Fluid models of gas discharges are typically based on continuity equations and drift-diffusion equations for plasma particle species. The boundary conditions for these equations are an important part of the description of the problem. In this Brief Report, we point out that the most commonly used boundary conditions do not describe the physics properly. We present improved boundary conditions that can be used instead.

The first direct observation of the velocity distribution of the metastable Ar+*(2G9/2) ions in the presheath of an inductively coupled plasma has been achieved by using the Doppler shifted laser induced fluorescence technique. Drift of the ions along the electric field in the presheath is observed and distribution functions of the velocity in both parallel and perpendicular directions, relative to the E field, are deduced at 5 and 40 mTorr. Present results show that in high density plasmas the velocity distribution of the metastable ions is directly related to that of the ground state argon ions. Neutral gas temperature of around 600 K is also measured from the absorption profile of a diode laser beam, set on one of the 772.4 nm argon lines.

The high-power density of a frequency quadrupled pulsed Nd-YAG laser has been used to photodetach electrons from negative ions in rf plasmas generated within a microwave cavity. Negative ion densities have been determined by measuring the frequency shift of the resonance transmission, the shift being caused by the photoelectrons created by irradiating the plasma with the laser pulse. By measurement of the shape of the resonance curve as a function of time and of microwave frequency, and consecutive fitting of a parabola to the top of the resonance curve, the negative ion density has been determined as a function of gas pressure, rf power, and position in the plasma. Measurements were performed in plasmas of CF4, CzFg, CHFs, and C3Fs. The results indicate that the negative ion densities are about one order of magnitude larger than the electron density, which is in good agreement with a fluid model calculation. The pressure and power dependence of the electron density and of the negative ion density gives insight in the relation between the electron temperature and the macroscopic plasma parameters. Measurements as a function of the laser wavelength, using a pulsed dye laser, show that in CF4 the negative ions mainly consist of F-, whereas in C2Fe significant densities of other negative ions may occur.

Electron densities hat,e been determined for RF plasmas that were generated within

Electron and negative ion densities in the afterglow and in the plasma initiation phase of a 13.56-MHz rf discharge in CF4 were measured by using a microwave cavity method and a laser photodetachment technique. Measurements were carried out at low rf powers ( 5 10 W) and in the pressure range from 100 to 300 mTorr. The electron density in the afterglow showed an enhanced decay rate due to the presence of negative ions. Electrons originating from negative ions through associative collisional detachment with neutral radicals were also detected in the afterglow. Decay curve analysis of the negative ion density gave an ion-ion (presumably CF: -F -) recombination rate constant of ( 5 f 2) x 10 -I3 m3 s -. ', and showed that, in the active plasma, the negative ion loss rates by associative detachment and ion-ion recombination are of the same order of magnitude. The behavior of the electron and negative ion densities in the plasma initiation phase indicates that molecules and radicals that slowly accumulate in the plasma do not play a significant role in the production of negative ions.