Annual Report (MS-Word)

Annual Report to IEA
Three Large Tokamak Cooperation

January to December 1992
Executive Committee

1. Cooperation activities

The cooperation among the three large tokamaks was carried out successfully in 1992. Results among the three tokamaks have been shared for deeper understanding of tokamak physics and hence the coordinated cooperation contributed much to achieve higher plasma performances in each device. Especially, since JET was shut down for the pumped divertor modification since March 1992, many prominent JET scientists sent to JT-60 made significant contributions for the achievement of eminent results on JT-60.
Two workshops were held in 1992 as shown in Attachment 1: one at JT-60 on the energy confinement under the intensive auxiliary heating in May and the other at JET on the tritium experiments in May. The number of personnel exchanges of which period exceeded four weeks was nine as shown in Attachment 2, while eight scientists participated in the review tours. Reports on these activities, written in Forms A, B and C, are listed in Attachment 3.

2. Meeting of the Executive Committee

The seventh Executive Committee meeting took place at JAERI on 16 and 17 July 1992. The names of attendee are as follows: Drs. A. Gibson and J-P Poff from JET, Drs. S. Tamura, A. Funahashi, H. Kishimoto and M. Azumi from JT-60, Drs. R. Hawryluk and K. McGuire from TFTR. The Committee elected Dr. S. Tamura as the chairman until the next meeting. JAERI acted as the secretariat, represented by Mr. H. Shirai.
The plans for workshops and personnel exchanges for coming one and a half year were agreed upon. The secretariat announced that Dr. T. Fukuda will succeed Mr. H. Shirai to represent the secretariat after this meeting.
The inclusion of non-member countries in the IEA collaboration was also debated, which was requested from Mr. E. Yamada of IEA. The executive committee agreed that although it would not be appropriate for non-member countries to attend the executive committee meeting, which is restricted to members from three laboratories operating large tokamaks, it will be beneficial for specific representatives from non-member countries to be invited to the workshops at the initiative of the workshop organizer.
The minutes of the seventh Executive Committee meeting is attached to this report as Attachment 4, and the next meeting of the Executive Committee will be held at TFTR on 25 and 26 May 1993.

3. Membership of the Executive Committee

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

4. Status of the Three Large Tokamaks

As is described in detail in the succeeding section, after the historical tritium experiment in 1991, JET was shut down at the end of February. However, even for the short period of six weeks in 1992, JET has obtained remarkable results of eighteen second H-mode discharge and high ripple loss experiment as well as the attainment of the 7 MA beam heated stable plasma discharge, all of which are directly relevant to the ITER design studies. 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. Recent research on TFTR has focused on the transport studies and fusion product physics in preparation for the D-T experiments, and the preparation for tritium experiment in 1993 is in steady progress.
The status and major results of the three large tokamaks are summarized below:

JET

The JET apparatus is presently in shutdown to permit the installation of a Pumped Divertor Coil system, intended to demonstrate reactor relevant impurity control. This modification to JET is the most extensive ever undertaken. It is expected that operation in the new configuration will start in the fourth quarter of 1993. The main elements of progress during the 1991 and 92 experimental campaign are listed below.
(a) Fusion Performance
The first demonstration was made of the controlled production of significant Fusion Power. This was possible because of the high performance obtained with JET plasmas - e.g. # 26087.
i = 18.6 keV
e = 5 x 1019 m-3
= 1.2 s
= 2.2 %
D

i = 9 x 1020 m-3keV s - within a factor of 6 or so of the value required in a reactor.The experiment was the first demonstration of tritium Neutral Beam injection into a fusion plasma. It resulted in a peak D-T neutron production rate of 6 x 1017 neutrons / sec in a neutron pulse lasting about 2 s. The total 14 MeV neutron production was 7 x 1017 neutrons. The neutrons came roughly equally from beam - plasma and thermal processes. These levels are equivalent to total fusion releases of 1.7 MW peak power and 2 MJ energy. Injection of more tritium ( 8 injectors instead of 2 ) into a similar plasma would have produced 10 MW of fusion power with Pfuse / (Pin - ) ~1.
(b) Impurity Control
Optimum shaping of the wall interaction tiles together with dynamic sweeping of the interaction region has excluded any large impurity influx at the highest performance conditions for about 2 sec. Extension of this time is the key aim of the JET Pumped Divertor Programme.
At lower currents ( Ip = 2 MA ) high performance conditions were maintained in essentially steady state for 18 sec. Impurity accumulation in the plasma was prevented by manipulating the edge conditions, without destroying the good confinement ( strong gas puffing to control ELMs and off axis ICRH to produce miniature rather than massive sawteeth ).
With strong gas puffing, gas targets have been produced which are stably maintained and which reduce the power to the target to a very small value. Control of this behaviour using feedback between the temperature and gas puff has been demonstrated.
(c) Pellet Fueling
Pellet fueled discharges have been produced with good profile peaking. They have very good confinement properties with equal electron and ion temperatures, similar to those needed in a reactor.
e.g. # 22490.
i = e = 11 keV
e = 6 x 1019 m-3
= 1 sec
D i = 8.2 x 1020 m-3 keVs
Zeff = 2(d) Long Pulse Operation
Very long pulse or continuous operation will be required for a Fusion Reactor. In JET there are two important advances in this area.
d.1 Using ICRH, LHCD and bootstrap current: (1) 1.5 MA has been driven with no applied voltage; (2) a pulse with 2 MA current has been sustained and heated for 1 minute.
d.2 Two cycle alternating current operation at the 2 MA level with heating has been demonstrated, similar parameters were obtained on each of the half cycles.
d.3 The current drive systems have also been used to modify the spatial current profile to improve plasma stability.(e) Heating
JET plasmas have been heated to temperatures in excess of those needed for a Fusion Reactor ( up to 30 keV ) using both Neutral Injection Heating and Ion Cyclotron Resonance ( ICRH ) heating. Each system has delivered 20 MW to the plasma and the two operating together up to 30 MW. ICRH regimes relevant to a reactor with predominantly ion heating have been demonstrated ( up to 50 % H minority in He3 and in D ).
(f) High
High operation with = 6 % has been obtained, ~ 20 % above the Troyon limit ( = 2.8 Ip / B . a ). There are indications of a soft limit which would permit burn control in a reactor.
(g) High current operation
Pulses with 7 MA current for 8.6 sec have been obtained. 30 MW of heating has been applied for 3.5 s. These limiter discharges do not have the high performance mode possible with divertor discharges, but the confinement times continue to increase with current, compared to similar discharges at smaller current. This supports confidence in extrapolations from JET to ITER.
These large plasma currents on JET which involve using the full JET cross section with careful shape and position control are higher than those for any other existing or approved device and higher than will be possible in JET in the future.

JT-60

Intensive heating experiment at BT = 2 - 4 T, IP = 1-4 MA, PNBI = 5 - 25 MW was carried out after the deliberate wall conditioning including the boronization in 1992. The production of the hot-ion mode plasmas as well as attainment of the enhanced poloidal beta regime improved the plasma performances to a significant extent. Progress has also been made in understanding the H-mode transition as well as plasma confinement in L-mode and ohmically heated plasmas including the isotope dependence, current profile effect, significance of the aspect ratio, thermal confinement scaling and ion transport anomaly. Following are the elements of highlights from the 1992 campaign:
(a) Hot-ion mode plasmas
In the hot-ion mode discharges, the confinement enhancement factor over the ITER L-mode scaling of up to 2.2 has been obtained at BT = 4 T and PNBI = 16 MW. The plasma stored energy of 7.7 MJ and fusion triple product of D i = 2.5 x 1020 m-3keVs have also been achieved. The maximum ion and electron temperature respectively reached 32 keV and 9 keV, and the D-D neutron emission rate was 2.3 x 1016 / s.
(b) High plasmas
In the high enhanced confinement regime of which dia exceeds 0.6, the energy confinement was enhances by a factor of 3 with a large fraction ( < 75 % ) of the bootstrap current. The plasma performance in the high bp enhanced confinement regime was characterized by high temperature of Ti ~ 38 keV and Te ~ 12 keV and the fusion neutron rate of 2.8 x 1016 n / s. Fusion triple product also reaches 4.4 x 1020 m-3 keVs, and QDD = 1.9 x 10-3. Contrary to the JET high performance discharges, The termination of the enhanced confinement phase was due to collapses with ballooning-like precursors occurring at a stability limit for ideal low-n kink-ballooning modes substantially below the Troyon limit. A workshop is being organized in May 1993 at JET to discuss this issue. The confinement enhancement appears to be insensitive to the current profile, but the stability limit is rapidly reduced by decreasing li. These results are expected to be promising for the future high tokamak reactor.
(c) Impurity control and divertor physics
The divertor heat flux was analyzed to establish a scaling law on the scrape-off-layer thickness. The experimentally derived scaling suggests that high density and high q operation alleviates the heat removal problem.
(d) High experiment
Discharges with peaked current density profile ( li ~ 1.5 ) and broad pressure profile, which are respectively obtained by the growing plasma operation and larger portion of tangential NB injection, have improved value of up to 3.4. Contrary to high plasmas, the value of was limited by the robust m / n = 2 / 1 activity.
(e) LHCD
A maximum driven current of 2.0 MA was achieved with the efficiency of 2.6, and it was found that increases with the electron density and plasma current. The installation of additional new LHCD launcher was just finished in December to start 10 MW LHCD experiment in early 1993.
(f) Disruption studies
Causalities of all the disruptions observed in JT-60 fall into three categories: locked mode, density limit and high li. Stabilization technique was also developed to enable 4 MA / s current decay.

TFTR

TFTR is making final preparations for the D-T campaign in September 1993. Recent research on TFTR has emphasized optimization of performance in deuterium plasmas, transport studies and studies of energetic ion and fusion products physics. The following is a list of highlights from our 1992 campaign.
(a) D-D reactivity
During 1992 we obtained a new record for D-D neutrons per second of 5.6 x 1016 n / s. Simulation of 50 / 50 D-T plasma for that condition would produce 11.6 MW of fusion power. Extensive Lithium Pellet conditioning on TFTR significantly improved performance of D-D reactivity and fusion triple product
ne(0)Ti(0)

= 5.8 x 1020 m-3 keVs
ne(0) = 0.98 x 1020 m-3
Ti(0) = 29 keV
= 0.205 sec
Zeff = 1.65(b) Fusion product losses
Observation of a new anomalous delayed loss of trapped D-D fusion products was observed. The anomalous loss peaks at a higher pitch angle and a lower energy than the normal first-orbit loss. This anomalous loss feature can be strongly modulated by large MHD activity, but also persists without any large coherent MHD activity.
(c) ICRF experiments on TFTR
The present ICRF antenna configuration consists of four antennas mounted on adjacent outboard midplane ports, six RF generations feed the antennas. To date a fixed frequency of 47 MHz has been used at power levels up to 11.4 MW.
Experiments that deploy hydrogen minority ICRF heating of helium plasmas have recently been extended to RF power level above 11 MW.
(d) RF direct electron heating
Observation of ICRF direct electron heating has been made on TFTR. ICRF direct electron heating results from electron Landau damping (ELD) and transit time magnetic pumping (TTMP). In this initial experiment approximately 1.5 MW was coupled to the plasma resulting in a 1 keV increase in the electron temperature. These results will be used to optimize Supershots in TFTR.
(e) Toroidal Alfvn eigenmodes (TAE activity)
A TAE like-mode, which is driven by the ICRF heating system at powers > 3 MW, was discovered. All the normal signatures of a TAE mode are observed as recently predicted by H. Biglari et al:
- density dependence
- frequency dependence VA / 2qR
- loss of fast ionsTheoretical calculations by G. Y. Fu and C. Z. Cheng, now shows that the ion Landau damping is the most important damping mechanism for the TAE modes driven by neutral beams on TFTR. The stability thresholds agree within a factor 2 or 3 with the experimental results.
(f) Observation of the ballooning modes in TFTR A significant result of FY92 work is the observation of medium-n ( 4 < n < 10 ) ballooning modes that appeared in a series of high TFTR discharges. These ballooning modes occurred during a slow degradation in the plasma and preceded a sudden partial collapse in the central plasma pressure. The amplitude of the mode is larger on the outboard (low-field) side of the torus and has a fast growth rate, both characteristics of an ideal MHD ballooning instability. This is the first reported observation of a ballooning mode in the interior of a large, collisionless tokamak plasma.
(g) Nondimensional transport studies
Scans of * on TFTR, holding the other nondimensional parameters fixed, showed better agreement with Bohm than gyroBohm scaling. This result indicates that heat transport in tokamaks is driven by turbulence whose characteristic size is determined by the tokamak size, rather than by purely local quantities such as ion gyro radius or temperature gradients.
In FY92 a scan in plasma was carried out at approximately constant * and n*. The variation of thermal energy confinement with was deduced from temperature and density profile measurements. th decreased by a factor ~ 3 as increased a factor of 4.6, implying an average deterioration as ~ -0.74 across the scan. This is the behavior expected for ITER-P scaling (i.e., Bohm scaling with an additional 1/2 deterioration).
(h) High beta poloidal operation
Recent TFTR high experiments have produced plasmas with qo < 2.5 and < 1.25. High qo was produced by injecting co-only beams into a low density, large major radius (R=2.6 m) plasma. Preliminary modeling shows that discharges with high and qo have substantially improved ballooning stability and are forming a plasma in the second stable region.

5. Contribution to the ITER project

As documented in the previous sections, the three large tokamaks have made best efforts to organize their research programs and to carry them out intensively to meet the requirements from the ITER. Many of the results produced in this program definitely encourage the ITER development. Along this line and complying with the physics R & D tasks discussed in the conceptual design activity, each party shall continue to contribute to the ITER project.

Appendix 1
Executive Committee Members

JET
Dr. A. Gibson : Associate Director and Head of Plasma Heating and Operation Department
Dr. J-P Poff : Head of Staff Office

Alternates
Dr. E. Bertolini : Deputy Head of Machine and Development Department and Head of Magnet and Power Supplies Division

TFTR
Dr. R. J. Hawryluk : TFTR Project Head, PPPL
Dr. J. W. Willis : Director, Division of Confinement Systems, Office of Fusion Energy, USDOE

Alternates
Dr. K. M. McGuire : Head of Physics Program Division, PPPL
Dr. S. A. Eckstrand : Division of Confinement Systems, Office of Fusion Energy, USDOE

JT-60
Dr. S. Tamura : Director, Department of Fusion Plasma Research
Dr. A. Funahashi : General Manager, Large Tokamak Experiment Division I

Alternates
Dr. H. Kishimoto : Deputy Director, Department of Fusion Plasma Research
Dr. M. Azumi : General Manager, Plasma Analysis Division