arXiv subjects

Benoit Labit

Publications and source records attributed to Benoit Labit.

3 recordsLinked to original sources

arXiv2026

The L-H transition in tokamaks: power threshold, density minimum and toroidal-field asymmetry

The physical mechanism underlying the L--H transition in tokamaks has remained an open problem for over forty years. We present three-dimensional flux-driven two-fluid simulations in a diverted geometry that exhibit a confinement transition at lower power in the favourable toroidal-field configuration. The simulations show that electromagnetic drift-wave turbulence spontaneously generates a sheared $\bm{E}\times \bm{B}$ flow responsible for transport suppression. The toroidal-field-direction asymmetry arises from time-reversal symmetry breaking by finite collisionality, as demonstrated by a quasilinear calculation of the turbulent momentum flux. First-principles scaling laws are derived for the L--H power threshold in both density branches, the density minimum, and the minimum power, all matching or surpassing existing empirical scalings.

physics.plasm-ph↗

arXiv2025

Survey of the Edge Radial Electric Field in L-mode TCV Plasmas using Doppler Backscattering

A Doppler backscattering (DBS) diagnostic has recently been installed on the Tokamak \`a Configuration Variable (TCV) to facilitate the study of edge turbulence and flow shear in a versatile experimental environment. The dual channel V-band DBS system is coupled to TCV's quasi-optical diagnostic launcher, providing access to the upper low-field side region of the plasma cross-section. Verifications of the DBS measurements are presented. The DBS equilibrium $v_\perp$ profiles are found to compare favorably with gas puff imaging (GPI) measurements and to the $E_r$ inferred from the radial force balance of the carbon impurity. The radial structure of the edge $E_r \times B$ equilibrium flow and its dependencies are investigated across a representative set of L-mode TCV discharges, by varying density, auxiliary heating and magnetic configuration.

physics.plasm-ph↗

arXiv2018

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

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 (It), or 'roll-over', 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 impurity-seeding) 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, Precl, and concurrent increase in the energy required per ionisation, $E_{ion}$: often described as 'power starvation' (or 'power limitation'). The detachment threshold is found experimentally (in agreement with analytic model predictions) to be $\sim P_{recl}/I_t {E_{ion}} \sim 2$, corresponding to a target electron temperature, $T_t \sim E_{ion}/{\gamma}$ where ${\gamma}$ 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 core density is varied.

physics.plasm-ph↗