Laser Produced Plasmas


Laser Produced Plasmas

When the intense laser light irradiates the solid target, the electric field of the laser heats up the electrons at first and those electrons heat up the ions through collisinos to form a solid density plasma. The incoming laser light interacts with the plasma to generate various physical phenomena (Brillouin scattering, Raman scattering, etc.).The critical density is defined as n=mw^2/4pie^2 with the laser frequency w, electron mass m and charge e. Since the laser cannot propagate beyond the critical density surface formed inside the plasma, strong light pressure is given to the surface. This results in the higher harmonics generation, density profile steepening, and so on. Although a laser produced plasma is not in an equilibrium state, this the only way to study a high density plasma experimentally, otherwise it can be observed only inside stars.

Chirped Pulse Amplification

The development of CPA (Chirped Pulse Amplification) technique fascilitated the research on the interaction between matter and high intensity laser light.
The basic consept of CPA is as follows.
  1. A seed pulse with a very short pulse duration ( < 1ps ) and a low energy ( ~microJ ) is produced at first.

  2. The pulse is temporally streched by a factor of about 1000 or more, with a diffraction grating dispersive line to avoid the nonlinear phase retardation, caused by intensity-dependent refractive index, during amplifications.

  3. A selected pulse is optically amplified with regenerative amplifier to ensure diffraction limited beam quality. The pulse is further amplified to get the energy of ~a few J.

  4. Finally, the pulse is recompressed to the original pulse duration by a grating pair compressor which is complementary to the strecher.

If a 1ps pulse with energy of 1 J is focused into 10 x 10^{-6} cm^2, the intensity of 10^17 W/cm^2 can be easily achieved.

rigid rotor

Experiments with CPA Laser

When a strong laser light irradiates various target, interesting physical processes ocurr with creating a plasma.
The followings are a few topics which might be interesting for undergraduates.
  1. Higher hermonics generation

  2. When a high intensity short pulse laser irrradiates a gas or solid target, coherent XUV and soft X-ray radiation can be geneated as higher hermonics. One interesting application is so called "water window" ( 2.3 nm ~ 4.4 nm). The light wave within this regime can be absorbed not by water but by carbon. It is thought that the light can be used to diagnose DNA of living cells.
  3. Fast ignition scheme in ICF (Inertial Confinement Fusion)

  4. Conventional ICF try to form a hot central core of the fuel to get fusion burn. However, it requires extreme symmetricity of laser radiation. To avoid its difficulty caused by R-T instability, fast ignition utilizes the high intensity short pulse laser to heat up the pre-warmed fuel to get ignition.
  5. Table Top Neutron Source

  6. Recently, the production of neutrons were confirmed by TOF, when D-T gas target was irradiated with high intensity laser. This opens up the new way for the compact neutron source.
  7. Etch rates of optical materials

  8. The maximum intensity obtained with a table top CPA laser is determined by the damage threshold of optical components. Thus, it is important to investigate the damage threshold and related issues. Etch rates of SiO2 was investigated in a wide rage of parameters with a picosecond CPA laser and 300 picosecond laser pulses (details).


References
  • D.Strickland and G.Mourou, Opt.Commun. 56 (1985) 219
  • M.D.Perry and G.Mourou, Science, 264 (1994) 917
  • T.Ditmire, Science, 264 (1999) 917
  • "The Physics of Laser Plasma Interactions", W.L.Kruer, Addison-Weseley, Redwood City, 1988

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