Soft X-ray multiplexing at LCLS

The design of third-generation light sources, namely synchrotrons with undulators, allows multiple endstations to be operated simultaneously with independent energy tuning. However, this is currently not possible in X-ray free electron lasers such as LCLS, severely limiting beamtime available for each endstation. A proposal for implementing multiplexing across soft X-ray (SXR) beamlines at LCLS is evaluated. Two beams are produced by dividing undulator cells into two segments. The first, longer set of cells produces a fully lased (‘main’) beam. Up to seven remaining undulator cells are angularly displaced and the same electron bunches are used to produce a lower power (‘parasitic’) beam with independent energy tuning. Some scientific uses for the lower-power, incoherent ‘parasitic’ beam are anticipated, such as RIXS and ARPES. By means of ray tracing of optical elements in the FEE and NEH sections of LCLS we have considered multiple geometries which could implement multiplexing, with an emphasis on minimizing cross-beam contamination and modification to existing beam paths. We have concluded that two geometries are most promising:

1. The vertical mirror MR1K1 may be left in position to send both beams up towards NEH 2.2/2.1. The parasitic beam is produced at an angle of 75 mrad southwards, such that it misses the horizontal mirror MR1K2 and propagates to NEH 2.1 (currently not in use). The main beam reflects off MR1K2 and continues on its current path to NEH 2.2. Both the positions of the main beam and MR1K2 are unchanged. Because both beams will have to pass through the same monochromator assembly SP1K1, a new monochromator will have to be designed or true two–colour control will be compromised.

2. MR1K1 is lowered by 9.4 mm and moved forward by 34.9 cm such that the main beam (whose path is unchanged) misses it and continues straight on to NEH 1.1. The parasitic beam is produced at an angle of 72 mrad downwards of the main beam, reflects off MR1K1 and proceeds to NEH 2.2/2.1 with the same beam path after MR1K1. There is no interference in energy tuning in this case.