M. S. Tillack IFE Technology Research at UC San Diego MAE Departmental Seminar 6 October

Many people have contributed to this research Faculty and Staff: C. V. Bindhu Z. Dragojlovic A. C. Gaeris S. S. Harilal F. Najmabadi T. K. Mau J. E. Pulsifer, MS’98 A. R. Raffray X. R. Wang M. R. Zaghloul Students: N. Basu, MS’98 D. Blair, PhD’03 L. Carlson S. Chen B. Christensen, MS’04 K. Cockrell M. Mathew J. O’Shay K. Sequoia New kids on the block: K. Boehm J. Hanft J. Mar R. Martin E. Simpson

The inertial confinement fusion concept

The goal of “ ICF ” research is to ignite DT targets in order to explore high energy density physics Indirect Drive Direct Drive Z-pinch Omega “fast ignition” NIF 60 beams/40 kJ 192 beams/2  2 MJ of x-rays Z

The goal of “ IFE ” research is to generate power economically HAPL: Laser driver (DPSSL or KrF) with direct drive targets and dry walls HI-VNL: Ion accelerator, indirect drive targets, liquid chambers Z-IFE: Z-pinch driver In addition to target physics, key issues include efficient rep-rated drivers, target mass production, target injection, reliable chambers and optics

Our IFE research is focused on the key issues for IFE chambers and chamber interfaces Prometheus-L Reactor Building 1. Chamber walls that survive long-term exposure 2. A residual chamber medium which allows propagation of targets and beams through it 3. Final optics that survive long-term exposure 4. Cryogenic targets that survive injection and are properly illuminated

1.Prompt transport of energy through and deposition into materials (ns-  s) 2.Radiation fireball & shock propagation, mass loss from walls (1-100  s) 3.Afterglow plasma & hydrodynamics (1-100 ms) 4.Liquid wall dynamics (ms-s) 5.Long-term changes in materials F ollowing target explosions, several distinct stages of chamber response occur: Wall protection and target/driver propagation depend on the details of target emissions Fireball forms from captured x-ray and ion energy, re-radiates on a slower timescale MJ released per target

Our chamber wall research simulates thermomechanics of armor and energy transport from ablation plumes High-cycle fatigue of tungsten armor –simulations with short-pulse lasers –phenomena similar to optics damage Laser plasma expansion dynamics –modeling of laser plasma –ablation plume experiments – magnetic diversion

We use laser ablation plumes to provide a surrogate plasma to study IFE target emissions 1.5 cm Time-resolved imaging and spectroscopy are performed with 2-ns gated camera and PMT

An aluminum ablation plume is confined by a moderate magnetic field 5 GW/cm 2, 8 ns, Al target 0.64 T RbRb

free expansion velocity v=6x10 6 cm/s 5 GW/cm 2 The plasma beta initially is large, but falls quickly Similar to results without B, the initial ns is ballistic, followed by plume drag The expansion is slowed after the thermal beta falls

Our IFE research is focused on the key issues for IFE chambers and chamber interfaces Prometheus-L Reactor Building 1. Chamber walls that survive long-term exposure 2. A residual chamber medium which allows propagation of targets and beams through it 3. Final optics that survive long-term exposure 4. Cryogenic targets that survive injection and are properly illuminated

We seek to understand the residual chamber medium and the propagation of targets and beams through it Chamber dynamic response modeling and “chamber clearing” Target transport through the perturbed chamber Aerosol generation in liquid- protected walls –explosive phase change (evaporation) – homogeneous nucleation in laser ablation plumes (condensation) Laser propagation in background gas Spartan simulation

Rapid condensation of vapor ejected from liquid- protected IFE chamber walls was modeled numerically and experimentally 0.15 Torr These processes also occur in laser machining, pulsed laser deposition, and other applications Again, lasers are used to simulate ion & x-ray deposition and response

The homogeneous nucleation rate and critical radius depend on saturation ratio & ionization # of atoms Ion jacketing (dielectric behavior of vapor) reduces the energy barrier Without ionizationWith ionization Si, n=10 20 cm –3, T=2000 K High saturation ratios result from rapid cooling during plume expansion Extremely small critical radius and high nucleation rates result Si, n=10 20 cm –3, T=2000 K, Z eff =0.01

The condensate size distribution was measured at stagnation using atomic force microscopy 500 mTorr He 5x10 8 W/cm 2 5x10 9 W/cm 2 5x10 8 W/cm 2 5x10 7 W/cm 2 Correlation between laser intensity and cluster size is observed. Is it due to increasing saturation ratio or the presence of ions?

Plasma temperature and density were measured spectroscopically using Stark broadening and line ratios Saturation ratio and ionization state were computed using these measurements and assuming local thermodynamic equilibrium The saturation ratio is inversely proportional to laser intensity As laser intensity increases, ionization increases but saturation ratio decreases Maximum charge state at 50 ns, 1 mm from Al target, as derived from spectroscopy and assuming LTE. Saturation ratio at 1 mm, derived from spectroscopy and assuming LTE.

Our IFE research is focused on the key issues for IFE chambers and chamber interfaces Prometheus-L Reactor Building 1. Chamber walls that survive long-term exposure 2. A residual chamber medium which allows propagation of targets and beams through it 3. Final optics that survive long-term exposure 4. Cryogenic targets that survive injection and are properly illuminated

The final optic in a laser-IFE plant sees line-of-sight exposure to target emissions Laser-induced damage x-rays ions neutrons and  -rays contaminants Damage threats: 5 J/cm 2 2 yrs, 3x10 8 shots 1% spatial nonuniformity 20  m aiming 1% beam balance Mirror requirements:

We are developing damage-resistant final optics based on grazing-incidence metal mirrors The reference mirror concept consists of a stiff, light-weight, radiation-resistant substrate with a thin metallic coating optimized for high reflectivity (Al for UV, S-polarized, shallow  ) Al reflectivity at 248 nm

Laser damage is thermomechanical in nature: high-cycle fatigue of Al bonded to a substrate S-N curve for Al alloy Basic stability High cycle fatigue Differential thermal stress

Testing is performed at the UCSD laser plasma and laser- matter interactions laboratory 400 mJ, 25 ns, 248 nm

Pure Al can have large grains, resulting in slip plane transport and grain boundary separation (data at 5 J/cm 2, 50 shots)

Several fabrication techniques have been explored to enhance damage resistance Monolithic Al (>99.999% purity) Thin film deposition on polished substrates –sputter coating, e-beam evaporation –Al, SiC, C-SiC and Si-coated substrates Electroplating Surface finishing –polishing, diamond-turning –magnetorheological finishing –friction stir processing Advanced Al alloys –solid solution hardening –nanoprecipitation hardening

Finer-grained electroplated Al withstands higher fluence, but eventually goes unstable At 18.3 J/cm 2 laser fluence:  Grain boundaries still separate  Damage is “gradual” at 18.3 J/cm 2  Mirror survived 10 5 shots At 33 J/cm 2 laser fluence:  Rapid onset (2 shots)  Severe damage (melting)  probably starts with grains

High shot count data extrapolates to acceptable LIDT; end-of-life exposures are still needed In addition, we are continuing to develop improve- ments such as “Al-on-Al”, hardened alloys, etc.

Our IFE research is focused on the key issues for IFE chambers and chamber interfaces Prometheus-L Reactor Building 1. Chamber walls that survive long-term exposure 2. A residual chamber medium which allows propagation of targets and beams through it 3. Final optics that survive long-term exposure 4. Cryogenic targets that survive injection and are properly illuminated

Targets play a central role in many of the critical issues for IFE R 6.5m T ~ 1000C 1. Mass production 500,000/day, $0.25/target, sub-  m uniformity 2. Injection 400 m/s, 1-5 mm accuracy 3. Tracking/steering 20/200  m accuracy, ~64 beams 4. Survival 18˚K target in a 1000˚C turbulent chamber

We collaborate with General Atomics on several target-related tasks 1.Target fabrication indirect drive target layering via external thermal control 2.Target injection sabot transport capsule steering 3.Target tracking/beam steering interface with beam steering system 4.Target survival

Target steering is possible in the chamber using a short-pulse guide laser Use shortest pulse possible for minimum ablation depth: 15 fs Use highest pulse energy to achieve maximum impulse (subject to total power and rep rate constraint) assume 100 kW for 15 ms, 100 kHz – 1 J pulses assuming 1 mm 2 contact, W/cm 2 if the full beam hits W/cm 2 is a more likely value Instead of steering 64 heavy mirrors, why not steer one 4-mg target? Can be accomplished using an annular guide-laser beam in the chamber Biggest concern is amount of ablation needed and degradation of target surface due to that ablation

Analysis of ablation depth and impulse was performed using the 1D Hyades rad-hydro code 850-nm laser pulse, 15 fs FWHM, W/cm nm Au coating, 100  m CH substrate run code until plume heating of surface is negligible and acceleration phase is complete (1 ns) DTCH Au feathered grid 3 nm 16 nodes

Ablation depth and expansion velocity of Au d~2.5 nm total ablation m~2.5  g/cm 2 accelerated to ~2x10 5 cm/s mv~0.5 (g-cm/s)/cm 2 assuming 1 mm 2 contact and 5 mg target, each “kick” results in 1 cm/s correction of the target transverse velocity

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