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Forum: Documentation and Examples
Re: Question Angular Distribution for Gaussian Electron Beam (Nathan)
Date: 08 Sep, 2010
From: Tom Roberts <Tom Roberts>

On Wed, 08 Sep 2010 13:12:23 GMT, Nathan wrote:
> Are there any examples that can help set up the angular distribution of
> a guassian electron beam that will traverse a lattice of dipoles and
> quads? I tried using a gaussian angular distribution however it doesnt
> produce the correct rms beam spot. Correct meaning matching results
> found with MAD-X. If i set it to produce the correct rms beam spot size
> at the entrance and exit of the first element (dipole magnet) it will
> not be correct further downstream. I tried looking at the methods in the
> microbeam and nanobeam examples but they didnt seem to work for a
> gaussian distribution in position. Any help will be greatly appreciated.

The most difficult aspect of this is making sure that MAD-X and your Geant4 program are simulating the same system. In particular, models of fringe fields are often difficult to translate exactly. But it's also true that the programs use completely different ways to characterize beamline elements (e.g. in MAD-X a quad's field is expressed as a K value, while in Geant4 it must be expressed as a B field in Tesla with the correct spatial distribution). Moreover, the change in the centerline of the beam-line due to a bending magnet is implicit in MAD-X, but must be handled explicitly in a Geant4 program; such rotations are difficult to get right and are a common source of error, so be sure to visualize some tracks in Geant4, making sure they actually go through the apertures of the elements. Also be sure the correlations among track parameters of your input beam are the same for the two programs, and be sure you measure the beam distributions in the same way.

Tracking in single-particle codes like Geant4 is an order-of-magnitude slower than in matrix codes like MAD-X. If you really need such tracking (e.g. for muon beams, or for quantitative evaluation of scraping), you might find G4beamline sufficient for your needs: it is an open-source program based on Geant4 that reads an input file to define the beamline and simulation parameters; it is much simpler and more directly related to the beamline than the corresponding C++ code.

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1 Question: Re: Angular Distribution for Gaussian Electron Beam   (Nathan - 08 Sep, 2010)
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