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.

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.

Interlocks

The DPS system is supposed to pump down the gas entering through the XGM. If it fails, the first thing to do is to close the XGM’s dosing valve, in order to prevent the entire beamline from venting. If the pressure still rises, we have to switch off the gate valves between the chambers, but not before the XFEL beam is switched off.

As a last prerequisite, the roughing valve RV1 should be open if we want to open the gas inlet. This may be closed because we are evacuating the gas lines, in which case the turbo pumps have no roughing valves, so we do not want unnecessary gas load.

Pressure interlock

In case of bad pressure, the first thing we want to do is to close the SCV valve, and to switch off the high voltage HV_LOCK.

The pressures are all a bit lower than they are on the gate valves that close, so that the gas stops flowing first, maybe then we don’t even need to close the valves anymore, as the pressure will most likely drop.

We use the gate valves of the DPSs as a proxy for their entire pumping system: if the turbo pumps behind them are not fine, or the roughing pressure, they are closed by the interlock system. As they are not closing immediately, we are not looking at the IsClosed bit, but at Value, which switches immediately to FALSE should their interlock trigger.

Number Device Parameter Operator Limit
c1 SCS_BLU_XGM/VALVE/GV3.state.IsClosed EQ FALSE
c2 SCS_BLU_DPS-2/VALVE/GV4.state.IsClosed EQ FALSE
c3 SCS_BLU_XGM/GAUGE/LV1_PVALVE.Value GT 1e-3
c4 SCS_BLU_XGM/VALVE/GV1.state.Value EQ FALSE
c5 SCS_BLU_XGM/VALVE/GV2.state.Value EQ FALSE
c6 SCS_BLU_DPS-1/GAUGE/PCC5.Value GT 1e-4
c7 SCS_BLU_DPS-1/VALVE/GV3.state.Value EQ FALSE
c8 SCS_BLU_DPS-1/GAUGE/PCC4.Value GT 1e-6
c9 SCS_BLU_DPS-1/VALVE/GV2.state.Value EQ FALSE
c10 SCS_BLU_DPS-1/GAUGE/CC3.Value GT 5e-8
c11 SCS_BLU_DPS-1/VALVE/GV1.state.Value EQ FALSE
c12 SCS_BLU_DPS-2/GAUGE/PCC7.Value GT 1e-4
c13 SCS_BLU_DPS-2/VALVE/GV3.state.Value EQ FALSE
c14 SCS_BLU_DPS-2/GAUGE/PCC8.Value GT 1e-6
c15 SCS_BLU_DPS-2/VALVE/GV2.state.Value EQ FALSE
c16 SCS_BLU_DPS-2/GAUGE/CC9.Value GT 5e-8
c17 SCS_BLU_DPS-2/VALVE/GV1.state.Value EQ FALSE
c18 SCS_BLU_XGM/GAUGE/LV1_PVALVE.AValueSemiRaw LT 1

If the needle valve controller is switched off, it gives a perfect pressure. This is condition c18.

We close the SCV if pressure is too high. Certainly, we don’t care whether the pressures are bad if the valves to the DPS are closed, which makes the conditions a bit complicated.

Number Logic Action
a1 c3 or c4 or c5 or c18 or (c1 and (c6 or c7 or c8 or c9 or c10 or c11)) or (c2 and (c12 and c13 and c14 and c15 and c16 and c17)) COff

And we put the HV inhibit on. This is weird for a safety interlock, but worst case DOOCS can switch off the HV.

Number Logic Action
a1 c3 or c4 or c5 or c18 or (c1 and (c6 or c7 or c8 or c9 or c10 or c11)) or (c2 and (c12 and c13 and c14 and c15 and c16 and c17)) COn

SCS_BLU_XGM/VALVE/RV2

The roughing line for evacuation of the gas lines must be closed if any of the gases are open.

Number Device Parameter Operator Limit
c1 SCS_BLU_XGM/DCTRL/GS_PV_AR.state.Value EQ TRUE
c2 SCS_BLU_XGM/DCTRL/GS_PV_KR.state.Value EQ TRUE
c3 SCS_BLU_XGM/DCTRL/GS_PV_XE.state.Value EQ TRUE
c4 SCS_BLU_XGM/DCTRL/GS_PV_N.state.Value EQ TRUE
Number Logic Action
a1 c1 or c2 or c3 or c4 CClose

SCS_BLU_XGM/VALVE/GV1 and /GV2

We want to close the butterfly valve over the turbomolecular pump in case they or their roughing vacuum goes bad. We do not want to close them just because the rouging vacuum is closed, because we may simply be evacuating the gas lines.

Number Device Parameter Operator Limit
c1 SCS_BLU_XGM/TPUMP/TP1.state.RPM80 EQ FALSE
Number Device Parameter Operator Limit
c1 SCS_BLU_XGM/TPUMP/TP2.state.RPM80 EQ FALSE

As the line merges into one, all other conditions are the same for both gate valves.

Number Device Parameter Operator Limit
c2 SCS_BLU_XGM/GAUGE/P2_1.Value GT 3e-2
c3 SCS_BLU_XGM/VALVE/RV1.state.IsOpen EQ FALSE
c4 SCS_BLU_DPS-2/VALVE/SP31_RV2.state.IsOpen EQ FALSE
Number Logic Action
a1 c1 or c2 or c3 or c4 CClose