Photoluminescence-based detector measures pulse energy of x-ray FELs

X-ray free-electron lasers (FELs) promise improved x-ray imaging and nonlinear matter interaction and time-resolved studies; however, their short, intense, small-wavelength pulses are difficult to measure. The pulse energy of the soft-x-ray FEL source in Hamburg, Germany called FLASH (with a 6-48 nm wavelength range) has been measured by applying an electric field across a rare-gas cell that produces strong electron and ion currents at low pressures and a correspondingly strong signal that can be detected with Faraday cups. A simpler detection technique has recently been developed by researchers at Lawrence Livermore National Laboratory (Livermore, CA) and Stanford University (Stanford, CA), who both modeled and experimentally demonstrated an electrode-less photoluminescence-based pulse-energy detector. It has been successfully used with the hard-x-ray Linac Coherent Light Source at Stanford and produced a strong signal.

The simple and inexpensive detector infers the total pulse energy from the UV radiation generated by passing the FEL beam through a weakly absorbing gas (in this case, nitrogen gas, since its UV luminescence behavior is very well characterized). The gas is contained in a 30 cm long, 8 cm diameter cylinder at a pressure between 0.1 and 2 Torr–adequate to overcome system noise. The FEL beam induces photoionization of the nitrogen gas, creating UV photons and measurable luminescence that can be quantified and correlated with total laser pulse energy. Contact Stefan P. Hau-Riege at [email protected].