Stationary flow conditions in pulsed supersonic beams

Abstract

We describe a generally applicable method for the experimental determination of stationary flow conditions in pulsed supersonic beams, utilizing time-resolved electron induced fluorescence measurements of high pressure jet expansions of helium. The detection of ultraviolet photons from electronically excited helium emitted very close to the nozzle exit images the valve opening behavior-with the decided advantage that a photon signal is not affected by beam-skimmer and beam-residual gas interactions; it thus allows to conclusively determine those operation parameters of a pulsed valve that yield complete opening. The studies reveal that a "flat-top" signal, indicating constant density and commonly considered as experimental criterion for continuous flow, is insufficient. Moreover, translational temperature and mean terminal flow velocity turn out to be significantly more sensitive in testing for the equivalent behavior of a continuous nozzle source. Based on the widely distributed Even-Lavie valve we demonstrate that, in principle, it is possible to achieve quasi-continuous flow conditions even with fast-acting valves; however, the two prerequisites are a minimum pulse duration that is much longer than standard practice and previous estimates, and a suitable tagging of the appropriate beam segment.

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

“…One way that this could be accomplished would be to use a short duration pulsed and focused electron beam, fired at the appropriate time to excite a narrow group of atoms in the central portion of the pulse. A fraction of the atoms will be excited to neutral, metastable excited states (usually triplet states) that have long radiative lifetimes [41]. However, here we present results that were obtained by creating metastable atoms using a soft discharge mechanism inside the nozzle itself [37,40].…”

“…This is illustrated in Figure 17, where the skimmer transmission is plotted as a function of skimmer entrance diameter for conical skimmers (20 ∘ full opening angle) subjected to gas densities ranging from 3 × 10 21 to 1 × 10 23 atoms/m 3 . The necessity of using narrow conical skimmers with large diameter entrance apertures, positioned far downstream, is a very unorthodox result that will require the redesign of source chambers for a large number of currently employed high intensity supersonic sources.…”

“…The time response, intensity, and the speed ratio of the supersonic jets, generated by this valve, were measured [40,41]. The operating gases and stagnation conditions (pressure and temperature) were varied.…”

“…Simulations of gas flow [21] as well as measurements of actual beam parameters are presented. Previous designs of molecular beam skimmers [5,20,[22][23][24] as originally developed for the earlier low-intensity continuous beams cannot be copied for high intensity beams because skimmer interference (also known as skimmer clogging) is simply too severe with the more intense pulsed beams that are now available. In this paper, we describe recent results on nozzle design (Section 2), pulsed valve design (Section 3), molecular beam properties (Section 4), skimmer design (Section 5), and conclusions (Section 6).…”

“…The development of this valve and its operating characteristics are described here, along with the changes in skimmer design that are required in order to make use of the higher on-axis beam intensities. A comprehensive review of our pulsed valve was recently published [20]. Simulations of gas flow [21] as well as measurements of actual beam parameters are presented.…”