Coherent population trapping as a magnetic-field diagnostic for hydrogen plasmas

Coherent population trapping (CPT) is theoretically examined as a magnetic-field diagnostic for high-β hydrogen plasma. Time-dependent quantum mechanical Bloch equations, which describe the evolution of the 2s and 3p level populations of the hydrogen atom under CPT conditions, were solved numerically. When the frequency difference of two copropagating lasers equals the energy difference between the atoms' levels subject to the local magnetic field, a discernable CPT dark line in the Hα emission is predicted, enabling the possibility of noninvasive, localized magnetic-field measurements. The effects of fine and hyperfine level structure, Doppler broadening, plasma-generated electric fields, and degree-of-hydrogen ionization are included in the model. A shift in dark-line position of 15% of the linewidth is predicted to be caused by contributions from the entire Hα manifold. The laser-induced Hα fluorescence is estimated to be an order of magnitude stronger than the background Hα emission.