13.1.1 Appendix - Jet Theory - Introduction
A nozzle, or free jet, molecular beam is defined as a neutral beam extracted
from an underexpanded supersonic, continuum jet expansion from a high-pressure
gas source into a low-pressure ambient background. The term nozzle
is used because of the similarity to expansions occurring from converging-diverging
supersonic rocket nozzles. Most molecular beam researchers have done away
with the diverging portion of the nozzle, and the term free jet
is frequently used instead. The discussion in this section follows closely
the article by David R. Miller (Ref. 21), focusing on issues relevant to
our apparatus and operating conditions.
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