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In the moderate magnetic field of TCV (1.5 T), the recently installed X3 system (3 gyrotrons, 118 GHz, 0.45 MW each, 2 s) broadens the operational space with the possibility of heating plasmas at high density, well above the cutoff density of the X2 system. To compensate for the significantly weaker absorption coefficient compared to the absorption of X2, the top-launch injection allows the ray path to maximize along the resonance layer thereby maximizing the optical depth. To maintain the maximum absorption in plasma discharges with a dynamic variation in both density (refraction) and temperature (relativistic shift) a real-time control system on the mirror injection angle, based on a mirror modulation technique (synchronous demodulation), has been developed and successfully tested on TCV. Comparisons of the absorption calculated with the TORAY-GA ray-tracing code and the ECWGB beam-tracing code, which includes diffraction effects, are presented. An experimental study of the X3 absorption versus plasma density in an L-mode plasma shows that with a total injected power of 1.35 MW full single-pass absorption is reached with a significant fraction of the absorbed power associated with the presence of suprathermal electrons. Compared with ohmic/low-power-heating of ELMy H-modes, it has been possible to enter into a different ELMy regime with an injected power of 1.35 MW.

L-mode to H-mode transitions (LH transitions) are obtained in a wide range of plasma parameters in TCV Ohmic discharges. LH transitions either occur directly, indicated by a single abrupt drop in D_α emission, or present an intermediate ‘dithering’ phase before the H-mode state is attained, in which D_α emission shows rapid excursions between L-mode and H-mode levels. The type of transition, with or without a dithering phase, depends on the plasma current and plasma triangularity. It is less sensitive to plasma density and divertor geometry. The relative direction of the ion ∇ B drift also plays a role in the characteristics of the LH transition.The time delay between an LH transition and the preceding sawtooth crash has been measured in every TCV H-mode discharge. Most LH transitions are found to occur in the first 30% of the sawtooth cycle, i.e. in the first 2 ms after a sawtooth crash. Moreover, LH transitions often occur soon after a large crash of a ‘double sawtooth’. The distribution function of these time delays clearly indicates a synchronization of these two processes. However, the heat pulse released during a sawtooth crash has a transit time from the q = 1 surface to the plasma edge, which is much shorter than the time delay between the sawtooth crash and the LH transition.

The extreme shaping capabilities of the TCV tokamak have been used to investigate the effect of plasma geometry on the confinement of non-recycling trace impurities injected by means of the laser blow-off technique. The progression of the injected silicon in the core of TCV Ohmic limiter plasmas was followed by the 200-channel soft x-ray (SXR) photodiode array with good spatial and temporal resolution. The results show that the plasma triangularity and elongation play an important role in the impurity confinement time, τ_imp. Remarkably, the increase of elongation from κ = 1.6 to 2.3 produces a threefold reduction of τ_imp while the electron energy confinement time, τ_Ee, remains almost constant. τ_imp is fairly constant in the triangularity scan for δ > 0.2, while there is a marked increase for lower values, leading to τ_imp > 100 ms for negative triangularities. The increase of the toroidal magnetic field, B_T, from B_T = 0.92 to 1.47 T produces a decrease in the confinement time by almost a factor of 2. Simulation of the evolution of the line-integrated SXR signals, performed by the one-dimensional code STRAHL, provided both central and peripheral values of the transport coefficients together with estimates of the radial profiles. The simulations show that anomalous transport is dominant over neoclassical transport, except near the plasma centre. Interestingly, the convective velocity is positive (outwardly directed) in all limiter cases.

Moderately peaked electron density profiles are observed in virtually all plasma conditions in TCV. The existence of an anomalous pinch is unambiguously demonstrated by the observation of peaked density profiles in stationary, fully relaxed, fully current driven electron cyclotron current drive (ECCD) discharges with V_loop&equal;0. The behaviour of the density profiles from a database of 300 Ohmic L- and H-mode, as well as electron cyclotron heating and ECCD discharges, is compared to predictions of models based on the Ware pinch, the curvature pinch and anomalous thermodiffusion. Best overall agreement throughout the database is obtained with models combining an anomalous pinch mechanism, such as the curvature pinch, with the Ware pinch.

L-mode to H-mode (LH) transitions are observed in a large variety of plasma conditions in TCV Ohmic plasmas. The power flowing through the separatrix at the LH transition differs by up to a factor of two from the predicted H-mode threshold power. Variations in this deviation are due to the large operational domain covered by the transitions as well as from the different scenarios identified to access the H-mode regime. LH transitions obtained during the plasma current ramp at low plasma current, elongation and triangularity have a threshold power below the standard expression while transitions at high current and triangularity or during the stationary phase of the discharge have a threshold power above the standard expression.

An experimental study of the extraordinary mode (X mode) absorption at the third electron cyclotron harmonic frequency has been performed on the TCV tokamak in plasmas preheated by X mode at the second harmonic. Full single pass absorption of injected X3 power was measured with X2 preheating in co-current drive (CO-ECCD). The measured absorption exceeds that predicted by the linear ray tracing code TORAY-GA by more than a factor of 2 for the CO-ECCD case. Experimental evidence indicates that a large fraction of the X3 power is absorbed by electrons in an energetic tail created by the X2 ECCD preheating.

An analysis of the effect of magnetic geometry on heat flux in a wide range of single null diverted discharge configurations with 1.5<κ₉₅<2.2, 0.2<δ₉₅<0.45 suggests that elongation is beneficial for confinement. If heat diffusivities are assumed not to depend on plasma shape, the predicted exponent for a power law scaling with elongation is α_κ≈0.74, based on the configurations in the study. This is very close to dependences from the ITER database for discharge conditions, such as ELMy H-modes, obtained from a large number of experiments in various tokamak devices.