Heating up the beam
Monday, November 17, 2008
A laser-electron-heater system has been recently installed alongside the LCLS injector system. It is like a small "inverse" free electron laser, or more precisely it is made to generate an uncorrelated energy spread in the electron beam. If you don't do this you may have subsequent micro-bunching instabilities in the electron beam that deteriorate its qualities.
 The heater laser beam actually comes from part of the UV drive laser system of the injector gun. A small piece of the IR beam that is not converted in UV (like a waste beam) is re-used for the heater. But as with the UV drive laser, it starts upstairs in the laser bay and needs to be transported 30 feet down into the injector vault. The laser group, and in particular Sasha Gilevich and Alan Miahnahri, were in charge during this fall downtime to bring the laser heater beam parallel to the heater chicane where there is a very special undulator. As you can see on the picture, the heater chicane is at the very end of the injector vault. (Not a very comfortable place!) The ceiling is low; it is fairly crowded with pipes, waveguides, cable trays, etc. But even in this "cave" type conditions Alan and Sasha were able to get the beam there safely.The other challenge was to have the laser beam focused close to the middle of the undulator chamber and perfectly parallel to the chamber axis (defined by the heater undulator chicane). The beam has to travel almost 30 meters from the output of the laser to the heater undulator chamber. There, its size has to be less than 200 micrometers and it has to be perfectly aligned. If you touch just a tiny bit one actuator of a mirror upstairs in the laser bay, because of the lever arm, the beam is gone! That's why we have a beam steering stabilization feedback loop. It uses CCD cameras that watch the beam at some strategic places and the loop keeps it always in the same spot by controlling mirror actuators. Hopefully the pointing stability of the laser is good enough (around 20 micrometers) so that the loop is only running to compensate for some slow drifts of the beam direction due to temperature changes for example. So as long as nobody bumps a mirror, the laser heater beam should be always there to help the LCLS electron beam to run at its highest performance. posted by Philippe Hering @ 11:10am
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