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...

The aim of this work is to provide an understanding of detachment at TCV with emphasis on analysis of the Balmer line emission. A new Divertor Spectroscopy System has been developed for this purpose. Further development of Balmer line analysis techniques has allowed detailed information to be extracted from the three-body recombination contribution to the n=7 Balmer line intensity. During density ramps, the plasma at the target detaches as inferred from a drop in ion current to the target. At the same time the Balmer $6\rightarrow2$ and $7\rightarrow2$ line emission near the target is dominated by recombination. As the core density increases further, the density and recombination rate are rising all along the outer leg to the x-point while remaining highest at the target. Even at the highest core densities accessed (Greenwald fraction 0.7) the peaks in recombination and density may have moved not more than a few cm poloidally away from the target which is different to other, higher density tokamaks, where both the peak in recombination and density continue to move towards the x-point as the core density is increased. The inferred magnitude of recombination is small compared to the target ion current at the time detachment (particle flux drop) starts at the target. However, recombination may be having more localized effects (to a flux tube) which we cannot discern at this time. Later, at the highest densities achieved, the total recombination does reach levels similar to the particle flux.Comment: Article accepted for publication in Journal of Nuclear Materials and Energ

Divertor detachment is explored on the TCV tokamak in alternative magnetic geometries. Starting from typical TCV single-null shapes, the poloidal flux expansion at the outer strikepoint is varied by a factor of 10 to investigate the X-divertor characteristics, and the total flux expansion is varied by 70% to study the properties of the super-X divertor. The effect of an additional X-point near the target is investigated in X-point target divertors. Detachment of the outer target is studied in these plasmas during Ohmic density ramps and with the ion ∇B drift away from the primary X-point. The detachment threshold, depth of detachment, and the stability of the radiation location are investigated using target measurements from the wall-embedded Langmuir probes and two-dimensional CIII line emissivity profiles across the divertor region, obtained from inverted, toroidally-integrated camera data. It is found that increasing poloidal flux expansion results in a deeper detachment for a given line-averaged density and a reduction in the radiation location sensitivity to core density, while no large effect on the detachment threshold is observed. The total flux expansion, contrary to expectations, does not show a significant influence on any detachment characteristics in these experiments. In X-point target geometries, no evidence is found for a reduced detachment threshold despite a Nuclear Fusion Results from recent detachment experiments in alternative divertor configurations on TCVInternational Atomic Energy Agency a See the author list of 'Overview of progress in European Medium Sized Tokamaks towards an integrated plasma-edge/wall solution' by H. Meyer et al, to be published in the Nuclear Fusion

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.

Detachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma–atom and plasma–molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. Both plasma–atom as well as plasma–molecule interactions can result in excited hydrogen atoms which emit atomic line emission. In this work, we investigate a new Balmer Spectroscopy technique for Plasma–Molecule Interaction—BaSPMI. This first disentangles the Balmer line emission from the various plasma–atom and plasma–molecule interactions and secondly quantifies their contributions to particle (ionisation and recombination) and power balance (radiative power losses). Its performance is verified using synthetic diagnostic techniques of both attached and detached TCV and MAST-U SOLPS-ITER simulations. We find that H 2 plasma chemistry involving H 2 + and/or H − can substantially elevate the Hα emission during detachment, which we show is an important precursor for Molecular Activated Recombination. An example illustration analysis of the full BaSPMI technique shows that the hydrogenic line series, even Lyα as well as the medium-n Balmer lines, can be significantly influenced by plasma–molecule interactions by tens ofpercent. That has important implications for using atomic hydrogen spectroscopy for diagnosing divertor plasmas.

This paper shows experimental results from the TCV tokamak that indicate plasma-molecule interactions involving D + 2 and possibly D − play an important role as sinks of energy (through hydrogenic radiation as well as dissociation) and particles during divertor detachment if low target temperatures (< 3 eV) are achieved. Both molecular activated recombination (MAR) and ion source reduction due to a power limitation effect are shown to be important in reducing the ion target flux during a density ramp. In contrast, the electron-ion recombination (EIR) ion sink is too small to play an important role in reducing the ion target flux. MAR or EIR do not occur during N 2 seeding induced detachment as the target temperatures are not sufficiently low.The impact of D + 2 is shown to be underestimated in present (vibrationally unresolved) SOLPS-ITER simulations, which could result from an underestimated D 2 + D + → D + 2 + D rate. The converged SOLPS-ITER simulations are post-processed with alternative reaction rates, resulting in considerable contributions of D + 2 to particle and power losses as well as dissociation below the D 2 dissociation area. Those findings are in quantitative agreement with the experimental results.

This paper shows experimental results from the TCV tokamak that indicate plasma-molecule interactions involving D + 2 and possibly D − play an important role as sinks of energy (through hydrogenic radiation as well as dissociation) and particles during divertor detachment if low target temperatures (< 3 eV) are achieved. Both molecular activated recombination (MAR) and ion source reduction due to a power limitation effect are shown to be important in reducing the ion target flux during a core density ramp. In contrast, the electron-ion recombination (EIR) ion sink is too small to play an important role in reducing the ion target flux. MAR or EIR do not occur during N 2 seeding induced detachment as the target temperatures are not sufficiently low.The impact of D + 2 is shown to be underestimated in present (vibrationally unresolved) SOLPS-ITER simulations, which could result from an underestimatedThe converged SOLPS-ITER simulations are post-processed with alternative reaction rates, resulting in considerable contributions of D + 2 to particle and power losses as well as dissociation below the D 2 dissociation area. Those findings are in quantitative agreement with the experimental results.

Using SOLPS‐ITER, we model a TCV conventional divertor discharge density ramp to understand the role of various processes in the loss of target ion current. We find that recombination is not a strong contributor to the rollover of the target ion current at detachment. In contrast, the divertor ion source appears to play a central role in magnitude (the source of most of the ion target current) and time, apparently dropping during the density ramps as a result of a drop in power available for ionization.