Laser-induced-fluorescence observation of ion velocity distribution functions in a plasma sheath

Abstract

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Collisionless “thermalization” in the sheath of an argon discharge

“…This hypothesis is supported by the experimental measurement of the ion drift velocity profile near the wall. 5 In the case of a 50 V discharge (which will be considered in the remainder of this work), the ion maximum drift velocity in the sheath reaches values of about 6c s , implying a potential drop of the order of the discharge potential. In comparison, using a model including only bulk electrons with T e ¼ 2:5 eV would lead to a peak ion drift velocity of only 3c s .…”

“…Vlasov codes are particularly adapted to this kind of study, because they display a fine resolution in all regions of the phase space, including the sheath, where the plasma density is very low. We shall concentrate on the experimental results obtained by Claire et al 5 using Laser Induced Fluorescence (LIF) measurements. As the sheath is very thin-just a few millimeters in a standard laboratory plasma dischargemeasuring the IVDF inside the sheath is a daunting experimental challenge.…”

“…Among plasma diagnostics, LIF is a privileged technique to obtain the IVDF with good spatial and velocity resolution without perturbing the plasma. [7][8][9][10][11][12] The results of Claire et al 5 show that the IVDF, after having developed a prominent asymmetric tail in the presheath, becomes again symmetric in the sheath. Here, we will show that this phenomenon can be explained using purely collisionless arguments, without invoking either ordinary or instability-enhanced 13,14 collision rates.…”

Experimental studies of transverse metastable ion velocity distribution functions in the presheath of a weakly collisional argon plasma

“…They found that the broadening was considerably greater at 40 mTorr in Ar, than at 5 mTorr. On the other hand, Claire et al 19 measured tIVDF profiles throughout the presheath and sheath region at low pressure ͑3.6 mTorr͒ in a dc Ar discharge plasma with multipolar magnetic confinement, and found no broadening whatsoever. The boundary plate in that case was an electrically floating metal plate.…”

“…Spatially resolved and in situ measurements of the tIVDFs were performed in various sources of plasmas [13][14][15][16][17][18][19] using the LIF technique, but some of these measurements were made at a single point in the plasma, and none were correlated with plasma potential measurements along the presheath. In the vicinity of a glass boundary plate, however, Sadeghi et al 18 measured tIVDFs at a variety of locations, which demonstrated considerable broadening ͑heating͒ as the ions transited the presheath.…”