“…However, while providing shielding from X-rays and charged particles, the presence of nearby conductive material results in some local perturbation of the field 1 , 7 . KDP is a good choice of crystal for this application, as it possesses a reasonably high electro-optic coefficient of r 63 = 9.7 pm/V 55 , and does not scintillate strongly in the absence of impurity ions 56 , so we can neglect X-ray-induced scintillation as a source of unwanted “non-electric field driven” intensity changes. Furthermore, by using a free-space setup within the interaction chamber and placing the fibre-injection optics outside, the coupling of optical self-emission from the target and any scintillation from the crystals into the fibres is minimised, and can be resolved as a prompt event coincident with the arrival of the drive laser, distinct from the longer timescale chamber ring-down.…”