High order harmonics from relativistic electron spikes

Abstract

A new regime of relativistic high-order harmonic generation has been discovered (Pirozhkov 2012 Phys. Rev. Lett. 108 135004). Multi-terawatt relativistic-irradiance (>10 18 W cm −2 ) femtosecond (∼30-50 fs) lasers focused to underdense (few × 10 19 cm −3 ) plasma formed in gas jet targets produce comb-like spectra with hundreds of even and odd harmonic orders reaching the photon energy of 360 eV, including the 'water window' spectral range. Harmonics are generated either by linearly or circularly polarized pulses from the J-KAREN (KPSI, JAEA) and Astra Gemini (CLF, RAL, UK) lasers. The photon number scalability has been demonstrated with a 120 TW laser, producing 40 μJ sr −1 per harmonic at 120 eV. The experimental results are explained using particle-in-cell simulations and catastrophe theory. A new mechanism of harmonic generation by sharp, structurally stable, oscillating electron spikes at the joint of the boundaries of the wake and bow waves excited by a laser pulse is introduced. In this paper, detailed descriptions of the experiments, simulations and model are provided and new features are shown, including data obtained with a twochannel spectrograph, harmonic generation by circularly polarized laser pulses and angular distribution. S Online supplementary data available from stacks.iop.org/NJP/16/093003/ mmedia Keywords: relativistic laser plasma, high-order harmonics, relativistic electron spikes, gas jet target Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2 New J. Phys. 16 (2014) 093003 A S Pirozhkov et al

High Power Laser Facilities at the Kansai Photon Science Institute

“…By using an f/10 optic, electron acceleration using a gas-jet, gas-cell, and capillary can be conducted for stable electron acceleration [4], and relativistic high-order harmonic generation experiments [5,6] are currently under consideration. In addition, two laser beam experiments will be easily implemented because our large compressor can accept dual beams with a small diameter (~100 mm).…”

Characterisation of a laser plasma accelerator x-ray source size using a Kirkpatrick-Baez microscope

“…Laser plasma accelerators can produce x-rays with unique properties and with energies ranging from ∼ 50 eV -∼ 10s KeV. [2][3][4][5][6] Previous experiments have used the SCAPA 40 TW Ti:Sapphire laser to irradiate a gas jet with density ∼ 10 18 cm −3 , producing x-rays with a critical energy ∼ 13 KeV. These hard x-rays are emitted from the plasma bubble due to the injection and subsequent oscillation of electrons along the laser polarization in the cavity.…”

“…7 Soft x-ray harmonic comb emission has also been reported from the interaction of the bow wave and the high density electron sheath in a laser plasma accelerator. 3,4 This investigation reports on the characterisation of a Kirkpatrick-Baez microscope developed in collaboration with the Centro de Laseres Pulsados (CLPU) to image the plasma cavity bubble and provide an insight into the bubble dynamics of a laser plasma accelerator.…”

Laser Requirements for High-Order Harmonic Generation by Relativistic Plasma Singularities

“…In the new regime of coherent X-ray generation [1,2], intense (>10 18 W/cm 2 ) high-power (multi-TW) femtosecond laser pulses focused onto gas targets propagate through underdense plasma (electron density n e~1 0 19 cm −3 ) and produce an electron-free cavity and bow wave [3]. The resulting multi-stream relativistic plasma flow leads to the formation of density singularities (cusp catastrophes) located at the joining of the cavity wall and the bow wave front.…”

“…This results in constructive interference, i.e., X-ray yield proportional to the number of electrons squared. The large number of photons together with the small source size and attosecond duration provide very high brightness, e.g., 10 27 photons/mm 2 •mrad 2 •s in 0.1% bandwidth at a wavelength of 18 nm [6].…”