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

Abstract

The physics of divertor detachment is determined by divertor power, particle and momentum balance. This work provides a novel analysis technique of the Balmer line series to obtain a full particle/power balance measurement of the divertor. This supplies new information to understand what controls the divertor target ion flux during detachment.Atomic deuterium excitation emission is separated from recombination quantitatively using Balmer series line ratios. This enables analysing those two components individually, providing ionisation/recombination source/sinks and hydrogenic power loss measurements. Probabilistic Monte Carlo techniques were employed to obtain full error propagation -eventually resulting in probability density functions for each output variable. Both local and overall particle and power balance in the divertor are then obtained. These techniques and their assumptions have been verified by comparing the analysed synthetic diagnostic 'measurements' obtained from SOLPS simulation results for the same discharge. Power/particle balance measurements have been obtained during attached and detached conditions on the TCV tokamak.

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

“…We first provide a brief summary of our analysis techniques and nomenclature for inferring the recombination sink [31], ionisation source and hydrogenic power loss. The manuscript on this analysis technique can be found at [2], while its code is available at [48]. Figure 2 illustrates the various steps in the analysis process, eventually resulting in estimates of both local plasma characteristics (weighted over the Balmer line emission profile along each viewing chord) and line integrated plasma parameters.…”

“…In this work we utilize new techniques for achieving these measurements (see [2]) which allow us to directly infer a) both the recombination sink/the ionization source and b) the total energy 'cost' per ionization, E ion per ionisation event as profiles along the outer divertor leg. The results of those studies (section 3) for L-mode TCV discharges quantitatively verify that the loss of target ion current is primarily due to an ionisation source loss for the TCV cases shown.…”

“…To obtain ionisation sources and sinks, we utilised the newly developed TCV divertor spectroscopy system (DSS) [1,2,31]. The DSS consists of vertical and horizontal viewing systems, each employing 32 lines of sight ( Figure 1a).…”

“…Our analysis is based on the horizontal system, which provides full coverage for the divertor shapes studied in this work. Full details on the analysis can be found in [1,2] and a summary can be found in section 2.1.…”

“…The values of power entering the recycling region, P recl , can be sensitive to this underestimation during detached conditions where P recl is small. This is reflected in our results, as we assume that 80-90% of all radiative losses is detected by the bolometry, for which we assume a uniform probability density function in the probabilistic analysis used for quantities which require P recl (or q recl ) estimates [2]; which includes the analytic model investigations in chapter 4.…”