Subject: Re: two questions re: PHP manuscript From: Otto Landen Date: Wed, 18 Feb 2004 10:01:04 -0800 To: dcohen1@swarthmore.edu > David, Paper nicely written, and by reading, I have been forced to learn things I was vague on. I especially like idea of disk outside halfraum and explanation of hohlraum vs halfraum Dante Tr. My comments by Page number: 1) First sentence should say single (also known as halfraums) and double-ended hohlraums in ICF and High Energy Density Physics (reference Lindl and Rosen paper above) 2) Give % LEH size here and elsewhere. Also, beam focus conditions. Explain that VISRAD assumes Lambertian optically thick surface source. This does ignore fact that there is Tr gradient in wall which means re-emission Tr will increase as view wall at more and more oblique angles. This is least serious at late times (high albedo) when Marshak wave has penetrated ‰ 1/(1-albedo) mfps. Rough estimate of effect (which I first heard from M. Rosen) given by assuming flux drops linearly into wall over 1/(1-albedo) mfps and ray at angle beta to wall sees into average depth mfpsin(beta): T(beta)/T(normal) = [(1-(1-albedo)sin(beta))/(1-(1-albedo)]^0.25 where beta is angle from wall surface. This gives T(45°)/T(normal) = 1.02 for albedo = 0.75. I guess you mean XCE accounts for plasma motion by implicitly assuming energy lost to hydro? 3) When explaining Fig. 4 trends, the size of LEH lip should be mentioned as does factor into Dante view as pointing changes. I would combine top 2 Figs. 2 and make fonts bigger. 4) Reference Back, PoP 7 (2000) 2126 for halfraums. Also, I enclose Suter paper on hohlraum drive worth referencing. I would mention that finding that Tr on sample does not depend on sample orientation or position is why symmetry can be achieved in cylindrical hohlraums and why ICF is attempted in cylindrical hohlraums. Discussion on LEH vs wall-facing samples at end of penultimate paragraph seems contradictory. Which has harder spectrum? 5) Specify disk is also made of Au. It would be worth going through simple power balance model of why adding the divider wall only drops sample Tr by 1%. Offhand, I would have expected more since assuming 50% of energy lost to walls and increase wall area by 20% should increase losses by 10% which is >2% in Tr. For explanation on higher Dante Tr for hohlraum, I would be more specific and say that Dante also sees laser hot spot emission from beams entering the opposite LEH of the hohlraum. This is to distinguish from case where low angle beams can cross hohlraum midplane before striking wall. In that case, Dante views of hohlraums and halfraums could be fairly similar as a low angle beam that would cross a hohlraum mid-plane would illuminate the divider disk in a halfraum visible by Dante (could actually make halfraum appear hotter as low angle beams would hit divider more head-on than hohlraum wall). I would add this discussion in here as very relevant to NIF where 1/3 of beams are low angle and cross. 6) Role of LEH lip size should be included. I am puzzled why you say solid angle of hot spots decreases as move closer to back-wall sample? Fig. 8 shows spots looking bigger as move closer to sample, and I calculate solid angle varying roughly as 1/z^2 where z is axial distance between wall and spots. I am assuming we are considering a sample point element which does not weight radiation flux arriving on it by angle which it arrives? (This is not so when considering ablation pressure on finite sized sample with finite gradient where arrival angle of x-rays does matter). I would further explain similarity in Tr at low albedo by fact that distinction between Dante seeing walls and sample seeing walls and LEH is lost as both walls and LEH are much colder relative to laser spots and hence unimportant contributors of drive. 7) In section V, make clear that pointing is kept fixed wrt to LEH. This brings up suggestion for another set of simulations I have often wondered about: How does Tr vary as pointing relative to end package kept fixed but halfraum length varied (to the extent possible while avoiding clipping of beams at LEH lip)? My guess is that Tr on sample will go up as halfraum lengthened (which, if true, is interesting as goes against simple-minded power balance formula which says more wall area decreases Tr). 8) Exterior/interior sentence grammar problem. 9) Sentence " In standard halfraum...", I would say temperatures are closer and talk about a 5 eV difference at 200 eV rather than disagreement, since when accounted for, no discrepancy. ...because Dante sees some of the hot spots from laser beams entering the opposite LEH of the hohlraum. Nino -- Otto L. Landen Acting Associate Program Leader for Ignition Physics Experiments Lawrence Livermore National Laboratory P.O. Box 808, L-473 Livermore, CA 94550 tel: (925) 424-5581 fax: (925) 423-6319 email: landen1@llnl.gov