[en] In this paper a spectroscopic investigation of an expanding surface wave produced plasma is presented. The electron temperature is evaluated by means of emission spectroscopy. The results show a strong and sharp increase of the electron temperature toward the end of the discharge, where the plasma is expanding into a region with lowered gas pressure

[en] Modulated beam mass spectroscopy is applied as a quantitative method for the measurement of the atomic nitrogen concentration in an expanding remote plasma. The plasma is created in a quartz tube by an electromagnetic surface wave (the upstream pressure ranges between 10 and 20 mbar). The recombined plasma with its N radicals is expanded into a reaction chamber where the pressure is about 10^-3 mbar. In order to take into account the losses of atomic nitrogen by sticking and recombination at the inner walls of the mass spectrometer, a chopper has been inserted between the orifice of the mass spectrometer and the ionization chamber. Hence, the measured signal is separated into a beam and a background component, with different dependencies on the expansion properties. The beam-to-background ratio is found to depend on discharge parameters such as total pressure or gas mixture composition. A correction factor for the wall losses which is independent of these parameters allows the quantitative measurement of the N flux

[en] The aim of paper was to investigate the effect of wall cleaning by ICWC on KSTAR. To achieve goal, the optical diagnostic systems consist of H-alpha monitors, visible survey spectroscopy (VSS) and filter scope (FS) has been utilized. In this paper, the experiment setup, experiment results and summary are introduced. Ion Cyclotron Wall Conditioning (ICWC) experiments have been conducted in 2012 Korea Superconducting Tokamak Advanced Research (KSTAR) campaign. Wall conditioning is essential task to make high-quality condition for plasma experiments. Especially, by using ICWC which is operable under strong magnetic field

[en] This paper gives an overview of the experimental and modeling activity on Ion Cyclotron Wall Conditioning (ICWC), with hydrogen as working gas, in order to assess the applicability of this technique on ITER for recovery from disruptions, vent or air leak, recycling control and mitigation of the tritium inventory build-up. Experimental results obtained on TORE SUPRA, TEXTOR, ASDEX-Upgrade, JET, KSTAR and LHD are presented. The conditions for safely producing RF plasmas with conventional ICRH antennas have been carefully investigated. Discharge homogeneity has been improved by adding a small poloidal component to the toroidal field. Proper choice of the duty cycle RF on/off allows mitigating re-implantation of wall desorbed particles reionized in the ICWC discharge. A 0-D model of ICWC plasmas in He and H₂ has been developed which reproduces experimentally determined electron density, neutral and ion fluxes. The installation in 2010/11 of the ITER like wall (ILW) in JET, allows now assessing the efficiency of ICWC for fuel removal on the ITER material mix. Experimental observations in carbon devices and modeling seem to indicate that ICWC, like other plasma-based conditioning techniques, interacts preferentially with transient reservoirs rather than in co-deposited layers. An attempt is made to extrapolate the found fuel removal rates to ITER. (author)

[en] The Tore Supra tokamak is devoted to long-duration, high-performance plasma research. The real-time measurements and control (RTMC) system has been developed to address the basic tokamak controls, such as plasma equilibrium and density control. Over the years, more and more sophisticated and demanding controls have been implemented, allowing the improvement of plasma performance and machine protection. This includes current profile control to enhance the performance and to avoid magnetohydrodynamic instabilities, infrared monitoring of plasma-facing components to prevent overheating, and disruption detection and mitigation techniques. Most of these improvements are relevant to the plasma operation in a full steady-state regime. This paper describes the present status of the Tore Supra RTMC system, detailing recent progress and highlighting the advantages of the various control schemes implemented so far. (authors)

[en] A shielded residual gas analyzer (RGA) system on Tore Supra can function during plasma operation and is set up to monitor the composition of the neutral gas in one of the pumping ducts of the toroidal pumped limited. This ''diagnostic RGA'' has been used in long-pulse (up to 6 min) discharges for continuous monitoring of up to 15 masses simultaneously. Comparison of the RGA-measured evolution of the H₂/D₂ isotopic ratio in the exhaust gas to that measured by an energetic neutral particle analyzer in the plasma core provides a way to monitor the evolution of particle balance. RGA monitoring of corrective H₂ injection to maintain proper minority heating is providing a database for improved ion cyclotron resonance heating, potentially with RGA-base feedback control. In very long pulses (>4 min) absence of significant changes in the RGA-monitored, hydrocarbon particle pressures is an indication of proper operation of the actively cooled, carbon-based plasma facing components. Also H₂ could increase due to thermodesorption of overheated plasma facing components.

[en] Ion Cyclotron Wall Conditioning (ICWC) is envisioned in ITER to clean the wall from impurities, to control the wall isotopic ratio and the hydrogen recycling in the presence of the toroidal magnetic field. Various experiments and modelling are advancing to consolidate this technique. In this contribution the modeling of ICWC is presented, which can be divided in two parts: plasma description and plasma wall interaction. Firstly a 0D plasma model, based on a set of energy and particle balance equations for Maxwellian Hydrogen and Helium species, is presented. The model takes into account elementary collision processes, coupled RF power, particle confinement, wall recycling, and active gas injection and pumping. The RF plasma production process is based mainly on electron collisional ionization. The dependency of the plasma parameters, the Hydrogen and Helium partial pressures and neutral or ionic fluxes on pressure and RF power are quantitatively in good agreement with those obtained experimentally on TORE SUPRA. Secondly an extension of the 0D model including the description of the wall interaction is presented and compared to TORE SUPRA multi-pulse ICWC discharges.

[en] In carbon dominated devices, the in vessel D inventory obtained from post-mortem analyses of plasma facing component samples is generally smaller by a factor of ∼4 than that estimated from gas balance measurements. However, for an accurate evaluation of the wall inventory, gas balance measurements must be done not only during discharges and conditioning procedures, but also in between discharges and during vents. From the analysis of the whole Tore Supra database for the 2002–2007 period, we show that long term outgassing during nights, weekends and vents is essential for evaluating the deuterium release. Taking these contributions into account reconciles the gas balance and post-mortem estimations of fuel retention

[en] Tritium (T) retention constitutes an outstanding constraint for ITER. It has been proposed that the end of the discharge could be used for reducing the amount of tritium trapped in the device by switching to He or H₂ injection during the ∼200 s of plasmas following the burning phase (power and plasma current ramp down). Thanks to the long discharge capabilities of Tore Supra, long pulse experiments (> mn) have been carried out to evaluate the effectiveness of such a scenario in reducing the tritium inventory during plasma operations. Starting with the device operated only in D₂, series of changeover experiments from D₂ to He and from D₂ to H₂ have been carried out in Tore Supra. The results demonstrate that with He the amount of D recovered after 130 s is limited to 0.8 × 10²² D whilst no further gain is foreseen. From these experiments, it is demonstrated that He injection will not contribute to the drop of the tritium inventory in the vessel. In contrast, with H₂ injection the amount of D recovered after 250 s is ∼4.2 × 10²² D with no limitation observed in the amount that could be removed from the vessel. The higher efficiency in removing D from the vessel by H₂ injection compared to He is attributed to the H charge-exchange (CX) flux (four to six times larger than the He CX flux) allowing for a significantly stronger plasma wall interaction with carbon deposition and layer areas. In Tore Supra, since most of the D retention through co-deposition with eroded material (C) takes place in these areas, H plasmas result in a better removal efficiency of D(T) from these regions. These experimental observations are supported by the results obtained using the EIRENE code for evaluating both the ion and CX fluxes for He and H plasmas. Finally, the consequences of removing D(T) from the vessel for the next discharges are unfavourable for both the He and H₂ removal methods. Indeed, in both cases, twice the amount of D(T) removed through the isotope exchange has to be re-injected since co-deposition of the re-injected D(T) will also take place in addition to the plasma wall isotope exchange. In these conditions, the low efficiency of the H₂ gas injection for controlling the plasma isotopic ratio inhibits a recovery of the initial plasma isotopic ratio over a time scale in the range of 200 s. (paper)

[en] Disruptive plasma termination was proposed for controlling long-term fuel retention because the subsequent increase of the post-discharge outgassing could help in minimizing the fuel stored in the vessel after each discharge. The efficiency of this method is investigated by analyzing the Tore Supra database for the period 2004-2010 (450 disruptions). Results are shown for the plasma motion during the current quench and the heating of deposits at the Toroidal Pumped Limiter surface. Dependences of the outgassing with internal and magnetic energy contents, increment of the wall inventory and cumulated time between two disruptions are characterized. It is shown: (1) that the desorbed gas originates from D-rich deposits close to the plasma when those located in remote areas are weakly affected and (2) that the amount of released gas saturates with discharge duration and time delay between two disruptions, which limits the efficiency of the method for long discharge operation. However, due to the larger volume/surface ratio, the efficiency of the method is expected to increase with the size of the device.