Count Rate Optimizations for TES Detectors at a Femtosecond X-ray Laser

Transition-edge sensor microcalorimeters have found success as X-ray detectors at synchrotron light-sources, due to a unique combination of high collecting area and good energy resolution. However, the upcoming generation of free-electron lasers (FELs), such as the Linac Coherent Light Source II, is designed to deliver more than $$10^{10}$$ photons in a 100 fs pulse at a 100 kHz rate, potentially leading to severe pulse-pileup issues. We will demonstrate that, for most relevant science cases, it is possible to mitigate pulse pile-up using simple X-ray filters in a way that takes advantage of the substantial increase in X-ray flux at modern FELs.