Evidence of supersonic flow in an arc-heated diamond film deposition reactor

Abstract

Abstract A Pilot impact tube was used to measure the flow velocity in a laboratory d.c. arc-heated diamond deposition reactor at 35 Torr. Speeds up to Mach 1.32 ± 0.09 were found, with the bracketing error in the measurement representing alternative interpretations according to equilibrium or frozen chemistries. Radial traverse across the reactor showed peak 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 centerline Mach number at the substrate location drops to M ≈0.3. Theory has indicated that optimal diamond film growth rate and quality will result in arc jet reactors if the Mach number is around sonic, M = 1. Here we have tested the flow field downstream of an arc jet issuing from a converging-diverging nozzle, establishing experimentally that regions in our diamond reactor indeed have M > 1. We believe that this is the first reported experimental measurement of such a high velocity in a diamond reactor.