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Forum: Physics List
Re: Question Need help starting to put together a Physics List for an RFQ linac (Marlin Mixon)
Date: 12 Jul, 2010
From: Tom Roberts <Tom Roberts>

It's not worth your time to worry about it. Just use QGSP_BERT. Use G4PhysListFactory to construct it. Sure it creates a lot of particles, but if you want a realistic simulation you'll need them (beam particles do hit apertures).

For an accelerator like this, don't forget to call G4FieldManager::SetFieldChangesEnergy(true), and use G4EqMagElectricField.

The challenge, of course, is to get the electric and magnetic fields for the complex geometry of the RFQ into your program. Note that the geometry of an RFQ depends on the initial KE of the particles, their charge/mass, and the RF frequency; the spatial distributions of the fields depend on the geometry. So it's not just as simple as applying different frequencies, you need different geometry and field maps for each frequency. You'll need a good EM modeling program to generate the field maps (unless you have a simple enough geometry for an analytical solution, which I doubt).

As for dd fusion, I'm not sure that it is in any physics list. In general, particle-particle interactions are not included in Geant4 physics lists, only particle-material interactions are there. So if it is available, it will be for a deuteron beam traversing a flask of deuterium, not for particle-particle interactions within the beam. Think about how Geant4 tracks particles and this will be obvious. If you are interested in modeling particle-particle interactions, I have implemented this in my program (G4beamline), but it is non-trivial and involves re-writing the RunManager, EventManager, TrackManager, and SteppingManager, so they track all of the tracks in parallel; plus adding a whole new class of interactions between particles, plus modeling particles as macro-particles (it's infeasible to simulate the 10^9 particles of a realistic bunch). Also think about what it takes for dd fusion to occur; a deuteron accelerator is not going to do it in the beam, you'll need to put the beam into a flask of deuterium for it to happen, and the beam must be above threshold. If you want to simulate a tokamak, plasma physics is extremely important and not present in Geant4.

Finally, you should realize that space charge will probably be important in the RFQ, and that Geant4 cannot simulate it. The motivation for implementing particle-particle interactions in G4beamline was to be able to model space charge (as its name implies, it is a Geant4-based program for modeling beam lines). As I said, this is non-trivial. G4beamline is open source, and you can look at how I did it and even re-use my code: http://g4beamline.muonsinc.com

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1 Question: Re: Need help starting to put together a Physics List for an RFQ linac   (Dragos Constantin - 13 Jul, 2010)
1 More: Re: Need help starting to put together a Physics List for an RFQ linac   (Tom Roberts - 16 Jul, 2010)
2 None: Re: Need help starting to put together a Physics List for an RFQ linac   (Marlin Mixon - 13 Jul, 2010)
3 Question: Re: Need help starting to put together a Physics List for an RFQ linac   (Marlin Mixon - 14 Jul, 2010)
1 More: Re: Need help starting to put together a Physics List for an RFQ linac   (Marlin Mixon - 14 Jul, 2010)
2 Warning: Re: Need help starting to put together a Physics List for an RFQ linac   (Tom Roberts - 15 Jul, 2010)
4 Question: Re: Need help starting to put together a Physics List for an RFQ linac   (Marlin Mixon - 17 Jul, 2010)
1 None: Re: Need help starting to put together a Physics List for an RFQ linac   (Tom Roberts - 19 Jul, 2010)
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