The Free Electron Laser FLASH at the German Electron Synchrotron (DESY) in Hamburg is a linear accelerator, which uses superconducting technology to produce soft x-ray laser light ranging from 4,1 to 45 nm. To ensure the operation stability of FLASH, monitoring of the beam is mandatory. Among various detectors located at the beam pipe, two Ionization Profile Monitors (IPM) detect the lateral x and y position changes. The functional principle of the IPM is
based on the detection of electrons, generated by interaction of the photon beam with the residual gas in the beam line. The newly designed IPM enables the combined determination of the FEL’s horizontal and vertical position as well as the beam’s profile. This is made possible by a compact monitor, consisting of a cage in a vacuum chamber, two microchannel plates (MCP) and two structural repeller plates with toggled electric fields at the opposite sides of the MCPs.
The electrons created by the FEL beam, drift in a homogenous electrical field towards the respective micro-channel plate, which produces an image of the beam profile on an attached phosphor screen. A CCD camera for each MCP in combination with a computer is used for the evaluation. This indirect detection scheme operates over a wide dynamic range and allows the detection of the center of gravity and the shape of the photon beam without affecting the FEL beam.
Exact knowledge of the path taken by the electrons permits a recursive determination of the beam position. Within a beam variance of less than 10 mm, an accuracy better than ±8 um seems to be possible.