Ion velocity distribution function observations in a multipolar argon discharge

Abstract

Observations of ion velocity distribution functions in two different configurations of a multipolar device with a laser induced fluorescence diagnostic in argon plasma are presented. The first experiment, carried out in the magnetic field closing the two ends of the device, shows the influence of this field on the ion trajectories. In the second experiment performed without the magnetic field closing the two ends of the device, the influence of the electrostatic boundary potential of the plasma is observed. Longitudinal ion velocity distribution functions have been recorded until inside the plasma sheath. The shapes of these distribution functions are in qualitative agreement with the results of a theoretical model and give an experimental justification for the ‘‘flux’’ shape of the ion velocity distribution source function retained in this model. However, to reach this qualitative agreement, it is necessary to adjust the ionic temperature for each distribution function suggesting that, for the authors’ weakly ionized plasma, another heating mechanism is missing in this theory.

Plasma energy transport to an electrically biased surface

“…This model without accounting for kinetic mechanisms of collisions is based on the fact that the ions would be a Maxwell-Boltzmann distribution in the absence of an electrostatic field far from the wall. This proposition has been confirmed by measurements of the ion distribution functions from Bachet et al [19], and successful comparisons between the measured ion density and velocity in argon plasma from Goeckner et al [20] and this work, as can be seen later. Since inert gases and low densities of charged particles are considered, chemical reactions and the existence of negative ions can be neglected.…”

A close-form solution to predict the total melting time of an ablating slab in contact with a plasma

“…This model is based on the fact that the ions would be a Maxwell-Boltzmann distribution in the absence of an electrostatic field far from the wall. This proposition has been confirmed by measurements of the ion distribution functions from Bachet et al [25], and successful comparisons between the measured ion density and velocity in argon plasma from Goeckner et al [26] and theoretical prediction by Yeh and Wei [9]. 5.…”

“…(25) and (26) into Eq. (24), the dimensionless total melting time is expressed as Substituting n = 0 into Eq.…”

Collisionless “thermalization” in the sheath of an argon discharge

“…This type of "convolution heating" (which can lead to an overestimation of the ion temperature) was observed in Ref. 3 and interpreted in Ref. 4.…”

“…1 Under the combined action of the electric field and the ion-neutral collisions, the ion velocity distribution function (IVDF) in the plasma-wall transition region (presheath and sheath) can deviate significantly from a Maxwellian distribution, as was observed many times in experiments as well as numerical simulations. 2, 3 In particular, the IVDF develops a broad and asymmetric low-velocity tail in the presheath, which can lead to an overestimation of the ion "temperature" if the latter is naively defined as the width of the IVDF. 4 Unexpectedly, it was observed in recent experiments that the IVDF becomes again symmetric and almost Maxwellian (within the apparatus resolution) inside the sheath 5 or within the presheath.…”