How to incorporate fluctuating magnetic field in the input file

[deSUBHAM]

Hi,

I would like to implement a decay turbulence set up with the fluctuating magnetic field given by the formula in the attached file. Is there any way to incorporate the fluctuating magnetic field as given in the figure, in the input file? Or, do we have to change it in the source file itself? In addition, I would like to know how to add the forcing term in the system to study the kinetic turbulence in collisionless plasma, in general. Thank you in advance.

[deSUBHAM]

Hi,
Any help in this regard will be highly appreciated.

[RemiLehe]

@deSUBHAM Sorry for the late reply.

You could probably load the above magnetic field by writing it into an openPMD file (using the Python package openpmd-api for instance), and then have WarpX read that file and use it as the initial magnetic field in the simulation, see this part of the documentation: https://warpx.readthedocs.io/en/latest/usage/parameters.html#external-fields, and in particular the option read_from_file.

Regarding generating the initial field map and writing it into an openPMD file, you can find examples here:
https://openpmd-api.readthedocs.io/en/0.16.1/usage/firstwrite.html
https://github.com/openPMD/openPMD-example-datasets/blob/draft/scripts/femm/openpmd_create_3d.py

Regarding the forcing term: I am not very familiar with this concept - do you have a link to more info? (e.g. is this a force to be applied to the particles at each iteration?)

[deSUBHAM]

@RemiLehe Thank you so much for your reply. I am sorry for the late reply.

Regarding the forcing term, what I meant is the following:

1. As I am interested in the turbulent simulation, one way to drive the turbulence is to use fluctuating external current density as oscillating Langevin antenna as mentioned in reference 1 (https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.118.055103) and in reference 2 (https://academic.oup.com/mnras/article/493/1/603/5718390). Please refer to the third paragraph of Method section in reference 1 and the first paragraph of section 2.2 in the reference 2. I do not know how to implement it in WarpX, as I do not know to drive the current in WarpX. I would appreciate if you help me in that.
2. The second method is to apply a body force to the particles consistently to drive a bulk motion at the large scales. It is implemented in the reference 3 (https://doi.org/10.3847/1538-4357/ac222e). Please have a look at the third paragraph of section 2.2 of reference 3. My query is how to implement it in WarpX.
3. In reference 4 (https://arxiv.org/pdf/2504.14000), they use a thermodynamic forcing by modifying the electric field and corresponding particle pusher. If I want to implement it do I have to change the code?

Thank you.