Motional Stark Effect Measurements Of The Local Magnetic Field In High Temperature Fusion Plasmas

Abstract

The utilization of the Motional Stark Effect (MSE) experienced by the neutral hydrogen or deuterium injected into magnetically confined high temperature plasmas is a well established technique to infer the internal magnetic field distribution of fusion experiments. The different properties of the Stark multiplet allow inferring, both the magnetic field strength and the orientation of the magnetic field vector. Besides recording the full MSE spectrum, several types of polarimeters have been developed to measure the polarization direction of the Stark line emission. To test physics models of the magnetic field distribution and dynamics the accuracy requirements are quite demanding. In view of these requirements, the capabilities and issues of the different techniques are discussed, including a newly developed Imaging MSE system which has been tested on the ASDEX Upgrade tokamak.

Motional Stark Effect measurements of the local magnetic field in high temperature fusion plasmas

“…However, the positions of those mirrors do not necessarily fulfil the condition ∆α = 0. In order to minimize the influence of the mirror on the polarization measurement, in ASDEX Upgrade the mirror is provided with a dielectric coating which for the wavelength range and the angular range given maximizes the reflectivity [49]. In addition, in ASDEX Upgrade the mirror is protected against coatings by an exchangeable glass window in front of the mirror.…”

Investigation of fast ion pressure effects in ASDEX Upgrade by spectral MSE measurements

“…The magneto-hydrodynamic force balance → → ∇ = × p j B describes a condition for stationary magnetic equilibria. Changes in the fast ion population can cause diamagnetic effects which decrease the toroidal magnetic field by about 1% [3,4], and these small effects, though difficult to detect, are of great importance for the local state of the plasma. Even more difficult is the measurement of changes to the local poloidal magnetic field (due to current profile reconfiguration) [3,5,6].…”