well.
Following the divertor septum removal, the three Experimental Campaigns on JET allowed a wider range of plasma configurations. At high triangularity, quasi-steady ELMy H-mode discharges with reduced disruptive forces allowed levels of normalised confinement, density and plasma pressure reach, or exceed, those required for ITER, both with and without impurity seeding. A diagnostic optimised configuration allowed pedestal density and temperature profiles to be resolved for ELM studies. Profile stiffness and existence of critical normalised temperature gradient demonstrated by modulating mode conversion ICRF power in discharges with different levels NB heating. Near DN configurations established for Type II ELM studies. 50s divertor pulse developed for studying long time plasma and wall processes. NTMs shown to be meta-stable, their -limits can be raised by ICRF destabilisation of sawteeth. Disruption mitigation experiments showed large argon or neon puffs accelerate current decay and reduce vertical plasma movements and halo current vacuum vessel forces. Neutral point for argon induced disruptions identified and used to bench-mark plasma position control equilibrium models. Measured divertor thermal loads during JET disruptions could modify ITER assumptions. Both monotonic and non-monotonic q profiles controlled in real time using polarimetry for feedback of LHCD power in extended (up to 10s) prelude phase of advanced tokamak discharges. Wide ITBs at 3.65m produced at 2.8MA/3.45T. Clear link between rational q values and formation of ITBs established. High-density ITBs with Te~Ti obtained with pellets between pre-heat (strongly reversed current profile with LHCD) and high performance (NB heating). Alfvnic instabilities with non-monotonic q (Alfvn cascades) allow qmin to be determined. Alpha particle simulations with ICRF accelerated 4He NBI ions provide a new tool for fast particle and MHD studies. Erosion and redeposition studies with quartz micro-balance probes measuring pulse-to-pulse in entrance to inner divertor pumped duct show reduced co-deposition. In January/February 2003, the third Campaign of 2002 will resume following the replacement of a faulty rotary valve in one of the NBI systems. Five Campaigns will then address issues related to high performance, trace levels of tritium and reversed magnetic field with upgraded NB and diagnostic capability and a new pellet track and extruder. Two Campaigns with helium and hydrogen plasmas in early 2004 will be followed by a major shutdown to install further enhancements, notably an ITER-like antenna and further diagnostic improvements for both deuterium and D-T.
Three major U.S. experiments, i.e. DIII-D, C-MOD and NSTX, produced substantial results (extensively discussed at the FEC2002 conference in Lyon). DIII-D made progress in Advanced Tokamak scenarios obtained with off-axis ECCD current profile control and of interest to ITER. Other important results are feedback control of Te, and current profile, and stabilization of NTM, RWM and disruption mitigation with high pressure gas puff. Inter-machine comparisons on QH-discharges and TAE modes were made. C-MOD has been in maintenance most of the year; new results include: Internal Transport Barrier with off-axis ICRF heating; arrest of density increase with additional on-axis RF heating, EDA modes and plasma rotation without momentum input. NSTX operated for about 12 weeks, achieving 35 % toroidal beta. 5-Year Planning Workshop was held for U.S. Tokamaks (DIII-D and C-MOD), both of which focus on advanced tokamak (AT) research. The U.S. Community continued developing a BP Strategy through the Snowmass Meeting, Burning Plasma Strategy Panel and FESAC, reaching the conclusion that U.S. should join ITER as a full partner; if ITER does not happen, FIRE should be pursued internationally. National Research Council established a study committee to assess a program of burning plasma experiments. A progress report is expected in December 2002. FESAC and NRC recommendations are critical for Ray Orbach's decision on advocating U.S. participation in ITER negotiations.
4. Collaboration Activities and Achievements
Five Task assignment Programs have been conducted intensively as follows.
4.1 Research on Transport/Confinement
Research on reversed-shear and optimized shear plasmas with internal transport barriers (ITBs) has continued on JT-60U and JET. The ITB formation condition between JET and JT-60U was compared and transport/confinement of double transport barriers in JET and JT-60U were discussed. The newly found equilibrium of current hole at the extreme condition of reversed shear plasma has been studied in JT-60U and JET extensively. The comparison of the current hole has been begun.
Significant activity from US collaborators has continued with focusing on analysis of ITB discharges via TRANSP, NCLASS, FULL and GS2 codes. Theˇˇanalyzed results of JT-60U ITB plasma were presented at 19th IAEA Fusion Energy Conference by a US scientist.
A US collaborator participated in pellet fueling experiments in H-mode plasmas on JET to investigate the physics of high-density operation. A high resolution spectrometer for measuring spectra from pellet light emission was installed with the assistance of the US collaborator and used to collect initial data from centrifuge accelerated pellets injected from the inner wall at speeds of 150 m/s. These experiments provided new inner wall (HFS) pellet data for comparison with theory.
Two US collaborators have participated in Helium Transport experiments of JET. This work has been presented at the EPS and APS Meetings. A paper is in preparation for Nuclear Fusion. One of the collaborators has also begun the design of a new diagnostic for Helium Ash Detection in DT plasmas and will be available for JET-EP. This new diagnostic will be jointly funded from the US and JET.
During the past year several important confinement issues have been addressed both by inter-machine collaborative experiments and experiments on single devices. An example of inter-machine collaboration was the attempts mode on DIII-D and JET to reproduce the enhanced mode seen on C-MOD. It was found that by reproducing the edge dimensionless physics parameters of C-MOD one could indeed obtain discharges in the larger machines which had similar features to those of the enhanced mode.
The IEA-LT workshop on "Electron Transport" was held at Annapolis on 3-6 April 2002. The workshop was held within the U.S. Transport Task Force Meeting.
4.2 Macroscopic Stability
Collaboration has concentrated on studies related to neoclassical tearing modes (NTMs) and resistive wall modes (RWMs), and other modes, such as Edge Localized Modes (ELMs).
The study of NTMs now concentrates on comparison of the marginal beta limit for 3/2 and 2/1 NTMs, using power ramp-down experiments once the modes are triggered, as it has been seen that this method avoids the dependence on the mode trigger mechanism. In addition, the results of the 3/2 NTMs have been obtained on JT-60U and systematic studies of the 2/1 NTMs has started on JET, DIII-D (and ASDEX-UG), but need to be completed in 2003 to allow scalings to be extracted. In addition, studies of real time feedback control are continuing and DIII-D has demonstrated ECCD stabilization of the 2/1 mode for the first time. In JT-60U, the advanced real time control system, which consists of the detection of the island by ECE measurement, the equilibrium reconstruction and the injection of EC waves by a steerable mirror, has been developed and used successfully to stabilize NTMs. The recovery of without any additional heating has been obtained.
It has been seen, in particular in DIII-D and NSTX, that the effect of error fields on plasma rotation are crucial for resistive wall mode stabilization and long pulse scenarios at values of above the ideal no-wall limit. Therefore error field studies in large tokamaks and their effects at high are studied in more detail. However, feedback control of RWMs still needs to be demonstrated in plasmas with low rotation. Following discussions at the IEA-LT workshop on "Real Time Control of ITB Discharges Approaching Steady-state" combined with ITPA meetings and the Japan-US workshop in Naka in February 2002, similarity studies of RWMs on JET, DIII-D and AUG will commence in 2003. Comparisons of RWMs in DIII-D and JT-60U related to plasma rotation will also be pursued. ELMs in JT-60U were analyzed in collaboration with DIII-D.
4.3 Divertor and Plasma Boundary
The IEA-LT workshop on "ELMs" was held at JET to compare and discuss the ELM behaviour in the three large tokamaks, with 2 participants from JT60 and 3 from US side.
The effect of plasma drifts on the boundary plasma of JET, JT60-U, and DIII-D has been examined using the UEDGE code. The simulation results have been compared with detailed probe measurement, particularly on the JT60-U experiment. The results were presented at the 2002 PSI meeting. The simulated plasma parallel flow was reasonably consistent with experiment on the high field side of the plasma, but somewhat poorer on the top and low field side. Work is continuing to better understand the differences between measurement and simulation. In addition, a code benchmarking effort has been begun to make detailed comparison of simulations of a JET discharge with UEDGE, SOLPS, and EDGE2D.
The divertor experiments without the septum in JET was discussed between scientists of JT-60U and EU. The septum was removed from MKII-GB divertor to increase flexibility at high- and high-triangularity operation. Discussion was also done on the error field correction coils (the saddle coils and the in-vessel coils) and their power supply system. The information is very useful for design of the error field correction coils and the power supplies in the JT-60 modification program (JT-60SC).
An EU scientist calculated the soft-X ray emission rate coefficients for Kr and Xe. The rate coefficients were used for analysis of Kr and Xe transport at internal transport barriers in JT-60U reversed shear plasmas. By the analysis, it was found that high-z impurity accumulated inside the internal transport barriers.
The analysis on thermal transport and micro-instability characteristics of high-density H-mode plasmas with and without impurity puffing in JT-60U was done by a US scientist. These plasmas had a normal divertor configuration and a configuration with the outer divertor strike point on the dome. Mechanisms leading to improvement of confinement in Ar-puffed plasmas were investigated with TRANSP code and GS2 code. The results were presented at 19th IAEA Fusion Energy Conference.
4.4 Fast Particle and Current Drive
Experiments on JT-60U and JET have shown that plasma configurations with shear reversal are prone to the excitation of unusual Alfvn Eigenmodes by energetic particles. These modes emerge outside the TAE frequency gap, where one might expect them to be strongly damped. US scientists have worked with JET and JT-60U scientists to develop a theory that explains the key features of the observed unusual modes including their connection to TAE's as well as the modifications of TAE's themselves near the shear reversal point. Another new model of the reversed-shear-induced Alfvn Eigenmode (RSAE) provided by JT-60U and experiments with accurate q-profile measurement in JT-60U can explain fast frequency chirping Alfvn Eigenmodes (AEs) observed in reversed shear plasmas. By using neutron profile monitor, redistribution of fast ions in weak shear (WS) plasmas was observed when large bursting AEs were destabilized. These results were presented as a joint paper by JAERI and PPPL in the 19th IAEA Fusion Energy Conference. Another collaborative paper between JAERI and PPPL on NNB AEs was published in Nuclear Fusion. The effect of fast particles in JET has been studied in non-monotonic q-profiles, typical of Advanced Tokamak (AT) regimes. An important observation is the presence of so called Alfvn Cascades (ACs). A theory of these modes, developed by US and EU collaborators, explains their frequency chirping.
A major diagnostic initiative for a lost alpha detector on JET has been initiated this year. Detailed design of an energetic ion loss ("Lost Alpha") diagnostic was started. This work is being performed by PPPL and EU collaborators. The diagnostic is scheduled for installation in 2004.
Also on JET, US collaborators have identified experimental evidence for the existence of the theoretically predicted odd TAE from the simultaneous appearance of odd and even TAEs.
The NNB performance was progressed dramatically by research and development of the NNB ion source in collaboration between JAERI and PPPL. A 10 second injection of 2.6 MW at 360 keV was realized by modifying electrodes of the ion source, which improved the convergence of the beamlet near the edge of the electrodes.
4.5 Tritium and Remote-Handling Technologies
The main long-term tritium retention mechanism in large tokamaks, especially JET and TFTR, is the co-deposition of tritium (and deuterium) with eroded wall material (principally carbon). Temporary trapping of tritium of up to 60% has been observed.
The co-deposited layers can become so thick that spalling occurs and flakes are generated. The D:C ratios of TFTR flakes (coming from the plasma facing inner bumper limiter and non plasma facing components) were in the range between 0.11 to 0.25, far lower than the values of 0.7 to 0.8 determined at JET. The vessel temperature, extent of interaction with the plasma and discharge history all play a role in determining the stoichiometry of the deposits.
With a Nd-YAG laser system developed at TFTR detritiation tests of JET MkIIA divertor tiles were performed which were installed during the Deuterium-Tritium Experiment (DTE1) in 1997. Up to 87 % of the tritium in the sample was removed in a rapid scan over the surface with the laser, most of the tritium being released as tritiated hydrogen and the carbon based films remaining basically intact.
Further detritiation activities were done by scientists of EU, US, and JAERI involve the use of an oxygen methane flame, the application of specially developed PIN diodes to measure surface tritium contaminations, the construction of a special facility for the detritiation of flakes and the determination of the release rate and of the type of the released tritiated gas species as a function of temperature, the use of ozone for the detritiation of plastic and similar soft waste, etc.
Six TFTR D-T plasma facing tiles were delivered to JAERI, and their tritium analysis and detritiation study were started comparing JT-60 D-D tiles.
Systematic experiments on the behavior of tritium in JT-60U have started. Various cleaning techniques of the vacuum vessel were tested for removal of hydrogen isotopes. Studies on retention and removal of tritium using graphite tiles of the JT-60U were also conducted. Collaborative activities under this framework will be considered.
5. Workshop on "Implementation of the ITPA Coordinated Research Recommendations":
This IEA-LT workshop was held at MIT on 18-19 November 2002. In addition to the IEA-LT members, the leaders of major world tokamaks and the chair of Coordinating Committee of ITPA have participated to the workshop. The purpose was to discuss the implementation of the ITPA proposals that would benefit from coordination of joint experiments among the major world tokamaks. Such coordination would add great value to the experiments on individual facilities, and would require personnel and some hardware exchanges. The exchanges would be carried out by using the existing IEA-IAs on tokamaks, such as the 'Large Tokamak Agreement between the U.S., Japan, and Europe, and the 'Poloidal Divertor Agreement between the U.S. and Europe, which provide the legal framework scientific exchanges.
About thirteen participants at MIT discussed 40 ITPA experimental proposals. Eleven of these were well-developed for consideration for joint experiments on various tokamaks in the coming year. Most of the proposals involve 3 or more tokamaks. The tokamak leaders indicated that they would be interested in considering another 16 proposals for joint experiments if the proposals are further developed in time to be discussed in the 'research forums' of the major tokamaks in December to develop their experimental program plans for 2003. The remaining proposals were considered to be ongoing programmatic activities that have not yet developed plans for joint experiments.
This was indeed a unique and successful meeting that provided a productive opportunity for enhanced communication among the major world tokamak leaders, the ITPA leadership, and the IEA-IAs members. These discussions and implementation of joint experiments will enhance progress on the tokamak burning plasma physics issues, and contribute to the success of future burning plasma experiments such as ITER.