Other Constraints on the IMS Gas Pressure

FIGURE 1.5 Paschen curves for N2, H2, and air. (From Cobine, J.D., Gaseous Conductors, Dover, 1941.) 

While electrical breakdown constraints set the lower limit for IMS gas pressure, there also is the upper limit. At some point, the density of gas molecules makes their colhsions with an ion a many-body rather than binary interaction. Eventually, the dynamics becomes governed by laws of viscous fiiction appropriate for hquids. In that regime, the terminal velocity of ions is stiU proportional to E at low E, and mobihty is defined by Equation 1.8. However, formahsms such as Equation 1.10 that relate K to ion structure cease to apply, and becomes independent of gas pressure.  

The onset of that regime has been estimated by comparing the mean-free-path of gas molecules, Ap, with ion size. Using 

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