Tcv Team

Empirical scaling of sawtooth period for onset of neoclassical tearing modes

Experimental observations from a range of tokamaks show that neoclassical tearing modes (NTMs) are triggered at lower plasma pressure when the sawtooth period is longer. A multi-machine database from nine tokamaks has been established in order to extrapolate the acceptable sawtooth period to avoid triggering NTMs in ITER. It is found that the governing physics is best compared between machines by normalizing the sawtooth period to the resistive diffusion time and using the normalized beta as a measure of performance and global stability. A multi-parameter power scaling is determined from regression analysis of the complete dataset and compared favourably with experimental data from a number of machines.

Full absorption of third harmonic ECH in TCV tokamak plasmas in the presence of second harmonic ECCD

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.

Observation and empirical modelling of the anomalous particle pinch in TCV

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 Vloop = 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.

Third-harmonic, top-launch, ECRH experiments on TCV tokamak

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.

Poloidal asymmetry in the narrow heat flux feature in the TCV scrape-off layer

Heat flux profiles inferred from a reciprocating probe at the outer midplane of the TCV tokamak during inner wall limited discharges feature radial fall-off lengths that shorten near the last closed flux surface (LCFS) consistent with the so-called narrow feature. The narrow feature is significantly wider on the outboard side compared with that measured on the inner wall by infrared thermography, so it is difficult to discern from the main scrape-off layer feature. After small shifts were applied for alignment, the fraction of the power contained in the narrow feature matches between inboard and outboard measurements, and they scale together with plasma current Ip, suggesting that we are observing the same phenomenon. The outboard side fall-off length within the narrow feature is found to scale closely with the radial correlation length of the edge turbulence as expected if the narrow feature arises due to radially sheared E × B flows. This is found to hold true even for cases where the narrow feature is ...

Third harmonic X-mode absorption in a top-launch configuration on the TCV tokamak

In addition to the second harmonic X-mode (X2) electron cyclotron heating (ECH), the TCV ECH system has been completed with three 450 kW gyrotrons operating at the frequency of 118 GHz for third harmonic X-mode (X3) in a top-launch configuration. In the relatively low magnetic field of TCV (1.45 T), the X2 cutoff density is n(e,cutoff)((X2)) = 4 x 10(19) m(-3) and X3 extends the accessible plasma density range up to n(e,cutoff)((X3)) = 11.5 x 10(19) m(-3). The X3 absorption coefficient is lower than that for, X2 by a factor (k(B)T(e))/(m(e)c(2)) and a top-launch injection system has been installed to maximize the beam path along the resonance layer, thus maximizing the optical depth. Theoretical considerations based on a one-dimensional slab geometry model show that the X3 absorption depends mainly on the temperature and the density. It is shown, using a simple two-dimensional model, that in the presence of a suprathermal electron population, the resonance layer width is significantly increased owing to the relativistic shift. The specificity of the top-launch configuration implies that the absorption strongly depends on the propagation direction of the beam. Experimental results are compared with calculations using the linear ray-tracing code TORAY-GA. At the maximum available X3 injected power (1350M), full single-pass absorption is measured, increasing the global electron energy by a factor of 2.5, whereas TORAY-GA predicts only 50% absorption.

Effects of plasma shape on laser blow-off injected impurity transport in TCV

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, tau(imp). Remarkably, the increase of elongation from kappa = 1.6 to 2.3 produces a threefold reduction of tau(imp) while the electron energy confinement time, tau(Ee), remains almost constant. tau(imp) is fairly constant in the triangularity scan for delta > 0.2, while there is a marked increase for lower values, leading to tau(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.

Density profile peaking in the presence of ECRH heating in TCV

Experimental observations in stationary ECRH discharges in TCV show that additional heating has a strong effect on the electron density profile. In the absence of MHD activity or strong internal transport barriers, additional electron heating generally leads to a broadening of the density profiles with respect to the Ohmic target plasma profiles. In the ECRH L-mode and in the presence of weak electron internal transport barriers, the density peaking factor depends on the edge safety factor, the power deposition profile and ECRH power. Beyond a critical power of some 0.5 MW, the power dependence saturates. The edge safety factor dependence is supportive of turbulent equipartition (TEP) theory, which predicts inward convection in the presence of turbulence. The observation of a reduction in the peaking with central electron heating supports drift wave turbulence theory, which predicts the decrease of inward particle convection in addition to the inward convection by TEP, when trapped electron modes are destabilized, thereby reducing the net inward convection.

H-mode threshold power in TCV Ohmic plasmas

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.

Understanding the core density profile in TCV H-mode plasmas

Results from a database analysis of H-mode electron density profiles on the Tokamak a Configuration Variable (TCV) under stationary conditions show that the logarithmic electron density gradient increases with collisionality. By contrast, usual observations of H-modes showed that the electron density profiles tend to flatten with increasing collisionality. In this work it is reinforced that the role of collisionality alone, depending on the parameter regime, can be rather weak, and in these dominantly electron heated TCV cases, the electron density gradient is tailored by the underlying turbulence regime, which is mostly determined by the ratio of the electron to ion temperature and that of their gradients. Additionally, mostly in ohmic plasmas, the Ware-pinch can significantly contribute to the density peaking. Qualitative agreement between the predicted density peaking by quasi-linear gyrokinetic simulations and the experimental results is found. Quantitative comparison would necessitate ion temperature measurements, which are lacking in the considered experimental dataset. However, the simulation results show that it is the combination of several effects that influences the density peaking in TCV H-mode plasmas.