First Measurements of Electron Temperature Fluctuations by Correlation ECE on Tore Supra

Electron temperature fluctuation studies can help to understand the nature of the turbulent transport in to-kamak plasmas. At Tore Supra, a 32-channel heterodyne ECE radiometer has been upgraded with two chan-nels of 100 MHz bandwidth and tunable central frequencies allowing the shift of the plasma sample volume in the radial direction. With the sufficiently large video bandwidth and the long sampling time, it is possible to reduce significantly the thermal noise and to identify "true" high frequency components up to 200 kHz from the cross-correlation between these channels. First results of temperature fluctuation measurements on Tore Supra are reported in this paper.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Langmuir probe electronics upgrade on the tokamak a configuration variable

A detailed description of the Langmuir probe electronics upgrade for TCV (Tokamak a Configuration Variable) is presented. The number of amplifiers and corresponding electronics has been increased from 48 to 120 in order to simultaneously connect all of the 114 Langmuir probes currently mounted in the TCV divertor and main-wall tiles. Another set of 108 amplifiers is ready to be installed in order to connect 80 new probes, built in the frame of the TCV divertor upgrade. Technical details of the amplifier circuitry are discussed as well as improvements over the first generation of amplifiers developed at SPC (formerly CRPP) in 1993/1994 and over the second generation developed in 2012/2013. While the new amplifiers have been operated successfully for over a year, it was found that their silicon power transistors can be damaged during some off-normal plasma events. Possible solutions are discussed. (C) 2019 Author(s)

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Z impurity profiles. The L-H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β. The enhanced D α H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts

Mitigation of disruptions by fast helium gas puffs

In order to mitigate the effect of disruptions in tokamaks, it is proposed to inject quickly a relatively large amount of helium; first experiments on this topic have been performed on TEXTOR. For this purpose, a fast valve has been developed which releases 10 mbar L of helium gas within 1 ms; the valve is located at a vessel flange such that a fast response is guaranteed even if it is triggered at the onset of the disruption. The amount of gas is sufficient to exceed the density limit even with low density discharges. The intention of the proposal is to shorten the plasma current decay phase, to reduce halo currents, to suppress runaway electrons and to provide good conditions for the start of the following discharge. In particular, for achieving the last goal, helium is the optimum choice of all the elements. The experiments performed on TEXTOR have proven various of these mitigation aspects: the current decay time is shortened, runaway electrons are expelled by the gas puff and the conditions for the start of the next discharge have neither deteriorated with respect to gas release from wall components nor with respect to excessive impurity production

Reflectometry measurements during operation of the Dynamic Ergodic Divertor at TEXTOR

The O-mode correlation reflectometer at TEXTOR is used for the investigation of the propagation and the properties of the turbulence at the plasma edge. The multi-horn antenna set-up allows the investigation of the correlation properties of the turbulence and its rotation. Turbulence and plasma rotation for different plasma conditions are found to be in agreement and based on this comparison the radial electric field is estimated. In addition the density fluctuation level is estimated from the standard deviation of the phase of the reflected signal.The influence of a magnetic perturbation field, generated by the dynamic ergodic divertor (DED), on the turbulence and transport properties is studied and compared with plasmas without such a field perturbation. The strength and radial range of the perturbation field can be widely varied. Together with tangential neutral beam injection in co- and counter-current directions, the turbulent transport is investigated. The combination of neutral beam injection and the DED enables the modification of the plasma rotation profile. It can lead either to the generation of a locked mode or the formation of a transport barrier

Poloidal asymmetry in perpendicular plasma rotation and radial electric field measured with correlation reflectometry at TEXTOR

Measurements of plasma rotation and electric field are crucial for the study of plasma confinement and transport. The present paper is devoted to experimental observations of poloidal asymmetry in perpendicular plasma rotation with correlation reflectometry on TEXTOR