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物理 ： 実用的なレーザー核融合
レーザーで核融合を可能にする新たな方法を報告する。現在の発電の代替方法の一つとして、太陽や核爆発の反応と同じである核融合の利用がある。しかし、原子核が融合して莫大な量のエネルギーを放出する前に、5000万℃程度にまで加熱しなければならないため、この方法は困難である。人工の太陽を作り出すには、水泳プール大の量の、高いエネルギーをもつ、プラズマ形態のトリチウム（三重水素、通常の水素の3倍の質量をもつ）燃料を磁場トラップし、融合に必要な温度にまで加熱するというのが一つの方法である。可能性としてはるかに安価なもう一つの方法が、以前から考えられており、それは、高性能レーザーを用いて、少量の超高密度燃料（この場合は重水素）を融合することである。以前、大阪大学の児玉了祐らが、この技術の可能性を報告したが（Nature 412号,798-802; 2001）、大規模での可能性については触れていなかった。今回、最も強力なレーザーの一つを利用し、児玉たちは、工業スケールでの発電に必要な温度にまで核融合燃料を加熱することが、原理的に可能であることを示した。
29 August 2002
Nature 418, 933 - 934 (2002); doi:10.1038/418933a
Nuclear fusion: Fast heating scalable to laser fusion ignition
Rapid heating of a compressed fusion fuel by a short-duration laser pulse is a promising route to generating energy by nuclear fusion1, and has been demonstrated on an experimental scale using a novel fast-ignitor geometry2. Here we describe a refinement of this system in which a much more powerful, pulsed petawatt (1015 watts) laser creates a fast-heated core plasma that is scalable to full-scale ignition, significantly increasing the number of fusion events while still maintaining high heating efficiency at these substantially higher laser energies. Our findings bring us a step closer to realizing the production of relatively inexpensive, full-scale fast-ignition laser facilities.
In advanced laser ignition of fusion3, 4, high-density energetic electrons generated by petawatt lasers instantaneously heat a compressed fusion fuel to its ignition temperature with high coupling efficiency5. We tested fast heating by a petawatt laser6 and the GEKKO XII laser systems on targets in which a hollow gold cone (30° or 60° angle) was inserted into a deuterated polystyrene ('CD') shell (500 μm diameter, 7 μm thick)2.
The shell was imploded using nine beams of the laser system operated at a wavelength of 0.53 μm and with an energy of 2.5 kJ for 1.2-ns flat-top pulses. The fast-heating laser was injected into the cone's interior and generated energetic electrons at the end of the cone, at the stagnation of the shell compression with a power of 0.5 petawatts (PW). The imploded core plasma was created near to the centre of the shell, close to the tip of the cone; the compressed density was estimated by using an X-ray backlight method2 as 50ﾐ100 g ml-1 for cores of diameter 30ﾐ50 μm. A single laser oscillator7 was used for both laser systems to provide perfect synchronization between shell compression and fast electron heating.
To quantify the heating of these highly compressed plasmas, we measured the increase in thermonuclear neutron production as a function of the injection timing of the heating pulse, with respect to the time of peak compression. Neutrons are generated by fusion of two deuterium nuclei to form a helium nucleus (atomic configuration, d(d,n) 3He) in the compressed CD plasma.
Neutron energy spectra were obtained using time-of-flight scintillator/photomultiplier detectors. The coincidence of signals from detectors at different distances and angles confirmed that the neutrons were thermonuclear in origin. Neutron enhancement was about three orders of magnitude at 0.5 PW, compared with neutrons obtained with no heating pulse (2ﾐ5 104 for a 1.2 flat-pulse implosion).
Figure 1a shows this enhancement as a function of injection timing of the heating pulse. The timing of heating was checked with X-ray streak images, as well as the injection timing of the pulse to maximum compression, from hydrodynamic simulations of the shell implosion. Enhancement was evident during 40 ps, which corresponds to the stagnation time of the imploded plasma; the heating pulse is 0.6 ps, which is two orders of magnitude shorter than the stagnation time. These results indicate that heating for ignition might be achieved by using pulses that are close to the duration of stagnation.
Figure 1 Fast heating of highly compressed plasmas with a petawatt (PW)-class laser pulse. Fulllegend
High resolution image and legend (92k)
If a gaussian profile is fitted to the neutron spectrum (Fig. 1b), the spectral width of the 0.5-PW heating shot is 90 5 keV, which is greater than that (60 keV) for 0.1-PW heating. The width of the 0.1-PW-heating spectrum was similar to that for no heating pulse, which was less than, or close to, the spectral resolution corresponding to the ion temperature of 0.4 keV. Taking into account the spectral resolution, the width (90 keV) for 0.5-PW heating corresponds to an ion temperature of 0.8 0.1 keV, indicating that the temperature of core plasmas could be doubled by this heating.
This finding is consistent with the enhancement, by three orders of magnitude, of neutron yield through heating; this indicates that the temperature increases from 0.3ﾐ0.4 to 0.8 keV. Our results are also consistent with the change in intensity and spectra of X-rays from heated core plasmas. The intensity increases by a factor of 1.5ﾐ2.0 compared with the absence of a heating pulse, and a continuum slope of the X-ray spectra (3ﾐ4 keV), temporally resolved with an X-ray streak camera, shows that the increase in temperature (1 0.1 keV compared with 0.4 keV) is more than doubled.
Neutron yields are summarized in Fig. 1c for 0.6-ps laser pulses. Simple predictions of the neutron yield normalized to the yield without heating from energy conservation are also shown as a function of the heated laser energy, for the coupling from laser to the core plasma of 15% and 30%. The yield increases with the energy of the heating laser, implying almost constant coupling from the laser to the core plasma. However, there may be a small decrease in the coupling, from about 30% to 20%, as the laser power is increased from 0.1 PW to 0.5 PW. This could be due to an increase in the energetic electron temperature, resulting in a reduction in the stopping power of electrons for a fixed spot diameter.
Efficient fast heating of imploded plasmas has been accomplished with a petawatt laser at powers that are almost equivalent to those required in fast-ignition conditions. The period for sufficient heating is similar to the stagnation time (40 ps), suggesting that the heating laser's energy could be increased to ignite the fuel with a heating pulse of up to 10ﾐ20 ps or more at similar irradiance. It may eventually be possible to ignite a compressed deuteriumﾐtritium fusion plasma with a relatively inexpensive fast-ignition facility comprising a petawatt-class laser.
R.KODAMA*, H.SHIRAGA*, K.SHIGEMORI*, Y.TOYAMA*, S.FUJIOKA*, H.AZECHI*, H.FUJITA*, H.HABARA†, T.HALL‡, Y.IZAWA*, T.JITSUNO*, Y.KITAGAWA*, K.M.KRUSHELNICK§, K.L.LANCASTER†§, K.MIMA*, K.NAGAI*, M.NAKAI*, H.NISHIMURA*, T.NORIMATSU*, P.A.NORREYS†, S.SAKABE, K.A.TANAKA, A.YOUSSEF*, M.ZEPF¶ & T.YAMANAKA*
*Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita Osaka 565-0871, Japan
†Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UK
‡University of Essex, Colchester CO4 3SQ, UK
§Blacket Laboratory, Imperial College, London SW7 2BZ, UK
Institute of Laser Engineering and Faculty of Engineering, Osaka University, Suita Osaka 565-0871, Japan
¶Queen's University of Belfast, Belfast BT7 1NN, UK
The Fast-Ignitor Consortium
e-mail: ryo@ile.osaka-u.ac.jp
References
1. Tabak, M. et al. Phys. Plasmas 1, 1626-1634 (1994).|Article|ISI|
2. Kodama, R. et al. Nature 412, 798-802 (2001).|Article|PubMed|ISI|
3. Norreys, P. et al. Phys. Plasmas 7, 3721-3726 (2000).|Article|ISI|
4. Kodama, R. et al. Phys. Plasmas 8, 2268-2274 (2001).|Article|ISI|
5. Key, M. H. Nature 412, 775-776 (2001).|Article|PubMed|ISI|
6. Kitagawa, Y. et al. Phys. Plasmas 9, 2202-2207 (2002).|Article|ISI|
7. Yoshida, H. et al. Conf. Laser Elec. Opt. 2002 4, 402-403 (2002).
Competing financial interests: declared none.
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Nature Macmillan Publishers Ltd 2002 Registered No. 785998 England.
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