Wall conditioning techniques applicable in the presence of permanent toroidal magnetic field will be required for the operation of ITER, in particular for recovery from disruptions, vent and air leak, isotopic ratio control, recycling control and mitigation of the tritium inventory build-up. Ion Cyclotron Wall Conditioning (ICWC) is one of the most promising options and has been the subject of considerable recent study on current tokamaks. This paper reports on the findings of such studies performed on European tokamaks, covering a range of plasma-facing materials: TORE SUPRA, TEXTOR, ASDEX Upgrade and JET. IntroductionIn ITER and future fusion devices, the magnetic field, generated by superconducting coils, will be continuously maintained. In the presence of such a magnetic field, conventional DC-glow discharges are unstable and can therefore no longer be used between ohmic plasma pulses. During the non-active He or H phase of ITER, with divertor targets made of carbonfibre composite (CFC), interpulse wall conditioning will be required for reliable discharge initiation or recovery after disruptions. In the D:T phase, wall conditioning may also contribute to the control of the tritium inventory in ITER, of which the build-up is a major investigations are needed prior to its validation and its application to ITER, in particular for fuel removal, recovery after disruptions and isotopic ratio control. This paper reviews the results of recent ICWC experiments performed on current tokamaks, covering a range of plasma-facing materials: TORE SUPRA (CFC), TEXTOR (fine-grain graphite), ASDEX-Upgrade (all W-coated wall) and JET (CFC/Be). The relevance of ICWC, specifications for its application to ITER and the operational domain on current tokamaks are introduced in the first part. The optimization of ICWC discharges is the subject of the second part. The third part reports on the assessment of the efficiency of D 2 (or H 2 ) and He-ICWC discharges for isotopic exchange and fuel removal. The benefit of pulsed ICWC discharges is treated in this part. The last part is devoted to the discussions of the I-9 experimental observations. In particular the He retention in metallic plasma-facing components (PFC) and the role of the different species in wall conditioning are discussed.The operation of ICWC and its efficiency for fuel removal are finally extrapolated to ITER.1. ICWC experiments on the four tokamaks and ICWC discharge characterization a. Principle and relevance of ICWC for wall conditioningThe principle of ICWC discharge production, in the presence of the toroidal magnetic field, has been described elsewhere (see e.g. [7]). The coupling of the RF power to the ICWC discharge is non-resonant and mainly results from the absorption of the RF energy by the electrons. Plasmas with densities ranging from 10 16 and 10 18 m -3 (i.e. 4 to 6 orders of magnitude higher than in DC glow discharges) and temperatures 1 < T e < 10 eV can be produced in a "relay-race" regime of slow and fast wave excitation [7] .In the presence of ...

ITER as a superconducting fusion machine needs efficient wall conditioning techniques for application in the presence of the permanent high toroidal magnetic field for (i) reducing the in-vessel impurity content, (ii) controlling the surface hydrogen isotopic ratio and (iii) mitigating the in-vessel long-term tritium inventory build-up. Encouraging results recently obtained with ion-cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have raised ICWC to the status of one of the most promising techniques available to ITER for routine inter-pulse and overnight conditioning with the ITER main ICRF heating system in the presence of the permanent high toroidal magnetic field. This paper is dedicated to a milestone experiment in ICWC research: the first simulation of ICWC operation in an equivalent ITER full-field scenario and the assessment of the wall conditioning effect on the carbon wall in the largest present-day tokamak JET. In addition, we address in this paper the following topics: (i) an analysis of the radio frequency (RF) physics of ICWC discharges, (ii) the optimization of the operation of ICRF antennas for plasma startup and (iii) an outlook for the performance of ICWC in ITER using the ICRF heating system. Important operational aspects of the conventional ICRF heating system in JET (the so-called A2 antenna system) for use in the ICWC mode are highlighted: (i) the ability of the antenna to ignite the cleaning discharge safely and reliably in different gases, (ii) the capacity of the antennas to couple a large fraction of the RF generator power (>50%) to low-density (≈10 16 -10 18 m −3 ) plasmas and (iii) the ICRF absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. Successful optimization of the JET-ICWC discharge parameters

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Helicon waves are launched in a toroidal device of small aspect ratio, to form and sustain high-density plasma with 1018m−3 peak density and 12eV electron temperature. Owing to the strong poloidal asymmetry in the wave magnetic field structures, a nonresonant current is driven in plasma by the dynamo electric field, which arises due to the wave helicity injection by helicon waves. A study of the parametric dependence of plasma current driven in the electron-magnetohydrodynamic regime, at very high frequency, along with numerical estimations of nonresonant components, are presented here. The measured magnitude of plasma current is in good agreement with the estimated values. Numerical estimation, using experimentally measured variations of wave magnetic field components, clearly delineates the plasma current due to wave-induced helicity from other possible resonant or nonresonant sources in the present parameter regime.

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This contribution reports on isotope exchange studies with both Ion Cyclotron Wall Conditioning (ICWC) and Glow Discharge Conditioning (GDC) in TEXTOR and TORE SUPRA. The discharges have been carried out in H 2, D 2 (ICWC and GDC) and He/H 2 mixtures (ICWC). The higher reionization probability in ICWC compared to GDC, following from the 3 to 4 orders of magnitude higher electron density, leads to a lower pumping efficiency of wall desorbed species. GDC has in this analysis (5-10) times higher removal rates of wall desorbed species than ICWC, although the wall release rate is 10 times higher in ICWC. Also the measured high retention during ICWC can be understood as an effect of the high reionization probability. The use of short RF pulses (~1s) followed by a larger pumping time significantly improves the ratio of implanted over recovered particles, without severely lowering the total amount of removed particles.

Potential double layers are observed to form as nested cylindrical sheaths that energize charge particles in the absence of any externally applied magnetic field. These visibly glowing nested cylindrical sheaths are named as “Fire Tube” in the present work. Sheaths, in unmagnetized collisionless plasmas, have been observed to be functional in generating energetic charge particles. Two characteristically different plasma sources are compared in terms of the dependence of potential and density profiles on the plasma sheath formations near the source boundary. The sheath structures, complex fire tube formation, and space charge formations demarcate the two sources in terms of complex structure formations and motion of the charge particles. We present here the characteristic study of high pressure collisional boundary sheath formations through systematic experiments carried out during the transition from unmagnetized to magnetized charge particles, in the case of two different plasma sources designed. The cylindrical ambient plasma sheaths are observed to energize the charge particles, in the absence of any external magnetic field. The fire tube formations essentially represent a non-neutral region located radially away from the plasma source boundary, connecting the ion sheath with the electron sheath. The probe diagnostic results suggest a significant role of energetic electrons in the formation of multiple double layer structures in the present operational regime. Generation of nonthermal electrons during the formation of MDL in radio frequency capacitive discharges is subjected to interactions of the plasma with the electrodes as well as the confining wall material.

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