Recent results on Ion Cyclotron Wall Conditioning in mid and large size tokamaks

Abstract

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 ...

Plasma and antenna coupling characterization in ICRF-wall conditioning experiments

“…In JET (R 0 = 2.96 m, r w-h ≈ 1.10 m, r w-v ≈ 1.85 m) two ICRF antennas (A2 antenna arrays C and D, each with four poloidal current straps) were used for ICWC operation [11], both powered with (100-400) kW, at 26.06 MHz for antenna C and 25.21 MHz for antenna D, in two or three step pulse ( RF-tot = 9 s), in monopole (0 0 0 0) or in super-dipole (0 0 ) phase. Antennas operated in presence of B T = 3.3 T thus simulating ITER ICWC scenario at full field (40 MHz, 5.3 T) with on-axis location of the fundamental ICR for deuterons.…”

“…In AUG (torus major radius, R 0 = 1.65 m, wall minor horizontal radius r w-h ≈ 0.57 m and wall minor vertical radius r w-v ≈ 1.22 m) up to 4 double strap ICRF antennas were used for ICWC operation [8,11]. AUG antennas operate in pairs which contain antennas 1 and 2 and antennas 3 and 4 respectively.…”

0D model of magnetized hydrogen–helium wall conditioning plasmas

“…The coupled RF power P RF is included in the energy balance equation of the electrons (xP RF /V pl ) and protons ((1 − x)P RF /V pl ). In the present 0D model however, only collisional absorption is considered, the population of resonant ions (∼ 10 −4 n e ), determined from JET NPA measurements [5], modeled neutral densities and the charge exchange reaction rate (reaction 17, Table 1), being negligible. Fig.…”

“…Wall conditioning objectives are facilitating tokamak plasma start-up and improving its performance by e.g. reducing the amount of impurities on the wall or lowering the recycling of particles [5], and in future machines wall conditioning can be used to mitigate the tritium inventory build-up [6]. The envisaged plasma parameters for ICRF wall conditioning are low plasma density n e = 10 10 − 10 12 cm −3 and low electron temperature T e = 1 − 5 eV.…”

19th INTERNATIONAL CONFERENCE ON PLASMA SURFACE INTERACTIONS IN CONTROLLED FUSION DEVICES (PSI-19) (SAN DIEGO, USA, 24—28 MAY 2010)

“…Эффективность обезгаживания увеличивается при работе короткими и м-пульсами (0,3 с -импульс, 1,5 с -пауза). На конференции ИЦРК были посвящены пять отдельных докладов [11,[14][15][16][17].…”