Applications of a novel model-based real-time observer for electron density profile control experiments in TCV

Abstract

Real-time control of tokamak plasmas encompasses sustaining a high-performance stationary state, avoiding disruptions, and managing ramp-up and ramp-down phases. Real-time estimation and control of electron density is fundamental for monitoring and controlling particle confinement, heating efficiency, exhaust conditions, impurity concentration, fusion power, and proximity to the density limit. Building on the integration of a multi-rate observer based on RAPDENS into the TCV control system, this study explores its application to density profile control for detachment studies, ECH, and NBH L-mode plasmas, and high-performance H-mode scenarios. TCV experiments demonstrate the observer's capability to support detachment studies in complex divertor geometries, controlling the line-averaged density within the last-closed flux surface while rejecting interferometer pick-up from Scrap-Off Layer density in the divertor. The estimated density profile enables local control of central density in ECH/NBH L-mode plasmas below cutoff conditions; heating-induced profile peaking modification is treated as a disturbance to the control task. Real-time estimation of time-varying transport coefficients, such as the pinch velocity-to-diffusivity ratio, improves model predictive capabilities, and the underlying turbulent transport is characterized via linear and non-linear gyrokinetic simulations with GENE. Simultaneous control of edge-normalized density and toroidal beta in H-mode plasmas is then demonstrated, yielding good confinement, scenario reproducibility, and a diagnostics-independent edge-density metric, while avoiding density limits and diagnostic faults propagation.

Figures (26)

• Figure 1: The diffusion transport coefficient $D \ [m^{2}/s]$ heuristically tuned for L- and H-mode density profiles.
• Figure 2: Timeslices of the RAPDENS predictive simulation using an analytical target profile (in dashed red) to provide the $\nu/D$ ratio adopted in the simulation. RAPDENS profiles, in black and spline coefficients, in magenta, are shown. The density profile evolves from the initial condition, in Figure \ref{['fig:timeslices_synth_nu_D']}a, to the target profile in dashed red.
• Figure 3: Timetraces of the RAPDENS predictive simulation using an analytical target profile (in dashed red) to provide the $\nu/D$ ratio adopted in the simulation. RAPDENS timetraces, at different radial coordinates, are reported in black.
• Figure 4: Schematic of the implementation of the electron density observer inside SCD.
• Figure 5: Poloidal section of TCV. The LCFS configurations of the shots #80728 (short leg, in blue), #80719 (medium leg, in orange), and #80729 (long leg, in yellow) are reported. The FIR system is represented in cyan, with 14 vertical lasers crossing the vacuum chamber. The numbering order of each channel starts with FIR #1 on the LFS, moving inwards towards FIR #14 on the HFS. The channels #1 to #7, represented with solid lines, are employed in the RAPDENS EKF reconstruction. The remaining set of FIR channels, from #8 to #14, is represented with dashed lines. Thomson scattering system's 117 closely packed volume measurement points at R=0.900 m, aligned with FIR laser #6 at R=0.903 m, are depicted with red triangle markers.