Experimental tuning of electron energy gain via laser focus shift in a laser-driven plasma wakefield accelerator

We present a combined experimental and simulation study on electron acceleration in a laser-driven plasma wakefield accelerator utilizing the plasma telescope mechanism. By employing a 15-J laser with its focal spot smaller than the matching condition, and systematically tuning the laser focus position relative to the gas cell entrance, we generate an electron beam with energy of up to 1.4 GeV when the laser is focused a few millimeters upstream of the plasma entrance. We further observe that electron self-injection and subsequent acceleration occur only when the laser is focused within 2.2 mm upstream to 2.4 mm downstream of the gas-cell entrance. Particle-in-cell (PIC) simulations reproduce the experimental observations and predict a maximum electron energy of 2.2 GeV at the optimal focal position. Both the experiments and simulations reveal a clear injection threshold when the laser focus is placed too far upstream of the plasma entrance. In this case, excessive laser diffraction prevents efficient refocusing by the plasma telescope, thereby suppressing wakefield injection and acceleration. These results provide experimental evidence that the plasma telescope mechanism can support the generation of GeV-scale electron beams in laser wakefield accelerators. Furthermore, they identify the laser focal position on the plasma density up-ramp as a critical parameter governing electron injection and energy gain, offering important guidance for the optimization of future plasma-telescope-based acceleration schemes.