= Molecular beam skimmer
A <molecular beam skimmer> is a small aperture, commonly at the tip of a cone, that transmits the central part of an expanding <molecular beam> into a downstream chamber. It separates source and measurement regions so that <differential pumping> can maintain a lower background <pressure> after the source. Its geometry selects directions; that selection alone does not give all transmitted <molecules> the same <speed>.
The surface also perturbs the beam. At high source <number density>, particles reflected from the skimmer walls can collide with the incoming flow and create <shock waves>, reducing and heating the transmitted beam. A 2017 experiment cooled the skimmer and measured a beam-flux increase of about eightfold under its tested conditions. A pulsed <plasma> glow made the disturbed flow visible; cooling reduced reflected <momentum> and, at lower <temperatures>, promoted <adsorption>. This tests why a narrower geometric opening is not a complete model of a real skimmer.<Molecular beam brightening by shock-wave suppression by Yair Segev and others>[Fig. 1 and Results and Discussion: wall interference, discharge imaging, cooling and measured flux enhancement]{ref}
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