The Spectral Broadening of Motional Stark Effect in the HL-2A Tokamak

The spectral broadening of motional Stark effect (MSE) is simulated in the HL-2A tokamak, which is in good agreement with the experimental observation. The divergence angle of the neutral beam, finite lens size and beam energy dissociation are the main broadening factors, while the lens size is the most important one. The experimental spectra fitted by multiple Gaussian distributions reveal that the Stark splitting and its full width at half maximum (FWHM) are ～0.07 nm and ～0.19 nm, respectively. Therefore, the π and σ components of the MSE will overlap since the Stark splitting is much smaller than its spectral broadening. A narrowband filter with the FWHM of about 0.16 nm is good enough to isolate the two components for providing the polarization fraction over 50%, which guarantees reasonable measurement precision for magnetic field pitch angle.