Relative electron density distribution in a low nd helium dc discharge

The axial and radial variations of the relative electron density distributions have been measured in an obstructed helium dc discharge by a laser collision induced fluorescence (LCIF) from 4 D-3 to 2 P-3 state at 447 nm. For this 1.75 Torr, 6.5mm gap, 1.6mA helium obstructed gas discharge, the on-axis LCIF intensity is mostly determined by the neutral atom collision. On the basis of this information, we have plotted the LCIF data to obtain a normalized electron density variation in the inter-electrode volume by plotting the ratio, I - I-on=axis/Ion-axis, where I is the radial variation of the LCIF intensity and Ion-axis is the on-axis LCIF intensity. The normalized LCIF data show an annular current density distribution in this obstructed discharge with the current peaking near the radial boundary of the electrodes.