The JET experimental campaign has focused on studies in support of the ITER physics basis. An overview of the results obtained is given for the reference ELMy H mode and advanced scenarios, which in JET are based on internal transport barriers. JET studies for ELMy H mode have been instrumental in the definition of ITER FEAT. Positive elongation and current scaling in the ITER scaling law have been confirmed, but the observed density scaling fits a two term (core and edge) model better. Significant progress in neoclassical tearing mode limits has been made showing that ITER operation with q(95) around 3.3 seems to be optimized. Effective helium pumping and divertor enrichment is found to be well within ITER requirements. Target asymmetries and hydrogen isotope retention are well simulated by modelling codes taking into account drift flows in the scrape-off plasmas. Striking improvements in fuelling effectiveness have been made with the new high field pellet launch facility. Good progress has been made on scenarios for achieving good confinement at high densities, both with radiation improved modes and with high field side pellets. Significant development of advanced scenarios, in view of their application to ITER, has been achieved. Progress towards integrated advanced scenarios is well developed with edge pressure control (impurity radiation). An access domain has been explored showing, in particular, that the power threshold increases with magnetic field but can be significantly reduced when lower hybrid current drive is used to produce target plasmas with negative shear. The role of ion pressure peaking on MHD has been well documented. Lack of sufficient additional heating power and interaction with the septum at high beta prevents assessment of the beta limits (steady plasmas achieved with beta (N) up to 2.6). Plasmas with a non-inductive current (I(NI)/I(p) = 60%), well aligned with the plasma current, high beta and good confinement have also been obtained.

Recent experiments in D-T plasmas on the JET and TFTR tokamaks have evaluated a wide range of ITER relevant ion cyclotron heating scenarios. Absorption of fast waves at the second-harmonic tritium resonance has provided bulk ion heating in TFTR supershots and electron heating in JET H-mode discharges. In JET, deuterium minority heating has generated 1.7 MW of fusion power with 6 MW of radio frequency power giving a record steady-state Q-value of 0.22. Strong bulk ion heating has been achieved with He-3 minority heating with central ion temperatures up to 13 keV being produced in H-modes with a density of 3.6 x 10(19) m(-3) Hydrogen, deuterium and He-3 minority heating methods have produced plasmas with normalized confinement times greater than or equal to that required by ITER for ignition. These H-modes are characterized by small-amplitude, high-frequency ELMs, each of which transports less than 1.5% of the plasma energy content to the limiters. The heavy minority scheme of tritium in a deuterium plasma has been demonstrated both as a heating scheme and a generator of suprathermal neutrons. On TFTR mode conversion to an ion Bernstein wave has achieved central bulk ion heating in supershots with target ion temperatures greater than 20 keV.

Two pumped diverters have been installed and tested in JET under ITER relevant conditions. A closed divertor is found to increase the particle and impurity exhaust rate in agreement with code modelling. Excellent power handling is demonstrated, allowing high current discharges with record stored energy (15 MJ) and quasi-steady-state discharges with high fusion triple product (4 x 10(20) m(-3) s keV). The ITERH93-P confinement scaling law is confirmed over abroad range and a more favourable beta scaling is found. No hysteresis is found in the II-mode power threshold. A data base of highly radiating discharges including impurity seeding describes the relationship between radiated power, impurity concentration and density. First results of high performance with optimised magnetic shear are presented. Strong internal confinement barriers develop despite the relatively small input of toroidal momentum and particle fuelling from ICRH and NBI.

All major JET systems have been fully commissioned for D-T and the DTE1 series of experiments has started with the D-T fuel mixture and operating conditions foreseen for ITER. In the area of ITER physics, significant results have been produced in both D-D and D-T. In D-D, the LH threshold power database has been extended, the bounds on edge-electron temperature and density in ELMy H-modes have been defined and the advantages of Types I and III ELMy discharges have been compared. In D-T plasmas, the isotope effect on H-mode threshold power and transport has been determined so that a more accurate assessment can be made of the ignition margin and heating requirements for ITER. Trace tritium experiments have provided first particle transport measurements and an assessment of the ITER reference ion-cyclotron resonance-frequency heating scenarios has been started, In the area of fusion performance, record D-D neutron yields have been obtained by controlling the plasma and current profiles in hot ion ELM-free H-modes and optimized shear modes. In D-T, internal transport barriers have been readily established in optimized shear discharges and Alfven eigenmodes have been observed.

ICRH on JET is used for a wide range of experiments: confinement and H-mode threshold scaling experiments, combined heating experiments at high plasma current and heating in high performance plasmas which will be presented here. Experiments of combined NBI and ICRH in ELM-free Hot Ion K-modes have been carried out in the JET MKII campaign in 3.8 MA/3.4 T plasmas with up to 9.5 MW of ICRH Fewer together with 16 MW of NBI using minority hydrogen resonance. ICRH results in increased core electron temperature, increased core ion temperature and increased stored energy up to a record of 14.5 MJ. Various schemes have been used including polychromatic mode of operation. Data analysis suggests that a substantial part of the ICRH power is coupled to deuterons at the second harmonic resonance. This is the dominant heating mechanism in new experiments with shear optimisation where the power is injected during the current ramp-up phase of the discharge. Very peaked ion pressure profiles are achieved, up to 3.5 bar and central ion temperatures ranging from 25 to 32 keV are obtained, 5 to 10 keV higher in combined heating than with NBI only. The highest neutron yield has been achieved with pi/2 phasing and good coupling is obtained during the L-mode phase. Absolute fusion yield records in DD operation on JET (5.6 10(16) neutrons/s) have now been established with combined NBI and ICRH, the latter increasing the neutron yield by about 30 to 40%.

This paper presents an overview of results of the 1994/95 experimental campaign on JET with the new pumped divertor and draws implications for ITER in the areas of detached and radiative divertor plasmas, the use of beryllium as a divertor target the material, the confinement properties of discharges with the same dimensionless parameters (except for the dimensionless Larmor radius) as ITER and the effect of varying the toroidal magnetic field ripple in the ITER relevant range. Discharges with high fusion performance at high current, in steady-state with ELMs and in the ELM-free hot-ion H-mode, are also reported. Limits to operations are discussed and projections to D-T performance are made.

JET was extensively modified in the 1992/93 shutdown. The new pumped divertor and many new systems were brought into operation early in 1994. Operations have progressed to 4 MA plasma current and, with substantial additional heating, H-mode confinement results confirm the expected scaling. The high power handling capability of the pumped divertor with sweeping is estimated at 20 MW for 20 s. H-mode plasmas have large Type I ELMs. With lower hybrid heating alone, 2 MA full current drive has been achieved with good efficiency, and with ICRF power, effective heating and direct electron heating have been demonstrated.

Describes a series of experiments in the Joint European Torus (JET), culminating in the first tokamak discharges in deuterium-tritium fuelled mixture. The experiments were undertaken within limits imposed by restrictions on vessel activation and tritium usage. The objectives were: (i) to produce more than one megawatt of fusion power in a controlled way; (ii) to validate transport codes and provide a basis for accurately predicting 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 usage; and (v) to establish safe procedures for handling tritium in compliance with the regulatory requirements. A single-null X-point magnetic configuration, diverted onto the upper carbon target, with reversed toroidal magnetic field was chosen. Deuterium plasmas were heated by high power, long duration deuterium neutral beams from fourteen sources and fuelled also by up to two neutral beam sources injecting tritium. The results from three of these high performance hot ion H-mode discharges are described: a high performance pure deuterium discharge; a deuterium-tritium discharge with a 1% mixture of tritium fed to one neutral beam source; and a deuterium-tritium discharge with 100% tritium fed to two neutral beam sources. The TRANSP code was used to check the internal consistency of the measured data and to determine the origin of the measured neutron fluxes. In the best deuterium-tritium discharge, the tritium concentration was about 11% at the time of peak performance, when the total neutron emission rate was 6.0 × 1017 neutrons/s. The integrated total neutron yield over the high power phase, which lasted about 2 s, was 7.2 × 1017 neutrons, with an accuracy of ±7%. The actual fusion amplification factor, QDT was about 0.15

Preliminary experiments were carried out in the Joint European Torus (JET) using 120 keV helium (He) neutral beam injection (NBI). Injected power levels up to 5 MW with 3 He and 7 MW with 4 He, lasting up to 3 s were reached. The 3 s helium NBI produced efficient ion heating and similar global and local energy confinements to those obtained with deuterium (D) NBI, in both limiter and X-point plasma geometries, in L-mode and H-mode plasma regimes. The elimination of beam-plasma and beam-beam fusion reactions by replacing D NBI with He NBI extended the range for measuring ion temperatures with the JET neutron profile monitor and neutron spectrometers.

Preliminary experiments were carried out in the Joint European Torus (JET) using 120 keV helium (He) neutral beam injection (NBI). Injected power levels up to 5 MW with 3He and 7 MW with 4He, lasting up to 3 s were reached. The 3 s helium NBI produced efficient ion heating and similar global and local energy confinements to those obtained with deuterium (D) NBI, in both limiter and X-point plasma geometries, in L-mode and H-mode plasma regimes. The elimination of beam-plasma and beam-beam fusion reactions by replacing D NBI with He NBI extended the range for measuring ion temperatures with the JET neutron profile monitor and neutron spectrometers.

In 1990 JET operated with a number of technical improvements which led to advances in performance and permitted the carrying out of experiments specifically aimed at improving physics understanding of selected topics relevant to the "NEXT STEP". The new facilities include beryllium antenna screens, a prototype lower hybrid current drive system, and modification of the NI system to enable the injection of He-3 and He-4. Continued investigation of the hot-ion H-mode produced a value of n(D)(0)tau-E(T)(i)(0) = 9 x 10(20)m-3s keV, which is near conditions required for Q(DT) = 1, while a new peaked density profile H-mode was developed with only slightly lower performance. Progress towards steady state operation has been made by achieving ELMy H-modes under certain operating conditions, while maintaining good tau-E values. Experimental simulation of He ash transport indicates effective removal of alpha-particles from the plasma core for both L and H mode plasmas. Detailed analyses of particle and energy transport have helped establish a firmer link between particle and energy transport, and have suggested a connection between reduced energy transport and reversed shear. Numerical and analytic studies of divertor physics carried out for the pumped divertor phase of JET have helped clarify the key parameters governing impurity retention, and an intensive model validation effort has begun. Experimental simulation of alpha-particle effects with beta-fast up to 8% have shown that the slowing down processes are classical, and have given no evidence of deleterious collective effects.

During its 1990 operation, 2 large RF systems were available on JET. The ion cyclotron resonance heating (ICRH) system was equipped with new beryllium screens and with feedback matching systems. Specific impurities generated by ICRH were reduced to negligible levels even in the most stringent H-mode conditions. A maximum power of 22 MW was coupled to L-mode plasmas. A new high confinement mode was discovered. Experiments were performed with the prototype launcher of the lower hybrid current drive (LHCD) systems with coupled power up to 1.6 MW with current drive efficiencies up to R ICD/P=0.4*1020 m-2 A/W. The authors present the first observations of the synergistic acceleration of fast electrons by transit time magnetic pumping (TTMP) (from ICRH) and electron Landau damping (ELD) (from LHCD). The synergism generates TTMP current drive even without phasing the ICRH antennae.

The performance of high current discharges can be improved by applying central ICRF heating before or shortly after the onset of sawtooth activity in the plasma current rise phase. Long sawtooth-free periods have been obtained which result in a transiently-enhanced discharge performance. High Te(0)=9-10.5 keV with peaked profiles Te(0)/(Te)=3-4 were obtained giving values of Ne(0)Te(0) up to 6*1020 (keV m-3). Improvements in Ti(0) and neutron production are observed. A best value of nD(0)Ti(0) tau E=1.65*1020 (m-3 keV s) was achieved. Local transport simulation shows that the electron and ion thermal diffusivities do not differ substantially in the two cases of current-rise (CR) and flat-top (FT) heating, the performance of the central plasma region being enhanced, in the case of current-rise, entirely by the elimination of the sawtooth instability. The maximum D-D reaction rate is enhanced by a factor of 2 compared to the flat-top value. An appreciable part of the reaction rate is attributed to 2nd harmonic deuterium (2 omega CD) heating. In all current-rise discharges radiation amounts to 25-50% of total power and Zeff remains roughly constant.

Two antennae have been installed in JET and operated to the maximum design capability of the generators. 4.5 MW, 10 MJ have been coupled to the plasma which heated up to a maximum stored energy of 3 MJ with central temperatures of Te0=5 keV and Ti0=4 keV without increase of the relative impurity concentration. Degradation of energy confinement is observed according to an L mode scaling. The effect of k/sub //// shaping is discussed using a quadrupole antenna. Hydrogen and helium 3 minority heating regimes give similar results.

Heating the JET plasma well above temperatures reached in the ohmic phase is the aim of the two additional heating systems planned for JET: ion cyclotron resonance heating (ICRF) and neutral beam injection (NBI). Operations with the latter started in February 1986, initially with hydrogen injection, up to a power level of 7 MW. ICRF power has been delivered to the plasma by three antennae and has reached power levels of 6 MW for 2 s. In most experiments, the frequency of the waves is adjusted to position the minority ion resonance layer close to the centre of the discharge, resulting in a centrally peaked power deposition profile. Results have also shown that the increase of the volume averaged electron temperature was much less dependent on the radial position of the ion resonance layer than were the central temperatures of both ions and electrons which can both approach 5 keV. In cases of 'on axis' heating, large increases in the sawtooth activity is observed with sawtooth periods exceeding 0.3 s. Initial results with NBI have shown a decrease of the global energy confinement time with additional power similar to the one observed with ICRF heating.

As a first step in the JET ICRF programme, two antenna generator units have been installed at JET and operated up to the design specification of 3 MW coupled in the Torus for 1 second. After a brief description of the system, the experimental results of wave coupling to the plasma and matching the plasma loaded antenna are discussed. The first heating results are presented concentrating on the analysis of a case where the total power to the plasma increases by a ratio of 2.5.