Annual Report (MS-Word)

Progress of Cooperation among
The Large Tokamak Facilities

Annual Report (January to December 2001)
Executive Committee

I. Cooperative Activities

According to extension of the Implementing Agreement for another 5 years beginning from 15th January 2001, the Large Tokamak Collaboration has continued successfully in 2001, 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, 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 Plasma Research, JAERI, "Chairman until the next ECM"
Dr. M. Kikuchi : General Manager, Tokamak Program Division, Department of Fusion Plasma Research, JAERI
Dr. R. Yoshino : Senior Staff, Office of Planning, JAERI
Dr. K. Ushigusa : General Manager, Large Tokamak Experiment Division I, Department of Fusion Plasma Research, JAERI

2. Personnel Assignments, Workshops and Executive Committee meeting

The total number of personnel assignments in 2001 was fifty-nine (U.S.<-->JET (36), JET<-->JT-60 (6) and U.S.<-->JT-60 (17)), six of which were the long-term exchanges over four weeks. The major activities are as follows: collaborations on Transport/Confinement (21 assignments, 36%); Macroscopic Stability (5 assignments, 8%); Divertor and Plasma Boundary (7 assignments, 12%); Fast Particle and Current Drive (20 assignments, 34%); Tritium and Remote-Handling Technologies (6 assignments, 10%).
Two workshops were held: "Technical Plannning of Tokamak Experiments" (35 participants), and "Plasma Shaping" (30 participants).
The Sixteenth Executive Committee meeting was held at JT-60 for 31 May - 1 June 2001. 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
3.1 EU

During 2001, there was a single Experimental Campaign on JET from January to March. This comprised 58 days for Scientific and Technical experiments. Operation was with a lower, more ITER relevant vessel temperature (200 C rather than 320 C) and included a period of helium discharges.
The reduction of the temperature of the JET vessel from 320 C to 200 C reduced the temperature of the divertor tiles from about 220 C to 140 C. As a result the chemical erosion yields and the carbon ion line emission decreased by about 50% in the inner divertor but did not change in the outer divertor.
Due to the non-existence of chemical erosion processes with pure helium plasmas a significant decrease of the carbon impurity release was observed compared to similar deuterium plasmas. The power threshold for H-modes is about 50% higher in helium compared to deuterium. Energy confinement decreases by 15-20% compared with similar deuterium plasmas. Power deposition profiles on the divertor plates are broader by 30-80% in helium plasmas.
Long pulses with approximately 80% non-inductively driven current have been achieved. 40% of the total plasma current of 2MA consists of bootstrap current and another 40% of the current is driven by neutral beams and lower hybrid waves. Significant progress in the real time control of localised internal transport barriers was achieved by adjusting the heating power to the temperature gradient length as the barrier develops.
Initial experiments on the formation of runaway electron beams after plasma disruptions were carried out. Optimal conditions for the formation of such electron beams were established consistent with the studies carried out in other tokamaks.
Experiments have also been carried out for the first time in 4He plasmas with ICRF power applied at the third harmonic of 4He neutral beam ions, with the objective of creating a strong 4He tail for alpha particle studies. In the presence of a strong 4He tail with an effective temperature around 0.5 MeV, Alfvn eigenmodes, sawtooth stabilisation and H-modes induced by 4He heating were observed for the first time on JET.

3.2 US

Dramatic progress has been made in the US program on the key facilities DIII-D, NSTX and the C-MOD tokamak. On DIII-D the effort to reduce error fields has resulted in a plasma beta roughly twice the no wall limit. In addition the minimization of error fields has led to the sustainment of high performance plasmas at high density. These results point to the role of the plasma rotation damping by error field amplification as a key cause of plasma degradation in high performance regimes. On C-MOD, a surprising result of core internal transport barrier formation has been observed in plasmas with RF heating only (no neutral beam injection) and with steady sawtooth oscillations. On NSTX, strong progress in continuing in advancement of the plasma parameters above 1 MA plasma current. Effective electron heating has been observed with High harmonic fast wave, and H-mode regimes have been created with power levels much below the standard tokamak scaling prediction.

3.3 JT-60

JT-60 has made 13 weeks of extensive research operations from March to November in 2001. Research highlights are achievement of total number of shots 40000 in September, discovery of "current hole" in reversed shear discharge, successful EC heating - Te(0) up to 25keV, sustainment of high normalized =2.7 plasma for 7.4 second, 50% increase of fusion performance (ni(0)Ti(0)=3x1020keVsec/m3) for full non-inductive plasma, N-NBI injection power up to 6.3MW for 1.6 second, Electron Cyclotron injection power of 2.8MW for 2.4 second. These research achievements are strongly supported by the collaboration activities under the IEA Large Tokamak Agreement, especially on N-NBI development.
JT-60 research operation will be continued to CY2002 for 4 weeks especially on exploration of "current hole physics". We also plan to modify JT-60 to improve tokamak concept with respect to the economical and environmental aspect of the fusion reactor by developing high beta steady-state operation and by utilizing reduced activation ferritic steel as plasma facing components.

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. Substantial progress has been made on the analysis of access to ITBs and on new techniques to produce ITBs and high density plasmas at high triangularity. Significant activity from US collaborators has focused on analysis of these discharges via TRANSP, the NCLASS, and FULL codes. For JET, a model was developed to include the neoclassical poloidal rotation calculated by NCLASS and the bulk toroidal rotation velocity calculated by JETTO in the ExB shear damping of turbulence. It was found that the rotational shear damping is fairly weak in typical JET ITB plasmas, an observation that is similar to that found on JT-60U. Nevertheless, the reduced transport extends over a wide region and results in wide barriers. Exchanges to JET also focused on the confinement properties of pure helium plasmas during campaign C4. A number of US and Japanese collaborators also participated in working out the ExB shearing rate and turbulence linear growth rate for a range of JT-60U ITB plasmas throughout the year.
The full-wave analysis of correlation reflectometry was done for the ITB of JT-60U data. The results were presented by a scientist of PPPL in the APS meeting.
Confinement and transport issues were also investigated on JET via pellet fueling experiments for H-mode plasmas as a reactor relevant demonstration. Centrifuge accelerated pellets were injected from both the inner wall at speeds of 150 m/s and outside midplane resulting in density levels above the empirical density limit, however strong edge localized modes and tearing modes reduced the energy confinement at the highest densities. A new pellet spectrometer from PPPL was utilized in these experiments. Pellet fuelling had a major influence on ELM behavior. One observation was that the inner wall launched pellets caused the plasma to produce type III ELMs instead of strong type I ELMs. High upper triangularity configurations resulted in more benign type III ELMs and less of a decrease in confinement.
A number of activities between JT-60U, JET, and US in the transport area were the result of "Remote Participation". Remote participation activities covered the areas of (a) Confinement characteristics in ITB plasmas on JT-60U, (b) Tokamak transport model testing, and (c) Highly radiative regimes for power and particle control in improved core confinement plasmas.
A workshop on the effect of plasma shaping upon the confinement and stability was held at the JET facility from 25-27th June and it was attended by some 30 delegates from the EU, Japan and US.

4.2 Macroscopic Stability

Experimental results of RWM in JT-60U, associated with the current driven external kink mode, were discussed with scientists of PPPL. Effects of the flow on the polarization current term were investigated to clarify the trigger of NTM in JT-60U. Stabilizing effect of the rotation is consistent with the results of DIII-D. The results were reported at the EPS-2001 conference and at the Madeira meeting on MHD, Disruption and Control. Studies of JET shots 51845 and 51976 identified strong similarities with the observations on JT60-U for the beta-limiting MHD activity in optimized shear scenarios. These results were reported at the EPS-2001 conference by a US scientist. A dependence on the current density at the edge was also observed. This dependence was examined in an independent study and reported at the Madeira meeting on MHD, Disruption and Control.
An extensive range of NTM experiments have been performed on JET. This was part of an identity experiment with DIII-D on (2,1) thresholds. Low density low toroidal field discharges were run on JET in early 2001 which formed a suitable basis for these identity experiments. When a good match in non-dimensional terms was achieved similar normalised beta limits were found in JET and DIII-D. Further collaborative experiments between JET and DIII-D are planned in 2002 on the critical beta for (3,2) NTMs and on error field thresholds at high beta.
In early 2001 some dedicated disruption experiments were conducted on JET. A reliable regime, at low elongation (to avoid rapid VDEs), was developed to produce disruption induced runaways. It was found that the lower limits in q95 and toroidal field for inducing runaways are very similar to JT60-U. The quenching of disruptions with large helium puffs was also studied. It was found that this technique led to slower disruptions and induced on the machine disruptive forces as large as those of non-mitigated disruptions. In dedicated runaway electron prevention experiments, large He puffs proved effective. Collaborations are planned in 2002 with GA and LLNL on modelling disruptions.
MHD which limits performance in the reverse shear regime has been studied on JET. The most fundamental limit to performance, pressure driven disruptions, have been studied partly as a collaboration with PPPL. Other limits to performance are found to be due to n=1 double tearing modes. Other characteristics of the reverse shear regime are q>1 sawteeth and high frequency cascade modes - similar to those observed on JT-60U.

4.3 Divertor and Plasma Boundary

Helium exhaust studies performed in the JET 2000 experimental programme has been analysed by US collaborators. The He concentration is measured in the sub-divertor pumping plenum region with a novel species selective Penning gauge. At the same time, He-concentration measurements are made in the plasma edge and strike point region with a spectrometer viewing these regions. A global particle balance analysis has been done providing a time history of the wall loading which allows an estimation of the particle-induced desorption coefficient which governs the rate of change-over of the wall. This work is intended for publication for the forthcomimg PSI conference.
A scientist of MIT was involved in studying wall recycling and impurity fluxes by visible spectroscopy and neutral pressure measurements. This resulted in a contribution to the 2001 EPS conference. Another scientist of MIT was involved by further data analysis of recycling and neutral gas pressures properties in the divertor versus the main chamber and comparing these quantities with Alcator-C.
A scientist of DIII-D contributed to the Helium plasma programme in 2001. He is also involved in the analysis of the impurity release behaviour from the main chamber walls and divertor region based on visible and VUV spectroscopy and to compare the behaviour with that in deuterium discharge and also with similar observations made in a He-plasma programme in DIII-D previously. He contributed also to the analysis of the divertor radiation and detachment behaviour in He compared to D- operation in particular also from his previous He-experience in DIII-D. This work is intended for publication for the forthcomimg PSI conference.

4.4 Fast Particle and Current Drive

The effect of fast particles in JET has been studied in non-monotonic q-profiles. An important observation is the presence of so called cascade modes which chirp strongly in frequency. A theory of these modes, developed by US and EU collaborators, explains the frequency chirping and shows that these modes can be an effective diagnostic of the q-profile in a burning plasma experiment. Very fast 4He ions were produced by coupling ICRF at the third harmonic to 4He beam ions in a 4He plasma for the study of alpha physics.
Experimental advances in understanding fast ion transport induced by EPMs has been made on the JT-60U tokamak under collabolation of PPPL. In experiments with weak magnetic shear the strong redistribution of tangentially injected fast ions was observed from the broadening of the neutron profile during a high frequency burst mode. Further understanding of these modes and their extrapolation to burning plasma regimes is required. To achieve this, further information is needed on the internal structure of these modes and advances are also required in the modeling of the non-linear evolution of the wave-particle interaction taking into account continuum damping and coupling to Kinetic Alfvn waves.
To improve the spatial uniformity of the ion source on JT-60U N-NBI system, continuous efforts were devoted by JAERI and PPPL. Up to 8 seconds pulse with 2.3MW (the beam energy is 356keV) has been successfully injected into JT-60 plasma. N-NBCD experiment in high current high H-mode discharge has been continued. N-NB power of 5.7 MW with the beam energy of 402keV has been injected into 1.8MA high ELMy H-mode plasmas with =0.34, q95=4.1. Fully non-inductive plasma with IBS/Ip~0.5, nDoTio = 3x1020keVsm-3, HHy2=1.2, =2.4 (at 4T) has been demonstrated in this series of experiments.

4.5 Tritium and Remote-Handling Technologies

In each tokamak a greater percentage of the tritium fuelling is retained in the short-term than deuterium, throughout the periods of combined deuterium-tritium fuelling of the machines. The reason is believed to be isotope exchange of tritium in the first wall inventory (which prior to the DT experiments contains an amount of deuterium equivalent to many hundreds of times the plasma content). There is a dynamic exchange of the fuelling species between plasma and wall during the plasma discharge which reaches equilibrium in the plasma T:D ratio in 10-20 pulses, however in JET tritium continued to be retained at 40% of the input (compared to 12-15% for deuterium) throughout the DTE1 campaign. The configurations and operating history for JET and TFTR are quite different, but similarities in retention and clean-up behaviour are found that lead to information on the underlying mechanisms.
In both TFTR and JET, co-deposition of T (and D) with eroded wall material (principally carbon) is the main long-term T retention mechanism. In places the deposits become so thick that spalling to form flakes occurs. Flakes from both JET and TFTR have been analysed during the current year. The JET flakes come from the inner divertor, in areas shadowed from the plasma such as the ends of tiles and the louvres that protect the magnetic coil casing from radiated heat. TFTR flakes that have been analysed have come from the plasma-facing inner bumper limiter (tile KA7), and a non-plasma facing surface at a diagnostic penetration (on tile KC22). The D:C ratios in the TFTR flakes were in the range 0.11 to 0.25, whereas in the JET flakes were ~0.7 to 0.8. The vessel temperature, extent of interaction with the plasma, and discharge history all play a role in determining the stoichiometry of the deposits.
A paper was presented from the collaboration in EPS Conf in 2001.
By means of the Nd:YAG laser system developed at PPPL the relation between the detritiation rate and laser power was studied by a scientist of JAERI using TFTR graphite tiles with carbon co-deposited layers. The result was presented at the tritium conference in Tsukuba, Japan.

II. Planned activities in 2002

Exchanges for the first half of the year will be proposed and agreed electronically by January 2002.
In 2002, three workshops are planned. Those titles, planned dates and locations are as follows: "ELMs, June '02 (after EPS), JET", "Real Time Control of ITB Discharges Approaching Steady State, Spring '02, JT-60", and "Electron Transport, Spring '02 (after TTF), U.S.".
According to the 5 years extension of this Implementing Agreement beginning from 15th January 2001, the next Executive Committee meeting will be held in June 2002 at U.S.. The coordinated assignments in the year as well as the annual strategic work program will be reviewed in this Meeting.
Planning of activities in 2003 will be also discussed in this meeting.