General concept
[edit | edit source]For certain experiments, a single laser pulse is required. However, by its very nature the output of the FEL process consists of a train of light pulses. In 25 MHz mode operation, the optical micropulses form a train with 40 ns intervals. With a 10 µs macropulse, there are 250 micropulses in that train. It is possible to extract (slice) a single pulse from that train using the following setup. A piece ("slab") of silicon is inserted in the optical beam. This is normally transparent, so the FEL light is transmitted and lost. However, when illuminated with a Nd:YAG laser, the silicon becomes reflective. If that laser is operated with a very short pulse (< 40 ns), and timed just right, it is possible to extract a single micropulse from the FEL train.
There are two timers that control the Nd:YAG laser. One enabled the flash lamp, the other dumps the cavity [i.e. enables the output].
Procedure
[edit | edit source]- Operate FEL at 25 MHz
- Connect timers DS-T.1 (M68) and DS-T.2 (M69) to the Nd:YAG laser. Appropriate times should be set in timing system (Flash lamp: ~ -230 µs, Q-dump: +5 µs [somewhere in macropulse where optical output has saturated]).
- Insert a fast detector into the optical path, and connect it to an oscilloscope, in order to see the optical micropulses.
- Insert silicon slab, using the remote control system of the FELIX diagnostic station. Adjust the position to minimize the micropulses peak height.
- Turn on YAG, one micropulse should be reflected towards the detector.
- Adjust YAG power and timing as needed (it has been noted that these are not completely independent).
A detailed document outlining the steps to achieve pulse-slicing can be found here:
File:PulseSlicingFLX Instruction.pdf
