Application of eclipse laser photodetachment technique to electron sheath thickness and collection region measurements.

ABSTRACT

The laser photodetachment (LPD) technique, which has been used for negative-ion density measurements, is applied for the measurement of electron sheath thickness and the collection region of photodetached electrons (PDE's). By forming a thin laser shadow in the laser beam channel, the electron sheath can be observed in the temporal evolution of the LPD signals. The collection region of PDE's is determined from the response of the signal peak value in scanning the shadow position. The measurement is applied to the electron sheath formed around a cylindrical probe and a plane probe. The experimentally obtained thickness of the sheath in front of the plane probe agrees well with the one-dimensional Child-Langmuir sheath when the magnetic field exists. Further, the results of the sheath thickness around the plane and cylindrical probe at different magnetic field strengths indicate that the effect of magnetic field on the sheath structure is significant even in weakly magnetized plasmas. The length of the collection region of PDE's was measured, and it was confirmed that the region was in the laser beam channel under our experimental conditions. It is proposed that this method be applied to check the validity of the laser photodetachment technique.