SIMION®
The field and particle trajectory simulator
Industry standard charged particle optics software
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SIMION v8.1.3.0-TEST posted.
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Ion-Gas Collisions

Is it possible to simulate ions flying through non-vacuum conditions (i.e. ions colliding with background gas)?

Yes, a number of models can be specified for this.

First, why simulate collisions? Normally, SIMION assumes that ions fly through perfect vacuum, and often this is a valid approximation. However, this assumption is not always appropriate. For example, the effect is significant in ion mobility mass spectrometry. In ion traps, ion funnels, and similar devices, a non-negligible buffer gas is often used to kinetically cool and focus ions.

How are collisions simulated? A collision model can be introduced into a simulation via a SIMION user program. This works well in SIMION 8 though is also supported via a lower-level programming language in 6 and 7. A number of pre-built collision models are included as examples in SIMION 8, such as Stokes’ Law (drag), hard-sphere (HS1), and high-pressure (SDS), but sure to download the latest 8.0.x update. A number of models have also been described in the literature below and can be found for version 7. If desired, you can customize these models or program your own. The way the models work is that at each time-step, such a user program applies an adjustment to the ion motion (typically in other_actions or accel_adjust segment) according to some equation or algorithm depending on the desired collision model.

Note, however, that SIMION is not a Computational Fluid Dynamics (CFD) solver. SIMION will not in itself calculate the bulk pressure, temperature, velocity, and density fields for gas flow from first principles (Navier-Stokes). You may, however, input known bulk flows into the SIMION collision model either via arrays or analytic equations. These bulk flows could be determined by third-party CFD software (e.g. Fluent), approximated with analytic equations (e.g. Hagen-Poiseuille flow) in simpler cases, or obtained by experimental measurements. One of the SIMION distributors also has been developing a new CFD solver called Virtual Device Hydrodynamics which specializes in supersonic gas flow calculations (e.g. ESI) and has direct integration with SIMION. See Computational Fluid Dynamics (CFD) for further details.

The following main types of models are typically used:

  • Viscous damping
  • Hard sphere collision model

The viscous damping models apply a force that is a function of the particle velocity vector. Often, this is Stokes’ law in which the force is proportional to ion velocity vector, particle radius, and fluid viscosity (see Wikipedia: Stokes’ law).

The hard-sphere collision models are based on the kinetic theory of gases in which, unlike the viscous damping models, the individual collisions between ion and gas particles are modeled. The expected frequency of collisions, measured as a distance (the mean-free-path) is predicted by the kinetic theory of gases as a function of the known pressure, temperature, and collisional cross sections of colliding particles. Collisions between ion and gas particles result in positive and negative energy transfers as well as scattering (deflection of ion velocity vectors), or even absorptions (e.g. in electron-gas collisions). The energy transfers provide for the kinetic cooling of a fast moving ion as well as the kinetic heating of a slow moving ion. Usually, we treat the colliding particles as hard-spheres. Often we assume that the collisions are elastic. Generally, the background gas is non-stationary and has a Maxwell-Boltzmann distribution of velocities, which is a function of temperature. The proposed reference model for hard sphere elastic collisions is Collision Model HS1 (an updated version of which is included in SIMION 8).

Some newer models are hybrids and have been successful at higher pressures.

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