FLASH Wavefront Sensor

Compact Hartmann wavefront sensor with a large field of view 15.2 x 15.2 mm including movements (xy direction and tip/tilt) for alignment mounted behind user chamber

In the past years, DESY has developed wavefront sensors for the soft x-ray spectral range, approx. 6-30 nm, in collaboration with the Laser-Laboratorium Göttingen e.V. (LLG). The wavefront sensors could be demonstrated to be a very valuable tool for characterizing and aligning the different FLASH beamlines and the KB-optic at the FERMI permanent end stations LDM and DiProI. The technique has thus proven very useful for soft X-ray optics as well as beam characterization.

The detectors are adapted to the special beam properties of FLASH in terms of wavelength, pulse energy, and coherence (6-30 nm, ~15 µJ). The Hartmann plate divides the incoming FEL into sub-rays and illuminates a phosphor coated CCD chip. From lateral deviations in the beam spot pattern, the wavefront for single pulses is reconstructed using a modal approach. The measured intensity and phase are used to calculate second moment beam parameters such as beam width, divergence, Rayleigh length, waist position, waist size, and M2. A Zernike expansion of the wavefront provides information on the aberrations of the optical system, resolved into single coefficients such as astigmatism or coma. Intensity profiles in any plane along the optical axis are accessible via numerical propagation using the Fresnel-Kirchhoff integral.

Our recent project is the development of a compact wavefront sensor with a bigger field of view and a refresh rate of about 5 Hz. Hereby measurementsof the full unfocussed beam of FLASH as well as behind experimental chambers with highly divergent optics at the FLASH or FERMI beamlines will be possible with a high repetition rate.

Three wavefront sensors with different characteristics, such as spacial requirements, Hartmann plate sizes, camera repetition rates etc. are available at FLASH. Please inquire with the wavefront sensor contacts.

CONTACT

Keitel
Barbara Keitel
E-Mail: Barbara Keitel
Phone: +49 (0)40 8998 4245
Location: 28k / O2.006

Plönjes-Palm
Elke Plönjes-Palm
E-Mail: Elke Plönjes-Palm
Phone: +49 (0)40 8998 (9)2602
Location: 28k / O2.004

References:

Barbara Keitel et al., Hartmann wavefront sensors and their application at FLASH, Proc. PhotonDiag2015, J. Synchrotron Rad. 23, 43-49 (2016). Article

B. Keitel et al., Hartmann wavefront measurements at FLASH, Advances in X-ray Free-Electron Lasers II: Instrumentation, Prague, Czech Republic, 04/17/2013 - 04/18/2013 Proceedings of SPIE 8778, 877814-1--1877814-7 (2013). Article

B. Flöter et al., Beam parameters of FLASH beamline BL1 from Hartmann wavefront measurements, Nucl. Instrum. Methods A 635, 108-112 (2011). Article

B. Flöter et al.,EUV Hartmann sensor for wavefront measurements at the Free-electron LASer in Hamburg,New J. Phys. 12, 083015 (2010). Article