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Warwick-Plasma / epoch

Particle-in-cell code for plasma physics simulations
https://epochpic.github.io
GNU General Public License v3.0
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Low energy cut-off in qed photons #35

Open keithbennett opened 3 years ago

keithbennett commented 3 years ago

In GitLab by Andrew Longman on 11 Feb 2018, 22:52 UTC (GitLab issue GL#1632)

Hello all, I am currently simulating 3D wakefield acceleration in EPOCH driven by various laser modes. A problem I have encountered with all of the simulations so far is the betatron radiation spectrum seems to be cut-off for energies below ~2fJ (~6keV). I have attached an input deck for the wakefield driven by a standard Gaussian pulse and the corresponding spectrum. The expected radiation spectrum is given in Jackson 3rd Ed pg 682. I know that the minimum energy of photons tracked can be specified but mine is set to the default of zero. Is this an error on my part or a limitation with the QED package in the wakefield regime?

Thanks, Andrew

betatron_spec

input.deck

keithbennett commented 3 years ago

In GitLab by Andrew Longman on 11 Feb 2018, 22:53 UTC

changed the description

keithbennett commented 3 years ago

In GitLab by Christopher Brady (@csbrady-warwick) on 12 Feb 2018, 08:43 UTC

Hi Andrew,

the QED package uses certain assumptions (detailed in http://iopscience.iop.org/article/10.1088/0741-3335/53/1/015009/meta (also at https://arxiv.org/pdf/1010.4584.pdf)). The most important of which (here) is that it is in the strong field QED regime, which imposes an (approximate) lower limit on the eta parameter in that paper that the model applies for. The underlying tables that the QED routines use simply cut off at that point and you get a cutoff effect such as the one that you see. To get the lower energy tail you'd have to come up with a model that extends the one in that paper down to lower values of the eta parameter. If you know how to write such a model then I could explain how to write the tables as needed, but I don't really know if it would be possible to create a model like that. I believe that we have the scripts somewhere that generated the current tables, but I don't think that they'd give the right answer if simply extended to lower eta.

You might want to talk to Chris Ridgers at York, since he's the guy who wrote the EPOCH QED module, and he's the member of the dev team who's done the most on the underlying QED theory.

keithbennett commented 3 years ago

In GitLab by Andrew Longman on 14 Feb 2018, 19:36 UTC

Thankyou for the info Chris, I will look into how to incorporate such a table but the QED regime is a little beyond my current understanding. I have been reading that with other codes (ie Osiris), people have been post-processing the classical radiation fields from the electron momentum and grid variables. I think this will be the approach I will take and just use 10000 or so tracer particles to do the calculation. Are you aware of any codes or anyone who has attempted this already that I could contact?

Thanks, Andrew