Concept and preliminary experiment on protection of final optics in wet-wall laser fusion reactor T. Norimatsu, K. Nagai, T. Yamanaka and Y. Izawa Presented at Japan-US Workshop on Power Plant Studies and Related Advanced Technologies, Oct 9, 2003, San Diego USA
Introduction In a laser fusion reactor with a wet wall, the critical issue is protection of the final optics, especially from neutral metal vapor. Following each laser shot, more than 20 kg of liquid metal evaporates in a reactor of 5 m radius. We are going to use a set of rotating shutters having different rotational speeds to block the major portion of the blast wave from the opposite inner surface. However, some neutral vapor (9mg/shot) will enter the beam duct, possibly contaminating the final optics and promoting erosion of the surface by laser-induced plasma.
Cascade reactor for fast ignition scheme New Koyo has a cascade flow of LiPb to provide fresh, cold surface to obtain quick pumping. The reactor has a hybrid structure to simplify the LiPb flow. The distance between the ceiling and the firing position is set to balance the ablation with condensation. LiPb Compression beam Non condensable gas Ignition beam Thermal out put 400 MJ Rep-rate 3 Hz Target Injector
In wet-wall reactor, protection of final optics from neutralized particles seems most critical. Number density (cm-3) 10 23 10 22 10 21 10 20 10 19 10 18 10 17 10 16 Neutrons α particles Ions Ablated neutral gas Rotary shutters 583.27ns 583.59ns Distance > 30 m Electro magnet 583.63ns 583.68ns 583.72ns 1500m/s 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Position (µm) The speed of blast wave is estimated to be 1500 m/s. Neutralized energetic particles Neutron damage can be reduced by locating the optics 30 m apart. Alpha particles and ions can be removed by a Magnetic field. Ablated metal from inner surface can be shielded by rotary shutters.
Protection scheme of final optics by rotary shutters 0.05Torr Xe or H 2
The dimension and the rotational speed of the first shutter seem acceptable. Beam diameter Rotary shutter Port diameter 1000 mm 30 m 5 m 350 mm 200 mm Mode Central igintion Fast ignit ion Beam Number Laser Energy kj/ beam Beam diamet er (m) Port diamet er (cm) Diamet er of shutter (m) Rotarional speed of first shutter (rps) 32 125 1 17 1 21.65 Compressio n 32 25 0.5 10 0.5 25.47 Heating 1 70 4 60 2 38.21
Timing chart for blast wave and rotary shutters 2500 Temperatrue of inner surface (K) 2000 1500 1000 500 Blaster waves 1st shutter 28Hz 2nd shutter 14Hz 3rd shutter 3Hz Open Open Open v=3,000m/s @20,000K 1st bluster wave from the wall v=1500 m/s 10µ s 100µ s 1ms 10ms 100ms 1s Charged particlies from plasma 1~2 μ s Neutron, alpha, x-rays 10~20 ns Miss fired target Target injection 2nd shot 3rd shot 4th shot
To experimentally simulate Pb vapor diffusion in a beam duct, an electro furnace to make glass shells was employed.
Basic characteristics of furnace 150 Cf. MP. of Pb = 600 K Temperature (C) Mass of Pb (g) 1200 Operation history 20 100 50 P=1 Torr? Pb vapor Temperature (C) 1000 800 600 400 200 15 10 5 Mass of Pb (g) 0 1600 1400 1200 1000 800 600 Temperature (K) 400 200 0 0 0 20 40 60 80 100 120 Time (min) 0.1Torr H2
A lot of Pb particles, drops of dew, was observed on the surface of a quartz rod during a H2 10 Torr, 1400K, 1hr operation. To simulate diffusion of Pb vapor from wet-wall-reactor to the final optics, 25 g Pb was evaporated. No deposition of Pb at areas of T > 800K and T< 600K. A lot of Pb particles ranging < 300 mm at the area of 650K < T < 750K. Cf. MP. of Pb = 600 K 5mm 1 5 0 1 0 0 P=0.1 Torr? Pb vapor 5 0 5mm 0 1600 1400 1200 1000 800 600 Temperature (K) 400 200 0.1Torr H2 Quartz rod
Many Pb particles ranging 0.1 to 30 µm were deposited on the bottom of furnace filled with 10 Torr H 2. Pb deposition area Top view 100 µm Cro ss sectional view 1 µm
Lead deposited on the 500K to 800K surface during a H2 0.1 Torr, 1400K, 1hr run. No dew on the bottom rod. To simulate diffusion of Pb vapor from wet-wall-reactor to the final optics, 14 g Pb was evaporated. No deposition of Pb at bottom area of T > 800K and T< 600K and at top area of T > 750K and T< 500K. Hydrogen gas flow from bottom to top carried the Pb vapor toward colder region. Vacuum 5mm P=0.1 Torr? Pb vapor 5mm 1600 1400 1200 1000 800 600 400 200 Quartz rod 0.1Torr H2
No deposition of Pb was observed on witness plate in 0.1 Torr H 2. It seems that Pb vapor condenses on cold surface before forming aerosol. 3 cm 1400K 100 No increase in absorption was obserbed. Pb vapor rich 80 Pb deposition area 50 cm 800K 500K Transmittance (%) 60 40 20 Glass substrate Top Bottom H2 300K 0 0 200 400 600 800 1000 1200 Wavelength (nm)
Deposition area moved inward during a H2 0.1 Torr, 780K, 1hr run. But no deposition on witness plate No deposition on witness plates 5mm 9.5 g of Pb (1/3 of previous case) was evaporated during 1hr run. P=0.0001 Torr? Pb vapor 5mm Deposition area moved inward. No drop of dew was observed. No deposition on bottom 0.1Torr H2 Quartz rod This result indicates that contamination of final optics can be avoided if the first bluster wave is blocked.
For simplification, theoretical value of Diffusion constant for Pb was used instead of LiPb. 1000 100 10 鉛蒸気圧フィット Vapor pressure (Pa) Vapor pressure of Pb 22865 P T = EXP T T 5 ( ) (23.23 4.09 10 ) Theoretical diffusion constant 1 0.1 y = exp(m1-m2/x+m3*m0) 値 エラー 0.01 m1 23.234 3.0571 m2 22865 1817.8 m3-4.093e-05 0.001302 0.001 カイ2 乗 0.0092075 NA R 1 NA 0.0001 700 800 900 1000 1100 1200 1300 D = 2 v = 2 8kT 6nπδ π m Saturated evaporation rate P 2π mkt 1 2 Temperature
No deposition on >800K area can be explained by re-evaporation. Distribution of Pb vapor in the duct is dominated by wall temperature.(left) No deposition at >800K can be explained by re-evaporation.(right)
The speed of vapor at beam inlet is estimated to be 100 m/s. When the shutter opens, a radial gas flow still develops due to cryogenic effect of the camber wall. The LiPb vapor pressure is estimated to be <0.05 Torr. In the next calculation, 0.1 Torr, 100 m/s were used for safety. Y. Kozaki et al, Presented at IAEA FEC
Pb vapor whose initial speed of 100 m/s can not reach final optics in 0.1 Torr buffer gas. 0.5m 100m/s 100m/s 30m/s The velocity of Pb vapor decreases to 30 m/s before the amplitude of deformation exceeds the thickness of Pb vapor. The vapor enters into the duct by 6m.
Summary In a future laser fusion reactor, final optics at the end of 30m-long beam duct can be protected from metal vapor using a rotary shutter and 0.1 Torr hydrogen gas. The vapor (v=100m/s) will stop within 6m. When the hydrogen gas in the duct is sufficiently low, ( 0.5 Torr for 3 cm diameter duct, 0.05Torr for 30cm duct), formation of Pb aerosol seems to be not very serious.
Remaining issue Synchronization of target injection with rotary shutter. Since it is quite difficult to change rotating speed of the shutter disk, target injection must synchronize with the open timing of the shutter. For example, v=200+/-1 m/s Chamber clearance Because of thermal radiation transfer, blast wave becomes cold before it reach the chamber center resulting rot of particles when it collides at the chamber center.
Rotary shutters impose sever accuracy for the injection velocity. 1 m 1st rotary shutter 24 Hz, 1440rpm Fire timing of the target and open time of the rotary shutters must be synchronized with accuracy of 0.2 ms. Laser port 0.35 m This requires the injection velocity of 200 +/-2 m/s. 52 m/s Laser beam 0.2 m Allowable jitter, 5% of port radius