ICRF Wall Conditioning: Present Status and Developments for Future Superconducting Fusion Machines (original) (raw)

Simulation of ITER full-field ICWC scenario in JET: RF physics aspects

Plasma Physics and Controlled Fusion, 2012

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

ICRF physics aspects of wall conditioning with conventional antennas in large-size tokamaks

Journal of Nuclear Materials, 2011

This paper focuses on a study of the principal operation aspects of standard ICRF heating antennas in the ion cyclotron wall conditioning (ICWC) mode: (i) ability of the antenna to ignite the cleaning discharge safely and reliably in different gases including those most likely to be used in ITER -He, H 2 , D 2 and their mixtures, (ii) the antenna capacity to couple a large fraction of the RF generator power (>50%) to low density ($10 16 -10 18 m À3 ) plasmas and (iii) the RF power absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. The ICWC discharge optimization in terms of RF plasma wave excitation/absorption resulted in successful simulation of the conditioning scenarios for ITER operation at full field (JET) and half-field (TEXTOR, TORE SUPRA, ASDEX Upgrade).

Plasma and antenna coupling characterization in ICRF-wall conditioning experiments

Fusion Engineering and Design, 2012

Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the divertor tokamaks ASDEX Upgrade (AUG) and JET to simulate the scenario of ITER wall conditioning at half-field (AUG) and full-field (JET). ICWC-plasma and antenna coupling characterization results obtained during the Ion Cyclotron Resonance Frequency (ICRF)-Wall Conditioning experiments performed in helium-hydrogen mixture in AUG and helium-deuterium mixtures in JET are presented here. Safe operational regimes for optimum ICWC in ITER could be explored for different magnetic fields. Satisfactory antenna coupling in the Mode Conversion scenario along with reproducible generation of ICRF plasmas and reliable wall conditioning were achieved by coupling RF power from one or two ICRF antennas at two (AUG, JET) different resonant frequencies. These results are in qualitative agreement with the predictions of 1-D TOMCAT code. Present study of ICWC indicates towards the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (B V B T ) magnetic field on antenna coupling and plasma parameters. The results obtained from JET and AUG tokamaks, presented in this paper, emphasizes the proposed phenomenological schemes for further development of ICWC in superconducting tokamaks.

Simulation of ITER ICWC scenarios in JET

2010

Encouraging results recently obtained with alternative ion cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have elevated ICWC to the status of one of the most promising techniques available to ITER for routine interpulse conditioning in the presence of the permanent high toroidal magnetic field. The paper presents a study of ICWC discharge performance and optimization of the conditioning output in the largest tokamak JET using the standard ICRF heating antenna A2 in a scenario envisaged at ITER full field, B T =5.3 T: on-axis location of the fundamental ICR for deuterium, ω=ω cD+ . The perspective of application of the alternative technique in ITER is analyzed using the 3-D MWS electromagnetic code, 1-D RF full wave and 0-D plasma codes.

ICRH system performance during ITER-Like Wall operations at JET and the outlook for DT campaign

EPJ Web of Conferences, 2017

Performance of JET ICRH system since installation of the metal ITER-Like Wall (ILW) has been assessed statistically. The data demonstrate steady increase of the RF power coupled to plasmas over recent years with the maximum pulse-average and peak values exceeding respectively 6MW and 8MW in 2016. Analysis and extrapolation of power capabilities of conventional JET ICRH antennas is provided and key performance-limiting factors are discussed. The RF plant operational frequency options are presented highlighting the issues of efficient ICRH application within a foreseeable range of DT plasma scenarios.

RF physics of ICWC discharge at high cyclotron harmonics

2014

Recent experiments on Ion Cyclotron Wall Conditioning (ICWC) performed in tokamaks TEXTOR and ASDEX Upgrade with standard ICRF antennas operated at fixed frequencies but variable toroidal magnetic field demonstrated rather contrasting parameters of ICWC discharge in scenarios with on-axis fundamental ion cyclotron resonance (ICR) for protons, = cH+ , and with its high cyclotron harmonics (HCH), =10 cH+ . HCH scenario: very high antenna coupling to low density RF plasmas (P pl 0.9P RF-G ) and low energy Maxwellian distribution of CX hydrogen atoms with temperature T H 350 eV. Fundamental ICR: lower antenna-plasma coupling efficiency (by factor of about 1.5 times) and generation of high energy non-Maxwellian CX hydrogen atoms (with local energy E H keV). In the present paper, we analyze the obtained experimental results numerically using (i) newly developed 0-D transport code describing the process of plasma production with electron and ion collisional ionization in helium-hydrogen gas mixture and (ii) earlier developed 1-D Dispersion Relation Solver accounting for finite temperature effects and collision absorption mechanisms for all plasma species in addition to conventionally examined Landau/TTPM damping for electrons and cyclotron absorption for ions. The numerical study of plasma production in helium with minor hydrogen content in low and high toroidal magnetic fields is presented. The investigation of the excitation, conversion and absorption of plasma waves as function of B T -field suggests that only fast waves (FW) may give a crucial impact on antenna coupling and characteristics of the ICWC discharge using standard poloidally polarized ICRF antennas designed to couple RF power mainly to FW. The collisional (non-resonant) absorption by electrons and ions and IC absorption by resonant ions of minor concentration in low T e plasmas is studied at fundamental ICR and its high harmonics.

ICRF physics aspects of wall conditioning plasma characterization in TEXTOR

Fusion Engineering and Design, 2013

Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the divertor tokamaks ASDEX Upgrade (AUG) and JET to simulate the scenario of ITER wall conditioning at half-field (AUG) and full-field (JET). ICWC-plasma and antenna coupling characterization results obtained during the Ion Cyclotron Resonance Frequency (ICRF)-Wall Conditioning experiments performed in helium-hydrogen mixture in AUG and helium-deuterium mixtures in JET are presented here. Safe operational regimes for optimum ICWC in ITER could be explored for different magnetic fields. Satisfactory antenna coupling in the Mode Conversion scenario along with reproducible generation of ICRF plasmas and reliable wall conditioning were achieved by coupling RF power from one or two ICRF antennas at two (AUG, JET) different resonant frequencies. These results are in qualitative agreement with the predictions of 1-D TOMCAT code. Present study of ICWC indicates towards the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (B V B T ) magnetic field on antenna coupling and plasma parameters. The results obtained from JET and AUG tokamaks, presented in this paper, emphasizes the proposed phenomenological schemes for further development of ICWC in superconducting tokamaks.

Present Status of the ITER-like ICRF Antenna on JET

2009

The commissioning of the ITER-Like ICRF Antenna (ILA) [1] on JET plasmas from May 2008 to April 2009 in various conditions (33, 42 and 47 MHz, L- and H-mode, antenna strap-plasma separatrix distances of ~9 to 17 cm) has provided relevant information for future antenna design and operation. The maximum power density achieved was 6.2 MW/m2 in L-mode with strap to plasma separatrix distance of ~9-10 cm at 42 MHz on the lower half of the ILA extrapolating to 8 MW/m2 if the full generator power had been available. Efficient (trip-free operation) ELM tolerance was obtained both at 33 and 42 MHz on a large range of ELMs with strap voltages up to 42 kV and a maximum power density of 4.1 MW/m2. The paper reviews these achievements as well as remaining issues.