Laser induced fluorescence studies of ion acceleration in single and multiple species expanding plasmas

Summary form only given. Ion acceleration in single (Ar) and two positive ion species plasmas (Ar-Xe and He-Ar) has been investigated through laser induced fluorescence (LIF) measurements of the ion velocity distribution function (ivdf). The investigations focused on the expansion region of a helicon plasma source where, for certain operating conditions (pressure in the source less than 2 mTorr, rf driving frequency of 9.5 MHz, source axial magnetic field strength of 700 G, and expansion chamber magnetic field strength between 0 and 70 G), an electric double layer (EDL) was observed to form. In pure argon plasma, 4 cm upstream from the helicon source- expansion chamber junction, the Ar+ ivdf is bimodal - comprised of a slow, nearly stationary ion population (~2.3 km/s) and a fast, supersonic ion population (~6 km/s = 1.8c
_Ar+
) created by the acceleration through the EDL. As the divergence of the field is increased, 19 cm deeper in the expansion region the fast ion group peak velocity determined by oblique incidence LIF is shifted toward higher speeds (~10.5 km/s = 2.9c
_Ar+
), indicative of a second ion acceleration mechanism presence. In He-Ar plasma the Ar+ ivdf is also bimodal. As the helium fraction increases from 0 to 30%, the axial flow speed of the fast ion group increases from 5.3 to 7.8 km/s. Both the fast and slow argon ion group population densities (proportional to LIF amplitude), decrease as the helium fraction increases. The slow population almost completely disappears at a helium fraction of ~30%. Similar effects were observed for Ar-Xe plasmas in which the lighter ion was argon. Although no Xe
⁺
beam was observed, addition of argon led to an increase in the speed of the background ion population from 1.3 km/s in pure xenon to 2.2 km/s for an 87.5% Ar/(Ar+Xe) ratio. In pulsed argon plasma, time resolved LIF measurements showed a time lag in the appearance of the fast ion population. The time lag was found to be a function of the pulse repetition frequency and duty cycle. Two-dimensional LIF provided additional insights into the origins of the accelerated ion population: the nearly isotropic slow population is a locally created background population whereas the distorted velocity distribution of the fast population is consistent with an origin upstream of the measurement location.