R. Reichle

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.

Thermography of target plates with near-infrared optical fibres at Tore Supra

Abstract First spectroscopic near-infrared thermography measurements in the range 0.9–1.95 μm performed with optical fibres are reported. Two set-ups served as test-beds for physical and technical questions for a security system based on fibre optical thermography. It was found that for the interesting temperature range above 600°C atomic and molecular line emission is negligible in comparison with thermal radiation. The observed near-infrared spectra are however different from simple blackbody radiation curves. They are explained by the coverage of the surface with dust and flakes. The dust particles are identified by their spectral emissivity falling off with the square of the wavelength. On one set-up, flakes were identifiable by fast cool-down times and confirmed by post-experiment inspection. In the absence of flakes, surface temperatures on a ripple protection plate were measured, that allowed to determine the mean energy of ripple trapped ions to be 200–300 keV.

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.

Surface temperature measurements on Tokamak target plates with two types of infra red fibres

Abstract Tore Supra has chosen to protect the most heavily solicited and actively cooled target plates by thermography using optical fibres. In consequence to previous experience [J. Nucl. Mater. 290–293 (2001) 701], the prototype periscope for the 2001 experimental campaign was equipped not only with silica fibres but also with a ZrF 4 fibre, transparent up to 4 μm. As then, the measurements in the mid infrared range give lower temperatures than the ones in the near infrared range. The silica fibres are more robust and easier to calibrate, which may justify to use both types side by side using a cross calibration formula. Measurements with an silica fibre embedded in the interior of the target plate exhibited only during discharges with LHCD heating sufficient signal levels to be spectrally analysed. Under these conditions the same kind of additional near infrared radiation as on the silica fibres looking at the exterior of the target plates was observed.

Particle recirculation in the ergodic divertor of Tore Supra

The present paper addresses the issue of particle recirculation in discharges where low-energy flux to ergodic divertor target plates is achieved in highly-radiating detached ohmic plasmas. Plasma temperature and particle flux are measured by flush-mounted probes in the divertor plates and by an upstream fast scanning Mach probe. The scalings with core density of the ion flux and electron temperature are well described by the simple two-point model used in axisymmetric poloidal divertors. The detachment signature is a pressure drop that occurs when the edge temperature falls below 10 eV. The parallel ion flux gradient is always positive, indicating that recombination is unlikely to play an important role in detachment. Visible spectroscopy of a neutralizer plate shows that attainment of cold detached plasmas near the density limit coincides with an abrupt increase of fuelling efficiency for both deuterium and impurities. A feedback algorithm based on real-time Langmuir probe measurements has been developed to monitor detachment and avoid disruptions.

Ergodic divertor experiments on the route to steady state operation of Tore Supra

Ergodic divertor operation on Tore Supra is characterized by good performance in terms of divertor physics. Control of particle recirculation and impurity screening are related to the symmetry, both poloidally and toroidally, of the shell of open field lines and to its radial extent, Δr ≈ 0.16 m. Feedback control of the divertor plasma temperature has led to controlled radiative divertor experiments. In particular, good performance is obtained when the plasma is controlled to be at a temperature comparable to the energy involved in the atomic processes (15-20 eV). For standard discharges with 5 MW total power and ICRH heating, the low parallel energy flux ≈ 10 MW m-2 is reduced to ≈ 3 MW m-2 with nitrogen injection. This is achieved at a modest cost in core dilution, ΔZeff ≈ 0.3. Despite the large volume of open field lines ( ≈ 36%), the ergodic divertor does not reduce the possible current in the discharge since stable discharges are achieved with qsep ≈ 2. It is shown that the reorganization of the current profile in conjunction with a transport barrier in the electron temperature on the separatrix stabilizes the (2,1) tearing mode. Confinement follows the standard L mode confinement. In a few cases at high density and with no gas injection (wall fuelled discharges), `RI-like modes are reported with a modest increase in confinement ( ≈ 40%). Despite the lack of core fuelling on Tore Supra, high densities during ICRH pulses can be achieved with Greenwald fractions fG ≈ 1. Compatibility with both ICRH and LH is demonstrated. In particular, long pulse operation with a flat-top in excess of 20 s is achieved with LHCD.

Analysis of energy flux deposition and sheath transmission factors during ergodic divertor operation on Tore Supra

The magnetic deflection of field lines to dedicated wall components in the ergodic divertor of Tore Supra generates complex patterns of power deposition. In this paper, we analyze the energy flux deposition on neutralizer plates as measured by infrared cameras and Langmuir probes. Three important features will be discussed: (1) The energy deposition during helium shots is as much as twice that for deuterium shots, for a given input power level. (2) The sheath heat transmission factor, deduced experimentally by comparison between probes and infrared measurements, increases with input power independently of the working gas from ∼7.5 (P TOT = 1 MW) to ∼10-11 for P TOT = 5 MW). In ohmic discharges, the standard value of 7 is recovered except specific cases in helium where γ can decrease to 2 or 3. (3) These anomalous values put in doubt the validity of edge temperature measurements by Langmuir probes in detached plasmas and have led to the development of a promising infrared degree of detachment (Dod).

Active cooling, calorimetry and energy balance in Tore Supra

The active cooling of the Tore Supra plasma facing components, including its operational limits, is extensively described. The associated calorimetric diagnostic is found to be a powerful and accurate tool to study the energy balance of plasma discharges.

Radiation efficiency of high power ergodic divertor plasmas in Tore Supra

Abstract Highly radiative edge layer experiments in Tore Supra ergodic divertor (ED) plasmas are reported. In these experiments, up to 88% of the injected power is radiated in steady state attached plasma conditions. The observed radiated power is higher than the prediction of Matthews multi-machine scaling. Such behaviour is consistent with the fact that the ratio of the ergodic divertor volume to the main plasma volume is higher for an ergodic divertor than for axisymmetric divertors, or with the enhanced impurity screening observed in Tore Supra. Indeed, a new scaling is derived for impurity radiation in a region of parallel ion transport. It shows that impurity screening is the dominant mechanism controlling divertor radiation, impurity transport along the open field lines having only little influence on P rad .