Efficient Coherent Mode Decomposition for Physical Optics Simulations of Beamlines and Experiments

We present simulations of the partially coherent undulator radiation (UR) propagation through several hard X-ray beamlines at NSLS-II, performed with SRW code, adopting two different memory- and CPU-efficient strategies / algorithms for the coherent mode decomposition (CMD). One of these algorithms employs the subtraction of quadratic phase terms from 4D cross spectral density (CSD) at a non-zero radius of the radiation wavefront curvature, whereas the other one applies the CMD at / near a radiation beam waist, where the radiation wavefront is typically “flat”. We show that, when performing the CMD on the 4D CSD of the partially coherent radiation wavefront, the required numbers of grid points of radiation meshes are largely reduced in both cases, compared to standard approaches. This dramatically reduces memory requirements (from TBs typically to several GBs) and improves the time-to-solution of the decomposed modes (from many hours at a parallel execution to several minutes at a sequential execution). Thanks to the implemented algorithms, the propagation simulations of tens or hundreds of thousands coherent wavefronts generated by independent electrons for an undulator-based X-ray beamline at a low- or intermediate- emittance synchrotron light source is reduced to the simulation of over fewer than one thousand major coherent modes obtained from the CMD, which fully recovers the structure of the partially coherent UR before and after the propagation through a beamline. The efficiency and accuracy of the methods is further demonstrated with simulations for two hard X-ray NSLS-II beamlines: HXN and CHX.