Info: Space Charge

From SIMION

Space-charge is the charge in space due to charged particles in space.

Normally, SIMION assumes that space-charge is negligible, as it often is. That is, all charges are assumed to be on the electrode surfaces (not in space) as defined by the electrode voltages, and SIMION solves the potentials in space using the Laplace Equation, and from that it calculates electric field in space. The charge from the particle trajectories themselves is assumed sufficiently small (sufficiently small current). In practice, any space-charge in the particle trajectories would affect the fields, which in turn circularly affect the particle trajectories, etc. ad infinitum. Really quantitative methods of space-charge calculation, such as done by CPO (http://www.simion.com/cpo/spacecharge.html) solve the fields and trajectories together via an iterative method until convergence.

SIMION 7.0/8.0 do have support some "charge-repulsion methods" when particles are flown "in groups." This calculates trajectories with a Coulomb's Law-like ion-cloud correction to simulate repulsion between all the particles or beam lines in the group. It is not an iterative method nor does it use the Poisson equation (the potentials and fields in space are not recalculated). It is generally used to "estimate the onset of space-charge." See p. 2-5 of the SIMION 7.0 manual. Details of the calculation methods are in Appendix E as well as in this SIMION user group post (coulombic repulsion model (http://www.simion.com/discuss/viewtopic?t=238)). There is a further discussion of SIMION's charge-repulsion methods in the 2005 paper by Appelhans and Dahl (http://dx.doi.org/10.1016/j.ijms.2005.03.010).

One advantage of SIMION's methods is that, although it is an approximation, it can handle (even efficiently) time-dependent fields, which the "iterative convergence" method of CPO does not due to the convergence requirement. CPO space-charge methods are much more suitable for static fields, such as accurate calculation of space-charge effects near the cathode surface (e.g. pierce gun with Child's Law current limiting effect) or in beam repulsion.

SIMION 8.1 (in development) does have a true Poisson solver that is currently available to 8.0 users for preview in early access mode (See Poisson_Equation ).

See Also

• Space Charge in Mass Spectrometry (http://www.spectroscopymag.com/spectroscopy/article/articleDetail.jsp?id=97167) (K.L. Busch, Spectroscopy, June 2004)