The lower hybrid current drive experiment on JET, aimed at generating current in the plasma by launching a wave at 3.7 GHz into the torus, required the building of a large high power microwave generator and antenna. The generator can provide 15 MW for 10 seconds or 12 MW for 20 seconds. This power will be fed to a phased-array antenna consisting of 48 stainless steel modules assembled in an array of 384 waveguides. The antenna position will be controlled in real time in order to optimize the coupling to the plasma. The phasing between different modules can be set so as to vary the NI/ between 1.8 and 2.4. -INTRODUCTIONThe lower hybrid current drive experiment on JET (1) is designed to generate large currents inside the JET plasma in order to study the effects of the modification of the current profiie on energy and particle confinement. Broadening of the current profile is a way of supressing intemal instabilities (such as sawtooth) linked to excessive peaking of the current profile. It is therefore expected that the profile modification will improve the energy contentand reactivity of plasmas with additional heating. Another use of LHCD current drive is the saving of volts per second in the transformer, which should enable the operation of the JET device for longer flat top pulses.Installation of the full performance system is scheduled for the mid-1990 shutdown. Installation of a prototype antenna connected to the first two generator modules (total power 5.2 MW) is foreseen in decemberl989.The general characteristics of the generator can be seen in table l. The choice of the frequency is a compromise between physics considerations (which tend to push towards higher frequencies) and technological problems, as well as the availability of large power generating devices. Our final choice was 3.7 GHz. Large amounts of power are required, since the LHCD power injected must be in the same scale as the ohmic power.The total power output of the generator was fixed at 15 MW, and the pulse lenght to 20 seconds. Finally we were asked to include remote handling capabilities and built-in safety systems allowing operation during the tritium phase, as well as high reliability requirements, since any intervention near the JET device will be long and difficult once the tritium phase starts. This means extensive testing of all components to be installed next to the machine (antenna modules, RF windows, waveguide components, loads, etc.). This tests required the building of a test bed generator providing up to 1.3 MW, wich was in the same time used as a test bench for concepts of the main generator.The system also has an active real time feed-back control of the position of the antenna in order to achieve an optimal power coupling to the plasma, as well as feed-back control of the phasing of the different modules in order to achieve the narrow radiated spectrum required to maximise efficiency. This also allows changing the phasing of the antenna modules in order to modify the current deposition profile. -THE GENERATORThe LHCD gene...

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