Introduction

The pplPattern middlelayer device is designed to provide an easy interface to configure the pattern of the pulse probe laser (PPL).

A sequence of up to four patterns can be saved per XFEL train, according to the needs of the experiment being run in the instruments. In addition, a train configuration can be executed multiple times, and two distinct train configurations can be set.

The PPL pattern configuration is saved in the DOOCS server XFEL.UTIL/BUNCH_PATTERN/PATTERN_BUILDER.

Two device scenes are available in the package to setup either a periodic or an arbitrary pattern. The first one is configured by using the scene “Periodic SubPattern” which allows to generate patterns always consistent with the PPL grid. The scene “Arbitrary SubPattern” allows the user to configure very generic patterns, also not in the PPL grid.

This grid is the reference regular sequence of pulses and of empty bunches generated by the frontend hardware of the pulse-probe laser; a base grid element is considered to be the sequence made of one pulse and of its subequent empty bunches (before the next pulse). The pulse frequency is set in the frontend by choosing there the so called ‘base frequency divider’ which reduces the laser pulse frequency generated by the frontend from 4.5 MHz to a submultiple value.

Being a user-selected pattern on the PPL grid means eventually that the laser to be delivered to the instruments should have the position of pulses aligned with the pulses in the reference pattern as defined by the frontend configuration. In case a user-selected pattern is not on the grid, the pulses of the pattern might not be amplified in the laser system, resulting in a laser pattern delivered to the instruments different from what expected. Patterns of only empty bunches are considered to be on the grid if their length is an integer multiple of the base grid element.

The condition for patterns to be on the PPL grid condition is always fullfilled when using the scene for periodic patterns. This is not the case when building patterns with the other legacy scene.