Titan June 2008 Experiment Planning

1. Titan June 2008 Experiment Planning September 10, 2014

2. Titan June 2008 Experiment Most Important 3 bullets • Cone physics – NIF FI point design needs electron number at cone tip for NIF ARC focal coniditions • What is conversion eff. to useful electrons (1-3 MeV electrons passing through tip in a non-refluxing geometry)? • Do cone wall bounces help? Either through electron flow in cone, or enhanced laser energy/intensity at cone tip? • What are the tolerable limits for laser pre-pulse? • Can we test ponderomotive steepening - leave for later experiment? • Proton fast ignition • Focusing: Better understanding of focusing with hemis, measure focused energy density better. New shapes based on LSP modeling? • Electron conversion/Thot/transport • Double buried layer foils to resolve coupled Thot and transport problem • Diagnostic development • NIF diagnostic? New diagnostic development? Test EP diagnostics • Collaborations • LULI, LLE, LANL, RAL

3. To what extent does the cone increase the total energy input to the wire • Cu nail wire w/ head machined down to 30-40 micron flared end. • HOPG, SHCCD, Cu K-alpha imager • Would be simpler to use 40 micron Cu wire with no head glue Insert nail wire into open ended Al cone with thin walls. Maybe put dab of glue on outside. This has the same laser interaction surface with the only addition being the of the surrounding cone. Measure yield compared to bare flared wire. If laser is focused tightly the only differences will be: 1) energy (light or electrons) outside flare tip area and 2) confined preplasma effects. The big mass and thickness of Cu in the Nail head will have the same effect as it does for a bare nail - the coupling will be very small - not worth doing this in my view glue Insert nail wire into open ended Al cone with thick, non-refluxing walls. Maybe put dab of glue on outside. This has the same laser interaction surface with the only addition being the of the surrounding cone. Measure yield compared to above configurations. Now, electrons interact with the “external plasma” surrogate defined by the very thick Al walls. Easier if we can make conical hole in thick substrate to use in simpler way with fluor layer behind

4. What effect does pre-plasma have on electron conversion efficiency through cone tip? • Use same targets as in previous efficiency measurements. • Importance: Significant pre-plasma must reduce coupling to unacceptable level. We need to define minimum acceptable laser pre-pulse spec for NIF ARC • What do we know: RAL PW shots measure 1-15% coupling into 10-40µm diameter wires at standard pre-pulse level (not well known). LULI shots by Baton show that (i) cone reduces the laser to target coupling, (ii) 2 coupling is more than 1consistent with pre-plasma effect • Shots: Vary laser pre-pulse, measuring Ka from sample attached to cone as a function of laser pre-pulse • Scientific issues: Pre-plasma effect may be mitigated in real FI due to ponderamotive steepening - could be investigated at EP • Technical issues: Good to characterise pre-plasma in cone. How best to measure electron flow into sample? Advantages of wire vs. thin foil vs. thick foil?Buried layer in thick foil avoids problems of directly hitting Cu wire and refluxing. Why not rerpeat simple butt joint Cu wire expt where we have data already - with added prepulse ?

5. Does cone wall reflection work? • Importance: Affects focal spot quality requirements. Affects comparison of cone vs. slab data • What do we know: Slabs irradiated at 75deg S-pol reflect highly for Titan laser conditions. Cones irradiated with different size focal spots show similar K-alpha emission? • Shots: Can we block light directly hitting tip? Roughen inner surface to make it non-specular? • Scientific issues: Can probably answer many questions by modeling. Substantive modeling would guide experiments • Technical issues: Extensive simulations before experiments. WHO???? Oblique shots in Jan should give us good insight so absent those data it is premature to plan what to do

6. Do shaped cones improve light coupling to tip? • Shots: Try different cone shapes. Measure coupling to a wire, or buried layer? Use 2 different fluors for cone and wire? • Scientific issues: Is it detrimental to hydro design? Extensive simulations before experiments. Andrew/Ohio

7. What is the electron flux and Thot emerging from the cone tip? • Importance: Need to measure and optimize these quantities. They are the input into energy deposition and gain calculations • What do we know: RAL PW measure 15% coupling into 40µm wire with kT~750eV fitted with resistivity model. • Shots: Vary laser intensity, pulse length, and pre-pulse? Using Ag K measure total yield and 2D image. Variation could be thin Al target and use XUV heating of rear surface, but then we have refluxing issues again? Some targets with thick high Z layers to measure Thot more accurately Use of Ag not yet proven to work - Cu better for wire target. • Scientific issues: We can’t fully replicate FI conditions, this is useful data for comparison with PIC and LSP. • Technical issues: Needs to be a non-refluxing target. Requires higher energy fluor like Ag. Enough photons to spatially image with pinhole camera? Third target type difficult to make but potentially very good Divergence information ….. Thot from cannon? Need to come up with a standard target type like the flare wires … how do we do that here???? Medium priority

8. Can we improve our data set on conversion eff. and Thot from flat foils? • Importance: Required for PIC benchmarking. Required for LSP benchmarking • What do we know: We should obtain an extensive data set in Jan Titan run • Shots: Depends on Jan results, but double buried foils is likely to be worth doing since it helps constrain transport effects • Scientific issues: Necessary step in longer term study of electron transport (both in cold and hot plasmas) • Technical issues: Needs design work …. Has Cliff done enough? Who else is working on design simulations? What are the transport issues to be resolved with double layers? Design basis not yet adequate - need review of what we know

9. Measuring Ka Yield from “useful” electrons w/ filtering Use 100 mm Au to block electrons <300 keV - cannot ignore ohmic and magnetic inhibition !! Remaining electrons interact w/ Ag layer and produce Ka Ag Ka mfp ~1.5 mm in Al – use ~500 mm to mitigate refluxing but still allow viewing of Ag. • From ESTAR: • 300 keV electrons in Au stop in 100 mm • 1 MeV electron loses ~ 200 keV in crossing Au • Have we an Ag imager ? 100 mm Au 50 mm Ag • Issues: • Bremsstrahlung from Au too much? • Resistive slowing? 10 mm Al 500 mm Al 50 mm Al

10. Pre-pulse in cones Long Pulse Cu or Ag Short Pulse • Use Cu or Ag cones to study the effect of pre-pulse on: • Conversion efficiency • Hot electron temperature • Spatially resolved Ag imaging needs to be developed in order to use Ag cones. How useful is imaging cone, how do you measure conversion and Thot? Better to use Al cone + Cu fluor? Or Cu cone + Ag fluor? Lawrence Livermore National Laboratory

11. Cones with different tips and walls Ti tip Obtain 2D spatially resolved K images, absolute K yield, and Thot. Cu or Ag Use low Z for the cone and high Z for the tip. This way the bremsstrahlung from the tip will dominate the bremss. from the cone. This is a better indication of Thot at the tip and few shots will serve as a benchmark for the cannons. Au tip Al cone 2D imaging comprimised by K-alpha shifting What about refluxing? I think that cones that are just reflux traps do not give good info relative to targets that allow non reflux forward flux to be measured Lawrence Livermore National Laboratory

12. Cones/wire assembly Cu cone Ti, Ag Cu cones and Ti wires are preferable since we have two K imagers we can use to obtain 2D spatially resolved images. But we are limited to using thin Ti wires due opacity issues. If Ag pinhole imagers is developed and working, then Ag wires can be used with Cu cones or vice versa. Having to use thin Ti is too much of a constraint - we should use 40µm diameters Use Ag regardless of whether or not imaging works? Can we use Ti cones and Cu wires ? Lawrence Livermore National Laboratory

13. Cones with foams Cu cone foam Study Thot and conversion efficiency under different irradiation conditions. Does the low density foam increase absorption and lower Thot? Not important enough - what reasons are there to think foam will help? Lawrence Livermore National Laboratory

14. cone Cu foil or spectralon Same material as cone Cones with no tip Compare throughput in cones and flat targets. Effect of pre-plasma? Self-focusing? K yield in the Cu foil? Can we use spectralon? What does this tell you exactly? Problems of pre-plasma, pinhole closure, sensitivity to pointing… Lawrence Livermore National Laboratory

15. Laser drilled cones in wires Cu or Ag wire Laser-drilled cone This could be a cheaper alternative to cones. It also has the advantage of the absence of glue and discontinuities between cone and wire May be issues with surface transport? What are the shots? Lawrence Livermore National Laboratory

16. Additional ideas • Cu cone with Al get loss block. Compare born electron pattern with laser pattern • Cu cone with Al get loss block. Do specular walls work? Compare smooth vs. rough surfaces • Can you mimick plasma generated toward end of 10ps pulse?

17. Proton focusing & conversion efficiency • Importance: • What do we know: • Shots: • Scientific issues: • Technical issues:

18. Diagnostics • Ag imaging: Switch from Cu to higher Z K imaging • Al X-ray spectroscopy: Spatially-resolve temperature along wire if it gets hot enough (prob needs to be >600eV). Or shifted Al K might work? • Optical throughput: Measure light throughput with open ended cones? Compare tight focus, defocus? Use spectralon?