24E01
Ultra-short X-ray and electron generation via laser-plasma interaction
レーザープラズマ相互作用による超短パルスX線及び電子ビーム生成
H. Kotaki1, M. Kando1, A. S. Pirozhkov1, K. Kawase1, T. Zh. Esirkepov1, Y. Fukuda1,
H. Kiriyama1, H. Okada1, I. Daito1, T. Kameshima1, Y. Hayashi1, M. Mori1, J. K. Koga1, H. Daido1,A. Ya. Faenov1, T. Pikuz1, J. Ma1, L.-M. Chen1, E. N. Ragozin2, T. Kawachi1, K. Sato1,Y. Kato1, T. Tajima1, and S. V. Bulanov1
小瀧秀行1, 神門正城1, Alexander S. Pirozhkov1, 川瀬啓悟1, Timur Zh. Esirkepov1,福田祐仁1, 桐山博光1, 岡田大1, 大東出1, 亀島敬1, 林由紀雄1, 森道昭1,甲賀ジェームス1, 大道 博行1, Anatory Ya. Faenov1, Tatiana Pikuz1, Jinglong Ma1,Liming Chen1, Eugene N. Ragozin2, 河内哲哉1, 佐藤克俊1, 加藤義章1, 田島俊樹1,Sergei V. Bulanov1
1

Japan Atomic Energy Agency, 8-1-7, Umemidai, Kizugawa, Kyoto 619-0215, Japan
日本原子力研究開発機構 〒619-0215 京都府木津川市梅美台8-1-7
2
P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Leninsky prospekt 53, 119991 Moscow, Russia
Ultra-short quantum beams can be generated by laser-plasma interaction. Ultra-short X-ray pulses andelectron beams are generated via flying mirror and laser-acceleration. Laser light reflection by arelativistically moving electron density modulation (flying mirror) in a wake wave is investigated. A
counter- propagating laser pulse is reflected and upshifted in frequency with a multiplication factor of
37–66, corresponding to the extreme ultraviolet wavelength. A laser pulse oscillates a laser-accelerated
electron beam in the laser field. From electron oscillations in the image of the electron energy distribution,
the electron beam pulse width is estimated to be 1.7 fs (rms).

1. Introduction
Short, coherent x-ray pulses are important tools
for applications [1]. Reflection from flying mirror[2,3] is one of the schemes to generate ultrashortpulses. The flying mirror is dense electron shells
formed in a breaking wake wave, excited by theintense, short pulse laser in underdense plasma.
These shells partially reflect a counterpropagating
laser pulse. The reflected pulse inherits propertiesof the incident electromagnetic wave, such ascoherence, polarization, and pulse form. The
reflected radiation wavelength is shortened due tothe double Doppler effect.
Laser-acceleration [4] has the possibility togenerate an ultrashort electron beam [5] with highquality [6–9]. In applications of the electron beam,
it is necessary to characterize the electron beam.
When the pulse length of the electron beam isshorter than the plasma wavelength and the electron
beam is in the laser pulse, by using electron
oscillation, the energy spectrum is converted to thepulse width of the electron beam by the laser field.
In this paper, we present the result of the flying
mirror experiment in a configuration with thehead-on collision of two laser pulses, and the resultof the pulse width measurement of thelaser-accelerated electron beam.

2. Flying mirror
The experiment was performed with aTi:sapphire laser named J-KAREN [10]. The laser
pulses were split after the compression into driver(to excite the wakefield) and source (to be reflected)
pulses. The typical energies of the driver and source
pulses were 400 and 42 mJ. Pulse durations of thedriver and source pulses were 27 and 34 fs. The
laser pulses were focused onto a helium gas-jet. The
laser pulses were set to collide head on. The
estimated peak irradiances in vacuum were
6.5×1018 W/cm2 and 1.2×1017 W/cm2 for the driverand source. The maximum plasma density was
4.7×1019 cm-3. The reflected light was resolved by aFig,1 (a) Raw CCD image. (b) Spectra in thediffraction order of m=1 and m=-1. (c) CCD
counts within the 1st diffraction order.

normal incidence spectrograph (NIS). The NIS
covered the observation angles α from 9˚ to 17˚.
The imaging magnification was 2.37.
The observed signal in the NIS is shown in Fig. 1.
In the raw image of the CCD shown in Fig. 1(a).
The similarity of the spectra of m=1 and m=-1
shown in Fig. 1(b) confirms that the detected signal
was resolved by the transmission grating, and was
not from high energy photons due tobremsstrahlung by fast electrons. From theobserved spectrum, we determine the range of γph in
the actual shot. In order to obtain the same
wavelength range of 12.8–22.0 nm in theobservation angles of α=9˚–17˚, the γph should be in
the range of 5–7. The factor γph can also be
estimated from the simplified relationshipγph~(ncr/ne)1/2, where ncr and ne are the criticaldensity and the plasma density. If we use themeasured density, (ncr/ne)1/2~10. Because thisproduces a rough estimate of γph, this estimation isin accordance with the measurements.
3. Pulse width of laser-accelerated electron beam
The experiments have been performed with aTi:sapphire laser named JLITE-X [11]. The laser
pulse with 130 mJ energy was focused onto a3-mm-diameter Nitrogen gas jet. The pulse duration
was 40 fs. The peak irradiance in vacuum was
7.3×1017 W/cm2. The electron energy is measured
with a magnetic spectrometer composed of a dipole
magnet, a scintillating screen, and a CCD camera.
A quasi-monoenergetic electron beam isgenerated in the self-injection scheme by using aNitrogen gas target around the plasma density, ne,of 4.0×1019 cm−3 assuming 5 ionizations of N2. Theelectron beam oscillates in the electric field of thelaser pulse. In energy space, when the electron
beam is in the acceleration or deceleration phase,the electron oscillation can be observed and theelectron energy spectrum can be converted to theelectron pulse width. We can measure the pulse
width by measuring the energy spectrum.
Figure 2 shows the typical image of an energy
distribution at ne = 4.4×1019 cm−3 when the laser
pulse has S (vertical) -polarization. The oscillation
has an angle of 16 mrad. When the laser pulse has
P-polarization, the image of the energy distribution
has no oscillation, because the direction of the laser
field oscillation is parallel to the energy axis. Thewave structure of the energy spectrum depends on
the laser frequency. The pulse width (FWHM) of
the electron is 1.5-cycles at a wavelength of 800 nm.
The pulse width is 1.7 fs (rms).

Fig.2 (a) Typical image of an electron beam inenergy space and (b) a projection of the image
onto the energy axis at ne=4.4×1019 cm-3.

4. Conclusions
We have conducted a laser light reflection
experiment from a wake wave. With and without
the source, there is deference clearly. The flying
mirror signal is observed in the NIS. Stable andmonoenergetic electron beams have been generated
by using a Nitrogen target. In the image of theenergy spectrum, the electron oscillation by thelaser field is observed. The energy spectrum can beconverted to the electron pulse width. The result
indicates a 1.7 fs (rms) pulse width for the electronbeam.
Acknowledgments
This work was supported by the Japan Society
for the Promotion of Science under a Grants-in-Aidfor Scientific Research (Contract No. 23600010,20244065, and 19740252).
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