Plasma and electrode interactions in short gap discharges in air II. Electrode effects†

Electrodynamic Model of Initiation and Extinction of Emitting Sites Interacting Together in Cold Cathode Spots (Type A) in Air

“…Thus, the estimated lifetime in the range 30-60 nsec may be considered as the sum lifetime of consecutive randomly scattered 3 nsec (returning) emitting events. This interpretation is confirmed by the fact that for clean surfaces in air (and comparable surface conditions in vacuum) the authors who have worked with short-time cable discharges (to 5 1 nsec-20 nsec) report on emitting site lifetimes in the range of 1-5nsec (JOTTNEB [19,25,27,68, 761, GRAY, AUGIS (20,39,701). Thus, this representation again enforces the idea that the electron-phonon interaction model proposed here is applicable to such experimental conditions where type A microspot ensembles especially leave typical trumpet-like microcraters on the surface as described above (see Fig.…”

“…In the latter case it must be assumed that the 30 nm emitting sites are present in e great number on the surface. Obviously, this fact is the cause that the radial expansion of the cathode root or the damaged area is governed by the law of electronic diffusion (diffusion rate = 2/ploa0) of the current paths confirmed by the experiments of SLADE [14] and GUY [20] for such conditions. The cathode damage .due to this fast moving spot type is characterized by the appearence of the typical .trumpet-Iike microcraters with pronounced crater rims with diameters in the order ,of 0.5-2 pm described in the previous section.…”

“…where (l), (20) and ( Another form to represent the inininiuni arc current is which follows from (11) using (l), (16), (16), (22) and…”

“…J t can be stated or supposed surely that the type A is initiated and maintained if the breakdown occurs cold (dielectric) and cathode initiated a t ignition voltages > 400 V, that means without long-time heating (for instance > 0.1 psec) caused by prebreakdown currents, preceding niolten bridge stages (in contact breaking), gap bridging or contribution of anode processes [3,4,9,10,19,20,21,23,25], the wave impedance coupling between the current constriction of the microspot, the electrode parasitic circuit and the connected cables allows to exceed rapidly (< 1 nsec) the niininium arc current (see next sections), tiny ( N 30 nm) fieldeiiiitten or fiuldeinission sites are present on the cathode surface in a disperse spreading (for instance favoured by polished and cleaned surfaces, grain hori ndaries with gas contaiiiinat ions, oxide filn i i nhoniogeneit ies, segregation of wrt a i n alloying rrdiiiixtures and of hulk gases within tiny siirface sites [l-16, [19][20][21][22][23][24][25][26][27]), the external existence conditions of the discharge enforce a rapid directed or tindirected nioveiiient of the cathode root on a cold surface (for instance transverse magnetic field, nanocseond arc durations, low gas presiiires, micrometer gap widths,-one electrode moved) [l-271 and finally inlioiiiogeneities of the surface in a large area scale (coarse roughness) 2 2 p i are not present (for instance iiiacroscopic molten particles froiii preceding iiioltrn bridges or arc stages, dispersion of the electrode inaterial). Referring to point 1. i t iiiust be conipleted that in "vacuiini" [14] and for extreiiiely stiiall electrode distances in air [4,9,10] after a molten bridge ruptrire (contact breaking) a radial spot expansion and spot division from the contracted tvpe B to the disperse type A can occur after the spot has remained on the ignition site for a certain time.…”

“…The lifetime of nn individual niicrospt or emitting site digging tiny microcraters could he estimated within the region of 0.5 to 10 nsec using nanosecond cable discharges [15][16][17][19][20][21][22][23][24][25] (for instance for Cu, W, Ag, Pd, steel, AgCd). The estimated niicrospot currents I , vary between 0.01 and 15 Adepending on oxide thickness, gas pressure, are current, siirface contaiiiination (in vacuum), surface polishing state, dispersion of …”