The use of a gas cell as a target for laser weakfield acceleration (LWFA)
offers the possibility to obtain stable and manageable laser-plasma interaction
process, a mandatory condition for practical applications of this emerging
technique, especially in multi-stage accelerators. In order to obtain full
control of the gas particle number density in the interaction region, thus
allowing for a long term stable and manageable LWFA, real-time monitoring is
necessary. In fact, the ideal gas law cannot be used to estimate the particle
density inside the flow cell based on the preset backing pressure and the room
temperature because the gas flow depends on several factors like tubing,
regulators and valves in the gas supply system, as well as vacuum chamber
volume and vacuum pump speed/throughput. Here, second-harmonic interferometry
is applied to measure the particle number density inside a flow gas cell
designed for LWFA. The results demonstrate that real-time monitoring is
achieved, and that using low backing pressure gas (< 1 bar) and different cell
orifice diameters (< 2 mm) it is possible to finely tune the number density up
to the range well suited for LWFA