EVIDENCE OF SUPERSONIC-FLOW IN AN ARC-HEATED DIAMOND FILM DEPOSITION REACTOR

A Pitot impact tube was used to measure the flow velocity in a laborat ory d.c. are-heated diamond deposition reactor at 35 Torr. Speeds up t o Mach 1.32 +/- 0.09 were found, with the bracketing error in the meas urement representing alternative interpretations according to equilibr ium or frozen chemistries. Radial traverse across the reactor showed p eak Mach numbers along the centerline, as expected for a free jet into a relatively stagnant ambient. As the surrounding fluid is entrained, it slows the jet, cools it, and spreads it out, such that the centerl ine Mach number at the substrate location drops to M approximate to 0. 3. Theory has indicated that optimal diamond film growth rate and qual ity will result in are jet reactors if the Mach number is around sonic , M=1. Here we have tested the flow field downstream of an are jet iss uing from a converging-diverging nozzle, establishing experimentally t hat regions in our diamond reactor indeed have M > 1. We believe that this is the first reported experimental measurement of such a high vel ocity in a diamond reactor.