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X-ray laser calibrated

Especially interesting for
  • X-ray laser applications
  • investigations of materials’ structures
  • accelerator technology

Scientists from PTB – together with colleagues from the Deutsche Elektronen-Sychrotron (DESY), from the Japanese Research Centre RIKEN and from the Japanese metrology institute AIST/NMIJ – measured absolute photon fluxes and impulse energies in the photon energy range around 10 keV at the new 700 m long X-ray laser facility SACLA in Japan, using two independent methods. SACLA has been in operation since the summer of 2011 and is the first free-electron laser (FEL) worldwide that is suited for the hard X-ray range.

Gas monitor detector (GMD) for the measurement of the photon flux at an FEL beam (vertically to the image plane) by hotoionization of a gas at a gas pressure in the range of 10–3 Pa and electrostatic extraction (horizontally) and detection of photoions and photoelectrons.

The history of FELs – which are based on electron linacs, for the generation of femtosecond laser impulses in the soft and hard X-ray ranges according to the principle of “self-amplified spontaneous emission” (SASE) – took its course some ten years ago with FLASH – the Freeelectron LASer in Hamburg. At that time, it was still called “Tesla Test Facility” (FFT) and emitted electromagnetic radiation at 0.01 keV only. Within the scope of a cooperation between PTB, DESY (Hamburg), and the Ioffe Institute (St Petersburg), a gas monitor detector (GMD) has been developed and already tested at the TTF, in order to detect the strong intensity variations of the SASE process and to measure absolute impulse intensities. This GMD can detect X-rays by photoionization of atomic gases and by detection of photoions and photoelectrons. The gaseous detection medium is radiation-hard and, due to the low gas pressure in the range of 10-3 Pa, practical ly fully transparent (see PTB News 3/2011).

Today at FLASH, GMDs are operated as a permanent part of online photon diagnostics. Furthermore, different GMD versions have been used for intensity measurements at the five FELs worldwide over the past few years. The detectors were calibrated with synchrotron radiation in the PTB laboratories at the storage ring facilities BESSY II and MLS in Berlin, respectively.

In the past few years, besides the absolute measurement of photon flux, impulse energy, and beam position via linear photoionization, methods have also been developed to determine the diameter of an FEL microfocus or the duration of the femtosecond impulses via non-linear photoionization processes. Today, investigations also extend to fundamental issues of non-linear photoionization in the X-ray range. In the coming months, however, the cooperation partners’ agenda will focus more particularly on the characterization of GMDs for the 3.4 km long European XFEL in Hamburg, which is expected to be commissioned in 2015 at photon energies up to 25 keV.


Mathias Richter
Department 7.1
Radiometry with Synchrotron Radiation
phone: (030) 3481-7100
e-mail: mathias.richter(at)ptb.de

Scientific publication

Tiedtke, K. et al.: Gas detectors for x-ray lasers. J. Appl. Phys. 103 (2008) 094511