A 20 MW/5GHz Lower Hybrid Current Drive (LHCD) system was initially due to be commissioned and used for the second mission of ITER, i.e. the Q=5 steady state target. Though not part of currently planned procurement phase, it is now under consideration for an earlier delivery. In this paper, both physics and technology conceptual designs are reviewed.

A new ITER-relevant lower hybrid current drive (LHCD) launcher, based on the passive-active-multijunction (PAM) concept, was brought into operation on the Tore Supra tokamak in autumn 2009. The PAM launcher concept was designed in view of ITER to allow efficient cooling of the waveguides, as required for long pulse operation. In addition, it offers low power reflection close to the cut-off density, which is very attractive for ITER, where the large distance between the plasma and the wall may bring the density in front of the launcher to low values. The first experimental campaign on Tore Supra has shown extremely encouraging results in terms of reflected power level and power handling. Power reflection coefficient <2% is obtained at low density in front of the launcher, i.e. close to the cut-off density, and very good agreement between the experimental results and the coupling code predictions is obtained. Long pulse operation at ITER-relevant power density has been demonstrated. The maximum power and energy reached so far is 2.7 MW during 78 s, corresponding to a power density of 25 MW m−2, i.e. its design value at f = 3.7 GHz. In addition, 2.7 MW has been coupled at a plasma–launcher distance of 10 cm, with a power reflection coefficient <2%. Finally, full non-inductive discharges have been sustained for 50 s with the PAM.

A new concept of lower hybrid antenna for current drive has been proposed for ITER (Bibet et al 1995 Nucl. Fusion 35 1213–23): the passive active multijunction (PAM) antenna that relies on a periodic combination of active and passive waveguides. An actively cooled PAM antenna at 3.7 GHz has recently been installed on the tokamak Tore Supra. This paper summarizes the comprehensive experimental characterization of the coupling properties of the PAM antenna to the Tore Supra plasmas. In this paper, the electromagnetic properties of the antenna are measured at a reduced power (<1 MW) to allow a systematic comparison with linear wave coupling theory and the associated modelling based on the linear ALOHA code. In a wide range of edge electron densities at the antenna aperture (spanning a factor 20 from 0.5 × n c to 10 × n c where n c is the slow wave density cut-off, n c = 1.7 × 1017 m−3 at 3.7 GHz) and antenna phasing, the ALOHA simulations reproduce the experimental results observed on Tore Supra. In addition, reduced power reflection coefficients (<5%) are measured at a low edge density, close to n c, i.e. in the range 0.5–3 × n c. Measurement and analysis with ALOHA of the antenna–plasma scattering matrices provide explanation of the good coupling properties of the PAM antenna close to n c by highlighting the crucial role of the slow wave intercoupling between active and passive waveguides through the plasma edge. This detailed validation of the coupling modelling is an important step towards the validation of the PAM concept in view of further optimizing the electromagnetic properties of the future ITER antenna.

a b s t r a c tIn the frame of the EFDA task HCD-08-03-01, a 5 GHz Lower Hybrid system which should be able to deliver 20 MW CW on ITER and sustain the expected high heat fluxes has been reviewed. The design and overall dimensions of the key RF elements of the launcher and its subsystem has been updated from the 2001 design in collaboration with ITER organization. Modeling of the LH wave propagation and absorption into the plasma shows that the optimal parallel index must be chosen between 1.9 and 2.0 for the ITER steady-state scenario. The present study has been made with n || = 2.0 but can be adapted for n || = 1.9. Individual components have been studied separately giving confidence on the global RF design of the whole antenna.

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The ITER plasma position reflectometry diagnostic aims to provide measurements of the edge plasma to correct or supplement the magnetics for plasma position control. It consists of five reflectometers, two of which have components installed inside the vessel. One of these systems probes the plasma from the high-field side using a bistatic array of small pyramidal horns located in the gap between two blankets. Electromagnetic simulations have shown that the blankets shape the radiation pattern and need to be considered as part of the antenna. Full-wave plasma simulations have confirmed these results and have also shown that the first-wall geometry may induce measurement errors above the required margin. To further address these issues, we manufactured a prototype of the high-field side antenna, which includes a mock-up of the blanket modules. Here, we present the results of the prototype tests, with and without the blankets, using a metallic mirror as a target. The signals reflected from the mirror are used to derive the mirror distance and assess the precision of the measurements under different arrangements. The sensitivity to the blankets’ installation tolerances is also assessed by changing the antennas’ position with respect to the blankets’ surfaces and cut-outs.

Multipactor, the well-known electron avalanche phenomena occurring in waveguides when the amplitude of the RF electric field exceeds a threshold, is modelled for lower hybrid current drive (LHCD) antennas. The electric field threshold departures significantly from the expected linear scaling with the frequency when the width of the waveguide along the electric field increases. For 5 GHz waveguides, the multipactor does not occur when this width exceeds 26 mm, in very good agreement with analytical formulation. Effect of static magnetic field with typical values of tokamak environment is examined. It is found that the threshold is weakly reduced (−6%) for the front face of the antenna featuring the rows of narrow waveguides. Strong reduction in the threshold is found where the wave frequency equals the electron cyclotron frequency (0.178 T at 5 GHz) but this condition will not be met on ITER. After baking at 240 °C, wide waveguides at the antenna input (29 mm on ITER) have a threshold of ∼7 kV cm−1, assuming a homogeneous magnetic field. However, preliminary results indicate higher threshold when the gradient of the magnetic field along the waveguide is taken into account and no limitation, from this section, is expected for the ITER LHCD antenna.