Schematic of a FLASH dual GMD for the online measurement of the photon intensity and the beam position.
Diagnostics panel in the FLASH control system showing the GMD 2 intensity signals. The upper window shows the mean pulse energy taken from the ion-current signal, whereas the lower window shows the pulse energies of the individual pulse trains measured via the electron signal on a shot-to-shot basis (narrow bars for current values, thick bars for maximum values since the panel was turned on, and medium bars for averaged values since the panel was turned on).
Signal ratio for the ion current split electrode of GMD 2 as a function of the relative beam position (bottom). The symbols show two independently measured data sets and the solid line is a result calculated according to Eq. (3). At the top part, the deviation of the theoretical curve from the experimental data is depicted.
Schematic of the X-GMD within its ionization chamber.
Spectral responsivity of the X-GMD at 3.5 kV multiplier voltage, normalized to the xenon target particle density, with and without additional ion collection along the photon beam.
Photoion signal of the X-GMD with (a) and without (b) additional ion collection along the photon beam as measured at the SPPS in January 2006.
Mean photon number per pulse as measured in January 2006 at the SPPS.
Total photoionization cross section and mean charge created per absorbed photon as a function of photon energy for the rare gases helium to xenon. The values represent a compilation of the experimental data given in Refs. 18–28. denotes the elementary charge. The spectral regions of strong resonances are omitted, respectively. Relative standard uncertainties are below 5% for the argon cross section, below 3% for all other cross sections, and below 2% for the values.
values as the relevant calibration quantities of the four GMDs currently operating at FLASH obtained at two different ion extraction fields (200 and 400 V/cm). For a given photon energy , a total photoionization cross section , and a mean charge created per absorbed photon according to Table I and a temperature and a target gas pressure , the spectral responsivities of the detectors may be calculated according to Eq. (1) with a relative standard uncertainty of better than 7%, respectively.
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