Heavy Ion Target Physics and Design in the USA

1. Heavy Ion Target Physics and Design in the USA D. A. Callahan, D. S. Clark, A. E. Koniges, M. Tabak Lawrence Livermore National Laboratory G. R. Bennett, M. E. Cuneo, R. A. Vesey Sandia National Laboratories A. Nikroo General Atomics 15th International Symposium on Heavy Ion Inertial Fusion Princeton, NJ June 7-11, 2004

2. Our emphasis has been on validating our calculations with experiment and theory • The “hybrid” target, which allows a large beam spot, has become the target of choice • The large beam spot is easier for the accelerator may allow a modular driver with a small number of separate accelerators • The hybrid target uses some new target design features that need validation such as shine shields and shims to correct asymmetries • In a LLNL/SNL/GA collaboration, we did our first experimental test of shims in April ‘04 • We succeeded in reversing a P2 asymmetry! • Our next experiments are scheduled for September ‘04 • We are using theory and simulations to gain a better understanding of Rayleigh-Taylor instability and capsule performance

3. The “hybrid” target has become the target of choice because it allows a large beam spot Hybrid Target 6.7 MJ -- Gain ~ 60 3.8 x 5.4 mm Distributed Radiator Target 5.9 MJ -- Gain ~ 70 1.8 x 4.15 mm

4. The hybrid target uses shine shields and shims to control symmetry HI hybrid target • The distributed radiator target and the NIF point design use beam placement to control symmetry • The hybrid target uses internal shields to control symmetry • A shine shield controls P2 • A shim corrects the P4 • The hybrid target and the Z double-pinch target use similar methods for controlling symmetry • This results in a natural area for collaboration Z double-pinch target

5. In a LLNL/SNL/GA collaboration, we did the first three experiments to test shims in April 2004 General Atomics Capsule Fabrication LLNL Theory and Design SNL Experiment and Hohlraum Fabrication This collaboration worked very well!

6. Large thin-shell capsules in double z-pinch hohlraums provide null cases for P4 shimming tests Z double-pinch hohlraum 6.7 keV backlit images P4 vs. length and capsule size Capsule diameter: 2.15 mm 3.3 mm 4.5 mm 4.7 mm Secondary hohlraum Radius  Rsec Length  Lsec These experiments had a P2 asymmetry in addition to the P4 so the shim was designed to take out this combination 8/2003 • RAVesey

7. GA fabricated the capsules by rotating them under a coater with a mask to produce the layer profile See Abbas Nikroo’s talk this afternoon for more details on fabricating these capsules

8. These targets were first-of-a-kind so there were a few problems • As this was the first attempt at putting the layer on a capsule, the layer ended up being thicker than we had asked for • Translation from flat to sphere for the P4 mask was not as expected--this is certainly fix-able • Recent calculations show that the layer we asked for was too thick anyway • Our plan was to stalk mount the capsules in the hohlraum, using the stalk (200 micron diameter) that was used to hold the capsule under the coater • Due to a miscommunication, the stalks were removed and left small (30-200 micron) holes in some of the capsules • We did not see any ill effects from these holes! • The capsules were then mounted with formvar • Wrinkles in the formvar caused ripples in the images of the capsule -- this was also seen in previous SNL shots (Sept 03)

9. Pole Waist The first shot gave the best data and shows that we reversed the P2 asymmetry! No Shim With Shim Roger Vesey’s preliminary analysis of the images gives: With shim P2 = + 15% (pole high) P4 = - 3% No shim P2 = - 6% (waist high) P4 = - 6.9%

10. Since the image was caught early in time, we see evidence of the shim layer in the radiograph Simulation of shot 1276 Remnant of the shim layer Approximate location of the fiducials

11. A more detailed comparison of the calculation and experiment shows good agreement Assumes layer was 0.3 microns thick in the 20 degrees around the poles (no data from target fab in this region)

12. The second shot was a repeat of the first, but at higher convergence ratio Experiment • The image is clearly pole high so we have certainly reversed P2 • Rippling, which seems to be due to wrinkles in the formvar, is evident • There is an unexplained ‘divot’ near the south pole • The spot where the stalk was removed was at the north pole • Simulated image agrees qualitatively Simulated Expt

13. The third shot used a shim with half the thickness • This shot had two purposes: • A backup in case the backlighter contrast was poor in shots 1 and 2 due to the remnant of the shim layer • A further demonstration of the change in symmetry • The capsule was put into the coater for half the time • The image was clipped, making it difficult to determine P2 Full thickness Half thickness

14. Shots in September will remove a P4 and explore different hohlraum mounting techniques • Based on SNL experiments in Feb/March, we should be able to tune out the P2 asymmetry using hohlraum and shine shield geometry • First four shots in September will tune out P2 and serve as a baseline for the shim shots • Four shim shots will try to correct and reverse a P4 asymmetry • Because of the problems with capsule mounting, the first four shots will use two different mounting techniques • Formvar on a split frame to reduce wrinkles • Beryllium stalk mount along the direction of the backlighter • We will use these results to decide which mounting technique to use for the shim shots These experiments give credibility to the heavy ion hybrid target as well as exploring a new symmetry technique for all indirect drive targets

15. Gain curves for the hybrid target are useful for system optimizations Point design Modular and conventional drivers may not optimize at the same beam energy/spot size so we need gain curves to do a fair comparison

16. Understanding nonlinear Rayleigh-Taylor instability growth may be important for HIF • For a power plant, we want to push the capsule performance in order to minimize the constraints on other parts of the system • Moving to lower drive temperature reduces the peak power needed from the accelerator • Allowing a rougher ablator may allow cheaper target fabrication • These mean less margin and more Rayleigh-Taylor growth • A power plant is likely to optimize beyond the capsule performance needed for the baseline NIF • NIF does not need high gain or $ 0.25 targets! • Understanding nonlinear Rayleigh-Taylor instability growth may be needed • This is a difficult computational problem and theory can help us understand the limits of our models • Theory can also guide us to the right parts of design space

17. The model includes the effect of convergence on the nonlinear phase of growth • Model uses • a Layzer-like, nonlinear bubble model in spherical geometry • combined with a self-similar Kidder implosion model • Solve this by transforming to a coordinate system that converges with the implosion (quasi-Lagrangian solution) See Dan Clark’s poster on Wednesday (WP-01) for more details!

18. HYDRA numerical theory WKB Layzer Bubble growth in converging geometry is faster than predicted by Layzer formula Bubble curvature at apex agrees with HYDRA using ALE relaxation (model does not apply to spike) Model and 2-d HYDRA both predict faster bubble growth Initial bubble growth agrees with Layzer model (linear with time) followed by late time acceleration similar to that seen in HYDRA

19. Our emphasis has been on validating our calculations with experiment and theory • The “hybrid” target, which allows a large beam spot, has become the target of choice • The large beam spot is easier for the accelerator may allow a modular driver with a small number of separate accelerators • The hybrid target uses some new target design features that need validation such as shine shields and shims to correct asymmetries • In a LLNL/SNL/GA collaboration, we did our first experimental test of shims in April ‘04 • We succeeded in reversing a P2 asymmetry! • Our next experiments are scheduled for September ‘04 • We are using theory and simulations to gain a better understanding of Rayleigh-Taylor instability and capsule performance