Annual Report (MS-Word)

Progress of Three Large Tokamak Cooperation

January to December 1995
Executive Committee

1. Cooperation activities

Enhancement of effectiveness and productivity through the coordination of collaborative program having been intrinsically prerequisite in the recent fusion research, the co-operation among three large tokamaks was strongly pushed forward also in 1995 to produce remarkable achievements, and hence the Implementing Agreement functioned efficiently to contribute much to resolve the tokamak physics and to respond to the ITER Physics R & D needs as well.

Progress of the large tokamak research in 1995 was significant, as represented by the equivalent QDT of around unity transiently and 0.7 steady at JET, sustainment of 1 MA full current drive condition for 2 s with a high triangularity discharge at JT-60 and achievement of the fusion product of 8.3 x 1020 m-3 keVsm-3 at TFTR. In addition, ITER physics R & D tasks were also robustly implemented in individual programs. Collaborative program in 1995 was thereby coordinated in practice with two major guidelines. One was to strengthen the task-assignment programs, which can integrate the main thrust of research activities of contracting parties, and the other was to include the urgent research issues imposed by the ITER Physics R & D requirements.

As to the task-assignment programs, which were strongly emphasized in the past years as the most effective means of coordinating the collaborative work, two proposals were newly approved by the Executive Committee. Namely, they are (1) neutral beam current drive research and (2) remote participation in experiments. The former was partly combined with the high beta-poloidal task-assignment, which has been continued since 1993 under the same framework and recently developed to one of the admittedly most promising schemes to achieve steady-state high performance discharges. Discussions between members of the JT-60U and TFTR groups on collaborative experiments using the newly installed negative ion neutral beam system have been held. Optimization of the current density and pressure profiles for the improvement of stability limit, as well as the confinement under the intensive electron heating will be the major issues of investigation. The latter was also undertaken to mark a remarkable milestone in the first year. One of the JT-60 diagnostics was remotely controlled from the States, and it was also confirmed that data file was correctly transferred. Presently, over 700 discharges produced at JT-60 are made available to be remotely accessed from JET and TFTR. In addition, feasibility of the rudimentary video conferencing system among the contracting parties was recently examined.

In regard to the three other task-assignments, all of which are carried over since 1993, also continue to produce significant results, respectively in the main areas of recent tokamak research. (3) Optimization and physics understandings of shear reversal discharges has been merged into the high beta-poloidal task-assignment, due to its intrinsic high-beta features. Joint experiment between JET and JT-60 was carried out in August at JT-60, where application of lower hybrid current drive on shear reversed plasmas was one of the main topics. Here, related experience at JET contributed much to produce and optimize the shear reversal plasmas at JT-60. Exchange of information in this area of research has also been vigorous among the contracting parties. (4) In order to identity the key elements to induce the termination of high performance discharges as well as to seek for the amelioration scheme of major disruptions, both of which are classified as urgent ITER Physics R & D issues, a workshop was organized at JET in July '95. Here, causality of disruptive instabilities were categorized for further analysis, and counteractions to avoid the vertical displacement events and large heat flux onto the divertor plates were discussed. (5) The burnout critical heat flux of the divertor test section produced at JT-60, which has unidirectional high conductivity carbon fiber composite tiles brazed on the Cr Zr Cu Hypervapotron heat sink supplied by JET, was examined at JET in January '95. The test section successfully accepted the power density of up to 30 MW / m2 in thermal equilibrium. Based on this encouraging result, present brazing technology will be extended to other materials for the thermal fatigue measurement at JET in 1996.

Many of the personnel exchanges were coordinated, concerning the potential contribution to the ITER Physics R & D. Divertor physics, alpha-particle induced instabilities, as well as the H-mode power threshold were the major issues of investigation in 1995. In particular, comparative studies of various puffing gasses to achieve remote radiative divertor cooling compatible with the improved confinement were emphasized to evoke active discussions between JET and JT-60. In addition, initial results from the JET pumped divertor experiment have provided precious information for the JT-60 divertor upgrading program.

2. Meeting activities and personnel assignments

One workshop was held in 1995 under the Implementing Agreement at JET on "Termination of High Performance Regimes and Disruptions" on July 10-12. The number of personnel exchanges of which period exceeded four weeks was one, while 26 scientists participated in the review tour. The number of personnel exchanges decreased in comparison with the previous year. However, number of participants in the review tour was as many as 1994, which is indicative of the increasing demand of co-operative work in spite of the various limitations.

3. Meeting of the Executive Committee

The ninth Executive Committee meeting took place at JT-60 on May 18 and 19, 1995. The coordinated assignments in the previous year as well as the annual strategic work program were reviewed, and proposed workshops and personnel assignments for the coming year were discussed and authorized. The Executive Committee agreed to proceed with the preparatory work for the remote participation in experiments, which was originally proposed by USDOE. In addition, mutual data transfer among the contracting parties by means of the computer communication was discussed. The recent activities of the FPCC and CRD were also reported.

4. Status of the Three Large Tokamaks


Effective operation of the new divertor has been obtained in all its design configurations i.e., the plasma current and the combined heating power respectively reached 6 MA and 32 MW. The experimental program proceeded as planned and addressed issues directly relevant to ITER in the correct geometry, with the right plasma parameters and in the relevant size range. The divertor has proved effective in handling the plasma exhaust heat load and so preventing the rapid impurity influx which previously terminated high performance pulses. Thus, with the new divertor configuration, operation with detached plasma and radiative power transfer in the divertor has been obtained. Here, it was found that nitrogen radiates more in the divertor than in the main plasma but leads to large values of Zeff at high power levels. At low powers argon can radiate 80% of the input power with Zeff = 2, but significant radiation occurs in the main plasma. Confinement is reduced with rapid ELMs to give H = 1.5, and this is true even with the heating power far above the H-mode threshold. Steady state operation has been obtained with ne, Zeff and Prad constant for 20 s at 2 MA and 9 s at 3 MA in ELMy H-mode discharges. Here, H-factor was two. In the high fusion performance campaign, nT within 10% of the JET best of 9 x 10 20 m- 3 keVsm-3 was obtained together with (1) the equivalent QDT around unity transiently (0.7 for 1 s and steady), (2) D-D neutrons equal to 4.7 x 1016 s -1 and the stored energy of 12 MJ. Here, a power step down technique was utilized, in order to delay the ELM and MHD activity as well as to obtain higher H-factor and reduce dWp / dt fraction. It was also found that high shear (which is accomplished by increased triangularity) and high flux expansion are effective ways to extend the ELM-free period. These high values are obtained despite the smaller plasma volume with the 'new' JET with divertor compared to the 'old' JET. Moreover, they are sustainable towards a steady state. Previously, this type of discharge ended with a roll-over of performance followed by a massive impurity influx. With the new divertor the impurity influx is prevented, but the performance roll-over still occurs. In addition, high values of > 3% have been maintained for more than 3 s in steady state, utilizing similar approaches as JT-60. Here, > 3, > 2 and 2.0 < H-factor < 2.2 at 1 MA / 1.7 T.

Nondimensional scaling studies of thermal diffusivity, carried out for steady state ELMy H-mode discharges, were closer to gyro-Bohm scaling. However, Bohm-like feature was observed at the heating power close the H-mode power threshold. Physics interpretation of this phenomena has been under discussion between JET and JT-60. Studies of TAE mode were also carried out by utilizing the saddle coil as exciter, and KAE was first observed at JET. Experiments to assess the effect of controlled toroidal field ripple have been carried out by powering the 32 TF coils in two sets of 16. It was found that a slightly increased ripple fraction can reduce the H-mode threshold power. For larger ripple rate, however, degradation of the H-mode quality was observed. The CFC target plate was replaced by Beryllium target of similar design in April '95, and it was found that (1) Beryllium tiles do not limit the power handling capabilities, (2) general plasma behaviors with beryllium was very similar to that with CFC target and (3) at high energy deposition the plate shows melt damage, and the protection by a vapor shield is not apparent.

II. JT-60

Substantial portion of the current JT-60 program is devoted to the contribution to the broad range of ITER Physics R&D. Major efforts are focused on the improvement of confinement and exploration of the steady-state discharge, where (1) achievement non-inductive full current drive with a high bootstrap-current fraction and (2) optimization of the divertor function to control the particle and heat flux are the principal issues of investigation. As to the former, installation of the negative-ion based 500 keV NBI system has progressed successfully, and preparatory work for the joint experiment with TFTR is in progress. The latter is represented by the pumped divertor modification program, a part of which engineering design has been completed and the manufacturing procedure has started in 1995.

The fusion performances of H-mode and high- H-mode discharges in JT-60 are in principle constrained by the edge density limit, which is determined by the appearance of ELMs. Optimization of the plasma configuration to make the ballooning instability benign, in particular the triangularity, was attempted to improve the edge MHD stability. The increase of edge density limit for the ELM-free discharges as well as improvement of beta limit have been accomplished with increased triangularity of < 0.45. Sustainment of 1 MA full current drive condition in high plasmas for 2 s was thereby achieved in high steady-state campaign in August '95. Here, albeit not optimized, = 2.3 - 2.8, = 2.4 - 3.1 and H-factor was 1.9 - 2.4. Further enhancement of the integrated fusion performances of quasi-steady-state high plasmas, as an alternative candidate to the low q long pulse scheme, are also discussed with JET, which produces similar discharges. In regard to the remote radiative cooling of divertor plasmas and the investigation of detached plasma characteristics, various species of working gasses were puffed in to the divertor region. It was found that neon puffing into the divertor was effective to reduce the power flow to the divertor plates without the deterioration of confinement properties. Chemical sputtering was found to be the dominant mechanism of carbon impurity generation at a high wall temperature of around 300 C. Water-cooled divertor operation has been successfully carried out, which apparently reduced the carbon impurity concentration in a plasma.

Exploitation of the high current regime was also undertaken in hot-ion H-mode discharges, where Ip was raised up to 4.5 MA. Here, 5 MW of ICRH power was coupled to the H-mode plasmas to successfully suppress sawteeth, and the stored energy of 8 MJ was obtained. However, the volume averaged fusion product was about half of the record value achieved in high H-mode plasmas, largely due to exceedingly broadened beam deposition profile and the effect of ripple induced fast ion losses. Instead, the capabilities of shear reversal discharges were examined later in the year, in which a JET scientist participated. Transition to the enhanced shear reversal mode similar to that of TFTR was observed. However in JT-60, reduction of thermal diffusivities both in ion and electron channels was observed at the region of qmin. Comparative studies of transport barriers observed in high discharges and shear reversal plasmas are in progress, with an emphasis on the current density profile, of which measurement was made available as a result of the collaborative work with TFTR.

Non-dimensional transport study has also been undertaken to respond to the ITER physics R&D requirements. The H-factor for the ELMy H-mode discharges was nearly independent of *, which indicates that the transport of these discharges is Bohm-type. Behavior of MeV ions produced by the ICRF acceleration and the suppression of TAE modes with the plasma rotation have also been intensively investigated. In addition, H-mode transition power threshold study, which is also one of the urgent ITER physics R&D issues, was pursued to demonstrate experimentally for the first time that edge neutral density influences the transition criteria.


TFTR has explored a broad range of physics issues involved in D-T discharges, in which number of scientists from JET and JT-60 participated, and a maximum fusion power of 10.7 MW was achieved using 39.5 MW of neutral-beam heating. Indications of alpha heating was also observed in April '95 as the increase of Te after NB is ramped down. The energy confinement increased in D-T, relative to D-D plasmas, by 20% and the nT product by 55%. Improvement in thermal confinement is ascribed primarily to a decrease in ion heat conductivity, and experimentally derived < A > -1.8 dependence is stronger than expected from the previous experiments and from the transport theory. Extensive lithium pellet conditioning of the graphite inner wall has resulted in highly peaked supershot profiles at higher Ip, and thereby increased the confinement time to 0.33 s with 17 MW of tritium NBI. As indicated in the high task-assignment work with JT-60, importance of the peaked deposition profile was herewith reconfirmed. Evaluated confinement time was approximately 2.4 times the prediction of ITER-89P scaling, based on an average ion mass of 2.7. In addition, central triple product nHyd * Ti was extended to the record value of 8.3 x 10 20 m -3 s keV in February '95, where * is defined as WTOT / PTOT. The central fusion power densities achieved in the high-performance TFTR supershots, which are 1.5 to 2.8 MWm- 3, are compa-rable to or greater than those expected in ITER.

Measurements of the confined alpha particles have been performed and found to be in good agreement with TRANSP simulations. Here, alphas in higher energy range of 0.5 to 2 MeV have been detected with ablating lithium pellet diagnostic, while lower energy alphas were measured by the charge-exchange recombination spectroscopy. Measurements of the alpha ash profile have been compared with simulations using particle transport coefficients from He gas puffing experiments. With the same transport coefficients, it was concluded that helium ash accumulation would not quench ignition in ITER provided the density of helium at the plasma edge can be controlled. In addition, loss of alpha particles to a detector at the bottom of the vessel was found to be well described by the first-orbit loss mechanism. Stochastic orbit losses in the toroidal field ripple have also been investigated. At major disruptions, losses of energetic alphas estimated to be up to 10% of the alpha population have been observed to occur in 2 ms during the thermal quench phase while the total current is still unperturbed, which could induce a serious impact on first-wall components in a reactor.

The initial DT experiments in TFTR showed no signs of instability in the TAE frequency range and the alpha-particle loss rate remained a constant fraction of the alpha production rate as the alpha pressure increased, suggesting that deleterious collective alpha instabilities were not being excited. Theory has since shown that the ion Landau damping of the mode in TFTR is generally stronger than the alpha-particle drive. Further experiments to assess the effect of NBI and ICRF induced TAE activity on the alpha population are in progress with collaboration with other large tokamaks. ICRF heating of D-T supershots at 2T has resulted in 60% absorption to ions, which is consistent with modeling. Electron heating as well as the current drive have been demonstrated using the mode-converted ion Bernstein wave. It is planned to use ICRF heating to increase the alpha-particle pressure and to investigate the possibilities for ICRF current drive to access "advanced" tokamak regimes with improved confinement and stability of the core plasma.

A sudden transition to a new regime of enhanced confinement has been observed in TFTR plasmas with reversed magnetic shear and balanced NB injection above 16 MW in February '95, and considerable number of discharges was thereafter devoted to the investigation of confinement properties and stability analysis of this heuristic and robust operating regime. These plasmas are characterized by a rapid increase of the central density in the shear-reversed region, rising to n e o > 1 x 1020 m-3 with Ip = 1.6 MA, BT = 4.8 T, Tio = 24 keV, Teo = 8 keV, low toroidal rotation velocity and a pressure peaking factor of 8. The calculated bootstrap fraction reaches two thirds of the total plasma current. As measured by the motional Stark diagnostic, qmin ranges from 1.8 to 3 at a / r = 0.35 and qo from 2 to 5. Transport analysis indicates that the electron particle diffusivity decreases by a factor of 50 after the transition, which is comparable with the neoclassical prediction including off-diagonal terms. The inferred ion thermal loss is substantially lower than predicted by neoclassical theory in reversed shear region. In addition, the ion pressure gradient scale length is comparable to banana width of the thermal ions, which violates the standard neoclassical ordering. However, the electron thermal diffusivity is not changed significantly, and it is much larger than i or De. In the high-performance reversed shear plasmas obtained so far, values of * = 3.8 have been achieved. Ideal stability analysis indicated that disruptions for the highest performance plasmas are near the stability boundary for n = 1 infernal modes. Code extrapolations indicate that 20 MW of fusion power may reasonably be obtained with the modification of qmin and the increase of rmin / a and extending the duration of the high confinement region in the plasma core. These calculations indicate that due to the highly peaked pressure profile, thermal runaway of the core pressure due to alpha heating may occur. Further, experiments are planned to explore this exciting new regime of operation.

5. Contribution to the ITER project

As documented in the previous sections, the three large tokamaks have made best efforts to co-ordinate their research programs and to carry them out intensively to make significant contributions to ITER EDA. The achievements hereby produced by the co-operative work under the Implementing Agreement have actually made and will continue to make remarkable contributions to understand the physics of and to define the operational regime of ITER.

6. Extension of the Implementing Agreement

Based on the consent that it is inconceivable for the progress of the world fusion program and to proceed with ITER without the contribution of the co-ordinated collaborative work of three large tokamaks under the Implementing Agreement, Executive Committee unanimously agreed to extend the agreement for another five years from January 15 '96, and its official procedure has been lately completed.