Enhancement of electron energies in the laser wakefield acceleration using a density up-ramp capillary gas-cell

In the laser wakefield acceleration (LWFA), the dephasing problem is a serious energy-limiting factor, which is caused by the velocity difference between the accelerated electrons and the laser wake wave. To overcome the dephasing problem, we developed a special capillary gas-cell with a density up-ramp along the laser propagation direction and used it for electron acceleration experiments. Our experiments, which were performed with a peak laser power of 15 TW at GIST, show that the electron beam energy was enhanced to 260 MeV compared with 98 MeV from a flat density profile, which is more than two fold by the density up-ramp due to the increase of the dephasing length in the density up-ramp. This is the first experimental demonstration for electron energy enhancement using the density up-ramp in a capillary gas-cell. Two-dimensional particle-in-cell (PIC) simulations were also performed to confirm the effect of the density up-ramp, which shows a good agreement with the experimental results. Compared with a gas jet, the capillary gas-cell can provide a more stable and longer plasma so that the density up-ramp method in the capillary gas-cell may produce high quality electron beams of energy up to GeV range.