XGM: The X-Ray Gas Monitor

The XGM (X-ray gas monitor) is used to determine both the intensity and position of the X-ray FEL beam by means of atomic photoionization processes. The XGM chamber is filled with a target gas (usually Ne, Ar, or Kr) at a pressure of about 10-5 mbar. When the FEL beam passes through the XGM, it ionizes the rare gas atoms. The reaction products, ions and electrons are then accelerated from the reaction volume by an electric field and detected in Faraday cups, in a time of flight spectrometer or in a HAMP detector (Huge area open multiplier detector). The measured ion current is proportional to the total energy of the FEL beam, which can be calculated from this data. The XGM of the SCS instrument is a full version of the XFEL XGM provided by WP-74 and DESY and therefore it is made of four units:

  1. Two beam intensity monitors to measure intensity along both x and y direction.
  2. Two beam position monitors to determine x and y beam positions.

Each monitor has a dedicated chamber, making 4 chambers in total. They are all connected without valves in between which allows them to share the vacuum and gas injection system.