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The gas monitor detectors provide non-invasive measurements of the shot-to-shot intensity. To the right: A Faraday cup counts the electrons and ions that are produced as the FEL pulse passes through the ionization chamber containing nitrogen or rare gases at very low pressure. To the left: The two split electrodes determine the horizontal position of the beam.
Intensity and Position
Most experiments need online information on the intensity, spectral distribution, and temporal structure of the FEL pulses. This is achieved using non-destructive diagnostic tools operating in parallel. Due to the SASE-specific shot-to-shot fluctuations, pulse-resolved diagnostics are mandatory.
For many users the most important parameter is the intensity, which varies from pulse to pulse. Depending on the operation conditions of the FEL, the energy per bunch, varying from shot to shot, is typically in the range of 10 μJ to 50 μJ. In fact almost all experiments need that information on a shot-to-shot basis. An example are experiments measuring the ablation threshold of a material, which need this in order to analyze and model these processes. The intensity monitors at FLASH have to cover the full spectral range of the FEL beam – from 6.5 nm to 60 nm – as well as the extended dynamic range from spontaneous undulator radiation to SASE in saturation. To accomplish these requirements a state-of-the-art gas monitor detector has been developed.
When an FEL pulse passes through the ionization chamber of the detector, the gas inside is ionized, and an electric field accelerates the ions upwards and the electrons downwards to be detected by Faraday cups. From the resulting electron and ion currents the absolute number of photons in each shot can be deduced with an accuracy of 15 percent. Furthermore,
the FEL pulse passes in between two split-electrode plates, allowing the pulse-resolved determination of the horizontal and vertical position of the beam.
Two pairs of gas monitor detectors are integrated into the FEL beamline on the way to the experimental hall as permanent intensity and beam position monitors. The gas in the ionization chamber has a very low pressure of some 10-6 mbar (10-5 hPa), and it is nearly transparent to the FEL pulse that proceeds unaltered to the experimental stations.
