Research ArticlePHYSICAL SCIENCES

Molecular beam brightening by shock-wave suppression

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Science Advances  08 Mar 2017:
Vol. 3, no. 3, e1602258
DOI: 10.1126/sciadv.1602258
  • Fig. 1 Skimmer clogging due to shock-wave interference.

    A cold supersonic plume is generated by expansion from a pulsed valve into a vacuum. A skimmer transmits the axial portion of the plume as a collimated beam into the detection chamber. With high beam densities, shock waves are reflected from the skimmer lip, interfering with the transmitted beam. In imaging experiments only, a discharge pulse from a high-voltage grid induces plasma glow in the denser portions of the flow, revealing the shock waves. A large split skimmer is used here for the observation of the internal shock structure.

  • Fig. 2 Measured beam densities of different species for varying values of skimmer temperature Tw.

    (A to D) Cooling the skimmer initially increases the peak density until a limit is achieved at the unclogging temperature. Time is measured from the trigger to the valve. The skimmer is conical. The source conditions are set to achieve clogging for a hot skimmer. Additional parameters are listed in Materials and Methods. a.u., arbitrary units.

  • Fig. 3 Visualization of the density field using a pulsed discharge.

    (A to I) Shock waves enveloping the skimmer recline and eventually vanish as the skimmer temperature is lowered. The internal shock structure exhibits similar phenomena. For each case, the species is indicated in the lower left corner, and skimmer temperature is indicated in the lower right corner. The source conditions are set to achieve clogging for a hot skimmer. Additional parameters are listed in Materials and Methods.

  • Fig. 4 External shock-wave angle as a function of skimmer temperature.

    The shock angle, defined on the right over an image with a helium beam, initially decreases gradually with the temperature. Near the unclogging temperature, the sensitivity increases until the shock becomes parallel to the wall and vanishes. Dashed lines serve as guides to the eye. The angle is defined relative to the line of maximum intensity within the shock. The source conditions are set to achieve clogging for a hot skimmer. Additional parameters are listed in Materials and Methods.

  • Fig. 5 DSMC calculations of a krypton beam impacting a conical skimmer.

    The density field is presented in logarithmic scale for cases of a hot (Tw = 300 K) (A) and a cold (Tw = 30 K) (B) skimmer. The transmitted beam consists of a dense bullet followed by a sparse cloud corresponding to the peaks and tails in experimentally measured beams. Changes in the shock structures with skimmer temperature qualitatively match the experimental observations above the unclogging temperature. (C) The centerline density 30 mm downstream of the skimmer entrance also exhibits a beam intensification trend at lower temperatures, but only the addition of significant adsorption to the surfaces enables complete unclogging. Additional parameters are listed in Materials and Methods.

  • Fig. 6 Peak measured density for a neon beam transmitted through hot (Tw = 300 K) and cold (Tw = 13 K) conical skimmers with varying beam density.

    The density at the skimmer entrance is controlled either by varying the source pressure (A) at a constant nozzle-skimmer distance (rns = 45 mm) or by varying this distance (B) at a constant stagnation pressure (P0 = 40 atm). Increasing the density by either method eventually clogs the hot skimmer, whereas the cold skimmer transmits a continuously increasing peak density with no observed limit. The source temperature is 300 K. Dotted lines serve as guides to the eye.

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Experimental setup for measuring the effect of skimmer temperature on the transmitted beam.
    • fig. S2. Schematics of the conical skimmer used in the quantitative experiments.

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