A motional Stark effect (MSE) diagnostic measures the local magnetic pitch angle in a tokamak which confines the plasma with the helical magnetic field made by toroidal and poloidal magnetic fields. Measurement of the magnetic pitch angle profile is an important issue in the advanced steady state tokamak studies in Alcator C-Mod where non-inductive current drive experiments using electromagnetic waves have been initiated. The local effect of the applied current drive and the current distribution across the poloidal cross section can be found by measuring the magnetic topology and the tokamak safety factor (q) profile. In addition to the advanced steady state tokamak research, knowledge of the internal magnetic structure is essential to understand various transport and stability problems such as transport barriers, MHD stabilities and the effects of a radial electric field.

The C-Mod MSE diagnostic utilizes polarization patterns from Doppler-shifted Balmer-alpha decay emission from the diagnostic neutral beam (DNB), a high energy neutral beam crossing the magnetic field perpendicularly. Due to the complex nature of the diagnostic, such as the combined effect of Zeeman-Stark transitions in E x B field environment; a long and complicated optical train that delivers the light signal from invessel to the external polarimeter, which may change the polarization properties of the incident light; and variable angular sensitivity among the channels, precise calibration activities and intensive hardware tests have been done.

Although a basic understanding of the diagnostic principles emerged from these exercises, there are still unresolved discrepancies between various calibrations and plasma measurements. Recently, however, a conjecture has been raised that the emission from the neutrals that are re-charge-exchanged from the fast ions in the beam with arbitrary Doppler shifts can contaminate the measured polarization angle. The main cause of this effect is the lack of the velocity components parallel to the magnetic field. The calculations show that some of the calibration discrepancies can be explained with this conjecture. Quantitative evaluations on this effect is underway and based on these studies, the DNB injection system will be tilted so that the beam can have a finite parallel velocity component.

Links:

MSE website: http://www.psfc.mit.edu/~jinseok

Personal website: http://web.mit.edu/jinseok/www

77 Massachusetts Avenue, NW16, Cambridge, MA 02139, info@psfc.mit.edu