Influence of magnetic field on laser-produced barium plasmas: Spectral and dynamic behaviour of neutral and ionic species

摘要

The expansion dynamics and spectral behaviour of plasma produced by a Nd:YAG laser (λ= 1.064 μm, pulse width: 8 ns) from barium target and expanding in 0.45 T transverse magnetic field in vacuum (10~(-5)Torr pressure) are investigated using time-of-flight optical emission spectroscopy. The experiments are carried out at various laser fluences from 12 to 31 J/cm~2. The temporal profiles of neutral (Ba Ⅰ 553.5 and 577.7 nm) lines are temporally broadened, while that of ionic (Ba Ⅱ 413.0 and 455.4 nm) lines show strong confinement in the presence of a magnetic field. In the absence of magnetic field, the temporal profile of Ba Ⅰ 553.5 nm is exactly reproduced by fitting with two Shifted Maxwell Boltzmann (SMB) Distribution components, while in the presence of a magnetic field the profile could only be fitted with three components. The field enhanced and field induced SMB components of neutral profile are correlated with populations of ground state, metastable states, and long-lived Rydberg states present in the barium plasma, while SMB components of ionic lines are explained on the basis of the presence of super-elastic collisions among the excited species in the plasma. The spatial variation of electron temperature and temporal variation of electron density are deduced and correlated to the different collisional processes in the barium plasma. The ionic profiles show efficient confinement in the presence of a magnetic field at higher fluences.