Field Precision title

Ion beam extraction in Trak and OmniTrak from sources with multiple species

We recently had a question about using Trak capabilities in the SCHARGE and PLASMA modes for ion guns with plasma sources that produce multiple ion species (e.g., H+ and H2+). In the present Trak and OmniTrak codes, a mixed ion beam can be defined through PRT files. Furthermore, both codes support multiple emission surfaces with the option for different species. The limitation is that only a single type of ion can be associated with each emission surface.

The problem is easily resolved without the necessity for changes in the codes for ion extractors without applied magnetic fields. For non-relativistic beams, the self-consistent electrostatic field distribution for space-charge-limited emission does not depend on the ion mass or charge. Although the space-charge density of the beam is invariant, the extracted current does depend on the particle parameters. To illustrate, consider the converging beam solution shown in Figure 1 for protons (H+). The space-charge limited current is 1.2866 A. If we change the ion mass to 2 amu, the current drops to 0.9077 A = 1.2866/sqrt(2) A, but the space-charge density and particle trajectories are identical.


Figure 1. Converging ion gun.

This observation suggests an approach to multiple species injectors. Suppose the plasma source produces H+ and H2+ and that we know that 15% of the extracted beam is carried by molecular ions (f++ = 0.15). Perform a solution in the SCHARGE or PLASMA modes with an emission surface of H+ ions. Let J+ be the resulting beam current. Then the current for the mixed beam is given by

It = (1 - f++)J+ + f++J+/sqrt(2).
For the example of Figure 1, the combined current is 1.230 A.

With some work, a mixed beam from an injector could be continued through a transport system via the following method:

  1. Using the output PRT file from the injector, create a new file where the particle entry lines are duplicated.
  2. For the first set of entries, multiply current values in the last column by (1 - f++).
  3. For the second set, change the RestMass values in Column 1 to 2.0, and multiply the values in the last column by f++/sqrt(2).
The operations could be performed easily with a spreadsheet, Python script,....

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