Maxwell Boltzmann Distribution in PICMI interface?

[yopology]

I have a rather basic question about particle initialization. When I use (density_expression, momentum_expressions, warpx_momentum_spread_expressions) to specify the particle distribution in picmi.AnalyticDistribution, the generated input file indicates that momentum_distribution_type is set to gaussian_parse_momentum_function. My code goes as follows:

        self.ions = picmi.Species(
            name="ions",
            charge="q_e",
            mass=self.M,
            initial_distribution=picmi.AnalyticDistribution(
                density_expression=self.den,
                momentum_expressions=[0, mom_iy_expr, 0], # Add y-drift
                warpx_momentum_spread_expressions=[mom_i_th] * 3  
            )
        )

Suppose I want to use maxwell_boltzmann instead (or maxwell_juttner for relativistic plasmas), how do I do that with the PICMI interface?

An unrelated question -- I noticed that I have to warpx_momentum_spread_expressions to get this working. If I use momentum_spread_expressions, then the initial velocities of particles are all zero (relative to the local drift frame).

I would appreciate very much your help on this issue.

Thanks,

Yi-Min

[ax3l]

Thanks for raising this.

The docs on our inputs file is confusing due to historical reasons. The gaussian_parse_momentum_function should be equivalent to maxwell_boltzmann: setting a Gaussian distribution on each momentum component will create a MW distributed particle momentum.

We have not added support for the momentum_distribution_type function maxwell_juttner to PICMI yet.

This can be added, do you like to give it a try? PRs would go:

• PICMI: https://github.com/picmi-standard/picmi/blob/0.33.0/PICMI_Python/particles.py
• WarpX: https://github.com/BLAST-WarpX/warpx/blob/25.08/Python/pywarpx/picmi.py#L566-L617