Laser wakefield accelerators are capable of generating gigaelectronvolt-energy electrons in centimetre-scale interaction lengths via the ponderomotive excitation of a relativistic plasma wave by a high intensity laser. Thus, these accelerators are promising candidates for compact sources of relativistic electron beams and have the potential to revolutionise high-energy radiation sources by reducing their scale by many orders of magnitude. By making accelerator facilities cheaper and more accessible, this could drive impact in areas ranging from laboratory astrophysics to biological imaging. 

Highly stable accelerator performance is required for applications of these electrons, but this is difficult to achieve due to the sensitivity of the injection and acceleration dynamics to initial conditions, resulting from the non-linear underlying physics. A key parameter in determining the quality of the accelerated electrons is the plasma density, often taken as a constant and controlled by the pressure of the gas target. By tailoring the density profile, such as introducing a sharp longitudinal density transition in the target, it may be possible to improve the shot-to-shot stability of the accelerator.

We present experimental results of the electron beams generated by a non-linear laser wakefield accelerator in a helium gas jet target with a density transition produced by a razor blade in the flow. The shot-to-shot variations in the plasma density profile for nominally equal conditions are diagnosed via interferometry. Through an in-depth sensitivity study using particle-in-cell simulations, the effects of fluctuations in the plasma density profile are investigated. The results suggest that blade motion is more detrimental to stability than gas pressure fluctuations, and that early focusing of the laser may reduce the detrimental effects of such density fluctuations.