Annual Report (MS-Word)
JET operations resumed in June 1991 using new uncooled continuous X-point dump plates equipped with carbon tiles at the top and beryllium tiles at the bottom X-point. The tiles were accurately aligned ( 1.5 mm) and the tile attachment fixtures reinforced. The experimental programme was geared to optimize plasma performance and to obtain information relevant to the future JET D-T phase and to the "Next Step" experiment.
The following results are highlighted.
(1) An extrapolated fusion amplification factor QDT=1.14 has been obtained in a high performance deuterium discharge in which the total fusion power would have exceeded the total losses in the equivalent deuterium-tritium discharge in these transient conditions.
(2) Using ICRF and LH current drive to enhance inductive drive, a one minute discharge was produce at 2MA.
(3) An H-mode lasting 18 seconds was also produced.
(4) Tokamak operation in AC (Alternating Current) Mode was demonstrated.
(5) The first tokamak discharge in deuterium-tritium fuelled mixtures were undertaken in JET within limits imposed by restrictions on vessel activation and tritium usage. The objectives were (i) to produce more than 1MW of fusion power in a controlled way; (ii) to validate transport codes and provide a basis for predicting accurately the performance of deuterium-tritium plasmas from measurements made in deuterium plasmas; (iii) to determine tritium retention in the torus systems and to establish the effectiveness of discharge cleaning techniques for tritium removal; (iv) to demonstrate the technology related to tritium in compliance with the regulatory requirements. A single null X-point magnetic configuration diverted onto the upper carbon target plates was chosen. Tritium was introduced, into hot ion H-mode discharges, from two neutral beam sources each delivering 0.75 MW within a total of 14.3 MW NB heating. This produced a tritium concentration of about 11 % at the time of peak performance, when the total neutron rate was 6x1017 n/s. The integrated total neutron yield was 7.2x1017 n (7%). The neutrons came about equally from beam plasma and thermal processes. These levels are equivalent to total fusion releases ( particles and neutrons) of 1.7 MW peak power and 2 MJ energy. The techniques used for introducing, tracking, monitoring and recovering tritium have been demonstrated to be effective. A valuable body of experience has been obtained which will be useful in planning the future more extensive tritium experiments.
The prolongation of the JET project up to the end of 1996 has been formally approved. A new phase of the project, "the pumped divertor phase" will start in March 1992, whose objective is to establish effective control of plasma impurities in operating conditions close to those of a Next Step device. This will be followed in January 1996 by the full D-T phase of operation.
Experiments of JT-60U were initiated in March and deuterium experiments started in the middle of July. Multivariable non-interacting control was adopted to reduce the strong mutual coupling between the ohmic heating coils and the vertical field coils. Well controlled divertor discharges were successfully obtained using this method and discharges with plasma current up to 5 MA were achieved. In NB heating experiments with PNBI < 22 MW, plasma stored energies Wdia of up to 5.1 MJ were obtained for 4 MA discharges. Results of L-mode discharges from JT-60U deuterium experiments and JT-60 hydrogen experiments show the isotopic dependence proportional to M0.5. The high regime with hot ions and enhanced confinement was investigated. The maximum reached 2.32, the maximum Ti of 20 keV was achieved and was improved to about 2.7 times as large as that of ITER89 L-mode scaling. The maximum neutron rate of 1.3x1016 n/s was also obtained in the highest discharge. The corresponding QDD and QDT were 8.9x10-4 and ~ 0.20, respectively. Localized heat load in both toroidal and poloidal directions was clearly identified as ripple loss and this was the first measurement consistent with calculated results. Divertor plasma profiles measured with Langmuir probes show significant in-out asymmetry.
Three people visited JET to attend the preliminary tritium experiment. Dr. P. Bertoldi (NBI Power Supply) and Dr. T. Bonicelli (Positional Instability) have made extended visits to JT-60 and made developments in these area. In 1992, seven people from JET and two people from TFTR are planned to visit and join the JT-60U experiment for over two weeks. These many joint researches will be of great use in the progress of plasma physics and operations.
The emphasis of the TFTR research program in 1991 was placed on modifications to TFTR machine, heating and diagnostic hardware during the extended shutdown period and on subsequent operations (planned for 1992 as well) to explore improving operational scenarios to support achieving the highest QDT and values possible during the D-T run period scheduled to begin in mid 1993. During the shutdown, additional poloidal limiters were installed to protect the ICRF launcher and the bumper limiter tiles were modified to reduce the heat load on the edge. Reflectometry, densely packed Mirnov coils and a motional stark effect diagnostic to measure the current profile have been added to support the studies of transport dependence on stability and current profile. During the present run, wall conditioning techniques have been extended and now include He ohmic conditioning, pellet/sputtering systems for LI/B coating and HeO glow conditioning to be used to reduce tritium retention. ICRF antenna modifications have been successful for demonstrating the feasibility of achieving > 12.5 MW power operation in 1992. The role of instabilities on confinement has received considerable attention. Of particular importance is the observed loss of energetic beam ions and fusion products by MHD and Toroidal Alfvn eigenmodes, the latter of which have been positively identified with Beam Emission Spectroscopy. The implications of these results for alpha confinement during D-T operation are been assessed.
Operation scenarios at high power and high current are under development and an IEA exchange group from JT-60 observed these in 1991. In addition, studies of MHD effects on transport have been enhanced by the extended exchange visit of T. Ozeki from JT-60 and IEA workshop on MHD effects held at PPPL in November 1991. Finally planning for D-T operations, including tritium inventory considerations, has profited from IEA sponsored visits to JET prior to and during the first tritium experimental period. TFTR staffs are looking forward to the extended period of tritium operation planned on TFTR during which key alpha physics issues will be addressed.