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Title: The Gas Attenuator of FLASH at DESY

Abstract

FLASH (Free electron LASer at Hamburg) as a part of the Deutsches Elektronen Synchroton DESY is the first Free Electron Laser (FEL) user facility for VUV and soft X-ray coherent light experiments. The SASE (Self Amplification by Stimulated Emission) process generates ultra short coherent radiation pulses on the femtosecond time scale with peak powers in the GW range. Several experiments need reliable means to reduce the FEL intensity over many orders of magnitude without changing the photon beam characteristics. Since a reduction of the FEL intensity by variation of machine parameters is not appropriate, a windowless gas-filled cell in combination with differential pumping units is used for attenuating the FEL radiation. This attenuator is placed in the beamline in outside the experimental hall. The total length of the gas cell is 15 m and the maximum gas pressure, which can be handled by the differential pumping units, is about 0.1 mbar. The attenuation range of Nitrogen covers at least 5 orders of magnitude in the spectral range of 19 to 120 nm due to its large absorption cross section. Between 19 and 9 nm and for shorter wavelengths Xenon and Krypton can be used, respectively.

Authors:
;  [1]
  1. Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22603 Hamburg (Germany)
Publication Date:
OSTI Identifier:
21043428
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 879; Journal Issue: 1; Conference: 9. international conference on synchrotron radiation instrumentation, Daegu (Korea, Republic of), 28 May - 2 Jun 2006; Other Information: DOI: 10.1063/1.2436055; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; ABSORPTION; AMPLIFICATION; ATTENUATION; BEAM PRODUCTION; COHERENT RADIATION; DESY; FAR ULTRAVIOLET RADIATION; FREE ELECTRON LASERS; KRYPTON; LASER RADIATION; PHOTON BEAMS; PULSES; SOFT X RADIATION; STIMULATED EMISSION; VARIATIONS; WAVELENGTHS; XENON

Citation Formats

Hahn, Ulrich, and Tiedtke, Kai. The Gas Attenuator of FLASH at DESY. United States: N. p., 2007. Web. doi:10.1063/1.2436055.
Hahn, Ulrich, & Tiedtke, Kai. The Gas Attenuator of FLASH at DESY. United States. doi:10.1063/1.2436055.
Hahn, Ulrich, and Tiedtke, Kai. Fri . "The Gas Attenuator of FLASH at DESY". United States. doi:10.1063/1.2436055.
@article{osti_21043428,
title = {The Gas Attenuator of FLASH at DESY},
author = {Hahn, Ulrich and Tiedtke, Kai},
abstractNote = {FLASH (Free electron LASer at Hamburg) as a part of the Deutsches Elektronen Synchroton DESY is the first Free Electron Laser (FEL) user facility for VUV and soft X-ray coherent light experiments. The SASE (Self Amplification by Stimulated Emission) process generates ultra short coherent radiation pulses on the femtosecond time scale with peak powers in the GW range. Several experiments need reliable means to reduce the FEL intensity over many orders of magnitude without changing the photon beam characteristics. Since a reduction of the FEL intensity by variation of machine parameters is not appropriate, a windowless gas-filled cell in combination with differential pumping units is used for attenuating the FEL radiation. This attenuator is placed in the beamline in outside the experimental hall. The total length of the gas cell is 15 m and the maximum gas pressure, which can be handled by the differential pumping units, is about 0.1 mbar. The attenuation range of Nitrogen covers at least 5 orders of magnitude in the spectral range of 19 to 120 nm due to its large absorption cross section. Between 19 and 9 nm and for shorter wavelengths Xenon and Krypton can be used, respectively.},
doi = {10.1063/1.2436055},
journal = {AIP Conference Proceedings},
number = 1,
volume = 879,
place = {United States},
year = {Fri Jan 19 00:00:00 EST 2007},
month = {Fri Jan 19 00:00:00 EST 2007}
}
  • More than 60 beam position monitors (BPM) are installed along about 350m of beamline of the Free Electron LASer in Hamburg (FLASH) at DESY. The room-temperature part of the accelerator is equipped mainly with stripline position monitors. In the accelerating cryo-modules there are cavity and re-entrant cavity BPMs, which will not be discussed here. In the undulator part of the machine button BPMs are used. This area requires a single bunch resolution of 10{mu}m. The electronics is based on the AM/PM normalization principle and is externally triggered. Single-bunch position is measured. This paper presents the methods used to determine themore » resolution of the BPMs. The results based on correlations between different BPMs along the machine are compared to noise measurements in the RF lab. The performance and difficulties with the BPM design and the current electronics as well as its development are discussed.« less
  • To take benefit from the improved brilliance of the laser-like source, proposed beamlines at Free Electron Lasers (FEL) require optical elements of excellent precision, characterised by slope errors beyond the state of the art limit of 0.5{mu}rad rms for plane and spherical shape. Part of the monochromator beamline for self-seeding at the vacuum-ultraviolet Free Electron Laser (FLASH) at DESY is a triple Variable Line Spacing (VLS) grating of spherical shape. The three grating structures on a common substrate will cover the wavelength range from 6.4 to 60nm The challenging specifications of these grating structures are characterised by a slope errormore » of less than 0.25{mu}rad rms and very stringent parameters for the VLS-polynomial. These grating structures have been measured by use of the Nano Optic Measuring Machine (NOM) at BESSY. Based on the principle of deflectometry the BESSY-NOM represents the latest generation of slope measuring metrology instruments. The NOM enables the inspection of optical components with a measurement uncertainty in the range of 0.05{mu}rad rms. This is a five to tenfold improvement compared to state of the art metrology tools of today. Here it is demonstrated how these outstanding metrology capabilities have been applied for a sound characterisation of a challenging precision optical component with error limits five times below the specifications. In the shown example the grating's figure accuracy has been characterised by linescans and surface mapping measurements of the optical active sections. In additional measurements under Littrow condition the higher order diffraction signals of the laser pencil beam have been traced to measure the groove density variation of the different grating-structures with a lateral resolution of 1mm. In contrast to the sparse and point like measurements of the manufacturer, these high resolution measurements yield a 'slope deviation equivalent' resulting from imperfections in the line density variation.« less
  • A general theoretical approach based on the decomposition of statistical fields into a sum of independently propagating transverse modes was used for the analysis of the coherence properties of the new free-electron laser source FLASH operated at 13.7 nm wavelength. The analysis shows that several transverse modes are contributing to the total radiation field of FLASH. The results of theoretical calculations are compared with measurements using Young's double-slit experiment. The coherence lengths in the horizontal and in the vertical directions 20 m downstream from the source are estimated at 300 and 250 {mu}m, respectively.
  • A sustained filamentation or density depression phenomenon in an argon gas attenuator servicing a high-repetition femtosecond X-ray free-electron laser has been studied using a finite-difference method applied to the thermal diffusion equation for an ideal gas. A steady-state solution was obtained by assuming continuous-wave input of an equivalent time-averaged beam power and that the pressure of the entire gas volume has reached equilibrium. Both radial and axial temperature/density gradients were found and describable as filamentation or density depression previously reported for a femtosecond optical laser of similar attributes. The effect exhibits complex dependence on the input power, the desired attenuation,more » and the geometries of the beam and the attenuator. Time-dependent simulations were carried out to further elucidate the evolution of the temperature/density gradients in between pulses, from which the actual attenuation received by any given pulse can be properly calculated.« less
  • Newtonian fluid dynamics simulations were performed using the Navier–Stokes–Fourier formulations to elucidate the short time-scale (µs and longer) evolution of the density and temperature distributions in an argon-gas-filled attenuator for an X-ray free-electron laser under high-repetition-rate operation. Both hydrodynamic motions of the gas molecules and thermal conductions were included in a finite-volume calculation. It was found that the hydrodynamic wave motions play the primary role in creating a density depression (also known as a filament) by advectively transporting gas particles away from the X-ray laser–gas interaction region, where large pressure and temperature gradients have been built upon the initial energymore » depositionviaX-ray photoelectric absorption and subsequent thermalization. Concurrent outward heat conduction tends to reduce the pressure in the filament core region, generating a counter gas flow to backfill the filament, but on an initially slower time scale. If the inter-pulse separation is sufficiently short so the filament cannot recover, the depth of the filament progressively increases as the trailing pulses remove additional gas particles. Since the rate of hydrodynamic removal decreases while the rate of heat conduction back flow increases as time elapses, the two competing mechanisms ultimately reach a dynamic balance, establishing a repeating pattern for each pulse cycle. Finally, by performing simulations at higher repetition rates but lower per pulse energies while maintaining a constant time-averaged power, the amplitude of the hydrodynamic motion per pulse becomes smaller, and the evolution of the temperature and density distributions approach asymptotically towards, as expected, those calculated for a continuous-wave input of the equivalent power.« less
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