Estimation on contamination of final optics with metal vapor in laser fusion reactor with liquid wall

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Formation of aerosol and contamination of final optics with metal vapor in a future laser fusion reactor with a liquid wall were experimentally and numerically simulated. The result indicated that formation of aerosol was not so critical if the back pressure in the reactor is less than 100 Pa. Contamination of the final optics at the end of a beam duct can be prevented with low-pressure buffer gas,In a future laser fusion rector KOYO-F, the final optics will be located at the end of a 30 m long beam duct to prevent damage due to neutrons. The radius of the reactor is 3m. In a case of fast ignition mode, the diameters of the beam duct for compression lasers and heating laser are 15 cm and 60 cm, respectively. After micro fusion explosion, energetic ions can be shielded with a magnetic filed. Lot of neutral metal vapor (10 kg in a case of KOYO-F) evaporates from the first liquid wall due to alpha heating from the micro fusion. The major part of the neutral metal vapor can be shielded wit … More h a set of three synchronized rotary shutters that opens 3 ms for laser irradiation every 4 Hz. The flow velocity of vapor toward the beam duct is 100 m/s at the time when the shutter opens. Contamination of the final optics due to this flow is important issue in the laser fusion reactor with a liquid wall.To experimentally simulate the influence of vapor coming into the beam duct, we made a 1/5 model of actual size. The model beam duct is connected to an electric furnace whose temperature is 1100 C to produce sufficient vapor pressure while the temperature of reactor is 600 C. The temperature at the other end of the beam duct was kept at room temperature. A witness glass plate was located at the room temperature end as the substitution of final optics. Since the phenomena depends on the ratio of the mean free path of molecules to the diameter of the beam duct, the pressure in the beam duct was kept 5 times higher than that in the actual reactor.As the result, 1)distributions of Pb in the beam duct were similar in both cases where Pb and LiPb were used as the vapor source. 2)No deposition of Pb was observed on the witness plate when the pressure in the beam duct was 10 to 100 Pa. No formation of aerosol, whose diameter is around 0.1 μm, was observed at this pressure range. 3)When the pressure was close to 1 kPa, formation of aerosol was observed. We conclude that contamination of the final optics in the future laser fusion reactor with liquid wall can be prevented if the pressure of buffer gas is 10 to 100 Pa. Less