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Input files
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EBeamDist00.GIN, EBeamDist00.MIN, EBeamDist01.GIN, EBeamDist01.MIN, EBeamDist02.GIN, EBeamDist02.MIN, EBeamDist03.GIN, EBeamDist03.MIN, EBeamDist04.GIN, EBeamDist04.MIN
Download MultistageGamBet.zip
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Description
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This example illustrates how to divide GamBet calculations into stages to derive detailed information on particle distributions at several positions. The top figure shows the geometry of the 2D cylindrical test calculation. A point beam of 2.25 MeV electrons enters an assembly of four thin aluminum foils (25 um) separated by voids of width 50 um. Beam particles lose energy and scatter passing through the foils. The goal is to determine the average beam energy and RMS (root-mean-squared) radius at the exit of each foil.
An initial calculation (EBeamDist00) modeled the full system. In the GamBet input file, the source particles were defined by the commands:
SList
E 2.25E6 0.001 0.001 0.001 0.00 0.00 1.00
End
NPMult = 1000
In response, the code creates 1000 primary electrons of energy 2.25 MeV near the axis moving in the z direction. There are two commands of interest of the Process section;
PlotOn 100 ElecP
EscapeFilter ElecP z>249.0
In response to the first command, GamBet generates a plot file which includes trajectories of the 100 primary electrons. The second command limits inclusions in the escape file to particles of interest (primary electrons that leave the downstream boundary).
The problem is that the run creates only a single escape file at position z = 250 um. The goal is to generate escape files that can be analyzed with GenDist at four positions, the foil exit boundaries. This can be accomplished by dividing the run into four parts:
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Results
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The top figure shows the full solution with 100 trajectories. The beam exhibits a smooth expansion with a halo of electrons that suffered a large-angle scattering event. The lower figure shows GenDist plots of the escape file distributions at the foil exits from the multi-stage calculations. The full calculation (EBeamDist00) gave the following results at z = 250 um: transmission fraction = 0.986, final energy = 2.207 MeV, RMS radius = 28.61 um. Parameters from the escape file of the final multi-stage calculation were (EBeamDist04) transmission fraction = 0.989, final energy = 2.209 MeV, RMS radius = 27.59 um. There is a difference of about 3.6% in the RMS radius. In principle, they should be the same. A possibility is that the electrons that have suffered random large-angle scattering are skewing the calculation for the central core. If we add a filter to include only particles within a radius of 60 um, then the results for RMS radius are 24.90 um for the full calculation and 24.93 um for the multistage calculation.
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Comments
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In the example, the length of the solution volume is comparable to the foil thickness, so there is no difficulty defining a mesh to score events. In a practical case, the propagation length may be much larger than the thickness of a vacuum foil. There are two possible approaches:
- Use a multi-stage approach with a calculation for the foil coupled to a solution for the propagation region.
- Reduce the density of the foil material and increase its thickness proportionally so that is has approximately the same effect on electrons.
In any case, in GamBet a small number of elements along the propagation direction is sufficient if material regions have simple shapes.
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