A. Lyssoivan

The dynamic ergodic divertor in the TEXTOR tokamak: plasma response to dynamic helical magnetic field perturbations

Recently, the dynamic ergodic divertor (DED) of TEXTOR has been studied in an m/n = 3/1 set-up which is characterized by a relatively deep penetration of the perturbation field. The perturbation field creates (a) a helical divertor, (b) an ergodic pattern and/or (c) excitation of tearing modes, depending on whether the DED current is static, rotating in the co-current direction or in the counter-current direction. Characteristic divertor properties such as the high recycling regime or enhanced shielding have been studied. A strong effect of the ergodization is spin up of the plasma rotation, possibly due to the electric field at the plasma edge. Tearing modes are excited in a rather reproducible way and their excitation threshold value, their motion and their reduction due to the ECRH/ECCD have been studied. The different scenarios are characterized by strong modifications of the toroidal velocity profile and by a reduced or enhanced radial transport.

0D model of magnetized hydrogen helium wall conditioning plasmas

In this paper the 0D description of magnetized toroidal hydrogen?helium RF discharges is presented. The model has been developed to obtain insight into the ICRF plasma parameters, particle fluxes to the walls and the main collisional processes, which is especially relevant for the comprehension of RF wall conditioning discharges. The 0D plasma description is based on the energy and particle balance equations for nine principal species: H, H+, H2, , , He, He+, He2+ and e?. It takes into account (1) elementary atomic and molecular collision processes, such as excitation/radiation, ionization, dissociation, recombination and charge exchange, and elastic collisions, (2) particle losses due to the finite dimensions of the plasma volume and confinement properties of the magnetic configuration, and particle recycling, (3) active pumping and gas injection, (4) RF heating of electrons (and protons) and (5) a qualitative description of plasma impurities. The model reproduces experimental plasma density dependences on discharge pressure and coupled RF power, both for hydrogen RF discharges (ne ? (1?5) ? 1010?cm?3) and for helium discharges (ne ? (1?5) ? 1011?cm?3). The modeled wall fluxes of hydrogen discharges are in the range of what is estimated experimentally: ~1019?1020?m?2?s?1 for H atoms, and ~1017?1018?m?2?s?1 for H+ ions. It is found that experimentally evidenced impurity concentrations have an important impact on the plasma parameters, and that wall desorbed particles contribute largely to the total wall flux.

Self-consistent application of ion cyclotron wall conditioning for co-deposited layer removal and recovery of tokamak operation on TEXTOR

This paper presents a demonstration experiment of ion cyclotron wall conditioning (ICWC) on TEXTOR covering all ITER wall conditioning aims and discusses the implications for ITER. O2/He-ICWC applied to erode carbon co-deposits removed 6.6xa0×xa01021 C-atoms (39 pulses, 158xa0s cumulated discharge time). Large oxygen retention (71% of injected oxygen) prevented subsequent ohmic discharge initiation. Plasma operation was recovered by a 1h47 multi-pulse D2-ICWC procedure including pumping time between pulses with duty cycle of 2xa0s/20xa0s, cleaning the vessel from oxygen impurities, followed by a 23xa0min He-ICWC procedure (2xa0s/20xa0s), applied to desaturate the deuterium-loaded walls. A stable ohmic discharge was established on the first attempt right after the recovery procedure. The discharges showed improved density control and only slightly increased oxygen characteristic radiation levels (1–1.5 times). After the recovery procedure 36% of the injected O-atoms remained retained in the vessel, derived from mass spectrometry measurements. This amount is in the estimated range for storage in remote areas obtained from surface analysis of locally exposed samples. The removed amount of oxygen by D2 and He-ICWC obtained from mass spectrometry corresponds to the retention in plasma-wetted areas estimated by surface analysis. It is concluded that most of the removed oxygen stems from plasma-wetted areas while shadowed areas, e.g. behind poloidal limiters, may feature net retention of the discharge gas. On ITER, designed with a shaped first wall, the ICWC plasma-wetted area will approach the total surface area, reducing consequently the retention in remote areas. A tentative extrapolation of the carbon removal on TEXTOR to tritium removal from co-deposits on ITER in the 39xa0×xa04xa0s O2/He-ICWC discharges, including pumping time between the RF pulses, corresponds on ITER to a tritium removal in the order of the estimated retention per 400xa0s DT-burn (140–500xa0mgT (Shimada and Pitts 2011 J. Nucl. Mater. 415 S1013–6)).

Status of the ITER IC H&CD System

The ITER Ion Cyclotron Heating and Current Drive system will deliver 20 MW of radio frequency power to the plasma in quasi continuous operation during the different phases of the experimental programme. The system also has to perform conditioning of the tokamak first wall at low power between main plasma discharges. This broad range of requirements imposes a high flexibility and a high availability. The paper highlights the physics and design requirements on the IC system, the main features of its subsystems, the predicted performance, and the current procurement and installation schedule.

ICRF Wall Conditioning: Present Status and Developments for Future Superconducting Fusion Machines

ITER and future superconducting fusion machines need efficient wall conditioning techniques for routine operation in between shots in the presence of permanent high magnetic field for wall cleaning, surface isotope exchange and to control the in-vessel long term tritium retention. Ion Cyclotron Wall Conditioning (ICWC) based on the ICRF discharge is fully compatible and needs the presence of the magnetic field. The present paper focuses on the principal aspects of the ICWC discharge performance in large-size fusion machines: (i) neutral gas RF breakdown with conventional ICRF heating antennas, (ii) antenna coupling with low density (similar to 10(17) m(-3)) RF plasmas and (iii) ICWC scenarios with improved RF plasma homogeneity in the radial and poloidal directions. All these factors were identified as crucial to achieve an enhanced conditioning effect (e.g. removal rates of selected marker masses). All the observed effects are analyzed in terms of RF plasma wave excitation/absorption and compared with the predictions from I-D RF full wave and 0-D RF plasma codes. Numerical modeling and empirical extrapolation from the existing machines give good evidence for the feasibility of using ICWC in ITER with the main ICRF antenna.