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Spatial and pulse shape dependence of K$$_alpha$$ source from high contrast fs laser plasmas in regime of Relativistic Engineering

Chen, L.-M.; Koga, J. K.; Kando, Masaki; Kotaki, Hideyuki; Nakajima, Kazuhisa; Bulanov, S. V.; Tajima, Toshiki; Xu, M. H.*; Li, Y.-T.*; Dong, Q. L.*; Zhang, J.*

Interaction of intense Ti: Sapphire laser with Cu foil targets has been studied by measuring hard X-ray generation. Hard X-ray spectroscopy and K$$_alpha$$ X-ray conversion efficiency ($$eta_K$$) from Cu plasma have been studied as a function of laser intensity via pulse duration scan (60 fs $$sim$$ 600 fs), laser pulse energy scan (60 mJ $$sim$$ 600 mJ) and target displacement scan from best focus. For intensity I $$>1times 10^{17}$$ W/cm$$^2$$, the Cu $$eta_K$$ keep on increasing to reach a maximum value of $$1times 10^{-4}$$ at an intensity $$I = 1times 10^{18}$$ W/cm$$^2$$. The focusing is varied widely to give a range of intensities from 10$$^{15}$$ W/cm$$^2 sim 10^{18}$$ W/cm$$^2$$. Comparing to a recent publication, two individual emission peaks are obtained, one is at best focal spot and the other is at larger target offset corresponding to $$sim 10^{15}$$ W/cm$$^2$$. Each peak is corresponding to different energy absorption mechanism. In addition, when we introduce slightly detuning of compressor gratings at the best focal condition, it shows $$eta_K$$ generated by negatively skewed 100 fs pulse width laser irradiation reach $$5times10^{-4}$$ and almost 7 times greater than the case of positively skewed pulse. Vacuum Heating is greatly stimulated in this case and preciously control of pre-plasma is the key factor in tuning control of X-ray emission in relativistic fs regime.



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