Ion and electron cyclotron wall conditioning in stellarator and tokamak magnetic field configuration on WEGA

Abstract

The influence of magnetic field configuration on an electron cyclotron resonance ion sourcea) Rev. Sci. Instrum. 79, 02A310 (2008); 10.1063/1.2812343Effect of magnetic-field configuration on the beam intensity from electron cyclotron resonance ion source and RIKEN superconducting electron cyclotron resonance ion source Rev. Sci. Instrum. 77, 03A304 (2006); Abstract. Discharge wall conditioning is an effective tool to improve plasma performance in tokamaks and stellarators. RF Discharge Conditioning (RFDC) techniques are envisaged for use during operational campaigns on superconducting devices like the ITER tokamak and W7-X stellarator, as alternative to DC Glow Discharge Conditioning which is inefficient in presence of magnetic fields. This contribution investigates RFDC in both the ion and electron cyclotron range of frequencies (ICRF and ECRF) on the WEGA device (Max-Planck-Institute for Plasma Physics, Greifswald, Germany) as preparation for W7-X operation.ECRF discharges produced by localised absorption of RF power at EC resonance layers suffer from poor radial discharge homogeneity in the tokamak vacuum magnetic field configuration, severely limiting the plasma wetted wall areas and consequently the conditioning efficiency. The non-localised production of ICRF discharges by collisional RF power absorption features much improved discharge homogeneity making Ion Cyclotron Wall Conditioning (ICWC) the favoured RFDC technique for superconducting tokamaks. RFDC with the stellarator vacuum magnetic field needs to aim at sufficient plasma densities at and outside the last closed flux surface (LCFS), maximising the convective plasma flux along the open field lines to the wall. Whereas for ICRF discharges this condition is easily fulfilled, on WEGA for He-ECRF discharges this could be achieved as well by off axis heating close to the LCFS.In stellarator magnetic field configuration it is found that He-ICWC for wall desaturation is at least one order of magnitude more efficient than He-ECWC. Novel ECWC methods are proposed that can decrease this efficiency gap with ICWC to a factor 2-3. The efficiency difference is less pronounced in case of H 2 -ICWC and ECWC for isotopic exchange.

Development of helium electron cyclotron wall conditioning on TCV

“…With low absorption in X2 mode, ECWC discharges at second harmonic are known to be less homogeneous than at first harmonic (see e.g. [6,8]), Indeed, ionization occurs preferentially at the intersection of the EC beam and the ECR surface at ω = n • q • B T • (R)/m e , with q the charge of the electron, m e its mass and n = 2, resulting in poor discharge uniformity, small wetted area and interaction with inner wall surfaces. Discharge homogeneity can be improved by the application of a weak poloidal field, and several attempts aiming at extending the discharge radially by superposing a weak (<1% of B T ) horizontal magnetic field have been reported [5,7].…”

“…Discharge homogeneity can be improved by the application of a weak poloidal field, and several attempts aiming at extending the discharge radially by superposing a weak (<1% of B T ) horizontal magnetic field have been reported [5,7]. A recent paper [8] evidenced however in JT-60U a lower conditioning efficiency in ECWC discharges at second harmonic than in X1-mode, but there is up to now no demonstration of the effectiveness of ECWC at the second harmonic for the operation of a tokamak, i.e. of its ability to efficiently clean wall surfaces for reliable plasma initiation or recovery from disruption.…”

Wall conditioning in fusion devices with superconducting coils

“…Numerous experiments evidenced the effectiveness of ICWC in tokamaks. Experiments demonstrated (i) recovery of sustained ohmic breakdown and density control after disruptions [51] and after severe impurity exposure [52], (ii) full change-over of the isotopic ratio of the JET ITER like wall [53] and (iii) efficient depletion of the tungsten ASDEX Upgrade wall from deuterium by He-ICWC in comparison with H-mode plasmas in helium [54]. The ITER research plan considers ICWC as the preferred RF conditioning technique for the conditioning of the first wall surfaces in presence of the toroidal magnetic field [2].…”

Ion cyclotron resonance heating scenarios for DEMO

“…Numerous experiments evidenced the effectiveness of ICWC, demonstrating (i) recovery of sustained ohmic breakdown and density control after disruptions (realised e.g. on Tore Supra [56]) and after severe impurity exposure (evidenced in TEXTOR [57]), (ii) full change-over of the isotopic ratio of the JET ITER-like wall [58], (iii) full depletion of the tungsten ASDEX-Upgrade wall from deuterium by He-ICWC [59], (iv) higher density, temperature and purity of the main plasma in the U-3M stellarator after ICWC performance [60].…”