Refractory graphite skimmers for supersonic free-jet, supersonic arc-jet, and plasma discharge applications

Abstract

The fabrication of molecular beam “skimmers” from electrical discharge machining (EDM) graphite is reported. EDM graphite is highly refractory and is easily machined using conventional cutting and grinding techniques. In its most fine-grained form, EDM graphite can be machined free-standing to a knife-edge lip radius of ∼1 μm, providing excellent skimmer aerodynamics. Being refractory, such EDM skimmers are of particular interest in sampling or collimating high-temperature plasma discharges. Our explicit application is in skimming an electrical discharge supersonic free-jet of molecular nitrogen, forming a molecular beam of A 3Σu+ metastable N2 to be used in the heteroepitaxial growth of III–N wide-band-gap semiconductors. In view of the their economy, ease of manufacture, and excellent aerodynamics, the skimmers may also find use in skimming conventional, nondischarge supersonic free-jets. The performance of the EDM skimmers was tested in a conventional helium supersonic free-jet expansion, measuring the time-of-flight distribution and beam intensity as a function of nozzle pressure and nozzle-skimmer separation. Direct comparison with commercial nickel and copper skimmers showed the EDM graphite skimmers to perform nearly as well as the best commercial metal skimmers. The refractory properties of the skimmers were tested in a high-temperature arc-discharge supersonic free-jet expansion of 10% nitrogen in argon. Exposure to a plasma plume of ∼7500 K for over 6 h produced only relatively minor deterioration of the EDM graphite skimmer.

Generation and Propagation of Intense Supersonic Beams

“…Molecular beams, and later, supersonic beams, have played a major role in chemical physics research for almost a century . Due to significant technological improvements, the field has progressed as several recent textbooks prove. − If at the early stage small diameters (<0.1 mm), low source pressures ( p < 10 bar), and CW pinhole nozzles were the norm, − the invention of pulsed valves enabled a remarkable increase in achieved beam intensity while maintaining the same required pumping capacity. − Nozzle shape − and skimmer geometry − were recognized as crucial elements in determining beam properties. Recent development of fast acting valves, , with opening times of 20 μs, enabled the use of even higher pressures.…”

Pulsed Supersonic Beams from High Pressure Source: Simulation Results and Experimental Measurements

“…Since the early days of low intensity continuous beams, skimmers have been recognized as more than a simple bystander that extracts the central portion of the beam. The effects of molecules scattering from the finite skimmer edges have long been recognized [22,24,31,[46][47][48], and the reduced transmission of the beam through the skimmer was termed "skimmer interference. " At times the term "skimmer clogging" was used to describe the dramatic reduction in skimmer transmission.…”

“The Even-Lavie valve as a source for high intensity supersonic beam”

“…Since the early days of low intensity continuous beams, skimmers have been recognized as more than a simple bystander that extracts the central portion of the beam. The effects of molecules scattering from the finite skimmer edges have long been recognized [1,5,31,[42][43][44], and the reduced transmission of the beam through the skimmer was termed "skimmer interference". At times the term "skimmer clogging" was used to describe the dramatic reduction in skimmer transmission.…”