J.C. Vallet

Improved confinement in high li lower hybrid driven steady state plasmas in TORE SUPRA

The global energy confinement of combined ohmic and lower hybrid driven TORE SUPRA plasmas has been analysed at various densities. In contradiction to the L mode ITER scaling law, this analysis indicates that the global energy confinement time depends strongly on the plasma density. Furthermore, the thermal electron energy content of steady state discharges is found to be in good agreement with the global Rebut-Lallia-Watkins (RLW) scaling law. Current ramp experiments show an enhancement of the global energy confinement with the internal inductance, li. These results have been extended to steady state regimes with lower hybrid current drive. Improved confinement has been obtained in a high li steady state plasma (li=1.7), where the modification of the current profile by lower hybrid waves leads to an increase in the central value of the safety factor (qψ(0) ≈ 2). In this case, the global confinement time is shown to exceed the value predicted by the RLW scaling law by 40%

Progress in ergodic divertor operation on Tore Supra

Upgrade of the Tore Supra ergodic divertor (ED) has led to significant progress in ED physics. Pulse durations of 30?s with LHCD have been achieved demonstrating the heat exhaust capability of both the actively cooled technology at hand and of this specific divertor concept. The disruptive limit governed by the stochastization of the outer magnetic surfaces is found to occur for a value of the Chirikov parameter reaching two on the magnetic surface q = 2+(3/12). This experimentally observed robustness allows one to operate at very low safety factor on the?separatrix (q~2). Numerical analysis of ballooning turbulence in a stochastic layer indicates that the decay of the density fluctuations is associated with an increase of the fluctuating electric drift velocity. This results in an enhanced cross-field transport in the vicinity of the target plates. This lowering of confinement appears to be compensated by an intrinsic transport barrier on the electron temperature. The three-dimensional response of the temperature field is computed with a fluid code. The code can recover the intrinsic transport barrier at the separatrix, reported experimentally, together with small amplitude temperature modulations in the divertor volume. Experimental evidence for the three density regimes (linear, high recycling and detachment) is reported. The low critical density values for transitions between these regimes indicate that similar parallel physics governs the axisymmetric and ED, despite the open configuration of the latter. Measurement and understanding of these density regimes provide a means for feedback control of plasma density and an improvement in ion cyclotron radiofrequency heating coupling scenarios. Experimental data also indicated that particle control with the vented target plates is effective. Increase of both impurity control and radiation efficiency are reviewed. Global power balance has been analysed in order to account for non-axisymmetric radiation. These results, taken together, confirm the large radiation capability of the ED.

Magnetic perturbation effects on boundary plasmas during high power lower hybrid current drive in Tore Supra

Small time-independent magnetic perturbations (δ b r ), produced with the Tore Supra ergodic divertor coils, have been used to control thermal loads on plasma facing components, current density profiles, the transport of non-Maxwellian particles, and the confinement properties of thermal plasmas during high power ( P LH ≤3.3 MW) lower hybrid current drive (LHCD) discharges. MARFEs with 0.12 ≤ϱ m =π a 2 n e20 > I p −1 ≤0.22 (i.e., roughly a factor of 3 less than the smallest value of ϱ m previously reported) are obtained during the δ b r pulse when P LH >2.0 MW and the edge safety factor is slightly less than 3. These MARFEs generally appear to have the same characteristics as high ϱ m MARFEs and are positionally stable throughout the LHCD+δ b r pulse. Steady state conditions in which more than 90% of the total input power is radiated from a 0.15 m wide region near the high-field side wall were obtained.

Plasma decontamination during preliminary ergodic divertor experiments in TORE SUPRA

Preliminary experiments in the TORE SUPRA tokamak using the ergodic divertor configuration have shown a strong effect on plasma impurities. The result is a decontamination of the central plasma which is due to a decreased carbon content. This is the consequence of a screening effect of the peripheral ergodic layer which is due to an important increase of the recycling impurity flux, as well as of a modification of the impurity source terms.

Plasma edge control in TORE SUPRA

TORE SUPRA is a large superconducting tokamak designed for sustaining long inductive pulses (t approximately 30 s). In particular, all the first wall components have been designed for steady-state heat and particle exhaust, particle injection, and additional heating. In addition to these technological assets, a strict control of the plasma-wall interactions is required. This has been done at low power: experiments with ohmic heating have been mainly devoted to the pump limiter, ergodic divertor and pellet injection experiments. Some specific problems arising in large tokamaks are encountered; the pump limiter and the ergodic divertor yield the expected effects on the plasma edge. The effects on the bulk are discussed.

Ripple losses during ICRF heating in Tore Supra

The toroidal field coils in Tore Supra are supra-conducting, and their number is restricted to 18. As a result, the ripple is fairly large, about 7% at the plasma boundary. Tore Supra has consequently been equipped with dedicated ripple loss diagnostics, which has allowed ripple loss studies. This paper reports on the measurements made with these diagnostics and provides an analysis of the experimental results, comparing them with theoretical expectations whenever possible. Furthermore, the main heating source accelerating ions in Tore Supra is ion cyclotron resonance range of frequency (ICRF) heating, and the paper provides new information on the ripple losses of ICRF accelerated ions.

Diagnostic Systems on Tore Supra

Abstract Realizing high-power long-duration discharges puts specific constraints on diagnostics: Their front parts have to withstand important thermal loads, eventually requiring active cooling of critical parts, and drifts in measurements have to be avoided in order to supply reliable measurement during the whole discharge duration. Furthermore, the importance of diagnostics for missions other than physics understanding, such as machine operation or safety control, increases. The diagnostics system of Tore Supra consists of roughly 30 diagnostics, covering a large range of plasma parameters from the core to the edge. They have been designed for long-duration plasma discharges, which can last up to 1000 s. Their inner components have been dimensioned to endure continuous high-radiation fluxes, and most of them have been conceived to give a fair measurement all along the discharges.

Heat flux pattern on the toroidal pump limiter of Tore Supra: first observations and preliminary analysis

Abstract Tore Supra has re-started operation in 2001 with a partial toroidal pump limiter as the main plasma facing component. Assembled with actively cooled high heat flux elements, it has the shape of a toroidal ring. Thermal equilibrium is achieved within 4 s and the stabilised surface temperature is comparable to the design calculation. The observed heat flux pattern is similar to calculations realised beforehand with the heat deposition code TOKAFLU, and is dominated by the large toroidal field ripple. The power deposition is a combination of parallel convection and cross-field transport of similar magnitudes. The heat flux decay length is evaluated by three independent diagnostics to 10 mm. The excellent agreement between experiment and modelling indicates that the description of the power deposition mechanism is reasonable, although parameters such as the heat flux decay length and fraction of perpendicular power are still difficult to predict.

STABILIZATION OF TEARING MODES IN LOWER HYBRID CURRENT DRIVE DISCHARGES ON PETULA-B

The effects of the RF current drive on a hybrid RF-OH discharge, in which more than 70% of the plasma current is driven by the RF, have been extensively studied in the Petula-B tokamak. Observations of MHD activity indicate that the presence of RF-driven current cuppresses the growth of the MHD modes and allows stable low-q(a) (as low as 2) discharges with electron density and temperature profiles similar to those of the OH case.

Edge turbulence during ergodic divertor operation in Tore Supra

We report new measurements of turbulence during ergodic divertor (ED) operation. At low density, some de-correlation of the turbulence is observed with a decrease of the long timescale structures. It is shown that the typical time involved is compatible with a de-correlation mechanism through radial separation of the B field lines by the ED, with an associated parallel length of the order of the distance between two modules of the ED. This observation reinforces the conclusion drawn in [1] and based on computer simulations. The situation changes when the density is increased: the turbulence level is found to increase. At the highest density, the structure of the turbulent signal is modified and the bursty behaviour suppressed by the ED at low density reappears. These observations lead to the conclusion that the turbulence measured at high density is not sensitive to the ED stabilization effect. This indicates that it could be carried by the ions.