Ultra-intense lasers can generate the strongest electromagnetic fields in laboratory conditions, expected to perform tests of QED in yet unexplored parameter ranges. Such experiments require knowledge of the field strengths and all possible interaction pathways. The latter can be simplified if a perfect, particle-free vacuum is present, thereby excluding competing interactions. We propose a method to evacuate all residual gas particles prior to QED interactions, based on tunnel ionization by a preceding auxiliary laser pulse and a static electric field. We present modelling and experimental results of testing this method at a 0.5 TW CPA laser system. Experimental results match well the simulations for the given conditions and thereby provide valuable understanding to extrapolate this method to QED experiments with PW-class laser systems where it can likewise be employed for in-situ peak field strength characterization.