Numerical study of electron acceleration and betatron radiation base on the interaction of petawatt femtosecond laser with near-critical-density plasma

Laser-driven high-brightness betatron radiation has great potentials for its broad applications in the detection of ultrafast processes (such as shock waves or implosion processes) in high energy density physics. Here a tube-like gas-cell target is proposed to generate a near-critical-density (NCD) plasma, which has a sharped rising edge with a length scale of hundreds of μm. Such a gas-cell target has the advantages of low back pressure and small jet volume. Moreover, due to the confinement of the gas chamber walls, it can more stably generate a plateau-shaped gas density distribution. Particle-in-cell (PIC) simulations of the the petawatt femtosecond laser interacting with such a NCD plasma were carried out to study the electron acceleration as well as the betatron radiation. It was shown that, with the appropriate gas density and pulse duration, a steady plasma channel can be well formed. In the channel, the electrons firstly undergo the wakefield acceleration. Then these energetic electrons directly interact with the laser tail, where the efficient betatron resonance and the direct laser acceleration happen, thus resulting in the great enhancement of both the yield and cut-off energy. The transverse oscillation of energetic electrons in the plasma channel leads to the production of high brightness betatron radiation, which has a critical photon energy of 8keV and a brightness of $ 1.75\times {10}^{20}\;\mathrm{p}\mathrm{h}\cdot {\mathrm{s}}^{-1}\cdot {\mathrm{mm}}^{-2}\cdot {\mathrm{m}\mathrm{rad}}^{-2}\cdot {\left(0.1\mathrm{{\text{%}}}\mathrm{b}\mathrm{w}\right)}^{-1} $. The influences of the gas density and laser pulse duration on the betatron radiation were also identified. These results provide an effective path for optimizing the generation of high brightness betatron radiation using the petawatt femtosecond lasers.