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...

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 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

TCV experiments demonstrate the basic power exhaust properties of the snowflake (SF) plus and SF minus divertor configurations by measuring the heat fluxes at each of their four divertor legs. The measurements indicate an enhanced transport into the private flux region and a reduction of peak heat fluxes compared to a similar single null configuration. There are indications that this enhanced transport cannot be explained by the modified field line geometry alone and likely requires an additional or enhanced cross-field transport channel. The measurements, however, do not show a broadening of the scrape-off layer (SOL) and, hence, no increased cross-field transport in the common flux region. The observations are consistent with the spatial limitation of several characteristic SF properties, such as a low poloidal magnetic field in the divertor region and a long connection length to the inner part of the SOL closest to the separatrix. Although this limitation is typical in a medium sized tokamak like TCV, it does not apply to significantly larger devices where the SF properties are enhanced across the entire expected extent of the SOL.

The operation of a cascaded arc hydrogen plasma source was experimentally investigated to provide an empirical basis for the scaling of this source to higher plasma fluxes and efficiencies. The flux and efficiency were determined as a function of the input power, discharge channel diameter, and hydrogen gas flow rate. Measurements of the pressure in the arc channel show that the flow is well described by Poiseuille flow and that the effective heavy particle temperature is approximately 0.8 eV. Interpretation of the measured I -V data in terms of a one-parameter model shows that the plasma production is proportional to the input power, to the square root of the hydrogen flow rate, and is independent of the channel diameter. The observed scaling shows that the dominant power loss mechanism inside the arc channel is one that scales with the effective volume of the plasma in the discharge channel. Measurements on the plasma output with Thomson scattering confirm the linear dependence of the plasma production on the input power. Extrapolation of these results shows that ͑without a magnetic field͒ an improvement in the plasma production by a factor of 10 over where it was in van Rooij et al. ͓Appl. Phys. Lett. 90, 121501 ͑2007͔͒ should be possible.