An improved understanding of the roles of atomic processes and power balance in divertor target ion current loss during detachment

Abstract

The process of divertor detachment, whereby heat and particle fluxes to divertor surfaces are strongly diminished, is required to reduce heat loading and erosion in a magnetic fusion reactor to acceptable levels. In this paper the physics leading to the decrease of the total divertor ion current (I t ), or 'rollover', is experimentally explored on the TCV tokamak through characterization of the location, magnitude and role of the various divertor ion sinks and sources including a complete analysis of particle and power balance. These first measurements of the profiles of divertor ionisation and hydrogenic radiation along the divertor leg are enabled through novel spectroscopic techniques.Over a range in TCV plasma conditions (plasma current and electron density, with/without impurityseeding) the I t roll-over is ascribed to a drop in the divertor ion source; recombination remains small or negligible farther into the detachment process. The ion source reduction is driven by both a reduction in the power available for ionization, P recl , and concurrent increase in the energy required per ionisation, E ion : This effect of power available on the ionization source is often described as 'power starvation' (or 'power limitation'). The detachment threshold is found experimentally (in agreement with analytic model predictions) to be ~ P recl /I t E ion~ 2, corresponding to a target electron temperature, T t~ E ion /γ where γ is the sheath transmission coefficient. The target pressure reduction, required to reduce the target ion current, is driven both by volumetric momentum loss as well as upstream pressure loss.The measured evolution through detachment of the divertor profile of various ion sources/sinks as well as power losses are quantitatively reproduced through full 2D SOLPS modelling through the detachment process as the upstream density is varied.2. We show the equivalence of approaching detachment from momentum balance (e.g. target pressure losses) and power limitation arguments from combining the Bohm sheath criteria with power/particle balance (section 4.2 -equation 21). This is supported with experimental measurements which show that both power loss (in fact power-limitation of the ion source) and volumetric momentum loss occur after the detachment onset. In addition, upstream pressure loss occurs during detachment, which is shown to be consistent with analytic modelling. 4 3. The ∝ trend observed experimentally in TCV (where n eu is the upstream electron density) during attached conditions contrasts the often assumed ∝ 2 trend on which the Degree of Detachment (DoD) is based [3,7,24,[36][37][38]. The TCV observations are however supported with analytic predictions, when accounting for changes in the upstream temperature and divertor radiation. This illustrates deviations in upstream and divertor conditions need to be accounted for before the DoD can be used.Our measurements show that as further power limitation occurs (P recl gets closer to P ion ), volumetric momentum loss (estimated from inferred charge ex...

Novel inferences of ionisation and recombination for particle/power balance during detached discharges using deuterium Balmer line spectroscopy

“…Figure 11 provides a more general overview of the characteristic output PDFs, shown for 3 timesteps during a density ramp discharge (#56567) -respective of attached (~0.5 s), detachment-onset (~1.0 s) and detached (~1.2 s) operation. That particular discharge is discussed in more detail in section 6 and [11]. Over these three different times, the magnitude of the uncertainty and its asymmetry can vary strongly.…”

“…Over these three different times, the magnitude of the uncertainty and its asymmetry can vary strongly. In addition, as the fraction of emission due to recombination changes for a particular chord, the input parameters contributing most to the resulting uncertainty can change strongly; as has been investigated using a Kendall rank correlation technique in [11]. In other words: the main uncertainty cannot be attributed to a single (set of) parameters and depends strongly on the conditions present.…”

“…volumetric recombination -) shown for a typical core plasma density ramp discharge in Fig. 14 a. The ion target current first increases linearly during the attached phase, after which begins to deviate from the linear trend (dashed line) at the detachment onset, ultimately rolling over [10,11].…”

“…Hydrogenic power losses, in this case, are significantly smaller than the total radiation -suggesting divertor impurity radiation dominates over hydrogenic radiation. Divertor impurity radiation (Prad imp ) (which is estimated by subtracting hydrogenic radiation -estimated through spectroscopic analysis -from the total measured radiation by bolometry [10,11]) continually lowers the power entering the ionisation region, Figure 14: Power and particle balance of #56567 as function of core Greenwald fraction. a: Divertor particle balance comparing the ion target flux measured by Langmuir probes (It) against the divertor ionisation rate (Ii) and volumetric recombination rate (Ir) obtained through spectroscopic analysis.…”

“…the power entering the divertor minus impurity radiation) and the power required for ionisation (Pion) obtained spectroscopically. Precl = Pdiv -Prad imp [9][10][11]14] until it approaches the power loss associated with ionisation during detachment.…”