[FLASH-USERS] Suspected EOS issue in Biermann battery source implementation

[Benjamin Spiers]

Hi All,

I have recently been trying to use the source term implementation of the Biermann battery in LaserSlab-like simulations in order to study magnetic field generation in long-pulse laser interactions.

I found that with laser powers of 450GW I ran into issues with the timestep in my simulations becoming impractically small (<1fs), which I believe to be because the ions in these simulations reached temperatures of 10-100MeV, and so had enormous associated sound speeds. I also found that this issue still arose when I added a multiplicative factor to all updates to U(*_VAR,...) quantities in hy_uhd_biermannSource.F90, even when said factor was set to zero (disabling the source terms altogether). However, setting hy_useBiermann = .false. stopped the issue.

I believe that the issue is caused by the calls to `Eos_wrapped(MODE_DENS_EI, blkLimits, blockID)` at the start and end of the main loop in this file. This EOS mode doesn't properly perform a 3T update, and somehow flip-flopping between EOS modes like this seems to cause significant artificial heating of plasma ions. Switching to MODE_DENS_EI_GATHER seems to have fixed the issue for me: magnetic fields are generated as expected and the plasma remains at a sensible temperature with sensibly-sized time-steps throughout.

I believe what is required to solve this issue is to change the two cases of MODE_DENS_EI to the corresponding _GATHER mode, and also to move the #ifdef FLASH_UHD_3T from the next line to encompass the whole loop, so as to protect 1T simulations from the inverse issue (It already disables field generation in the 1T case so this shouldn't be a regression).

Best wishes,

Ben
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