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

[Mathieu Bailly-Grandvaux]

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: image.png][image: image.png][image: 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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