The plasma current has been increased to 7 MA for limiter-bounded plasmas and to 5 MA for single-null, magnetic separatrix operation. Additional heating power has been raised to 20 MW of neutral beam injection and 18 MW of ion cyclotron resonance heating. Ion temperatures up to 23 keV, central densities up to 1.8 x 1020 m-3 and energy confinement times up to I .2 s are achieved in different discharges. The level of carbon and oxygen impurities is a major problem with the ratio between deuterium and electron densities in the range O.15 - 0.80 depending on conditions. Several features of a-particle physics have been studied, notably by the RF heating of minority ions to mean energies of order 1 MeV. The trapping and slowing down rates for the ICRH minority ions and d-d tritons are close to the classical values. Plasma conditions in deuterium plasmas corresponding to an equivalent (D-T) thermonuclear Q 0.1 and total Q 0.25 (including non-thermal reactions) have been reached. Exploiting from present results, taking into account the operation limits on density and impurities a total Q approaching 0.6 is predicted for D-T operation corresponding to an -power of 2 MW.
JET has been shut down since October for a number of technical improvements and additions to the apparatus such as stiffening of the vacuum vessel, improved RF launching antennae with beryllium screens, single high speed pellet injector (~4-6 km/s), the replacement of carbon tiles and antennae protection by beryllium, water cooled X-point dump plates with carbon fibre material facing the plasma and upgrading one NBI box to 140 kV/D.
With the increased balanced NBI power available (maximum injected power of 30.5 MW, with a maximum beam voltage of 110 kV and pulse duration of 1 sec), experiments in 1988 have concentrated on increasing the plasma current at which the supershot characteristics of high fusion reactivity and improved confinement are obtained. Ion temperatures up to 32 keV and ne(0)(a)Ti(0) values up to 4.2 x 1020 m-3 s keV have been achieved in the supershot regime. A maximum value of QDD = (1.550.4) x 10-3 has been attained, which would project directly to QDT = 0.290.07 in a D-T plasma and would project to QDT* = 0.50.13 when allowance is made for ion-mass effects and -particle heating. A strong correlation is observed between the ratio of central to volume-average electron density and the magnitude of both the neutron source strength and energy confinement time. The limiter H-mode observed on TFTR during supershot-like plasmas does not lead to an increase in , but rather a modest drop from the supershot level. Two launchers for ICRH were installed on the large-major-radius side of the vessel. ICRF heating experiments have been conducted for powers up to 2.5 MW.
The performance of JT-60 has been extended with addition of a new divertor coil, installation of a multi-pellet injector and enhancement of neutral beam heating power up to 25 MW. An offset linear law of energy confinement has been confirmed in limiter, outer and lower X-point discharges. A new improved regime of energy confinement was observed in the lower X-point discharges at high heating power. The improvement is correlated with high particle recycling and enhanced power loss in the divertor region. Beam heating of plasmas with peaked density profile by pellets brings about improvement in energy confinement by up to 40% when heating power is moderate (~ 10 MW). Lower hybrid waves can heat pellet-fueled high density plasmas without exciting parametric instabilities. Current drive efficiency by lower hybrid wave (neRpIRF/PLH) increases with the volume averaged electron temperature up to 2.9 x 1019 m-2 A/W. A new launcher for LHCO and an improved multi-pellet injector were installed in December. The JT-60 upgrade program has started already, in which the vacuum vessel and the poloidal field coils will be replaced completely to enable high current discharges with 6 MA in a divertor and 7 MA in a limiter configuration.