Current-voltage characteristics in a flame plasma: analysis for positive and negative ions, with applications

A flat, cylindrical, laminar, H-2-O-2-N-2 flame in plug flow with velocity v(f) = 19.8 m s(-1) and cross-sectional area A = 1.05 X 10(-4) m(2), at 1 a tm and 2400 K, was doped with appropriate additives to give a weak, continu um, quasi-neutral plasma involving Cs+/e(-) or H3O+/e(-) or H3O+ /Cl-/e(-). The flame impinged on a planar, normal, water-cooled, conducting electrode designated N (for nozzle) located a variable distance z downstream from th e flame's luminous reaction zone which is separated by a dark space or gap delta = 1 mm from a water-cooled metallic burner B, the second electrode. N either electrode has the properties of a Langmuir probe. Two types of data were measured: i-V characteristics with the current i(N) collected by the n ozzle versus the applied voltage Delta phi between N and B at a fixed value of z (e.g. 20 mm); and profiles of i(N) versus z at a fixed value of Delta phi. For Delta phi < 0 (e.g. -50 V; N is negative), the electrons are stop ped in the flame and the constant saturation current is controlled by the c onvective flow of positive ions to the nozzle: i(+)(N) = eAn(+)(N)v(f) This provides a wonderfully simple and accurate measurement of the absolute den sity n(+)(N). Alternatively, if a weak solution of a Cs salt (e.g. 10(-4) M CsCl) is sprayed using a pneumatic atomizer such that Cs is completely ion ized, the delivery factor of the atomizer can be calibrated. Furthermore, i f the delivery factor giving n(+)(N) is known, v(f) can be determined. With <Delta>phi fixed (e.g. -50 V), a profile of n(+)(N) versus z can be obtain ed throughout the flame gas downstream; the density distribution is not aff ected by the application of the applied voltage. For Delta phi > 0 (e.g. +5 0 V, N is positive), the current through the flame i(+)(B) = -i(e)(N) - i(- )(N) (the latter term is included if negative ions are present), is control led by the flow of positive ions of mobility mu (+) to the burner across th e potential gradient in the burner gap g: i(+)(B) = eAn(+)(B)mu (+)del phi (g), When Delta phi is sufficiently positive to achieve a constant saturati on current, n(+)(B) can be determined; it represents the total ion producti on in the flame reaction zone. When negative ions are present replacing eve n a large fraction of the electrons, the effect on the i-V characteristic i s relatively minor; it does not appear possible to provide a separate analy sis for n(e) and n(-), However, profiles through the flame plasma clearly s how the effects of negative ion processes such as ion-ion recombination, fo r example. For a wide range of the applied voltage Delta (phi) both positiv e and negative, it is possible to calculate the potential distribution in t he burner gap, the bulk flame plasma and in the positive ion sheath at the nozzle. This provides a quantitative understanding of the ion and electron behaviour throughout the flame. (Int J Mass Spectrom 206 (2001) 137-151) (C ) 2001 Elsevier Science B.V.