Diagnostic development for current density profile control at KSTAR

The current density profile diagnostics are critical for the control of the steady-state burning plasmaoperations. A multi-channel motional Stark effect (MSE) diagnostic system has been implemented forthe measurements of the internal magnetic field structures that constrain the magnetic equilibriumreconstruction to accurately produce the tokamak safety factor and current density profiles for the KoreaSuperconducting Tokamak Advanced Research (KSTAR). This work presents the design and fabrication ofthe front optics and the filter modules and the calibration activities for the MSE diagnostic at KSTAR

A model-based approach for filtering magnetic pitch angles obtained by the Motional Stark Effect diagnostic

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Evolution of the central safety factor during stabilized sawtooth instabilities at KSTAR

\u3cp\u3eA motional Stark effect (MSE) diagnostic has recently been installed in the KSTAR tokamak. A difficulty faced at KSTAR and common to other MSE diagnostics is calibration of the system for absolute measurements. In this report we present our novel calibration routine and discuss first results, evaluating the evolution of the the central safety factor during sawtooth instabilities. The calibration scheme ensures that the bandpass filters typically used in MSE systems are aligned correctly and identifies and removes systematic offsets present in the measurement. This is verified by comparing the reconstructed safety factor profile against various discharges where the locations of rational q surfaces have been obtained from MHD markers. The calibration is applied to analyse the evolution of q \u3csub\u3e0\u3c/sub\u3e in a shot where the sawteeth are stabilized by neutral beam injection. Within the analysed sawtooth periods q \u3csub\u3e0\u3c/sub\u3e drops below unity during the quiescent phase and relaxes close to or slightly above unity at the sawtooth crash. This finding is in line with the classical Kadomtsev model of full magnetic reconnection and earlier findings at JET.\u3c/p\u3

Optimal MSE polarisation angle and q-profile estimation using Kalman filters and the plasma simulator RAPTOR

\u3cp\u3eAccurate q-profile reconstruction is of importance for the development of advanced scenarios, but continues to be a challenge in tokamak research. To constrain the q-profile in the plasma centre the motional Stark effect diagnostic (MSE) is often used, however achieving routine measurements with the required accuracy proves to be difficult in many devices. We present a novel approach to obtain accurate estimates of the q-profile using an observer based approach. The observer combines MSE measurements with a model-based prediction of the system. For this the plasma transport simulator RAPTOR is coupled with a fixed-boundary equilibrium solver to create a model-based prediction of the MSE measurements. An extended Kalman filter is used to merge profile evolution predictions from the RAPTOR code with measurements. Using synthetic data we demonstrate accurate q-profile estimations in situations where the model is purposely disturbed and only erroneous MSE measurements are available. For shots at ASDEX Upgrade we show that by constraining RAPTOR with MSE measurements, the evolution of the model's q-profile is in close proximity to reference profiles of reconstructed equilibria from an integrated diagnostic suite.\u3c/p\u3

Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade

\u3cp\u3eWithin the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (n\u3csub\u3ee,sep\u3c/sub\u3e/n\u3csub\u3eG\u3c/sub\u3e ∼ 0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened.\u3c/p\u3