Development of supersonic metastable helium pulsed beam source for plasma diagnostics

A supersonic metastable helium (2 ¹S) pulsed beam source, which consists of an electromagnetic valve, a collimation skimmer, and discharge electrodes for production of a plasma, has been constructed, which is essential for the direct measurement of electric fields in plasmas by means of polarization laser-induced fluorescence spectroscopy. A supersonic helium atomic beam with a short pulselength (∼300 μs), narrow divergence (∼1.1°), and high density of ∼1.4 × 10¹⁴ cm ^-3 has been achieved. In order to generate metastable atoms in this beam source, a Penning-type discharge was employed, which is suitable for producing stable plasma with high temperature, even at low gas densities, due to the particular configuration of electrodes together with a magnetic field. Spectroscopic observations indicated that the temporal behaviors of neutral atom and ion emissions were almost the same as that of the helium atom profile, and on increasing the discharge voltage applied between electrodes, the spectral intensity increased approximately linearly. Moreover, the fact that the ion emission can also be observed showed that it was probable that a high-temperature plasma was generated by the Penning discharge.