  The upper limit of the laser field strength in perfect vacuum is usually
considered as the Schwinger field, corresponding to ~10^29W/cm^2. We
investigate such limitations under realistic non-ideal vacuum conditions and
find out that intensity suppression appears starting from 10^25W/cm^2, showing
an upper threshold at 1026W/cm^2 level if the residual electron density in
chamber surpasses 109cm^-^3. This is because the presence of residual electrons
triggers the avalanche of quantum-electrodynamics cascade that creates copious
electron and positron pairs. The leptons are further trapped within the driving
laser field due to radiation-reaction, which significantly depletes the laser
energy. The relationship between the attainable intensity and the vacuity is
given according to particle-in-cell simulations and theoretical analysis. These
results answer a critical problem on the achievable light intensity based on
present vacuum conditions and provide a guideline for future 100's-Petawatt
class laser development.
