Generation of bright collimated vortex $γ$-ray via laser driven cone-fan target

We use numerical simulations to demonstrate that a source of bright collimated vortex $γ$-ray with large orbital angular momentum can be achieved by irradiating a circularly polarized laser with an intensity about $10^{22}\rm{W/{cm^2}}$ on a cone-fan target. In the studied setup, electron beam of energy of hundreds of MeV and vortex laser pulse are formed. And furthermore a high quality vortex $γ$-ray is yielded with small divergence of $5^{\circ}$ and high peak brilliance $\sim5\times10^{22}$ photons ${\rm\cdot s^{-1} \cdot mm^{-2} \cdot mrad^{-2}}$ $0.1\%\mathrm{BW}$ at $10\mathrm{MeV}$. A considerable fraction of angular momentum of laser is converted to electron beam and vortex $γ$-ray, which are roughly $27.8\%$ and $3\%$, respectively. And the conversion efficiency of energy from laser to electron beam and vortex $γ$-ray are around $41\%$ and $3.8\%$. Moreover, comparative simulations for different right radius of cone reveal that there exists an optimal size that makes the highest angular momentum of $γ$-ray photons to be around $2.8\times10^6\hbar$. The comparative simulations for different laser modes exhibit that it is more appropriate to choose the circularly polarized laser to generate vortex $γ$-ray than the Laguerre-Gaussian one.