Lower Hybrid Current Drive (LHCD) experiments performed at density close to that required for the steady state scenario are reported. On C-Mod, FTU and Tore Supra, a strong decay of the brehmsstrahlung emission is observed when the density is increased, much faster that the prediction of LHCD modelling. On JET, LH power deposition is also found to be sensitive to the plasma density: LH power modulation indicates that the power deposition moves to the very edge of the plasma (r/a ~ 0.9) when the density approaches the requirement of the JET SS scenario. From this experiment but also from the reconstruction of the electron cyclotron emission spectrum, the decrease of the LHCD efficiency with density is also found. From LHCD modelling of different JET pulses performed at different densities and wave parallel refraction indexes, it is concluded that the wave accessibility condition is not the key parameter for explaining the decrease of the efficiency. C-Mod, FTU and Tore Supra experiments indicate that the plasma edge parameters, namely density and temperature but also fluctuations, are affecting the efficiency via loss mechanisms which are likely to be collisional damping (C-Mod), parametric decay instabilities or wave scattering (FTU/ Tore Supra).

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.

Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device.Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high β p spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed the detailed information on three discharge phases: (i) In the initial current formation phase, linearity between the plasma current and the stored energy was confirmed. (ii) In the current jump phase, the initial closed flux surfaces cause a change in the current increasing rate, but the stored energy does not show such a change. (iii) The current sustained plasma is characterised by the fraction of the current inside the last closed flux surface to the total current, and the fraction seems to determine the ratio of the plasma current to the external vertical field. MHD instabilities often terminate the RF sustained plasma, but no such phenomenon was observed in the EC sustained plasma. IntroductionKey issues in spherical tokamak (ST) research are plasma current I p start-up and formation of the ST configuration without the use of a central solenoid (ohmic coil). Successful current generation, ST formation and sustainment have been achieved by injecting RF power (usually in the EC frequency range) to a configuration with a toroidal field and a weak vertical field. This scenario was developed in CDX-U using EC waves [1], and similar experiments were performed in the ST devices: LATE [2,3], TST-2@K [4], TST-2 [5], CPD [6], MAST [7]. A clear transition from open field line configuration to ST configuration, accompanied by a rapid increase in I p (so called current jump), was found in LATE. This phenomenon was also observed in TST-2 and in CPD. Experiments in these devices suggest that a higher vertical field B z and a higher EC power are preferable to achieve a higher I p as long as a current jump occurs. However, the current formation mechanism and the ST formation mechanism are still not clearly understood. Especially, equilibrium reconstruction was performed for only one case [4], and the time evolution and the variation in different operations remain unknown. The mechanisms for ST formation and the features of the plasma should be identified to extrapolate present results to next step ST devices. In TST-2, effects of various operational parameters are studied in [8]. It was found that the sustained current is roughly proportional to the vertical field strength B z , but dependences on other parameters are very weak. On the other hand, the initial current ramp-up rate depends on several parameters, and a scaling law was obtained. A current jump occurs when the initial current reaches a value proportional to B z , and the value is consistent with the condition to form closed flux surfaces. Recently, a low frequency RF source (21MHz) was used, and ST configuration was sustained by a non-EC heating method for the first time [9]. There is a threshold in the RF power, and the threshold for deuterium plasma was lower...