Beam parameters of FLASH beamline BL1 from Hartmann wavefront measurements Bernhard Fl ¨ oter a,n , Pavle Juranic ´ b , Peter Großmann a , Svea Kapitzki b , Barbara Keitel b , Klaus Mann a , Elke Pl¨ onjes b , Bernd Sch¨ afer a , Kai Tiedtke b a Laser-Laboratorium G¨ ottingen, Hans-Adolf-Krebs-Weg 1, D-37077 G¨ ottingen, Germany b Deutsches Elektronen-Synchrotron, Notkestraße 85, D-22603 Hamburg, Germany article info Available online 13 October 2010 Keywords: Wavefront Free-electron laser Hartmann sensor EUV abstract We report on online measurements of beam parameters in the soft X-ray and extreme ultraviolet (EUV) spectral range at the free-electron laser FLASH. A compact, self-supporting Hartmann sensor operating in the wavelength range from 6 to 30 nm was used to determine the wavefront quality of individual free- electron laser (FEL) pulses. Beam characterization and alignment of beamline BL1 was performed with l 13.5 nm /90 accuracy for wavefront rms (w rms ). A spot size of 159 mm (second moment) and other beam parameters are computed using a spherical reference wavefront generated by a 5 mm pinhole. Beam parameters are also computed relative to a reference wavefront created by a laser-driven plasma source of low coherence, proving the feasibility of such a calibration and reaching l 13.5 nm /7.5 w rms accuracy. The sensor was used for alignment of the toroidal focusing mirror of beamline BL1, resulting in a reduction of w rms by 25%, and to investigate wavefront distortions induced by thin solid filters. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Hartmann–Shack and Hartmann sensors are routinely used for real-time wavefront detection and laser beam characterization in the near infrared, visible and ultraviolet spectral region. Both the wavefront (directional distribution) and the beam profile (intensity distribution) of a radiation field can be recorded for a single pulse, enabling evaluation of paraxial beam parameters such as beam diameter d, divergence y, beam propagation factor M 2 , Rayleigh length z R , waist position z 0 and waist diameter d 0 for coherent radiation [1]. Solving the Fresnel–Kirchhoff integral allows also numerical propagation of the beam and thereby prediction of intensity distribution in any plane [2]. The Free-electron LAser in Hamburg (FLASH), operating in the extreme ultraviolet (EUV) spectral region, is based on the self-amplified spontaneous emission (SASE) process, which builds up laser emission from spontaneous undulator radiation. The photon beam characteristics relevant for user experiments can differ from pulse to pulse, leading to a strong requirement for single-pulse photon diagnostics and on-line characterization of the beam propaga- tion parameters [3,4]. Recently, we reported on a compact EUV Hartmann wavefront sensor that was jointly developed by Laser- Laboratorium G ¨ ottingen (LLG) and DESY for photon diagnostics, beamline alignment and monitoring of FEL radiation at FLASH [5]. The validity of beam parameters, computed in the framework of second moments, was confirmed using data from beamline BL2. In this paper, we present results from measurements at beamline BL1, including determination of beam parameters relevant for many user experiments as well as alignment of the toroidal focusing mirror. One of the major challenges using the Hartmann technique is to create a known reference wavefront. Two approaches are taken here: using a laboratory-scale laser-driven plasma source and spatial filtering at FLASH. Great effort is currently undertaken to design optical elements that preserve coherence and wavefront properties of FEL pulses. The gas attenuator, developed at FLASH [3], and thin solid filters are standard techniques for intensity attenuation at FELs. Both techniques were investigated at FLASH, and it was reported that in general the gas attenuator induces less wavefront aberrations [4]. Solid filters provide other features such as very high attenuation levels and spectral characteristics that may outweigh this disadvantage in certain applications. In this context the Hartmann sensor has proven to be a useful tool for pulse resolved beamline monitoring. 2. Hartmann sensor and reference wavefront generation The setup for the Hartmann sensor is based on the ideas Hartmann [6] presented in 1900. The essential parts are the Hartmann plate, a pinhole array consisting of a 7 mm thick tantalum foil with laser drilled holes (pitch 320 mm, diameter 65 mm), which divides the incoming beam into an array of smaller beams, and a 12bit camera with a Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.10.016 n Corresponding author. E-mail address: bernhard.floeter@llg-ev.de (B. Fl ¨ oter). Nuclear Instruments and Methods in Physics Research A 635 (2011) S108–S112