Help with unstable simulation using mesh refinement

[Cosimo89]

Dear WarpX community,

I am writing here because of a problem I am facing in what I thought would be a simple setup of laser-plasma interaction study.
I want to investigate the heating and interaction between electrons and layers of plasma with different material. The laser pulse regime is in the low-energy, long-duration, so not the typical application range for ultra-intense laser-plasma interactions.

I started by modifying the input script from the laser-ion acceleration case (https://github.com/ECP-WarpX/WarpX/blob/development/Examples/Physics_applications/laser_ion/inputs_test_2d_laser_ion_acc)

In my first setup, I wanted to have a slab of Carbon at a given density (say $\sim30n_c$) and a low intensity laser pulse impinging on it. On a later stage I wanted to add an under-dense hydrogen ($n_c\leq 1$) after the Carbon slab. Ideally, I wanted to have ionization and coulomb binary collisions included. I also want to use mesh refinement around the Carbon slab, to reduce the computational burden.

Below a sketch of the simulation setup:

The problem that I am facing is that in all investigated configurations, the simulation is unstable as non-physical electric fields seems to be generated at the interface between the refined and the coarse regions which lead to an abrupt explosion of the plasma (see images below). In all my trials, the problem seems to arise around the mesh refinement region, but it also appeared in a low-resolution case I performed without using mesh refinement.

Please note that this test is performed using on purpose a grid resolution of about 4 times coarser than a proper one for production. Can it be a resolution issue?

I performed many different trials to debug my input script (i.e. different boundary conditions, domain sizes, material, no ionization), but all simulations showed similar behavior.
I have the feeling I am committing some bad setup error, but I am afraid not be expert enough to spot it, so I invoke the help of the experts!

Any help/input/ feedback on the input file (attached below) is highly appreciated.

inputs_test_2d.txt

Thank you in advance and best regards,
Cosimo