Description

Beam Intensity monitors

The XFEL beam ionizes the target gas in the interaction zone. The produced ions are accelerated in an electric field to the upper section of the XGM, while the electrons are pulled to the bottom section. The necessary field for this is produced by two high voltages applied to the extraction electrodes.

Ion measurement

The ions are detected by a large Faraday cup electrode. The current signal from this electrode is measured by a Keithley 6487 electrometer. Before the signal is fed into the electrometer, it is integrated in a custom made circuit to prevent errors in the current measurement due to the pulsed structure of the signal. This current measurement is used to calculate the bunch intensity of the FEL. In addition to the Faraday cup an ion time-of-flight spectrometer is installed on the ion side. The ions can pass through a hole in the faraday cup electrode and enter a commercial ETP 14880 ion detection system, which consists of an entrance mesh and a detector with an electron multiplier. The signal of the electron multiplier is fed out of the vacuum system and then amplified by a Femto HVA-S2 low noise amplifier. The amplifier has two gain settings of 20 dB and 40 dB, which are switchable electronically by a voltage of +3 to 15V (1 to 6 mA).

The in time-of-flight signal is then fed into a digitizer. The signal is mainly used to monitor the charge-state distribution of the produced ions, from which a correction factor for the FEL intensity calculation can be obtained. This measurement can be performed on the 4.5 MHz pulse time-scale.

Electron measurement

The electron signal on the bottom side Faraday-cup electrode is coupled out capacitively, amplified and fed into a digitizer. This signal is not used for the beam intensity calculation and is only monitored during commissioning or maintenance.

Position intensity monitors

The position intensity monitor uses a HAMP (Huge area open multiplier detector) specially developed at DESY for this application. The signals are amplified and feed into a digitizer.