368. Inertial confinement: more on interaction of laser emission with matter

Posted by Dmitry

April 24, 2009

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Yeah… so, where did we stop last time? I’ve just said another triviality – that laser emission strongly interacts with material of the fuel capsule. There are several mechanisms of this interaction: deflection (ablator and fuel are almost transparent but not quite), absorption and scattering.

Wired Science visits LLNL and National Ignition Facility

As for absorption, the most important mechanisms that play role in the particular physics that we discuss are bremsstrahlung absorption, anomalous effects (like decay of photon into two plasmons) and resonant absorption. The later appears when the plasma density approaches the critical value

$n_{\rm cr}=\frac{m_e}{4\pi{}e^2}\left(\frac{2\pi{}c}{\lambda}\right)^2$ ,

where $\lambda$ is the wave length of laser emission.

Let us discuss absorption in a bit more details… Bremsstrahlung absorption is most important for short wavelength lasers (in what follows we suppose that the flux is of the order $f\sim{}10^{14}{\rm W}/{\rm cm}^2$ ). For $n_e<n_{\rm cr}$ absorption factor is

$\kappa\sim{}10^{-41}Zn_e^2\left(\frac{\rm keV}{T}\right)^{3/2}\left(\frac{\lambda}{10^{-6}{\rm m}}\right)({\rm cm})^{-1}$ .

Bremsstrahlung absorption becomes less important with increase of the flux, since $T\sim{}f^{2/3}$ and $\kappa\sim{}q^{-1}$ .

Resonant absorption is more important (and actually dominates in the interval of fluxes that is available for the present-day technology) for long wavelength lasers. It is called "resonant" since near $n\sim n_{c\rm cr}$ the frequency of plasma oscillations coincides with frequency of the laser beam. This leads to resonant amplification of oscillations in plasma and generation of fast electrons. In practice, more than 50% of the beam energy can be absorbed by this mechanism (it is established experimentally that about 25%-90% of the total energy of the beam gets absorbed for beams with $f\approx 10^{14}\frac{\rm W}{{\rm cm}^2}$ , $\tau_{\rm impulse}\approx{}10^{-9}{\rm sec}$ and wavelengths $(10.6-0.26)\cdot{}10^{-6}{\rm m}$ ). The distribution function of fast electrons is Maxwell-like, with effective temperature about 10 times higher than the temperature of "normal" electrons in plasma.

The most important mechanisms of scattering of laser emission on material of the capsule are Raman and Mandelstam-Brillouin scatterings.

Next time I’ll explain how energy of the beam is transferred at deeper, denser layers of the compressed fuel capsule.

Literature

1. J. Duderstadt, G. Moses, “Inertial confinement fusion”
2. S. Atzeni, J. Meyer-ter-Vehn, “The physics of inertial fusion”
3. For the Russian-speaking part of the audience, I would really recommend to read all articles on thermonuclear fusion in “Fizicheskaya Encyclopedia”, Moscow, 1990-98.

Fellow Craft, Nuclear physics

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