Conductive target influence on helium metastable production in a µs Plasma Gun discharge
Résumé
Any medical treatments with plasma jets implies plasma biological target interactions. In vivo, in most cases, the targeted tissue or organ is electrically conductive. Many studies have been devoted to the characterization of free jets (i.e without target in front of) but only a few concern plasma jets touching conductive targets. However, it has been shown that this contact leads to a strong increase of reactive species production [1] and helium metastable production in helium plasma jet [2]. Recently, numerical model evidence target effect depending of target nature ( dielectric or metallic) [3]. We studied the influence of conductive target on plasma properties in µs helium Plasma Gun discharge by a cross correlation of two diagnostics. Helium metastable concentration measurements by laser absorption spectroscopy (optical bench LPGP [4]) were correlated with two orthogonal components of electric field ( longitudinal and radial) measurements realized with electro-optic probe [5]. These two diagnostics are both spatially and temporally resolved. Comparing with free jet, it first appears that the helium metastable production is very dependent on target distance from capillary end, not only in the plasma plume, but also all along the capillary. With 1 cm target gap, the helium metastable concentration strongly increases by more than one order of magnitude close to target surface (1,8.1012 cm-3) compare to free jet plasma plume at same position (1,4.1011 cm-3). In the capillary the concentration follows a quasi linear increase of almost factor 3 between the beginning and the capillary end, whereas no increase occurs in free jet. With the correlation of metastable fast production and electric field components time evolutions, we have identified a mechanism triggered by the ionization front contact on the conductive target. This contact is immediately followed by a very fast counter propagation of a secondary front from the target surface to the high voltage electrode through the remnant plasma tail of the primary front. As observed, this lead to a greatly enhanced production of helium metastable inside the capillary and inside plasma plume.
Acknowledgments
This work is supported by ANR BLAN 093003 PAMPA, APR PLASMEDNORM,
TD is supported by MENSR
References
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