Abstract
During detachment a structure of strongly enhanced density develops close to the inner target. Its dynamics is approximated by those of radiative fluctuations appearing at a similar position and studied by means of a time-delay-estimation technique in the ASDEX Upgrade tokamak. Compared to theory the dynamics can be described as follows: at increasing density the ionization front moves upstream to reduce ionization radiation in order to balance the increased recombination radiation. The recombination zone stays close to the target strike point. The parallel motion of the ionization front is determined by the perpendicular neutral motion. The divertor nose constitutes an obstacle for the perpendicular neutral flux from the target to the region above the X-point. Passing into this shadow the neutral flux above the X-point is strongly reduced, the ionization front fades away and the heat flux from upstream can increase the temperature in the recombination region, subsequently reducing recombination and reforming an ionization front below the X-point. A cyclic reformation of the ionization front propagating from below to above the X-point occurs leading to a fluctuation as observed in the experiment.
Highlights
The detachment process as observed in ASDEX Upgrade (AUG) can be divided in three states [5] as explained in the following
Beside its relevance for ITER and DEMO, the dynamics of the HFSHD region may be critical for the proposed experiment ADX [7], where high current drive from RF launchers installed at the high field side are planed
This reference lines of sights (LOSs) has been chosen for two reasons, first the X-point fluctuations are most prominent for this LOS [5] and this LOS measures at the high-field-side (HFS) and allows to discriminate low-field-side (LFS) and HFS dynamics
Summary
For magnetically confined fusion devices such as ITER the power load on the foreseen divertor target material, tungsten, must be kept below P ≈ 10 MWm−2, which can be only achieved under detached or partially detached conditions [1]. Stable completely detached H-mode plasmas have been demonstrated in ASDEX Upgrade (AUG) [2,3]. In detached conditions modeling results show the development of a so-called virtual target regime, in which adjacent strong ionization and recombination regions are observed [4]. The strong volumetric recombination lets the virtual target act like the actual one at the wall and it is proposed to take advantage of this virtual target instead of the target plate. This investigation concerns the dynamics and stability of the virtual target, directly the stability of (partial) detachment, which is most crucial for the operation of magnetically confined fusion reactors. Based on consistency with this theory and previous experiments [2,5] a model for the fluctuating state of detachment has been developed
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