Quantum Beam Science Research

5-6

Relativistic Harmonics from Tenuous Plasma－ Discovery of a New Compact Coherent X-ray Source －(a)(c)

Multi-TWlaser(room scale)

eamCoherent X-ray beam

10λ0 y10λ0 x

Gas jet(b)700CCD Counts 250
80λ(nm)1690 100
1412150
108High-harmonicsourcez, ne
10λ0Laser b200 ℏω(eV)
6Electronspike

Cane

BowwavevityLaser

4Fig.5-14 New regime of high-order harmonic generation by relativistic laser focused onto gas jet target(a) Experimental setup. (b) Typical single-shot raw data. (c) 3D PIC simulations demonstrating electron density spike at the jointof the boundaries of the wake and bow waves and location of high-harmonic source.

Bright X-ray sources are necessary for fundamental research
and applications in the life sciences, material sciences,
nanotechnology, and other fields. The ultimate performancerequires temporally and spatially coherent X-ray pulsesthat are compressible to the shortest durations, focusable tothe tiniest spots, and capable of producing fine interference
and diffraction patterns. Two broad classes of bright X-ray
sources are available, those based on large-scale accelerators(synchrotrons, X-ray free-electron lasers) and those based oncompact lasers (e.g., laser plasma X-ray emission, plasmabased X-ray lasers, atomic high-order harmonics, and betatronsources). The great advantages of the laser-based sources
are their accessibility for university-scale laboratories andtheir ultrashort pulses with durations from picoseconds
down to <100 as (10-16 s). However, because of fundamental
limitations, it has been overwhelmingly difficult to achievelaser-based bright coherent X-ray sources with keV andespecially multi-keV photon energies. Our aim is to develop
a new generation of such sources to create new fields offundamental research and applications.
We discovered a new regime of high-order harmonic
generation by high-power (10–200 TW) relativistic irradiance(>1018 W/cm2) femtosecond lasers (~30–50 fs) focused on
gas jet targets, as shown in Fig.5-14(a). Comb-like spectrawith hundreds of even and odd harmonic orders (Fig.5-14(b)),

reaching a photon energy of 360 eV, including the “waterwindow” spectral range, were generated by either linearlyor circularly polarized pulses from the J-KAREN (KPSI,JAEA) and Astra Gemini (CLF, RAL, UK) lasers. A 120 eV
harmonic contained up to 4 × 109 photons. Using particle-incell (PIC) simulations and mathematical catastrophe theory,
we introduced a new mechanism of harmonic generation bysharp, structurally stable, oscillating electron spikes at thejoint of the wake wave and bow wave boundaries, as shown in
Fig.5-14(c).
Our compact bright coherent X-ray source can be built
on a university-lab-scale repetitive laser and accessible,
replenishable, and debris-free gas jet target. Importantly,
the photon energy and number of photons are scalable withthe laser power. This will affect many areas of fundamental
research and applications requiring a bright X-ray/extreme
ultraviolet source for pumping, probing, imaging, or
attosecond science.
We acknowledge the financial support from the Japan
Atomic Energy Agency (JAEA) (Research and Development
Adjustment Funding for Exploratory Research, “Relativisticextreme ultraviolet harmonics from underdense plasma,” 2009)
and the Japan Society for the Promotion of Science (JSPS)[KAKENHI Grant-in-Aid for Young Scientists (B) No.23740413,“New high harmonic generation mechanism,” 2011].

Reference
Pirozhkov, A.S. et al., Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes, Physical Review Letters, vol.108, issue 13, 2012, p.135004-1-135004-5.

JAEA R&D Review 201367