Improvements for real-time magnetic equilibrium reconstruction on ASDEX Upgrade

Abstract

Real-time magnetic equilibria are needed for NTM stabilization and disruption avoidance experiments on ASDEX Upgrade. Five improvements to real-time magnetic equilibrium reconstruction on ASDEX upgrade have been investigated. The aim is to include as many features of the offline magnetic equilibrium reconstruction code in the real-time equilibrium reconstruction code. Firstly, spline current density basis functions with regularization are used in the offline equilibrium reconstruction code, CLISTE [1]. It is now possible to have the same number of spline basis functions in the real-time code. Secondly, in the presence of edge localized modes, (ELM's), it is found to be necessary to include the low pass filter effect of the vacuum vessel on the fast position control coil currents to correctly compensate the magnetic probes for current oscillations in these coils. Thirdly, the introduction of ferromagnetic tiles in ASDEX Upgrade means that a real-time algorithm for including the perturbations of the magnetic equilibrium generated by these tiles is required. A methodology based on tile surface currents is described. Fourthly, during current ramps it was seen that the difference between fitted and measured magnetic measurements in the equilibrium reconstruction were larger than in the constant current phase. External loop voltage measurements and magnetic probe pairs inside and outside the vessel wall were used to measure the vacuum vessel wall resistivity. This is the first step towards including vacuum vessel currents during the plasma current ramp in the real-time equilibrium reconstruction. Fifthly, the introduction of a constraint of the safety factor on the magnetic axis is found to be a helpful method to improve the prediction of the location of rational surfaces for NTM stabilization and disruption avoidance experiments. Soft X-ray tomography is used to assess the quality of the real-time magnetic equilibrium reconstruction using this internal constraint.

Real-time diamagnetic flux measurements on ASDEX Upgrade

“…6, the measured diamagnetic flux is compared to that predicted by the TRANSP simulation for a discharge with neutral beam and electron cyclotron heating. The poloidal beta inferred from the diamagnetic flux measurement, 19 β DIA , is in good agreement with the values calculated from the real-time 20,21 and offline magnetic equilibrium reconstruction codes, β MHD , in the steady state phase of the discharges. In the discharge with electron cyclotron heating, the increase in poloidal beta with time is due to a steady increase in the nitrogen seeding introduced for reduction of the heat load to the divertor target.…”

Long term project in ASDEX upgrade: Implementation of ferritic steel as in vessel wall

“…Fictitious currents are modeled in the code as function of the eff calculated by the FEM. As reported in [8], with this approach a good agreement between the calculated magnetic field and the measured one is reached.…”

“…The real time magnetic equilibrium code handling the magnetic probes has been updated as described in [8]. Fictitious currents are modeled in the code as function of the eff calculated by the FEM.…”

RABBIT: Real-time simulation of the NBI fast-ion distribution

“…Currently, real-time T e and n e diagnostics are used, and there are plans to make the charge-exchange diagnostics (T i and rotation) real-time capable as well. The plasma equilibrium is reconstructed in real-time by the JANET code [45]. An extension of these activities and further applications to other tokamaks such as JET, DIII-D and TCV is planned.…”