Polariziation data of individual XFEL pulses collected with Ne 1s at hν = 914 eV. (a) Polar plot representation of intensity data from one pulse with fitted parameters PLin and ψ from equation (3). The arrow indicates the polarization vector of light, ∊. (b) Polarization parameters for 450 consecutive pulses when the machine is operated in single-bunch mode. A few sharp spikes can be seen – these are pulses with undesired properties.