Understanding the plasma dynamics in the edge and Scrape-Off Layer (SOL) regions of a tokamak is mandatory for future exploitation of fusion power as a source of energy, since the SOL transport properties determine the peak particle and heat fluxes flowing towards the plasma-facing components (PFCs). The inherently turbulent nature of SOL transport makes this assessment particularly challenging. In the first part of this contribution, an investigation is reported on a set of H-mode discharges on the TCV tokamak, focused on the role of gas fueling in determining the properties of transport and kinetic profiles in the near and far SOL. In all discharges additional NBI heating was used for H-mode access, while different gas settings allowed to span a wide interval of divertor neutral pressure ... . The analysis has been conducted by first estimating the SOL power width ?... from a fit of the upstream-remapped parallel heat flux profile as measured by IR cameras. This allowed to estimate the separatrix temperature T...,?... under the assumption of conduction-dominated parallel transport (justified by the considerably lower temperatures at the target than upstream). After fitting the Thomson Scattering density and temperature profiles, the separatrix position and density n...,?... could be estimated. The level of turbulent transport has been quantified by the ?? ? Z... R...q...?...? n...,?...T...,?... ? parameter introduced in [1], this being a measure of the effect of the interchange instability drive on drift waves and thus effectively an estimate of the interchange turbulence level, being directly related to the separatrix collisionality. An increase in p...,?...? is seen to translate into an increase of n...,?... and ultimately of ??. Limited to the high-density part of the database, a p...,?...? scan between 20 and 120 mPa produces a variation of ?? between 0.4 and 0.95. This is accompanied by a broadening of ?...by up to a factor ~2.5 (from ~4 to ~10 mm), coherently with recent modelling results predicting a larger SOL power width in H-mode at high separatrix collisionality [2]. A similar increase of the near-SOL electron density, temperature and pressure e-folding lengths is observed as well. In the far-SOL the density profile shows a progressive flattening at increasing ??, leading to formation of the so-called density shoulder. This observation can be accounted for by a progressive increase of filamentary transport into the far SOL at higher ??, as observed by wall-mounted and reciprocating Langmuir Probes. Given that ?? shows an explicit dependence on q...?... , a scan in upper triangularity has been performed in H-mode on TCV as well, where ?... has been varied between 0 and ~0.45. Preliminary results show a transition from a Type-I ELMy regime towards a small ELM/QCE regime as ??... increases at similar fuelling levels, coherently with previous experimental observations [3, 4]. In the second part of this work the effect of such a ??... variation on ?? and consequently on SOL profile and turbulence properties will be investigated.
Effect of Divertor Fuelling and Plasma Shaping on Scrape-Off Layer Profiles and Transport Properties in TCV H-mode Discharges
Vianello N;
2022
Abstract
Understanding the plasma dynamics in the edge and Scrape-Off Layer (SOL) regions of a tokamak is mandatory for future exploitation of fusion power as a source of energy, since the SOL transport properties determine the peak particle and heat fluxes flowing towards the plasma-facing components (PFCs). The inherently turbulent nature of SOL transport makes this assessment particularly challenging. In the first part of this contribution, an investigation is reported on a set of H-mode discharges on the TCV tokamak, focused on the role of gas fueling in determining the properties of transport and kinetic profiles in the near and far SOL. In all discharges additional NBI heating was used for H-mode access, while different gas settings allowed to span a wide interval of divertor neutral pressure ... . The analysis has been conducted by first estimating the SOL power width ?... from a fit of the upstream-remapped parallel heat flux profile as measured by IR cameras. This allowed to estimate the separatrix temperature T...,?... under the assumption of conduction-dominated parallel transport (justified by the considerably lower temperatures at the target than upstream). After fitting the Thomson Scattering density and temperature profiles, the separatrix position and density n...,?... could be estimated. The level of turbulent transport has been quantified by the ?? ? Z... R...q...?...? n...,?...T...,?... ? parameter introduced in [1], this being a measure of the effect of the interchange instability drive on drift waves and thus effectively an estimate of the interchange turbulence level, being directly related to the separatrix collisionality. An increase in p...,?...? is seen to translate into an increase of n...,?... and ultimately of ??. Limited to the high-density part of the database, a p...,?...? scan between 20 and 120 mPa produces a variation of ?? between 0.4 and 0.95. This is accompanied by a broadening of ?...by up to a factor ~2.5 (from ~4 to ~10 mm), coherently with recent modelling results predicting a larger SOL power width in H-mode at high separatrix collisionality [2]. A similar increase of the near-SOL electron density, temperature and pressure e-folding lengths is observed as well. In the far-SOL the density profile shows a progressive flattening at increasing ??, leading to formation of the so-called density shoulder. This observation can be accounted for by a progressive increase of filamentary transport into the far SOL at higher ??, as observed by wall-mounted and reciprocating Langmuir Probes. Given that ?? shows an explicit dependence on q...?... , a scan in upper triangularity has been performed in H-mode on TCV as well, where ?... has been varied between 0 and ~0.45. Preliminary results show a transition from a Type-I ELMy regime towards a small ELM/QCE regime as ??... increases at similar fuelling levels, coherently with previous experimental observations [3, 4]. In the second part of this work the effect of such a ??... variation on ?? and consequently on SOL profile and turbulence properties will be investigated.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
