SIMION®
The field and particle trajectory simulator
Industry standard charged particle optics software
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SIMION 8.0 Issues/Bugs/Enhancements

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Issue I362 [80,example,hs1collisionmodel,high,resolved]
  HS1 collision model did not accurately thermalize.
  Reported by davidl-200702.
  Description:
    It was reported that in the HS1 collision model, ions reach an
    equilibrium temperature somewhat lower than that of the gas
    temperature, when in fact the two should be equal (thermal
    equilibrium).  For example, at 300K, mass 100 amu, pressure
    0.1mbar, over 1ms, HS1 gives <KE> = 0.0259eV (~33% lower than gas
    KE = 0.0388eV), while a slightly different collision method gave
    0.0385.  HS1 also slightly underestimated dispersion.

    The problem was in the way impact_angle and gas velocity for the
    collision were handled. To be truly correct, one can first translate
    the velocity reference frame so that the gas particle is
    stationary.  Fixing that gives a much a more correct ion
    temperature at equilibrium (e.g. ~300K).

    This is fixed in REV-4 of the Lua code in 8.0.2 (as well as REV-4
    of the SL code on simion.com).

    Another unrelated change in REV-4 (of Lua version) is a slightly
    more accurate calculation of gas velocities at collisions.  REV-3
    assumed Maxwell-Boltzmann, which is a reasonable approximation,
    but it should actually deviate a bit from Maxwell-Boltzmann at low
    ion velocities.  This issue is explained in the new notes.pdf.  In
    practice, this probably doesn't makes much difference.

    Also, various other improvements were made to REV-4 (of Lua
    version), such as additional documentation, improved code
    structure, and enhanced statistical reporting.  See the new
    notes.pdf in SIMION 8.

  Affects: 8.0.0-8.0.1 (and version 2 of the SL version on simion.com).
    Resolved in 8.0.2 (and version 4 of the SL version on simion.com).

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