feasibility of the inertial confinement fusion reactor is the chamber that should ... behavior of the chamber wall under simulated laser fusion condition by the ... ablated from ICF target shell and fuel as the energy deposition ranges of these ions ...
ABLATION AND ITS PRODUCT OF THE FAST IGNITION INERTIAL CONFINEMENT FUSION REACTOR CHAMBER Y. Takashima a, Y. Yamamoto a, Y.Sakawa b, H. Nishimura c, H Azechi c, S Fujioka c and S. Konishi a a b
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Graduate School of Engineering, Nagoya University, Nagoya, Aichi 464-8603, Japan c Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan
Abstract—One of the major problems in the technological feasibility of the inertial confinement fusion reactor is the chamber that should accept pulsed load of radiation, ion particles and debris and be pumped out in the repeated pulsed operation cycle of several Hz. Especially in a fast ignition scenario, Lead (or lithium lead) cone attached to the target fuel pellets adds specific material transfer issues such as deposited on the chamber wall, ablation, and formation of clusters that is suspected to affect the pumping characteristics. Ablated metal particles from the wall are suspected to form various sizes of clusters that fly slower and more difficult to evacuate. Therefore, it is necessary to investigate behavior of the chamber wall under simulated laser fusion condition by the experiments as well as numerical studies. In this study, we have made the preliminary experiments to investigate formation of cluster of plastic and metals. The YAG laser (2J, 10ns) was irradiated on the targets, and the ablated particles are measured using the Thomson parabola, the charge particle collector, and the quadrupole mass spectrometer. Results show formations cluster of ethylene (-CH2-)n. and metals, and ablation of metals by bombardments by ethylene debris. Keywords-ablation; cluster; vaccum; chamber
I.
chamber inner wall, generation and removal of the metallic vapor, cluster, and mist will become important problems [3]. But until now, there is no experiment to investigate and evaluate formation of cluster by ablated particles. Therefore, in this study, we have tried to investigate behavior of ablated particles in the chamber. II.
SCHEMATIC OF LASER ABLATION
High heat load by the intense laser pulse immediately heats up target surface and generates fume of target materials (ablation). These atoms may collide each other and make cluster that is the feature of several atom connected. Figure 1 illustrates ablation and formation of cluster in the ICF chamber. From the simulation studies made by Furekawa et al., it is shown that the liquid metal on the chamber wall is ablated by the heat load deposited by C, H, O, D, T ions ablated from ICF target shell and fuel as the energy deposition ranges of these ions are much shorter than higher energetic X-
INTRODUCTION
In order to realize Inertial Confinement Fusion (ICF) power plant, it is necessary to develop blanket and other system for converting fusion output to electricity, other than the intense laser drivers. From a conceptual design studies of ICF power plat, it is pointed out that pulse repetition rate of more than 310 Hz is required for economically attractive plant[1]. To achieve this objective, the exhaust of the gases and materials from the fuel pellet and vaporized wall materials between pulses becomes an important problem. Especially in a fast ignition scenario, there is a core attached to the target fuel pellets made of lead (or lithium lead) to guide the heating laser to compressed pellet. It adds specific material transfer issues such as deposited on the chamber wall, ablation, and formation of clusters that is suspected to affect the pumping characteristics [2], as ablated metal particles are suspected to form various sizes of clusters that flies slower and more difficult to evacuate. Also when liquid metal is used as
This work is supported by interactive coordinated researches of National Institute of Fusion Science and collaboration researches of Institute of Laser Engineering, Osaka Univ.
1-4244-0150-X/06/$20.00 (C) IEEE
Theory of Laser ablation Laser
CH Target
Pb Pb Pb
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Pb Pb Pb
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Pb Clusters are made of other particles
Figure 1. Schematics of laser ablation ray or α-particles.
In this study we have made two types of irradiation experiments, (1) “direct irradiation” experiments in which intense laser ablated target (plastic and metals), (2) “indirect irradiation” experiments which uses second target, which is supposed to be irradiated by ablated particles from the main target.
As for the main target, plates made of plastic, Pb, Al, and Sn were used, and Pb and Sn were used as for second target. Al target is used to detect cluster ions within measurable mass range of the Thomson parabola and QMS. Sn target is used as it is used in the EUV laser experiments. (A) 1.0E-09
III.
< Plastic target > 0sec 12sec 24sec 35sec 47sec 59sec
EXPERIMENT SETUP 1.0E-10
Figure 2 shows the schematics of experimental setup. We used the EUV laser facilities at Institute of Laser Engineering, Osaka University for these experiments. The pulse YAG laser (2J, 10ns) was irradiated on the target.
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We prepared three types of measurement method, i.e. • Thomson parabola is aimed to measure energy and charge state of ablated ion in 2-D photograph by electric and magnetic deflection picture. It is synchronized to the laser pulse and only catches ablated ions. •
•
Quadrupole mass spectrometer (QMS) will measure ablated particle mass, and can be measure up to 100 amu particle. It is aimed to measure mass of cluster particle and their decay by pumping. This mass scan does not synchronized to the laser pulse as the scanning time (~30sec for 1~100 amu) is much longer than the laser pulse (10ns). Charge particle collector is a simple Faraday cup to measure ion current along time. This measurement is synchronized to the laser pulse.
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(c) Figure 3. Results of quadrupole mass spectrometer. The targets are plastic, plastic and Al respectively for (a), (b) and (c). Laser shot was irradiated between 0 to 12 seconds for (a) (b), .and between 0 to 30 seconds for (c). The plots shown in the graphs are difference from base date without laser pulse. Figure 2. Experimental setup
IV.
RESULTS AND DISCUSSIONS
A. Mass spectrometer Results of QMS measurements are shown in Figure 3. Figure 3(a) shows mass spectrum scan results after the laser shut and (b) shows decay of specific mass signals with time for direct irradiation of the plastic target. All plots shown in figure 3 are difference from the base measurement before laser shut. From figure 3(a), mass signals of (14n± 2) which are corresponds to (CH2 )n were clearly observed. It is found in the Figure 3(b) that the lightweight cluster decrease quickly than heavyweight one. This suggests that clusters happen to make recombination at surface or adsorb to the chamber wall of the surface. The results for direct irradiation of Al target are shown in Figure3 (c), where existent of small amount of 27, 54, and 81 mass number are observed. These masses are corresponds to Al(1-3) . Al clusters decrease as quickly as plastic cluster.
B. Thomson parabola Photographs obtained from the Thomson parabola are shown in figure (a), (c), (d) for direct irradiation of plastic, Al, and Pb target, respectively. High energy ions with different charge states are observed in each measurement. Figure 4(b) shows calculated carbon ion trajectories corresponding to the figure 4 (a) setup of electric and magnetic fields, and shows that the measured bright lines are corresponding to different charge state of carbon ions. C. Charge Collector Figure 5 (a), (b) show signals of charge collector for indirect irradiation experiments for Pb, and Sn secondary target, respectively. In these experiments, the secondary target is setup at 45° angle and a few mm apart from the main target, and the charge collector is placed 102° angle so that it may not collect ablated particles of the main target directly. The setup is shown in figure 2. It is seen in figure 5 that • peaks corresponding to ablated particle from the main target (Plastic (CH2)n) are detected,
Plastic
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(b) Al
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Pb
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Figure 4. Photographs of Thomson parabola measurements (a)plastic target, (c)Al target, (d)Pb target, and result of carbon ion trajectory calculation (b) corresponding to the measurement conditions of (a).
• ablation of secondary target (Sn, Pb) by ablated plastic ions are detected, and • Pb particle is detected slightly later than Sn particle because of mass difference.
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V.
SUMMURY
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Preliminary experiments for measurement of cluster formation of ablated ions were carried out using the intense YAG laser. From results of mass spectrometer scan, cluster of ethylene (-CH2-)n. is observed in irradiation of plastic target, and indication of Al cluster is observed in irradiation of Al target. Ablation of metal surfaces bombard by ethylene debris formed by irradiation of plastic target is measured by the charge collector. From the Thomson parabola detector, existence of various charge state ions ablated from metal surfaces are observed. Further investigation to identify the amount and energy of ablated particle is underway to evaluate the effects of cluster formation to the pumping speed.
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REFERENCES [1]
[2]
[3]
H.Furukawa et al., “Simulation on interaction of X-ray and charged particles with first wall for IFE reactor.” in Fusion Engineering and Design 2005 Elsevire Science B.V. A.Hassanein et al., “Chamber wall response to target implosion in inertial fusion reactors;new and critical assesements.” In Fusion Engineering and Design 2002 Elsevire Science B.V. J.J.R Reperant et al., “Studies of tubrent liquid sheets for protecting IFE reactor chamber walls” in Fusion Engineering and Design 2002Elsevire Science B.V.
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(b) Figure 5. Waveforms measured by charge particle collector for irradiation of plastic target with metal secondary target. The material of secondary target is (a) Sn, (b) Pb, respectively.
