Validation of equilibrium tools on the COMPASS tokamak

J. Havlíček, M. Komm, M. Peterka (IPP Prague)

L.C. Appel, I. Lupelli (CCFE)

J.-F. Artaud (CEA)

B. Faugeras (U. Nice)

Abstract

Preliminary contents

  1. FREEBIE inverse mode calculation for a given shape, p and .
  2. Reconstruction by EFIT from FREEBIE with available diagnostics (Rogowski coils, flux loops, TS)
    • accuracy vs noise
    • accuracy vs p', ff' degrees
  3. Rapid reconstruction by EQUINOX radial basis functions
  4. Possible enhancements -- synthetic dianostics
    • MSE
    • LCFS position (e.g. probes)
  5. FREEBIE in inverse mode for COMPASS as a shape designer, scenario designer)

Optional

  1. VDE simulation compared to experiments
  2. METIS/FREEBIE comparison to experiments

Introduction

\label{sec:introduction}

We report here on validation and verification of tokamak equilibrium tools used for the COMPASS tokamak (Pánek 2006). We particularly focus on fundamental global plasma parameters and the shapes of magnetic flux surfaces, which are crucial in diagnostics interpretation and other analyses. EFIT++ (Appel 2006) is used for routine equilibrium reconstruction on COMPASS. FREEBIE (Artaud 2012) is a recent free-boundary equilibrium code; FREEBIE enables predictive equilibrium calculation consistent with the poloidal field (PF) components of the tokamak. In this study, FREEBIE is used in the so-called inverse mode, which predicts PF coils currents from a given plasma boundary and profiles. The third code employed in this study is VacTH (Faugeras 2014), which provides a fast reconstruction of the plasma boundary from magnetic measurements using a toroidal harmonics basis.

In order to verify and validate the aforementioned tools, we analyse EFIT++ and VacTH reconstructions of equilibria constructed with FREEBIE. Synthetic diagnostics (e.g., magnetic probes or flux loops) with optional artificial errors provide inputs for the reconstructions.

Verification and validation procedure

\label{sec:procedure}

Reliable MHD equilibrium reconstruction is very important for tokamak exploitation. Numerous diagnostics and subsequent analyses require as inputs equilibrium properties such as flux surface geometry, magnetic field, stored energy, internal inductance etc. We have set up a set of benchmarking tasks, which verify and validate equilibrium tools that are currently employed on COMPASS. The procedure is fundamentally following:

  1. Equilibrium reconstruction of selected experimental cases using EFIT++.

  2. Recalculate the equilibria using FREEBIE in inverse mode.

  3. Optionally alter the equilibria in FREEBIE using e.g. experimental pressure profiles.

  4. Reconstruct FREEBIE equilibria using EFIT++ and VacTH with various parameters and artificial input noise.

The first step employs a routine EFIT++ set-up for COMPASS with heuristically