[FLASH-USERS] Defining an extended focal spot / Rayleigh length in LaserSlab

[Meirielen Caetano De Sousa]

Good morning, 



I started working with FLASH from the LaserSlab simulation. 

Now I adapted it to model the optical shaping of gaseous targets with a low-intensity laser (~10¹³ W/cm²) and nanosecond duration. 

To define the laser focusing, we specify the position and width of the lens, and the parameters ed_target and ed_targetSemiAxis , then define the position and size of the focal spot. If I understand correctly, FLASH treats the rays as originating from the lens and converging toward the target point specified by ed_target , with an aperture determined by ed_targetSemiAxis . 




My problem is that, since I am using gaseous targets (with density below the critical density), the laser keeps propagating beyond the position defined by ed_target and ends up focusing into a single cell . As a result, I obtain a single focal point rather than a focal spot with a finite longitudinal extent. I therefore do not obtain a focal volume corresponding to the laser’s Rayleigh length. 




For example in this simulation, my target is gas (ne = 5% nc) and I have defined ed_target_X = 0.3 cm, ed_target_Y = 0.16 cm and ed_targetSemiAxis = 100 µm 

Indeed, the laser beam has a radius of 100 µm at the position [x, y] = [0.3, 0.16] but the laser beam keeps propagating until a single focal point (one cell). 









How can I configure FLASH so that the laser does not converge into one cell (a single focal point), but instead forms an extended focal spot, i.e. , something that reproduces the Rayleigh length of the laser? Otherwise, is it possible that the laser "stops" at the ed_target position ? 

Thank you in advance for your help ! 

Meirielen Caetano de Sousa 
Postdoctoral researcher - CELIA UMR5107 
Université de Bordeaux 
43 rue Pierre Noailles - Bât C6 
33405 TALENCE 

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