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Polychromatic x-ray production in helium from afemtosecond high-intensity laser system

K. Ta Phuoc, A. Rousse, L. Notebaert, M. Pittman, J. P. Rousseau, V. Malka, S. Fritzler,S. Sebban, P. Balcou, and D. Hulin

Laboratoire dOptique Appliquee, Ecole Nationale Superieure des Techniques Avancees, Centre National de laRecherche Scientifique, Ecole Polytechnique, Chemin de la Huniere, 91761 Palaiseau, France

J. R. Marques and P. G. David

Laboratoire pour lUtilisation des Lasers Intenses, UMR7605, Centre National de la Recherche Scientifique,Ecole Polytechnique, Commissariat a lEnergie Atomique, Universite Paris VI, 91128 Palaiseau, France

Received April 17, 2002; revised manuscript received October 18, 2002

Polychromatic x-ray radiation has been produced during the relativistic interaction of a 50-TW femtosecondlaser with a helium gas jet. We have characterized the spectrum and the angular distribution of the x-rayemission as well as its dependency on the laser polarization and on the plasma electronic density. We have

observed a broad continuous spectrum peaking at 0.15 keV with a significant tail up to 2 keV. The radiationwas fairly collimated. 2003 Optical Society of America

OCIS codes: 300.6560, 350.5400, 350.5616, 320.7090, 340.7480.

The x-ray radiation ( less than a few kiloelectron volts)produced during the interaction of a femtosecond laserwith matter has been widely studied at moderate laser in-tensities below the relativistic threshold.1 The genera-tion of high-order harmonics from atomic ensembles,2 op-tical field ionization x-ray lasers,3,4 K radiation,

5,6 andthermal x rays7 from strongly ionized plasma have wellbeen characterized. At much higher laser intensities,close to 1020 W/cm2, no x-ray studies have been under-taken in gaseous targets. Many radiative processes areexpected during the relativistic interaction between an ul-trafast laser and matter. Electrons oscillating inside thelaser field, trapped in the plasma wave by self-modulatedlaser wakefield (SMLWF) acceleration or accelerated di-rectly by the ponderomotive force, can participate in theoverall x-ray emission through collisional process. TheLarmor radiation directly emitted by the electrons oscil-lating in the laser field can produce x-rays whose spatialand spectral behaviors are fully governed by thetrajectory811 of the electrons and the laser strength pa-rameter a 0 . Here we report the experimental character-ization of the x-ray emission produced during the interac-tion of a 50-TW femtosecond laser with a supersonic

helium gas jet.The experiment was conducted at the LaboratoiredOptique Appliquee, where we used the 30-fs and 1-J (33-TW) Ti:sapphire laser system. The laser beam, 55 mm indiameter, was focused with an f /5.45 off-axis parabolicmirror onto the front edge of a supersonic gas jet of he-lium or argon. The diameter of the focal spot was 6 mfull width at half-maximum (FWHM) in intensity andcontained 60% of the laser energy. The maximum inci-dent laser intensity (I) was 7 1019 W/cm2, correspond-ing to a laser strength parameter a0 5.6 for the linearpolarization, where a0 eE/m00c 8.5 10

10

m(IW/cm2)1/2. E is the amplitude of the laser field, e is

the electron charge, m0 is the electron mass, 0 is the la-ser frequency, and is the laser wavelength. At such la-ser intensities the helium gas was fully ionized early inthe pedestal of the laser pulse. The x-ray radiation wascollected with grazing-incidence metallic mirrors (nickelor gold) over a solid angle of 1.5 102 sr and was fo-cused onto a back-illuminated CCD camera. Filters werepositioned between the plasma and the detector to block

the laser infrared light. A static magnetic field was in-serted between the plasma and the x-ray spectrometer todeflect the charged particles accelerated in the forwarddirection out of the detector. We selected spectral bandsof the incoming polychromatic x rays by combining thespectral absorption of the filters and the spectral reflec-tivity of the mirrors. Our x-ray spectrometer was sensi-tive to x rays in a bandwidth from 20 eV to 2 keV. Theangular dependence of the x-ray emission was obtainedby rotation of the whole spectrometer around the laser fo-cal spot. A second arm of the laser system was used tooptically probe, at each x-ray acquisition, the propagationof the beam inside the cylindrical gas jet (3 mm in diam-eter). At such high laser intensities, the interaction was

characterized and controlled through the analysis of for-ward Raman scattered light, time-resolved shadowgra-phy, and side Thomson scattering. An electron spectrom-eter was also positioned in the forward direction and wasset to cover the energetic range from 1 to 60 MeV.12

The spectral x-ray distribution obtained on axis (the di-rection of the laser propagation) is presented in Fig. 1.The spectral bandwidths were selected with aluminum,zirconium, titanium, and beryllium filters, which respec-tively provide spectral ranges centered on 45, 150, 350,and 1500 eV. We can see that the output spectrum peakson the bandwidth centered at 0.15 keV with a significant

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tail up to 2 keV. This spectral distribution was recordedwhere the produced x-ray intensity was the highest. Thex-ray intensity actually shows a strong dependency on theposition of the focal spot relative to the front edge of thegas jet (z0 , not shown). The x-ray signal up to 350 eVhas two maximums separated by a minimum when the la-ser is focused right onto the front edge of the nozzle. Thetwo maximums move closer to z0 as the x-ray energy be-comes higher; the x-ray signal peaks on z0 for the spectralbandwidth centered at 1.5 keV, where the laser intensityis the highest.

The angular distribution for the bandwidth centered at 350 eV and at the maximum x-ray intensity is dis-played in Fig. 2 for linear horizontally, vertically, or circu-

larly polarized light. The radiation peaks in the forwarddirection and decreases smoothly down to 40. The di-vergence is found to be independent of the polarization;however, we can see in Fig. 2 that the x-ray intensity issignificantly higher in circular polarization.

We can see in Fig. 2 a significant increase of the signalin the case of circularly polarized light for the spectralband centered at 350 eV. A similar result has been alsoobserved at all the x-ray wavelengths. This feature is incomplete contrast to the process of high-order harmonicsgeneration from atomic ensembles that could occur in thewings of the focal spot where the laser intensity is suffi-ciently weak. In that case the signal would be expectedto completely vanish for circular polarization.2 Brems-

strahlung radiation can be produced following the inelas-tic collision of the electrons with the ions during the in-teraction. The spectral distribution is predicted to beflat, which is clearly not the case in the experiment. Fur-thermore, the on-axis x-ray signal was also found to in-crease linearly with electronic density (Ne) as shown inFig. 3. This linear increase of the x-ray emission cannotaccount for bremsstrahlung radiation. In that case thesignal would increase as the square of the electronic den-sity. However, angular distributions obtained for thelargest angles of observation show a signal that does notdepend on the angle of observation. The inset of Fig. 3

shows that the distribution becomes flat from 35 to 55(the largest angle of observation attainable in the experi-ment), which may be consistent with bremsstrahlung ra-diation. As mentioned previously, a different behavior ofthe x-ray signal is then expected when the electronic den-sity is increased. Figure 3 shows that the x-ray yield forlarge angles of observation has a component increasing as

Ne2. In addition, experiments done in argon at the same

electronic density as in helium did not change the x-rayflux measured on axis, whereas bremsstrahlung emissionwould have scaled as the square of the atomic number.For all the reasons listed above, we can safely rule outbremsstrahlung radiation as the dominant radiative pro-cess in our experiments on axis.

Atomic lines from the high-temperature plasma gener-ated in the interaction region could also participate to thex-ray emission. However, such processes must be takeninto account only for sufficiently high-Z elements. Incase of helium, the electron binding energy is limited to25 eV, which is less energetic than the observed radiationspectrum. Furthermore, the observed spatial distribu-tion is collimated in the forward direction, completely ex-cluding all the isotropic radiative processes such as theemission from this thermal plasma.

Fig. 1. Energy spectrum of the x-ray emission obtained at fulllaser energy (on the front edge of the gas jet). The polarizationof the incident laser is linear. The bars correspond to the spec-tral bandwidth associated with each measurement of the spec-trum. The electron density is 5 1019 cm3.

Fig. 2. Spatial distribution of the x-ray emission at a 0 0.5(front edge of the gas jet) in the spectral bandwidth centered at0.35 keV. Filled diamond, horizontal linear polarization; opendiamond, vertical linear polarization; circle, circular polariza-tion.

Fig. 3. Intensity of the x-ray emission as a function of theplasma electronic density for angles of observation of 0 (dot) and40 (triangle). The inset shows the spatial distribution of thex-ray intensity obtained at an electronic density of 5 1018

cm3. The fits are linear for 0 and second-order polynomial for40.

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The previous observations rule out the collisional pro-cesses and the generation of high harmonics from theatomic ensemble, as well as the radiation from atomiclines. Additional analysis are necessary to understandwhat the physical process is at the origin of the observedx-ray radiation. One of them, the Larmor radiation, isexpected to produce a large flux of radiation under theserelativistic interaction conditions11,13 and could be onecredible candidate.

ACKNOWLEDGMENT

This work was supported by the European Communityunder contracts TMR ERB-FMGE-CT95-0019 and ERB-FMRX-CT96-0080.

A. Rousses e-mail address is rousse@enstay.ensta.fr.

REFERENCES1. I. C. E Turcu and J. B. Dance, X-Rays from Laser Plasmas

(Wiley, New York, 1998).2. T. Brabec and F. Krausz, Intense few-cycle laser fields:

frontiers of nonlinear optics, Rev. Mod. Phys. 72, 545591(2000).

3. B. E. Lemoff, G. Y. Yin, C. L. Gordon III, C. P. J. Barty, andS. E. Harris, Demonstration of a 10-Hz femtosecond-pulse-driven XUV laser at 41.8 nm in Xe IX, Phys. Rev. Lett. 74,15741577 (1995).

4. S. Sebban, R. Haroutunian, Ph. Balcou, G. Grillon, A. Rousse, S. Kazamias, T. Marin, J. P. Rousseau, L.

Notebaert, M. Pittman, J. P. Chambaret, A. Antonetti, D.Hulin, D. Ros, A. Klisnick, A. Carillon, P. Jaegl, G. Jamelot,and J. F. Wyart, Saturated amplification of a collisionallypumped optical-field-ionization soft x-ray laser at 41.8 nm,Phys. Rev. Lett. 86, 30043007 (2001).

5. A. Rousse, P. Audebert, J. P. Geindre, F. Fallies, J. C.Gauthier, A. Mysyrowicz, G. Grillon, and A. Antonetti, Ef-ficient K x-ray source from femtosecond laser-producedplasmas, Phys. Rev. E 50, 22002207 (1994).

6. Ch. Reich, P. Gibbon, I. Uschmann, and E. Frster, Yield op-timization and time structure of femtosecond laser plasmaK sources, Phys. Rev. Lett. 84, 48464849 (2000).

7. T. Mocek, C. M. Kim, H. J. Shin, D. G. Lee, Y. H. Cha, K. H.Hong, and C. H. Nam, Soft-x-ray emission from small-sized Ne clusters heated by intense, femtosecond laserpulses, Phys. Rev. E 62, 44614464 (2000).

8. J. D. Jackson, Classical Electrodynamics, 3rd. ed. (Wiley,New York, 1998).

9. S. Y. Chen, A. Maksimchuk, and D. Umstadter, Experi-mental observation of relativistic nonlinear Thomson scat-tering, Nature 396, 653655 (1998).

10. E. S. Sarachik and G. T. Shappert, Classical theory of thescattering of intense laser radiation by free electrons,Phys. Rev. D 1, 2738 (1970).

11. Y. Ueshima, Y. Kishimoto, A. Sasaki, and T. Tajima, LaserLarmor x-ray radiation from low-Z matter, Laser Part.Beams 17, 45 (1999).

12. V. Malka, J. Faure, J. R. Marques, F. Amiranoff, J. P. Rous-seau, S. Ranc, J. P. Chambaret, Z. Najmudin, B. Walton, P.Mora, and A. Solodov, Characterization of electron beamsproduced by ultrashort-30 fs-laser pulses, Phys. Plasmas 8,26052608 (2001).

13. E. Esarey, S. K. Ride, and P. Sprangle, Nonlinear Thomsonscattering of intense laser pulses from beams and plasmas,Phys. Rev. E 48, 30033021 (1993).

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