[FLASH-USERS] Unexpected instable shell interface in cylindrical implosion simulation

[Fernando Garcia Rubio]

Hi Mathieu,

I also noticed that the laser package in FLASH does not currently have energy smoothing techniques, and as such might incur in significant laser imprint on targets. Some times ablation is enough to smooth these perturbations out, but for very thin targets,  this results in an artificial enhancement of RTI and subsequent target shred. 

What has worked for me in the past is to increase the “vacuum” temperature and impose a tanh(x) density profile for the large edge so that thermal conduction can naturally smooth the imprinted perturbations.

Hope this helps!

Fernando 


> On Dec 20, 2024, at 11:04 AM, Mathieu Bailly-Grandvaux <mbaillygrandvaux at ucsd.edu> wrote:
> 
> Dear FLASH Users,
> 
> I am using FLASH to simulate a cylindrical implosion for the National Ignition Facility (NIF). The simulations are performed in 2D r-z geometry.
> 
> Initially, I utilized the 3D-to-2D ray tracing algorithm, incorporating the 128 beams of the experimental configuration. I observed a rapid growth of instabilities at the gas-shell interface and initially considered that this might be a (potentially physical) result of azimuthal asymmetry (mode-8 is particularly expected) due to the beam imprint pattern projected in 2D. However, the asymmetries were far more pronounced than anticipated.
> 
> To rule out this effect, I simplified the simulations by switching to a 2D irradiation model using an azimuthally averaged "gaussian1D" function. Surprisingly, the implosions remained highly unstable, indicating that the projected azimuthal asymmetries were not the cause this time. I experimented with varying the number of rays, ticks, and grid resolution, but the instability persisted across all configurations.
> 
> At this point, I am uncertain about how to resolve this issue, and would greatly appreciate your advice and insights.
> 
> I have attached the flash.par and setup_call files for the simulation, which was conducted using FLASH version 4.8. Below, I include mass density plots ("dens" in g/cm³) at several time steps (2.5 ns, 5 ns, and 7.5 ns out of the 10 ns pulse duration) to illustrate the instabilities observed:
> 
> 
> <image.png><image.png><image.png>
> 
> Additionally, I enabled the MHD capability with an axial B-field, but it is currently set to B = 0 T in this simulation.
> 
> Thank you in advance for your time and assistance. I look forward to hearing your thoughts and potential suggestions.
> 
> Best regards,
> Mathieu Bailly-Grandvaux
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