A gas baffle will soon be inserted in the vessel of the tokamak à configuration variable (TCV) [Reimerdes, Nucl. Mat. and Energy 2017]. This upgrade aims at achieving more favorable conditions for the onset of detachment. In this work, we simulate the effect of the baffle on the TCV edge plasma with the SolEdge2D-EIRENE code [Bufferand, Nucl. Fusion 2015], which couples a fluid plasma model to a kinetic model for neutrals and impurities. Firstly, a specific TCV shot with a baffle-compatible shape is simulated. This comparison allows to tune perpendicular transport coefficients in order to match upstream experimental profiles, and results in a good agreement with the experiment at the targets. The same simulation is then carried out including the baffle. The neutral compression ratio, namely the ratio between divertor and upstream neutral pressure, is shown to improve by a factor of order 10, resulting in bigger power and momentum losses in the divertor plasma. Next, we perform a scan in upstream density to access different divertor regimes, revealing that the neutral compression increases as we approach detachment. Finally, in view of possible future optimizations, the level of baffle closure is varied in the simulations and the feedback on plasma properties is discussed.

The effect of magnetic geometry on scrape-off layer (SOL) transport and detachment behaviour is investigated on the TCV tokamak with the goal of assessing the potential of alternative divertor geometries and for the validation of theoretical models. L-mode experiments reveal that increasing connection length and hence divertor volume by either increasing poloidal flux expansion or divertor leg length have different effects on the boundary plasma. In attached conditions, the SOL heat flux width q inferred from target infrared thermography measurements is weakly dependent on poloidal flux expansion but increases approximately with the square root of the divertor leg length. The divertor spreading factor S shows no clear trend with leg length but decreases with flux expansion. TOKAM3X turbulence simulations of the leg length scan are in qualitative agreement with the experiment and can explain observations by a strongly asymmetric (ballooning) transport at and below the X-point. Evidence for increased transport in the region of low poloidal field is obtained in the Snowflake minus geometry. The presence of an additional X-point in the low-field side SOL increases the effective SOL width by approximately a factor two. Increasing flux expansion and leg length both result in enhanced divertor radiation levels, with the effect being much larger in the latter case. This behaviour, together with the observed trend in q, is consistent with a substantial drop in the density threshold for divertor detachment with increasing leg length and a weak variation with flux expansion. Novel spectroscopic techniques reveal that the drop in target ion current and access to detachment is caused by a reduction of the divertor ionization source due to power starvation, while volume recombination is only a small contributor. This interpretation is confirmed by SOLPS modelling. TCV alternative divertor studies are being extended to neutral beam heated H-mode plasmas. The H-mode power threshold is found to vary weakly between standard, X-, and Super- X geometries. In all cases, ELMy H-mode is obtained at intermediate current, while the discharges are ELM-free at high current. Signs of detachment have so far only been observed in the latter case. Ongoing experiments further investigate H-mode detachment in these plasmas and will be extended to Snowflake configurations.

The fully non - inductive sustainment of high normalized beta plasmas ( ? N ) is a crucial challenge for the steady - state operation of a tokamak reactor. In order to assess the difficulties facing such scenarios, steady - state regimes have been explored on the Tokamak à Configuration Variable (TCV) using the newly available 1MW Neutral Beam Injection (NBI) system. The operating space i s extended towards plasmas that are closer to those expected in JT - 60SA and ITER, i.e. with significant NBI and Electron Cyclotron Resonance Heating and C urrent D rive (ECRH/CD) , bootstrap current and F a st I on (FI) fraction . ? N values up to 1.4 and 1.7 are obtained in lower single null L - mode (H 98 (y,2)~0.8) a nd H - mode (H 98 (y,2)~1) plasmas, respectively, at zero time averaged loop voltage and q 95 ~6 . Fully non - inductive operation i s not achieved with NBI alone, whose injection c an even increase the loop voltage in the presence of EC waves. A strong contribution to the total plasma pr essure of bulk and FIs from NBI is experimentally evidenced and confirmed by interpretative ASTRA and NUBEAM modelling , which further predict s that FI charge - exchange reactions are the main loss channel for NBH/CD efficiency. Internal t ransport b arriers, which are expected to maximize the boo t strap current fraction, are not formed in either the electron or the ion channel i n the plasmas explored to date, despite a significant increase in the toroidal rotation and FI fraction with NBI, which are known to re duce turbulence. First results on scenario development of high - ? N fully non - inductive H - mode plasmas are also presented.

Detachment is predicted to be of paramount importance in handling the power exhaust for future fusion devices, such as ITER. However, a direct experimental quantification of the role and spatial profile of the various atomic processes controlling the loss of divertor ion current during detachment has, until now, not been available due to lack of ionisation measurements. The physics of the target ion current loss, a defining and important feature of detachment, is studied in TCV using density ramp, or N2 seeded, L-mode discharges with various plasma currents. Novel spectroscopic analysis techniques utilising the hydrogen Balmer series have been developed to infer the ionisation source magnitude and distribution. This provides, together with the ion sink in the plasma (recombination), electron density and divertor power balance measurements, a detailed picture of particle and power balance along the outer divertor leg. The results for a conventional single-null topology show the divertor ion source tracks the ion target flux both in magnitude and in time: both the ion target current and ion source decrease together at detachment. Surprisingly, the volumetric recombination ion sink - commonly thought to be the primary detachment ion loss mechanism - is only a small (sometimes negligible) portion of the ion current losses. New evidence from TCV shows the driver of the divertor ion source decrease is a decrease in power flowing into the ionisation region combined with an increase in the energy required per ionisation - essentially 'starving' the ionisation region of power. SOLPS modelling of a TCV density ramp discharge with a conventional divertor configuration has reproduced the general characteristics of the ion source reduction. 'Power starvation' of the ionization source, therefore, appears to be central to loss of divertor target ion current. The divertor plasma sink for ions, recombination, is maximised at the highest divertor densities, which are achieved at the highest core densities. Nitrogen seeding enables detachment by 'power starving' the ionisation region at lower core densities and hence lower recombination sink. The ratio between the recombination ion sink and the ion target flux increases when poloidal flux expansion is increased (x-divertor) under constant core conditions. Understanding these changes may be key to understanding the role of magnetic geometry on detachment and possibly the detachment process itself.