Relativistic self-focusing not observed in underdense plasma

[Shikhamis]

Description

Description:
I am running a simulation in Smilei with an intense laser pulse propagating through underdense plasma. Based on theoretical expectations, relativistic self-focusing or ponderomotive focusing of the laser beam should occur, but I do not observe it in the results.

Simulation details (benchmark: tst_2d_laser_wake):

Laser beam: 6 µm focal spot size (w0)

Plasma: underdense regime (ne =0.005 nc)

a0: 4

Observed behavior:

The laser enters the plasma and initially creates a bubble-type structure.

Soon after, the field becomes more linear due to the lower a0.

The laser undergoes defocusing instead of relativistic self-focusing.

Expected behavior:
Relativistic self-focusing (or at least ponderomotive focusing) of the laser beam in the underdense plasma, consistent with theoretical predictions.

Questions:

Is this behavior expected for my parameter regime (6 µm spot, low a0, underdense plasma)?

Are there recommended parameter ranges (a0, w0, ne/nc) or simulation settings in Smilei to observe self-focusing more clearly?

Could this be related to numerical resolution, moving window choice, or box size effects?

[mccoys]

I have converted this into a discussion (issues are for bugs and feature requests)

Have you checked the laser enters correctly from the side? Does it have enough space to avoid diffraction? Did you try to make a reference simulation without plasma?

[Shikhamis]

Laser enters from left side to the right side. laser waist was 6 micron and window width was 180 micron. yes i did it without plasma also. In the absence of plasma it naturally focuses to the minimum waist and then further diverges as per Rayleigh range.

[beck-llr]

Hi

In the 2D geometry all things related to laser focusing are badly reproduced because they are fundamentally 3D. If you want to simulate properly that kind of mechanism you need either 3D or AM geometry.

Please do not label as bug questions that are related to numerical or physical understanding of the problem and prefer the github discussions rather than issues.

[Shikhamis]

Okay next time will put it in discussions