As a result of experimental observations of localized heat flux on components magnetically connected to radiating waveguides in Tore Supra and in TdeV, the acceleration of electrons near lower hybrid (LH) antennas has been investigated. A simple analytical model has been developed to compute the dynamics of the particles in the near field approximation. Landau damping of the very high N|| (20 < N|| < 100) component of the launched spectrum on the thermal electrons of the scrape-off layer (SOL) is found to occur. Simulation of a typical LH pulse in Tore Supra indicates that the electrons can be accelerated up to 2-3 keV. Modelling of the interaction of this fast electron population with the edge plasma allows a calculation of the heat flux on plasma facing components that are magnetically connected to the antenna. Model results and the results of experiments in Tore Supra and TdeV are compared. The calculated heat fluxes are found to be fairly consistent when the variation of convective heat flux at the grill aperture is taken into account. From this analysis, it is concluded that, for an LH power density of 25 MW/m2, the resulting heat flux along the field lines (3.5 MW/m2) is manageable for the components connected to the antenna, provided that good coupling can be maintained at a low density in front of the grill.

The TORE SUPRA lower hybrid current drive experiments (8 MWi3.7 GHz) use large phased waveguide arrays, four rows of 32 active waveguides and two passive waveguides for each of the two grills, to couple the waves to the plasma. These launchers are based on the 'multijunction' principle which allows them to be quite compact and is therefore attractive for the design of efficient multi-megawatt antennas in NETIITER. Extensive coupling measurements have been performed in order to study the radiofrequency (RF) characteristics of the plasma loaded antennas. Measurements of the plasma scattering coefficients of the antennas show good agreement with those obtained from the linear coupling theory (SWAN code). Global reflection coefficients of a few per cent have been measured in a large range of edge plasma densities (0.3 X 10" m-3 I neg I 1.4 X 10l8 m-3) or antenna positions (0.02-0.05 m from the plasma edge) and up to a maximum injected RF power density of 45 MWlm'. When the plasma is pushed against the inner wall of the chamber, the reflection coefficient is found to remain low up to distances of the order of 0.10 m. The coupling measurements allow us to deduce the 'experimental' power spectra radiated by the antennas when all their modules are fed simultaneously with variable phases. Thus, the multijunction launcher is assessed as a viable antenna for high power transmission with good coupling characteristics and spectrum control.

A new concept-the reflector waveguide array-is proposed to improve and simplify the design of continuous wave lower hybrid (LH) launchers for steady state reactor applications. Mechanical robustness of the antenna and efficient heat removal are provided by a thick wall waveguide structure that can accommodate a large number of cooling ducts. The plasma facing mouthpiece could be made of the same material as the reactor first wall and could be easily replaceable through remote handling. In order to compensate for the increased horizontal distance between adjacent waveguides, the front ends of the thick septa are grooved to form short ( equivalent to lambda /4) passive waveguides that act as reflectors between the radiofrequency powered waveguides (drivers). Then, for an adequate phasing between the active waveguides, the total electric field at the reflector waveguide apertures varies coherently with the one in the drivers to launch a highly directional slow wave. It is shown that the coupling properties of such an array and the directivity of the radiated power spectrum are similar to those of present day launchers. Their dependences upon the depth of the reflector waveguides, and the electron density and its gradient are investigated. The effect of changing the phase between the drivers is also studied. The proposed reflector LH antenna would provide enough flexibility to vary the N// spectrum for plasma control purposes in a steady state fusion reactor

Parasitic absorption of the short wavelength modes of the LH spectrum is a probable reason for the hot spots seen in the grill region of several tokamaks. Experiments suggest that the heat loads on the wall structures depend on the coupled power. In this work, the parasitic absorption of LH power was studied with self-consistent particle-in-cell simulations. The launched spectra were obtained from the SWAN coupling code. The power and temperature dependences of the absorption in the near field of the LH grill were investigated with a series of simulations. The parasitic absorption was found to grow from 0.6 to 1.1% when the coupled power increased from 26 to 67 MW/m2. When the edge temperature rose from 12.5 to 100 eV, the absorption increased from 0.4 to 1.7%. The maximum kinetic energies were between 0.6 and 1.8 keV. Estimates for the heat loads and surface temperature of the grill limiter are also obtained. The absorption leads to heat loads between 1.5 and 13 MW/m2 and surface temperatures of 510-2390° C.

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The global energy confinement of combined ohmic and lower hybrid driven TORE SUPRA plasmas has been analysed at various densities. In contradiction to the L mode ITER scaling law, this analysis indicates that the global energy confinement time depends strongly on the plasma density. Furthermore, the thermal electron energy content of steady state discharges is found to be in good agreement with the global Rebut-Lallia-Watkins (RLW) scaling law. Current ramp experiments show an enhancement of the global energy confinement with the internal inductance, I,. These results have been extended to steady state regimes with lower hybrid current drive. Improved confinement has been obtained in a high I, steady state plasma ( I , = 1.7), where the modification of the current profile by lower hybrid waves leads to an increase in the central value of the safety factor (q$(O) 2: 2). In this case, the global confinement time is shown to exceed the value predicted by the RLW scaling law by 40%.

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The recent success in coupling lower hybrid (LH) waves in high performance plasmas at JET together with the first demonstration on FTU of the coupling capability of the new passive active multijunction launcher removed major concerns on the possibility of using LH on ITER. LH exhibits the highest experimental current drive (CD) efficiency at low plasma temperature thus making it the natural candidate for off-axis CD on ITER where current profile control will help in maintaining burning performance on a long-time scale. We review recent LH results: long internal transport barrier obtained in JET with current profile sustained and controlled by LH acting under real time feedback together with first LH control of flat q-profile in a hybrid regime with T e ∼ T i . Minutes long fully non-inductive LH driven discharges on Tore Supra (TS). High CD efficiency with electron cyclotron in synergy with LH obtained in FTU and TS opening the possibility of interesting scenarii on ITER for MHD stabilization. Preliminary results of LH modelling for ITER are also reported. A brief overview of ITER LH system is reported together with some indication