Info: Grid

From SIMION

Background

A grid is sheet of fine wires. Ideally, a grid provides an approximately equipotential surface (like a sheet of metal) yet allows particles to transparently pass through. In practice, the field between grid wires is not fully uniform but focuses and diverges particles to some extent, especially near the wires, and some fraction of the particles may hit the wires--preferably, these undesirable effects are small. The spacing between the wires is called the pitch. Grids with small pitches tend to reduce the field distortions at a cost of reduced ion transmission.

The figure above shows a small section of a square grid modeled to high resolution in SIMION (model taken from the Advanced ASMS Course). The effect on ions passing through is shown.

Grids have been used in TOFs, detectors, and elsewhere.

SIMION Methods

The following methods are available in SIMION for simulating grids:

• Ideal grids (100% transmission) are easy to specify. Just create a PA file with a one-grid point thick sheet of electrode points. See p. 5-3 to 5-5 in the SIMION 7.0 manual.
• Non-ideal grids (< 100% transmission) can be done in these ways:
  • The more complete methods is to model a small section of the grid (e.g. a small number of wires) to high resolution in a 3D potential array. This small section of the grid can be effectively repeated via either placing multiple instances of the grid element into a workbench or using a user program (ion jumping tricks). That requires Multiple_PAs, so you'll need to ensure that the fields match up between the PA instances. See the Non-ideal grids sections of the Advanced ASMS Course for details or Application notes 47 (http://www.sisweb.com/referenc/applnote/app-47.htm) and 52 (http://www.sisweb.com/referenc/applnote/app-52.htm).
  • If you have a known equation for the theoretical transmission and deflection of the ions at the grid, you can incorporate this effect using a SIMION user program. A simple method may be to randomly kill 10% of the ions that pass the grid and/or apply a small random deflection angle. This may be sufficient if you already know how the grid behaves.

Links

Below are some links related to using grids in SIMION.

• SIMION 7.0 Manual. p. 5-3 to 5-5 has some discussion on defining ideal grids in potential arrays. p. 9-7 has some discussion on simulating non-ideal grids via ion jumping tricks.
• [1] (http://www.simion.com/docs/asms_course.html) Non-ideal Grids sections of the Advanced ASMS Course. Uses ion jumping tricks. Files are included on the SIMION CD. See particularly grid2d*.gem and grid3d*.gem files.
• SIS Application Notes on grids:
  • [2] (http://www.sisweb.com/referenc/applnote/app-47.htm) The Application Of SIMION 6.0 To Problems In Time-of-Flight Mass Spectrometry (SIS Application Note #47)
  • [3] (http://www.sisweb.com/referenc/applnote/app-52.htm) Computer Modeling of Ion Optics in Time-of-Flight mass Spectrometry Using SIMION 3D (SIS Application Note #52)
  • [4] (http://www.sisweb.com/referenc/applnote/app-59.htm) Computer Modeling of a TOF Reflectron With Gridless Reflector Using SIMION 3D