<div dir="ltr"><div><p>Dear All,</p><p>Thank you for your responses and helpful insights!</p></div><div><a class="gmail_plusreply" id="plusReplyChip-2" href="mailto:rjfarber@umich.edu" tabindex="-1">@rjfarber@umich.edu</a>: Yes, I believe we can rule out grid artifacts due to resolution. I do expect RT growth as a physical phenomenon; however, the observed level of RT growth appears unphysically high when compared to both GORGON 2D simulations and experimental data. The shell seems to get "punctured" early (see at t = 2.5 ns it almost made a hole through its 50 um thickness), likely due to laser imprint artifacts. These artifacts are subsequently strongly amplified by RT growth, leading to what appears to be artificial and exaggerated shell shredding. Regarding the "ForceHydroLimit = .true." option, I recall trying this previously without observing significant changes, but I didn’t save that run. I will revisit this to confirm whether the MHD model could be contributing to the issue and will let you know the outcome.</div><div><br></div><div><a class="gmail_plusreply" id="plusReplyChip-3" href="mailto:fernando@pacificfusion.com" tabindex="-1">@fernando@pacificfusion.com</a>: Exactly—this aligns with my thoughts as well, as discussed above. I previously attempted to include a pre-heating beam to allow the shell to expand slightly before driving it, hoping that the resulting heated gradient scale length would mitigate the artificial RT growth. Unfortunately, this approach was not particularly successful. However, I haven’t yet tried directly modifying the initial density profile, and my "vacuum" is still set at room temperature. Thank you for this suggestion—I’ll explore that approach!</div><div><br></div><div><a class="gmail_plusreply" id="plusReplyChip-5" href="mailto:tjoshi@lle.rochester.edu" tabindex="-1">@Joshi, Tirtha</a>: When I employed the 3D ray tracing, I used the "statistical2D" configuration. However, for this case, since I am using 2D irradiation, I’ve switched to "statistical1D".</div><div><br></div><div>Thank you again for your input, and I’ll follow up with updates as I test these suggestions.</div><div><br></div><div>Best,</div><div>Mathieu</div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Fri, Dec 20, 2024 at 1:23 PM Joshi, Tirtha <<a href="mailto:tjoshi@lle.rochester.edu">tjoshi@lle.rochester.edu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div class="msg-3112335125472556805">
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Hi Fernando and Mathieu,</div>
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I had also observed similar effects in my laser-slab simulations, and these effects were reduced when I replaced the ed_gridType_1 = "radial2D"</div>
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by</div>
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ed_gridType_1 = "statistical2D". </div>
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Best,</div>
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Tirtha</div>
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<div id="m_-3112335125472556805divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> flash-users <<a href="mailto:flash-users-bounces@flash.rochester.edu" target="_blank">flash-users-bounces@flash.rochester.edu</a>> on behalf of Fernando Garcia Rubio <<a href="mailto:fernando@pacificfusion.com" target="_blank">fernando@pacificfusion.com</a>><br>
<b>Sent:</b> Friday, December 20, 2024 3:48 PM<br>
<b>To:</b> Mathieu Bailly-Grandvaux <<a href="mailto:mbaillygrandvaux@ucsd.edu" target="_blank">mbaillygrandvaux@ucsd.edu</a>><br>
<b>Cc:</b> <a href="mailto:flash-users@flash.rochester.edu" target="_blank">flash-users@flash.rochester.edu</a> <<a href="mailto:flash-users@flash.rochester.edu" target="_blank">flash-users@flash.rochester.edu</a>><br>
<b>Subject:</b> Re: [FLASH-USERS] Unexpected instable shell interface in cylindrical implosion simulation</font>
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<div>Hi Mathieu,
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<div>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. </div>
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<div>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.</div>
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<div>Hope this helps!</div>
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<div>Fernando </div>
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<div>On Dec 20, 2024, at 11:04 AM, Mathieu Bailly-Grandvaux <<a href="mailto:mbaillygrandvaux@ucsd.edu" target="_blank">mbaillygrandvaux@ucsd.edu</a>> wrote:</div>
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<p>Dear FLASH Users,</p>
<p>I am using FLASH to simulate a cylindrical implosion for the National Ignition Facility (NIF). The simulations are performed in 2D r-z geometry.</p>
<p>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.</p>
<p>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.</p>
<p>At this point, I am uncertain about how to resolve this issue, and would greatly appreciate your advice and insights.</p>
<p>I have attached the <code>flash.par</code> and <code>setup_call</code> 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:</p>
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<p>Additionally, I enabled the MHD capability with an axial B-field, but it is currently set to B = 0 T in this simulation.</p>
<p>Thank you in advance for your time and assistance. I look forward to hearing your thoughts and potential suggestions.</p>
Best regards,<br>
Mathieu Bailly-Grandvaux</div>
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