Section: New Results

Equilibrium reconstruction and current density profile identification

Participants : Jacques Blum, Cédric Boulbe, Blaise Faugeras.

EQUINOX is a real-time equilibrium reconstruction code. It solves the equation satisfied by the poloidal flux in a computation domain, which can be the vacuum vessel for example, using a P1 finite element method and solves the inverse problem of the identification of the current density profile by minimizing a least square cost-function. It uses as minimal input the knowledge of the flux and its normal derivative on the boundary of the computation domain. It can also use supplementary constraints to solve the inverse problem: interferometric, polarimetric and MSE measurements. Part of the work reported here has been done in the frame of a RTM-JET contract [2]

Direct use of the magnetic measurements

The code EQUINOX was not originally designed to take as magnetic inputs directly the magnetic measurements, as it should be the case in the ITM (Integrated Tokamak Modeling European platform), but some outputs from the real-time codes APOLO at ToreSupra and XLOC at JET. These codes provide EQUINOX with the values of the flux and its normal derivative on a closed contour defining the boundary of the computation domain (this contour can be the limiter for example). As a consequence the main difficulty arising in the objective of integrating the code EQUINOX in the ITM structure is to interpolate between the magnetic measurements (flux loops and poloidal B-probes) with a machine independent method. A solution to do this is to use toroidal harmonics functions as a basis for the decomposition of the poloidal flux in the vacuum region in complement to the contribution of the PF coils. The first version of the algorithm implementing this method for EQUINOX-ITM developed in 2012 has been updated and tested during 2013:

• WEST and JET: This method can provide an alternative tool, comparable to APOLO (for Tore Supra) and FELIX (for JET), to compute the plasma boundary in real time from the magnetic measurements. Some twin experiments for WEST have been successfully conducted. In a first step the equivalents of magnetic measurements were generated using the free boundary equilibrium code CEDRES++. In a second step these measurements were used by the toroidal harmonics algorithm to reconstruct the plasma boundary. Additional calculations aiming at validating the design of the WEST magnetic diagnostics have been performed. They consisted in checking the equilibrium reconstruction accuracy with respect to: (i) a reduced number of magnetic sensors; (ii) noise on magnetic sensor and/or current measurements. Then, experiments on the possibility to reconstruct not only the plasma boundary but also the current density have been conducted. A paper on this subject is accepted for publication [13] . The same algorithm has been tested using real JET measurements in order to provide an equilibrium reconstruction code that directly uses the magnetic measurements instead of using FELIX as an intermediate preprocessing of the measurements.

• EFDA-ITM (Task WP13-ITM-IMP12-ACT3): EQUINOX-ITM has been upgraded and tested on the new gateway machine of the ITM. The Kepler actor was tested and used on 3 different tokamaks (JET, Tore Supra and WEST) (with F. Imbeaux, T. Aniel, P. Moreau, E. Nardon (CEA)). A benchmark work is on going between the codes Equal, Efit and Equinox on JET shot 74221 (with Dimitriy Yadykin and Wolfgang Zwingmann).