Using laser-induced phetodetachment (LIPD), we investigate in some detail h
ow different discharge parameters affect the negative ion fraction in high-
density low-pressure SF6 magnetoplasmas sustained by the propagation of ele
ctromagnetic surface waves. A plane electrostatic probe is used for collect
ion of the photodetached electrons. Careful testing of the LIPD technique i
tself is carried out prior to systematic measurements and adequate laser fl
uence conditions are determined. Negative ions are found to outnumber elect
rons several times, even at mTorr and submTon pressures, indicating the imp
ortant electronegative character of the discharge. The dependence of the ne
gative ion fraction on gas pressure, argon admixture, microwave power, and
axial and radial position in the reactor is interpreted on the basis of dif
ferent negative ion formation and loss mechanisms. The negative ion fractio
n is found to be maximum in conditions and regions of minimal electron temp
erature and positive ion density. (C) 2000 American Vacuum Society. [S0734-
2101(00)00305-5].