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Kinetic Equilibrium Prediction at TCV using RAPTOR and FBT

C. E. Contré, A. Merle, O. Sauter, S. Van Mulders, R. Coosemans, G. Durr-Legoupil-Nicoud, F. Felici, O. Février, C. Heiss, B. Labit, A. Pau, Y. Poels, C. Venturini, B. Vincent, the TCV team, the EUROfusion Tokamak Exploitation Team

Abstract

We present results from a new Kinetic-Equilibrium Prediction (KEP) workflow and shot preparation for full TCV discharges, by coupling predict-first RAPTOR transport simulations with FBT inverse equilibrium calculations. RAPTOR is a 1.5D transport code which has been extensively used for plasma shot optimization and real-time modeling. We show that rapid pre-shot simulations can be performed directly using information from the pulse schedule across a wide range of plasma shapes and scenarios, given an estimate of the confinement quality factor H98(y,2) and line-averaged density. The resulting p' and TT' profiles are then provided to the pre-shot equilibrium computation performed by FBT - a static free-boundary solver routinely used at TCV - achieving convergence between the two codes in a few iterations. Finally, we show that this coupling, when integrated into the TCV shot preparation, improves the evaluation of the coil currents needed to match the target plasma shape; in particular providing an accurate estimate of critical quantities such as the internal inductance $l_i$ and normalized pressure $β_N$, giving more realistic information to tokamak operators about the expected pulse behavior and enabling them to adjust the plan correspondingly.

Kinetic Equilibrium Prediction at TCV using RAPTOR and FBT

Abstract

We present results from a new Kinetic-Equilibrium Prediction (KEP) workflow and shot preparation for full TCV discharges, by coupling predict-first RAPTOR transport simulations with FBT inverse equilibrium calculations. RAPTOR is a 1.5D transport code which has been extensively used for plasma shot optimization and real-time modeling. We show that rapid pre-shot simulations can be performed directly using information from the pulse schedule across a wide range of plasma shapes and scenarios, given an estimate of the confinement quality factor H98(y,2) and line-averaged density. The resulting p' and TT' profiles are then provided to the pre-shot equilibrium computation performed by FBT - a static free-boundary solver routinely used at TCV - achieving convergence between the two codes in a few iterations. Finally, we show that this coupling, when integrated into the TCV shot preparation, improves the evaluation of the coil currents needed to match the target plasma shape; in particular providing an accurate estimate of critical quantities such as the internal inductance and normalized pressure , giving more realistic information to tokamak operators about the expected pulse behavior and enabling them to adjust the plan correspondingly.
Paper Structure (20 sections, 14 equations, 20 figures, 1 table)

This paper contains 20 sections, 14 equations, 20 figures, 1 table.

Table of Contents

  1. Introduction
  2. Physics models and codes
  3. RAPTOR
  4. General assumptions.
  5. Transport and pedestal.
  6. Ion temperature.
  7. L-H transitions.
  8. Extension of the gradient-based model to negative triangularity plasmas.
  9. Line-averaged density.
  10. Runtime optimization.
  11. FBT
  12. Coil currents optimization.
  13. FBT-RAPTOR coupling
  14. Benchmark of the pre-shot prediction of 207 shots
  15. Experimental validation
  16. ...and 5 more sections

Figures (20)

  • Figure 1: Schematic overview of the Kinetic Equilibrium Prediction (KEP) workflow. The pulse schedule is given as input to RAPTOR and FBT, loosely coupled to provide the active coil currents and pulse profiles predictively. These new feedforward currents traces are then integrated into the TCV preparation system to control the plasma shape and position with its 16 separately powered PF coils. Examples of $\#82030$ (lower single-null, LSN, in red) and $\#83575$ (snowflake (SF) upper NT, in blue) shapes, achievable at TCV and simulated with the KEP (see Sec. \ref{['Sec:TCV_results']}), are also represented.
  • Figure 2: Comparison of two $T_i$ estimations, using Eq. \ref{['equ:PRETOR']} with $H_e^{98(y,2)} = 0.5$ (in blue) and solving for the full ion heat equation with $H^{98(y,2)} = 1$ (in red), with the measurements of 4 CXRS systems for shot $\#81882$, and comparison with $T_e$, in the L-mode ohmic phase (a) and in the NBI-heated H-mode phase (b). These models yield similar results, both predicting the CXRS measurements within their error-bars, except in the H-mode outer radii region, where measured values are biased by Edge Localized Modes.
  • Figure 3: Evolution of (a) $T_e$, (b) $n_e$ and (c) $T_i$ profiles before and after the onset of NBI. As the H-mode is triggered and reaches a QCE regime, the TS measurements (markers with error-bars) show a slightly better confinement ($H_{\text{exp}}^{98(y,2)} > 1$) than the one modeled in RAPTOR (plain line), particularly shown in $T_i$. The experimental $n_{e,l}$ was used as input to this simulation.
  • Figure 4: (a) $T_e$, (b) $n_e$ and (c) $T_i$ profiles of a negative triangularity (NT) shot. As the shaping decreases from $\delta = -0.37$ to $-0.30$, the NBI-1 power is switched on and ramped up to $516kW$, leading to an increase in density and total energy. With an $H^{98(y,2)}=1$ and taking the experimental $n_{e,l}$, RAPTOR (plain line) matches the high confinement properties of the L-mode NT plasma, slightly over-estimating $T_e$ in the ohmic (lower density) phase.
  • Figure 5: Result of the FBT-RAPTOR convergence for TCV shot #81882 after N iterations. $\alpha=p'_{\text{FBT}}/p'_{\text{RAPTOR}}$ and $\Delta\beta_{pol}=|\beta_{pol}^{\text{FBT}}-\beta_{pol}^{\text{RAPTOR}}|$. The y-axis shows the distance between these quantities and their final values at N=7.
  • ...and 15 more figures