7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Nonlinear Compton Scattering between a Laser-Accelerated multi-GeV ElectronBeam and an Ultrahigh Intensity Laser
Chang Hee Nam
Center for Relativistic Laser Science, Institute for Basic Science, Korea;
Dept. of Physics and Photon Science, Gwangju Institute of Science and Technology, Korea
chnam@ibs.re.kr
Ultrashort high power lasers, based on the chirped pulse
amplification technique, have prompted the research onstrong field physics. At CoReLS a 20 fs, 4 PW Ti:Sapphirelaser was developed in 2017 [1] and applied for producing
multi-GeV electron beams using the laser wakefield
acceleration scheme. By tightly focusing this laser after
wave-front correction, we demonstrated the recordbreaking laser intensity of 1023 W/cm2 in 2021 [2]. Withan ultrahigh intensity laser, physical processes in strong
field quantum electrodynamics (QED), such as electronpositron pair production from vacuum and light-by-lightscattering, can be tackled. These processes weretheoretically proposed a century ago, but could not berealized in experiments.
As part of strong-field QED research, we explorednonlinear Compton scattering between a multi-GeV
electron beam and an ultrahigh intensity laser in an alloptical setup in Fig. 1. We generated a multi-GeV electron
bunch with the CoReLS PW laser using the laser
wakefield acceleration (LWFA) scheme. When a 5-cm gas
cell of He, mixed with 3% Ne, was driven by a PW laser,
an electron beam could be accelerated by the wakefield
formed in the He plasma and stable electron beams ofenergy up to 3.5 GeV with an energy spread as small as
1% could be produced. We made this electron beam
scatter with an ultrahigh-intensity laser beam, producing
Compton gamma rays. As an ultra-relativistic electron
beam sees the laser photon as a Lorentz-boosted hard xray photon, this scattering is a Compton scattering. In thelab frame, the Compton scattered photon becomes a
gamma-ray photon.
The beam profile and energy spectrum of gamma raysfrom the Compton scattering were measured with two
detectors made of scintillating crystals, as shown in Fig. 1.
The gamma-ray signals from the Compton scattering weremeasured with LYSO scintillation detectors – a single
crystal LYSO for imaging and a pixelated LYSO forenergy spectrum. Using these gamma-ray detectors, weobserved broad gamma-ray spectra extending overhundreds of MeV. We could confirm the nonlinear
Compton scattering between a GeV electron and several
hundred laser photons by obtaining the gamma-ray signal
far exceeding the cutoff energy of the linear Compton
scattering between an electron and one laser photon. In
addition, the gamma-ray beams have a low divergence (≈1mrad), small source size and ultrashort duration, thus
exhibiting an ultrahigh brilliance. Such high energy
gamma beams open up new research possibilities infundamental physics and nuclear photonics. We present
our experimental results on NCS along with the analysisof measured gamma-ray signals.

Fig.1 Experimental setup of all-optical nonlinear
Compton scattering.

References[1] J. H. Sung et al., “4.2 PW, 20 fs Ti:Sapphire Laser at
0.1 Hz,” Opt. Lett. 42, 2058 (2017).
[2] J. W. Yoon et al., “Realization of laser intensity over1023 W/cm2,” Optica 8, 630 (2021).