Radial electric field and density fluctuations measured by Doppler reflectometry during the post-pellet enhanced confinement phase in W7-X

TL;DR

This work investigates turbulence suppression during the post-pellet enhanced confinement phase in the W7-X stellarator by measuring radially resolved $E_r$ and density fluctuations with Doppler reflectometry. It combines Doppler reflectometry measurements with neoclassical (DKES, KNOSOS) and gyrokinetic (STELLA, EUTERPE) simulations to disentangle the influences of $E_r$, kinetic profiles, and magnetic configuration. Key findings include a strong $E_r$-well (up to $-40$ kV/m) whose peak scales with $n_e$ and $P_{ECH}$, and a substantial reduction of density fluctuations toward the core, especially in the high iota configuration; neoclassical predictions reproduce the $E_r$ features, while gyrokinetic analyses attribute much of the suppression to post-pellet profile evolution and $E_r$-shear. The results demonstrate the diagnostic power of Doppler reflectometry for probing turbulence control in stellarators and offer guidance for achieving reactor-relevant confinement regimes.

Abstract

Radial profiles of density fluctuations and radial electric field, $E_r$, have been measured using Doppler reflectometry during the post-pellet enhanced confinement phase achieved, under different heating power levels and magnetic configurations, along the 2018 W7-X experimental campaign. A pronounced $E_r$-well is measured with local values as high as -40 kV/m in the radial range $ρ\sim 0.7-0.8$ during the post-pellet enhanced confinement phase. The maximum $E_r$ intensity scales with both plasma density and Electron Cyclotron Heating (ECH) power level following a similar trend as the plasma energy content. A good agreement is found when the experimental $E_r$ profiles are compared to simulations carried out using the neoclassical codes DKES and KNOSOS. The density fluctuation level decreases from the plasma edge toward the plasma core and the drop is more pronounced in the post-pellet enhanced confinement phase than in reference gas fuelled plasmas. Besides, in the post-pellet phase, the density fluctuation level is lower in the high iota magnetic configuration than in the standard one. In order to discriminate whether this difference is related to the differences in the plasma profiles or in the stability properties of the two configurations, gyrokinetic simulations have been carried out using the codes \texttt{stella} and EUTERPE. The simulation results point to the plasma profile evolution after the pellet injection and the stabilization effect of the radial electric field profile as the dominant players in the stabilization of the plasma turbulence.

Figures (10)

• Figure 1: Left: Radial profiles of $E_r$ measured during the post-pellet phase in two experimental programs (solid symbols) and those measured in gas fuelled reference plasmas (open symbols), with $n_e \sim 8 \times 10^{19}$ m$^{-2}$ and $P_{ECH} =$ 3 MW (in blue), and $n_e \sim 9 \times 10^{19}$ m$^{-2}$ and $P_{ECH} =$ 5.5 MW (in red). Right: Corresponding electron density and temperature profiles measured by Thomson Scattering during the post-pellet phase in the two pellet fuelled plasmas (solid lines) and in the reference ones (dotted lines).
• Figure 2: Local $E_r$ intensity measured by DR at $\rho \sim$ 0.7-0.75 (left) and diamagnetic energy (centre) during the post-pellet high confinement phase (solid symbols) and in gas fuelled plasmas (open symbols) as a function of the product $n_e \times P_{ECH}$. The relation between the $E_r$ intensity and the diamagnetic energy is shown in the right panel. Standard magnetic configuration EJM is represented in red and high iota configuration FTM in black.
• Figure 3: Electron density and temperature profiles measured by Thomson Scattering diagnostic and ion temperature profile measured by XICS diagnostic during the post-pellet phase in the standard EJM (a) and high iota FTM magnetic configuration (c), and in the reference gas fuelled plasmas in the two configurations (b) and (d), respectively. The solid lines represent the fits to the experimental values (symbols) measured during the time interval of the DR analysis.
• Figure 4: Comparison of experimental $E_r$ profiles normalised with $| \nabla r|$ (symbols) and neoclassical predictions obtained using the codes DKES and KNOSOS (lines) for the post-pellet enhanced confinement phase (solid symbols and thick lines) and for the corresponding gas fuelled plasmas (open symbols and broken lines); for the standard magnetic configuration EJM (left, in red) and for the high iota FTM configuration (right, in black). The error bars in the simulated $E_r$ profiles result from a sensitivity study performed assuming deviations of 10$\%$ from the measured density and temperature profiles and their gradients.
• Figure 5: Radially resolved density fluctuations measured during the post-pellet phase in two experimental programs (solid symbols) and those measured in gas fuelled reference programs (open symbols), with $n_e \sim 8 \times 10^{19}$ m$^{-2}$ and $P_{ECH} =$ 3 MW (in blue), and $n_e \sim 9 \times 10^{19}$ m$^{-2}$ and $P_{ECH} =$ 5.5 MW (in red). Same experimental programs as in figure \ref{['f:fig_1']}.