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On ITER, the LH system aims at supplying 20 MW CW for controlling the q-profiles in partially and fully non-inductive steady-state operation (9 MA). The designed LH system relies on a transmitter made of 24 5 GHz I MW klystrons linked to one antenna based on the passive active multijunction (PAM) concept, via a 60 m long oversized circular transmission line. Through a common European effort including several associations (CEA, ENEA, UKAEA, IST, TEKES and IPP-CZ) many outstanding problems have been solved: coupling has been achieved on JET in a physics scenario similar to ITER, a PAM has been tested with success on the FTU plasma. The reliability of LH for long pulse operation has been ascertained on Tore Supra. In order to routinely realise long pulse operation (1000 s), the Tore Supra LH system is being refurbished within the CIMES project framework where the transmitter is to be equipped with 16 klystrons 700 kW 1000 s at a frequency of 3.7 GHz. A new LH launcher based upon the PAM concept has been studied, designed and is actually under realisation. (c) 2005 Elsevier B.V. All rights reserved.

For simplification of Lower Hybrid Current Drive (LHCD) antenna system and reduction of power density in the antenna, three types of poloidal power divider which split the power in three, and 3 × 6 multi-junction module were developed. A good power dividing ratio of 33 ± 4 % was obtained for each of these dividers. In combination with two dividers connected to the 3 × 6 multi-junction module, a good high power handling capability was assessed: 420 kW-100 sec was transmitted without breakdown. Quasi stationary operation for r.f. injection time of 1000 sec at 300 kW was also demonstrated under water cooling. The outgassing rate at that time is in range of 10−7 Pa m3 s−1 m−2 with maximum temperature of 100°C, which requires no active vacuum pumping of the LHCD antenna.

A first conceptual antenna design based on the passive-active multijunction (PAM), was proposed in 93 in the frame of an ITER-TASK [ 1 ]. The present work accounts for the dimensional change of the ITER ports and also proposes a new arrangement of the hyperguides feeding the PAM to allow the flexibility of the N // spectrum which is desirable to cover the variety of envisioned scenarios. 35 to 55 MW of LH power could be injected in 1 port of ITER depending on the actual plasma reflection coefficient.

High power and energy performance have been achieved by the Tore-Supra LH system during the last experimental campaign. Far distance coupling as well as plasma pulses up to 120 s in steady state conditions with a power density of 24 MW/m2 were obtained. A new launcher, made with RF components such as mode converters has been designed in order to extend the present TS performance towards quasi continuous operations.

Outgassing, induced by very long RF waves injection (up to 6000s) at high power density, is studied with a module, able to be used for a lower hybrid frequency antenna.

In large tokamak such as JET, JT60, Tore Supra, the lower hybrid frequency antennae are composed of hundreds of waveguides and are fed by tens of klystrons. The N // spectrum which is then radiated depends on the geometry and on the wave amplitude and phase. It has to be checked when the total system is linked to the machine. Here four methods using the RF system are described which could be relevant for ITER: - The first method is the standard one. It uses measurements in laboratory to have the antennae electric lengths and measurement on the real system after its calibration. The incident wave is driven by the RF system for each klystron and the incident phase is then measured at the input of the antenna, computed at the antenna mouth and compared to the ones of neighbouring klystron. - The second one uses only the real system. The incident phase in the waveguide i fed by the klystron i is measured at the antenna mouth with the use of a magnetic loop. The other waveguides fed by the same klystron are matched with good loads. The calibration of the RF measurement system is not necessary here. - The third method uses the observation of the amplitude of the reflected signal of the klystron i and i+1 when they are fed together and the incident phase of i is constant whereas the one of i+1 is pulsed. The experiment has to be done on plasma. In this method only the measurement of amplitude is necessary, the components of the antenna must be identical from the reflection point of view. - The fourth one consist in the measurement of the scattering matrix at the input of the antenna on plasma. For N klystrons, N cases of feeding linearly independent must be done in such a way to have a linear system of N × N equations obtained from the measurement of the electric field at the input of the antenna (192 RF signals on Tore Supra). Then the S parameters can be obtained and by using the property of symmetry of the matrix, the measurement system calibration can be checked.

For Tore Supra lower hybrid current drive experiment a 8 MW, 3.7 GHz, 0.36 duty cycle RF transmitter made of 16 klystrons (efficiency 42 %, Uc = 58 kV, Ib = 19 A) with a modulating anode will be built. Two feedback systems will be set up to perform constant RF amplitude and phase for each klystron output. Control, slow protection of the transmitter will be done by control data acquisition, for fast protection an electronic device will be used. A 25 m long pressurized, water cooled transmission line will be built to link the transmitter to 2 antennas. The cooling of the whole system will be made by TS general water cooling system.