POSSIBILITY AND CURRENT STATUS OF ABSOLUTE XHV MEASUREMENT BY LASER IONIZATION

The current status and possibility of XHV measurement by laser ionizat ion using Second harmonics of a picosecond YAG laser are discussed. Un der the condition that incident laser energy was 30 mJ/pulse and the l aser beam was focused with a spherical lens (focal length f=250 mm) to a power density sufficient for the saturation of H-2 ionization at th e focal point, the minimum pressure in an XHV chamber was measured. Fr om the minimum pressure estimated by extrapolation (5 x 10(-11) Pa) an d the ion counting rate at that pressure (one count during 500 laser s hots), the volume of the ionization region was expected to be 1.7 x 10 (-4) mm(3). The ionization region was then measured experimentally und er the same laser beam conditions. It was found that the ionization re gion has a spindle shape with maximum diameter of 45 mu m and the leng th of 1.2 mm. The estimated volume is larger than the expected volume by a factor between 4 and 12. The ion detection efficiency of the elec tron multiplier is one of the major factors affecting the difference, and it can be reduced to within a factor of 3. On the basis of the res ults, the possibility of absolute measurement of XHV pressure by laser ionization is discussed. Copyright (C) 1996 Published by Elsevier Sci ence Ltd.