<html><body><div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: #000000"><div data-marker="__QUOTED_TEXT__"><div style="font-family:'arial' , 'helvetica' , sans-serif;font-size:12pt;color:#000000"><div><span style="font-size:10pt">Good morning, </span></div><div><div style="font-family:'arial' , 'helvetica' , sans-serif;font-size:12pt;color:#000000"><div><div style="font-family:'arial' , 'helvetica' , sans-serif;font-size:12pt;color:#000000"><br><div><span style="font-size:10pt"></span><p style="margin:0px"><span style="font-size:10pt">I started working with FLASH </span><span style="font-size:10pt">from the <strong>LaserSlab</strong> simulation.</span></p><p style="margin:0px"><span style="font-size:10pt">Now I adapted it to model the optical shaping of gaseous targets with a low-intensity laser (~10¹³ W/cm²) and nanosecond duration.</span></p><p style="margin:0px"><span style="font-size:10pt">To define the laser focusing, we specify the position and width of the lens, and the parameters <em>ed_target</em> and <em>ed_targetSemiAxis</em>, 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 <em>ed_target</em>, with an aperture determined by <em>ed_targetSemiAxis</em>. </span></p><p style="margin:0px"><br></p><p style="margin:0px"><span style="font-size:10pt">My problem is that, since I am using <strong>gaseous targets</strong> (with density below the critical density), the laser keeps propagating beyond the position defined by ed_target and ends up focusing into a <strong>single cell</strong>. As a result, I obtain a <strong>single focal point</strong> rather than a <strong>focal spot</strong> with a finite longitudinal extent. I therefore do not obtain a focal volume corresponding to the laser’s Rayleigh length.</span></p><p style="margin:0px"><br></p><p style="margin:0px"><span style="font-size:10pt">For example in this simulation, my target is gas (ne = 5% nc) and I have defined ed_target_X = 0.3 cm, <strong>ed_target_Y = 0.16 cm </strong>and <strong>ed_targetSemiAxis = 100 µm</strong> </span></p><p style="margin:0px"><span style="font-size:10pt">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).</span></p><p style="margin:0px"><br></p><p style="margin:0px"><img src="cid:1e3066941419d42f6709aa4d0e76a5ea3de5efe4@zimbra" data-mce-src="cid:1e3066941419d42f6709aa4d0e76a5ea3de5efe4@zimbra" width="438" height="337"></p><p style="margin:0px"><br></p><p style="margin:0px"><span style="font-size:10pt">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, <em>i.e.</em>, something that reproduces the Rayleigh length of the laser? Otherwise, is it possible that the laser "stops" at the ed_target position ?</span></p><span style="font-size:10pt"></span></div><br><div><span style="font-size:10pt">Thank you in advance for your help !</span></div><div></div></div></div></div></div></div></div><div data-marker="__QUOTED_TEXT__"><div><br data-mce-bogus="1"></div><div data-marker="__SIG_PRE__"><div style="font-size: 12pt; font-family: 'arial' , 'helvetica' , sans-serif; color: #000000;" data-mce-style="font-size: 12pt; font-family: 'arial' , 'helvetica' , sans-serif; color: #000000;"><div style="font-size: 12pt; font-family: 'arial' , 'helvetica' , sans-serif; color: #000000;" data-mce-style="font-size: 12pt; font-family: 'arial' , 'helvetica' , sans-serif; color: #000000;"><img src="cid:d785ee7249b0d19dc650d85a263bcf634f7afa4a@zimbra" width="65" height="51" alt="" data-mce-src="https://webmel.u-bordeaux.fr/home/meirielen.caetano-de-sousa@u-bordeaux.fr/Briefcase/LOGO%20CELIA%202023.PNG?v=6720" doc="Briefcase/LOGO CELIA 2023.PNG"></div><div><div><span style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;" data-mce-style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;">Meirielen Caetano de Sousa</span><br><span style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;" data-mce-style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;"><strong>Postdoctoral researcher - </strong>CELIA UMR5107</span><br><span style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;" data-mce-style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;">Université de Bordeaux</span><br><span style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;" data-mce-style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;">43 rue Pierre Noailles - Bât C6</span><br><span style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;" data-mce-style="font-family: 'trebuchet ms' , sans-serif; color: #333399; font-size: 10pt;">33405 TALENCE</span></div></div></div></div></div><div data-marker="__QUOTED_TEXT__"><br data-mce-bogus="1"></div></div></body></html>