Measurements of the poloidal magnetic and radial electric field profiles in ASDEX Upgrade and JET

Abstract

In magnetic confinement devices the plasma confinement and stability are determined by the magnetic field configuration consisting of toroidal and poloidal field components. In tokamaks the poloidal magnetic field is determined by the profile of the toroidal current. In addition to the Faraday rotation polarimetry [1, 2] the Motional Stark Effect (MSE) has become a routine diagnostic technique to determine the current density distribution [3, 4, 5, 6, 7, 8, 9, 10, 11]. The MSE diagnostic utilizes the Doppler shifted neutral beam emission spectrum which is dominated by the Stark effect due to the electrical field experienced by the fast atoms in their rest frame which is mainly the Lorentz electric field \( \overrightarrow v _{beam} \times \overrightarrow B \). The orientation of this Lorentz electric field is measured, and incorporated in an equilibrium reconstruction, which yields profiles of poloidal magnetic field, current density, safety factor q or poloidal flux. In this paper the diagnostic principle and its implementation on ASDEX Upgrade and JET will be presented in sections two and three. The results of the diagnostic will be discussed in section 4 and the influence of another beam source on the measurement in section 5. Since the MSE diagnostic is sensitive to electric fields in general this opens the possibility to measure also internal electric fields such as the radial electric field [12, 13]. If E r /u beam becomes of the order of B pol then E r can no longer be neglected which is the case in particular for discharges with an internal transport barrier (ITB). The measurement of E r may also be applicable to stellarators where the magnetic field configuration deviates only slightly from the known vacuum configuration. The stronger field line curvature could however increase the requirements regarding the spatial resolution of a MSE diagnostic. First measurements of the radial electric field will be presented in section 6 and a summary will follow in section 7.