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SIMION Time of Flight Example

This article demonstrates the simple reflectron time of flight (TOF) simulation distributed with SIMION.

The purpose of a TOF system is to separate ions by mass so that ion count for each mass may be tallied. In particular, the speed at which an ion travels though the TOF and hits the detector is a function of the mass of the ion.

Shown below, the TOF system consists of three components: a source (lower-left), a reflectron mirror (lower-right), and a detector (upper-right). Ions originate at the source, where they are accelerated through a potential difference across the two source electrodes, the latter of which is an ideal grid through which ions may fly through. Ions then travel into the reflector, which deflects the ions back and causes them to hit the detector.

Here is a cutaway view of the inside of the reflector:

Notice the data recording at the bottom of the first picture. Four ions, of two different masses and two different initial kinetic energies were flown. Each pair of ions with the same mass had roughly the same time of flight (TOF), and the higher mass pair had higher TOF values than the lower mass pair had. The initial ion definitions are shown below:

Potential energy maps of the source and reflector are drawn below. Note the effect of potential energy gradients on ion paths.

The potential energy displays provide a good intuitive understanding of the system behavior. At the source, ions start with approximately zero kinetic energy (1 eV and 51 eV in this example). Ions are then accelerated across a large, negative potential difference (-800 V). Regardless of mass, all ions will exit the source with approximately the same kinetic energy (801-851 eV) and will fly toward the detector. Since KE = (1/2)m v2, heavier ions will fly through the system at a lesser speed than lighter ions. The heavier ions will therefore hit the detector later.

But what is the purpose of the reflector? Indeed, the reflector can be omitted, and the TOF will still discriminate ions by mass. However, as simulated in this example, practical considerations require that ions will start out with a small, non-zero, and random initial kinetic energy. This KE variance persists when the ions exit the source. The problem is that when ion KEs are not kept equal, ions with very close mass can reach the detector in inverted order, thereby resulting in a loss of resolution. The reflector mitigates this problem. As shown in the potential energy map above, the potential (height on the potential surface) at each ion's entrance into and exit from the reflector is identical. Therefore, regardless of mass, the reflector causes no net change in KE on the ions. The noticeable effect is, however, that ions with higher KE travel deeper into the reflector than lighter ions. So, although additional KE would naturally propel an ion faster through the rest of the system, the added KE will slow the ion's travel through the reflector, thereby canceling the effect of KE variations. As seen in the data recording box in the first screenshot, the ions with ~6% difference in KE during flight but of same mass have near identical TOF values.

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