Penning type discharge was adopted to excite helium atoms. It is suitable for generating high density of metastables at a range from 0.1 mTorr to 0.5 Torr. The highest metastable density of 3.5 * 10^(10) cm^(-3) was observed at a static gas pressure of 0.5 Torr. The highest fraction of metastables (N_(2^(1)S) = N_(He)) of 10^(-3) in a low gas pressure was obtained. The variation of the magnetic field strength on the discharge does not result in a significant density change of the metastable helium atoms. When no magnetic field was applied, no discharge took place.
© 2005 Chinese Optics Letters
X. P. Feng and B. W. James, "An investigation of high fractions of metastable helium atoms," Chin. Opt. Lett. 1, 187-189 (2003)