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

Abstract

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

3D simulations of gas puff effects on edge density and ICRF coupling in ASDEX Upgrade

“…In the present of powered ICRF antennas, the RF fields can generate fast electrons which may hit the gas cloud and cause additional ionization. This ionization exist either in the region near the antennas where electrons gain energy from the slow wave, or in the region far from the antennas where electrons gain energy from the ponderomotive force [26,27]. To calculate the effect of a population of fast electrons, profiles of T e , n e and n D0 from the EMC3-EIRENE calculations at the position of the gas puff were used.…”

Advanced ponderomotive description of electron acceleration in ICRF discharge initiation

“…While in later stage (phase III and IV) collective plasma effects are playing an important role, in phases I and II the ionisation rate can be assessed from single electron motions in the antenna vacuum electric field component parallel to the toroidal magnetic field, E z (z, t). The electron acceleration by E z (z, t) is described by Lyssoivan [2]. The theory separates the motion of charged particles into a fast oscillation and a slower drift around the oscillation centre.…”

“…In the work of Lyssoivan [2], the Taylor series expansion retains the zeroth-and first-order term. Adding to this theory, for the purpose of this contribution, we introduce as well the quadratic term.…”

“…From eq. (2) the spatial oscillation amplitude can be derivedẑ 1 = E z (z 0 )q e /m e ω 2 as well as the minimum electric field value for which the kinetic energy of the oscillating electron overcomes the ionization potential ion [2]:…”

“…From eq. (3) follows the ponderomotive force responsible for the slow drift of the oscillation centre z 0 with potential [2]:…”