1,721,471 research outputs found
Understanding the core density profile in TCV H-mode plasmas
No full textResults from a database analysis of H-mode electron density profiles on the Tokamak à 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.CRPPSP
Effects of plasma shape on laser blow-off injected impurity transport in TCV
No full textThe 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, τimp. Remarkably, the increase of elongation from κ = 1.6 to 2.3 produces a threefold reduction of τimp while the electron energy confinement time, τEe, remains almost constant. τimp is fairly constant in the triangularity scan for δ > 0.2, while there is a marked increase for lower values, leading to τimp > 100 ms for negative triangularities. The increase of the toroidal magnetic field, BT, from BT = 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
Impact of Plasma Trangularity and Collisionality on Electron Heat Transport in TCV L-mode Plasmas
No full textThe impact of plasma shaping on electron heat transport is investigated in TCV L-mode plasmas. The study is motivated by the observation of an increase in the energy confinement time with decreasing plasma triangularity which may not be explained by a change in the temperature gradient induced by changes in the geometry of the flux surfaces. The plasma triangularity is varied over a wide range, from positive to negative values, and various plasmas conditions are explored by changing the total electron cyclotron (EC) heating power and the plasma density. The mid-radius electron heat diffusivity is shown to significantly decrease with decreasing triangularity and, for similar plasma conditions, only half of the EC power is required at a triangularity of -0.4 compared with +0.4 to obtain the same temperature profile. Besides, the observed dependence of the electron heat diffusivity on the electron temperature, electron density and effective charge can be grouped in a unique dependence on the plasma effective collisionality. In summary, the electron heat transport level exhibits a continuous decrease with decreasing triangularity and increasing collisionality. Local gyro-fluid and global gyro-kinetic simulations predict that trapped electron modes are the most unstable modes in these EC heated plasmas with an effective collisionality ranging from 0.2 to 1. The modes stability dependence on the plasma triangularity is investigated.CRPPSP
Divertor power load studies for attached L-mode single-null plasmas in TCV
Full text linkThis paper investigates the power loads at the inner and outer divertor targets of attached, Ohmic L-mode, deuterium plasmas in the TCV tokamak, in various experimental situations using an Infrared thermography system. The study comprises variations of the outer divertor leg length and target flux expansion, the plasma current and a reversal of the magnetic field direction. The direct impact of the divertor magnetic geometry on scrape-off layer (SOL) transport-parameterised by the SOL power fall-off length lambda(q,u,) the divertor spreading factor S-u and the in-out power asymmetry-is reported for constant core properties. The in-out power asymmetry increases, either with the divertor leg length, or the target flux expansion. The SOL width lambda(q,u) scales positively with divertor leg length, with a strength that depends on the field direction and differs between the inner and outer divertor. This implies a parametric dependence of lambda(q,u) that is not explicitly included in current multi-machine scaling laws. The divertor spreading factor at the target S = S(u)f(x), where f(x) is the target flux expansion, appears unaffected by changes in the divertor geometry and in the plasma current, is independent of the magnetic field direction and is similar between inner and outer divertor. Possible interpretations of these observations using an ad-hoc analytical purely conductive model for the SOL, by ion drifts or by asymmetric turbulent cross-field transport in the divertor are presented. The observed values of lambda(q,u) are related to existing L-mode and H-mode scaling laws and to similar studies in other tokamaks. Finally, potential implications of these findings for future larger fusion machines are discussed.SP
A new method for the inversion of interferometry data using basis functions derived from singular value decomposition of local measurements in tokamak plasmas
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