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. In their rest frame, the neutral atoms experience a Lorentz electric field, E L = v × B, which results in a characteristic line splitting and polarized line emission. 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 the influence of the Zeeman Effect and the sensitivity to radial electric fields. A newly developed Imaging MSE system, which has been tested on the ASDEX Upgrade tokamak, is presented. The sensitivity allows to resolve sawtooth oscillations.

Newly uncovered physics of MHD instabilities using 2-D electron cyclotron emission imaging system in toroidal plasmas

“…plasma density and temperature profiles) in tokamak plasmas is well established and will not be discussed in this paper. There are two principal methods for measurement of the current density (or equivalently, poloidal field) in tokamak plasmas and both methods are technically complex and many factors can limit the required accuracy for the stability analysis: polarimetric measurement based on Faraday rotation [13][14][15] and spectroscopy based on the motional Stark effect (MSE) [16][17][18][19][20][21]. Since the required accuracy level varies for different instabilities under study, it is important to be aware of limitations of these methods which are critical for the model validation process.…”

“…The spectroscopic method based on the Motional Stark Effect (MSE) [16][17][18][19][20][21] and Zeeman splitting [22] measures polarization angle of the MSE or Zeeman splitting and the resultant pitch angle measured depends on system geometry from equilibrium reconstruction. The nature of the MSE spectroscopy is the local measurement of the magnetic field line pitch angle by measuring the polarization of the emission of the injected neutral beam (which is usually the neutral beam used for heating the plasma) with known energy as illustrated in Figure 3.…”

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

“…The lines can be grouped in two components: πand σ-lines, which are polarised perpendicular and parallel to the electric field vector. The setup at AUG allows simultaneous measurement of both spectral components at similar radial and vertical positions [20][21][22].…”

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

“…Motional Stark effect (MSE) polarimetry, often denoted by MSE-LP, is routinely used in many fusion devices to measure the magnetic pitch angle [7,8,9,10]. In this paper spectral Motional Stark effect measurements of the internal local magnetic field [11,12] are performed. Spectrally resolved MSE measurements aim to employ the MSE multiplet.…”

“…Therefore, the polarization of the Stark lines is sensitive to the orientation of E L . From the line splitting, ∆λ, the Lorentz field and thus | B| can be deduced [13,14,11]. Spectral MSE measurements based on the line ratio are usually denoted by MSE-LR, whereas spectral MSE measurements applying the line splitting are called MSE-LS.…”