A method for producing coherent x-rays via high-harmonic generation using an intense laser interacting with highly-stripped ions in cavitated plasma wakefields is presented. Two laser pulses of different colors are considered for cavitation and harmonic generation, enabling laser harmonic generation in the sub-nm regime.
Coherent x-ray light sources are of interest for many applications. Laser high-harmonic generation (HHG) is a compact method for producing ultrafast, coherent light, but is limited to the extreme-ultraviolet to soft-x-ray spectral region. In HHG, an ultrashort intense laser is focused into a gas, generating multi-harmonics of the laser frequency. Physically, harmonics are generated by bound atomic electrons that tunnel through the effective potential barrier formed by the atom and laser field, oscillate semi-classically in the laser field, and recombine with the atom, emitting a high-energy photon. Photon energies up to keV have been produced via HHG. Hard x-ray generation is suppressed by ionization and plasma production, limiting the coherence length via plasma-induced phase slippage. Phase-matching to overcome the plasma-induced slippage has been a challenge to further development of HHG x-ray sources.
It this presentation a method for producing hard x-rays via high-harmonic generation using intense lasers is discussed. This method relies on plasma cavitation by an intense drive beam, producing a region without plasma electrons and with ions in a high-charge state, but not fully stripped of bound electrons. An ultra-short pulse laser co-propagating in the plasma-electron-free ion cavity generates laser harmonics. A charged-particle beam driver has been previously considered for ion cavity creation for HHG [Schroeder et al., Phys. Plasmas 15, 056704 (2008)]. In this presentation two laser pulses of different colors are considered: a long wavelength pulse to create an electron free ion cavity, and a short wavelength pulse to generate harmonics via interaction with the deeply-bound electrons. This method enables laser harmonic generation in the sub-nm regime. Examples of hard-x-ray HHG using existing and future laser systems will be presented.
Work supported by the US DOE under Contract No. DE-AC02-05CH11231.
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