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13Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2016
DIAGNOSTICS OF HOT ELECTRONS GENERATION USING HARD X-RAYS EMISSION FROM METAL TARGETS IRRADIATED BY INTENSE LASER PULSES
OF. Kostenko, N.E. Andreev, O.N. Rosmej*, A. Schönlein**
Joint Institute for High Temperatures RAS, Moscow, Russia, e-mail: olegkost@ihed.ras.ru *GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany **Goethe University, Frankfurt am Main, Germany
Experiments at the GSI with high intensity laser system PHELIX have been aimed at investigation of generation of the 22.1 keV Ag Ka radiation for radiographic applications. The s-polarized laser pulses with wavelength of 1.053 ^m, energy of 80-115 J and average duration of 0.78 ps were focused onto Ag targets under an angle of 10° to the target normal. Laser intensity was varied in a range of 3x1019-1018 W/cm2 by displacement of the targets out of the best focus position. In order to exclude the process of hot electron refluxing from the analysis, the bulk silver target of 3 mm thickness and silver foils of 10 and 100 p,m thickness, attached onto mm thick aluminium or plexiglass substrates, were used. Measurements of the Ka radiation, caused by ionization of silver atoms by laser-accelerated electrons, have been performed using a CCD camera in the single-photon-counting mode, which allows reconstructing a measured spectrum. Bremsstrahlung radiation, caused by collisions of the accelerated electrons with target atoms, was measured by means of a hard X-ray detector based on the radiation attenuation method.
We have developed models of characteristic [1] and bremsstrahlung [2] X-rays generation by laser produced relativistic electrons propagating in a metal target. The models take into consideration energy losses of the electrons, cross-sections of the X-rays generation, as well as an absorption of the X-rays. We consider intensity dependencies of the hot electron temperature, determined by Wilks et al. [3] and by Beg et al [4]. Modeling of a spectrum of bremsstrahlung X-rays from the bulk silver target and calculation of the detector response show good agreement with the measurement at the intensity of 1.6x 1018 W/cm2, at which both scaling lead to close values of the hot electron temperature. Similar modeling at the intensity of 2x 1019 W/cm2 reveal correspondence of the hot electron temperature with the Beg's scaling.
We have determined an intensity dependence of the conversion efficiency of laser energy into hot electrons using measurements of absolute Ka-photon yield per unit laser energy in given direction per unit solid angle. Intensity dependences of the Ka-photon yield from silver targets of different thickness, calculated using determined conversion efficiency with assumption of suppression of hot electron refluxing, describe well features revealed in the experiment. The modeling with refluxing electrons confirm the assumption about suppression of hot electron refluxing in foils deposited on the bulk substrates, both conducting and nonconducting.
Theoretical part of the work performed by the team of JIHT RAS was supported by the grant of the Russian Science Foundation (project No. 14-50-00124).
References
[1] Kostenko O F., Andreev N.E. Quantum Electron. 2013. V. 43. P. 237.
[2] Kostenko O.F., Andreev N.E., Rosmej O.N., Schönlein A. Phys. Plasmas (submitted).
[3] Wilks S.C., Kruer W.L., Tabak M., Langdon A.B. Phys. Rev. Lett. 1992. V. 69. P. 1383.
[4] Beg F.N. et al. Phys. Plasmas. 1997. V. 4. P. 447.
