“…In fact, for wider exploration and manipulation of electron dynamics in a vast spectrum of natural phenomena, attosecond or a few fs keV pulses are demanded. Several schemes have been proposed and/or demonstrated to generate an ultrashort keV xray pulse: the relativistic Doppler shift of a backscattered laser pulse by a relativistic electron beam Hartemann, 1998;Esarey et al, 1993a;Chung et al, 2009), the harmonic frequency upshift of a laser pulse by relativistic nonlinear motion of electrons (Vachaspati, 1962;Brown & Kibble, 1964;Esarey et al,, 1992;Chen et al, 1998Chen et al, , 2000Ueshima et al, 1999;Kaplan & Shkolnikov, 2002;Banerjee et al, 2002;Lee et al, 2003aLee et al, , 2003bLee et al, , 2005Lee et al, , 2008Phuoc et al, 2003;Kim et al, 2009), high order harmonic generation in the interaction of intense laser pulse with solids (Linde et al, 1995(Linde et al, , 1996Norreys et al, 1996;Lichters et al, 1996;Tarasevitch et al, 2000) and x-ray laser using inner shell atomic transitions (Kim et al, 1999(Kim et al, , 2001. Ultrafast high-intensity X-rays can be generated from the interaction of high intensity femtosecond laser via Compton backscattering (Hartemann et al, 2005), relativistic nonlinear Thomson scattering (Ueshima et al, 1999;Kaplan & Shkolnikov 2002;Banerjee et al, 2002) and laser-produced betatron radiation (Phuoc et al, 2007).…”