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Beamline Description

The LDM beamline together with the modern end-station opens an opportunity for scientists to study a broad range of target systems and perform a different type of experiments. The beamline has been commissioned and opened to users since 2012 and is undergoing rapid development.
The layout of FERMI and the main properties of its light (high-brilliance, short-pulse length, variable polarization, and coherence) have been described elsewhere [1-3]. The 100-4 nm wavelength range is covered by two distinct undulator chains: the long-wavelength FEL-1 (100-20 nm) and the short-wavelength FEL-2 (20-4 nm). The LDM beamline is operating in the full photon energy range of FEL-1 and FEL-2 and the main parameters of these ranges are summarized here.


FEL photons are delivered to the LDM end-station through the PADReS (Photon Analysis, Delivery, and REduction System) section of the FERMI FEL. A layout is sketched on the following Figure:



PADReS covers the section between the exit of the FEL and the beamlines it currently serves. A set of plane mirrors (PM1a and PM1b serving FEL-1; PM2a serving FEL-2) is installed in the so-called safety hutch. Each FEL source has its own beam-diagnostics and beam-conditioning instruments, such as the intensity and beam position monitors, beam defining apertures, and gas absorber. At the end of the safety hutch the two photon beam paths enter the PM1b chamber, where one or the other can be selected.
 
Outside the safety hutch, the energy spectrometer (PRESTO) records the spectrum of each pulse by diffracting and detecting 1–2% of the total intensity [4], while delivering the essentially unperturbed beam downstream. A split-and-delay line is installed after the spectrometer, for FEL pump – FEL probe experiments. The instrument is based on wavefront splitting and subsequent recombination of the incoming radiation on the edges of grazing-incidence plane mirrors, and can introduce a [-2,+30] ps-delay at all wavelengths.

The 3-way switching mirror chamber selects which beamline is in use. The plane switching mirror (SW) serving LDM deflects the beam in the horizontal direction, and has a dual coating (graphite and iridium, each covering half the width of the mirror), intended to maximise the reflectivity for FEL-1 and FEL-2, respectively. Downstream of the switching mirror, the beam undergoes three further reflections: on a vertical deflecting mirror (VD) and on two Kirkpatrick-Baez (K-B) mirrors [5].

The FEL light co-propagates with a certain amount of seed laser light, with a wavelength in the range 230–260 nm, and this light may interfere with the experiment. To suppress it, there are three retractable aluminium filters of thicknesses 200, 400 and 800 nm in the PADReS system.
The light enters the experimental station horizontally, where it can be routinely focused to a spot of about 30 μm diameter. Substantially better focusing has also been achieved when more time and/or other instrumentation was available.

Focusing performance


Calculations of the expected spot size and shape have been carried out for both sources (FEL-1 and FEL-2) in their whole wavelength range, using the codes SHADOW [6], based on ray tracing, and WISE [7], based on physical optics.
Calculated spot profiles in the case of smallest spot size (4 μm × 6 μm FWHM for FEL-1 and 3μm × 5μm FWHM for FEL-2) are represented in Figure. Here the effect of the non-ideal mirror shape can be seen as a broadening of the spots and the presence of some diffraction peaks.
The additional techniques have been adopted by PADReS to optimize focusing of the photon beam according to the needs of the user. For studies requiring microfocusing, the most informative diagnostic is a wavefront sensor (WFS): with it the focus down to an average spot size of 6.5μm (FWHM) has been achieved and it’s in a good agreement with the WISE simulations (see Figure) While for a larger spot size, above ≈20μm (FWHM), a simpler fluorescent screen (YAG, YAP, or phosphor) inserted at the nominal focal plane is used.



Simulated focal spots for the LDM beamline in the case of ideal (red) and real (blue) mirrors; WISE program was used. The intensity profiles are calculated for FEL-1 at 30nm and FEL-2 at 4 nm, and are displayed along the vertical and the horizontal directions. The diffraction effect is due to the finite size of the mirrors. The smallest achievable spot sizes (FWHM) are predicted to be ≈4μm×6μm for FEL-1 and≈3μm×5μm for FEL-2.

Beamline transmission



Due to the beamline geometry (five mirrors reflecting in the horizontal plane, two in the vertical one for FEL-1; four and two for FEL-2) the vertical and horizontal polarizations are not equally transmitted to the LDM endstation, thus the ellipticity varies across the whole wavelength range. The linear horizontal polarization suffers greater losses than the vertical one. While this effect is negligible below 40 nm, above this wavelength it rapidly increases. As an example the calculated overall transmission of the beamline (excluding geometrical losses) is shown in Figure. The choice of APPLE-2 undulators for FERMI allows a smooth control of the polarization of the emitted radiation: i.e., one can set the FEL polarization to be elliptical upstream of the transport optics, in order to have circular polarization radiation at the endstation [3].
The LDM beamline transmission is also affected by some geometric losses due to the finite size of the mirrors. For example, the geometrical acceptance at different wavelengths is reported in the following Table.



Wavelenght (nm) FEL Θrms (μrad) Geometrical acceptance (%) Total transmission (%)
65 1 81.3 41.3 22.7
52 1 65 54.5 31.6
43 1 53.8 68.1 38.8
32 1 40 86.9 51.3
20 1 25 99.3 56.6
20 2 30 97 55.3
<10 2 15 100 65.0
















Calculated transmission of the LDM beamline optics for the two FEL sources and the two linear polarizations (FEL-1: full (vertical) and dotted (horizontal) red curves; FEL-2: full and dotted blue curves). The geometrical losses have not been included in the graph.


[1] Allaria, E. et al., New J. Phys. 12(7), 075002, (2010).
[2] Allaria, E. et al., Nature Photon. 6(10), 699, (2012).
[3] Allaria, E. et al., Phys. Rev. X 4, 041040, (2014)
[4] Svetina, C.et al., Proc. SPIE, 8139, 81390J(2011)
[5] Svetina C et al, Proc. SPIE 8503, 850302, (2012)
[6] Sanchez del Rio et al., Journal of Synchrotron Radiation, 18 (5), 708, (2011).
[7] Raimondi, L. et al., Nucl. Instrum. Methods A, 710, 131, (2013).



The end-station consists of a differentially pumped vacuum system featuring interchangeable beam sources, three detectors that can be independently or simultaneously operated, all with single-shot capabilities, and several diagnostics units. Droplets and clusters can be further doped on-the-fly in a series of gas cells, ovens, and laser evaporation sources.

Chamber



The end-station itself is a modular design comprising six vacuum Chambers, attached to each other along the axis of the atomic/cluster beam in the following order: source chamber, doping chamber, differential pumping chamber, detector chamber, quadrupole mass spectrometer chamber, surface ionization detector chamber. This concept allows any chamber to be replaced to adapt to the needs of the experiments, which may require a non-standard configuration of the apparatus. Gate valves mounted between the chambers section the end-station off into four units providing additional flexibility and vacuum safety.





Schematic view of the experimental station: (1) Source chamber. (2) Doping chamber. (3) Differential pumping chamber. (4) Detector chamber. (5) Quadrupole mass spectrometer chamber. (6) Surface ionization detector chamber.


Beam sources

Beams of atoms, molecules, radicals and helium droplets as well as clusters of atoms, molecules and metals can be produced by different type of pulsed valves.

Pulsed Cryogenic Source

An Even-Lavie (E-L) valve based is used  for producing a dense sample of atoms or clusters particularly those of rare gases [1].


Temperature range 6-300K
Backing pressure range 10-100bar
Opening time 18-30 μsec
diameter of the nozzle 100 μm
diameter of the sealing gasket 50 μm


Pulsed High Temperature Source

This is a versatile source which can be used for the producing of atomic and molecular beams, water droplets beams, molecular clusters. In this source both the nozzle and an internal reservoir can be heated. Heating may be used to eliminate cluster formation of condensable gases, or to increase the vapour pressure of samples such as liquids. The construction of this E-L valve is similar to that of the cryogenic one. The main differences is a bigger gasket orifice (100 μm diameter) and the temperature range is 300-500K.

                                 

Pulsed Micro-Plasma Cluster Source

 


The PMCS (Pulsed Micro-plasma Cluster Source) is a pulsed-vaporization source of nanoparticles based on gasdynamic confined plasma sputtering process.  The clustering occurs in inert-gas atmosphere, but oxygen or other reaction species can be also introduced for the synthesis of compound clusters.
The method has been proven to be effective for the production of carbon or metallic clusters (form a large variety of metals: Mg, Al, Ti, Zr, W, Ta, Ni, Co, Pd Cu, Ag, Au, Pb), for oxide clusters (ZnO, TiO2) or other componds such as MoS2. Aerodynamic focusing of the clusters is exploited, yielding a particle beam intensity that can be in the range of 1019 s-1 sr-1.
A schematic representation of PMCS operation is shown in the figure and a detailed description is reported in [2].


 



Flash Pyrolysis and Laser Photolysis Source to generate radicals.

Doping


 The atomic and molecular beams may be seeded, and the clusters and    droplets may be pure, or doped with other atoms and molecules. For  these purposes 2 gas cells as wells as 2 ovens have been installed into  the doping chamber. Each doping oven has a water cooled shield and  can be heated up to 900 oC. The construction of the gas pick-up cell  allows direct measurement of the pressure inside its active volume using  a capacitance vacuum gauge which is insensitive to the type of gas.



Ovens and gas cells installed into the doping chamber. View from the side (a) and top (b).

Laser evaporation

here the laser and the ablated material can be adjusted to optimize the doping to the droplet beam. The ablation laser currently available at LDM is an Edgewave pulsed Nd-YAG laser [http://www.edge-wave.de/web/en/produkte/short-pulse-systeme/is-serie/]

  • Pulse energy: ~10 mJ
  • Wavelength: 532nm (SHG)
  • There is roughly 30% of the fundamental (1064nm) in the laser path.
  • Pulse length: low ns range 

Detection

The electrons, ions and x-ray scattering patterns produced during the interaction of the samples with intense light of FERMI can be analysed by the available spectrometers, to give mass spectra and energy as well as angular distributions of charged particles. The design of the detector allows simultaneous detection of electrons and ions using velocity map imaging (VMI) and time-of-flight (TOF) as well as diffraction images using photon scattering detectortechniques, respectively. The instruments have a high energy/mass resolution and large solid-angle collection efficiency.


Velocity map imaging spectrometer

The present VMI spectrometer consists of electrodes shaped as truncated cones, a flight tube and a detection system based on a Photonis MCP stack of 75 mm diameter, phosphor screen and the conventional CCD camera with a good spatial resolution (2560 ×2160 pixels, of size 6.5 microns) and the large dynamic range.[www.andor.com/scientific-cameras/neo-and-zyla-scmoscameras/neo-scmos].This custom-designed instrument has been built to detect charged particles in the kinetic energy range 1 to 100 eV, with calculated energy resolution of 2%, and experimental resolution of 4%.




(left) Cut-away view of the main chamber, showing the VMI and the TOF mass spectrometer. (right) Photograph of the detectors: (1) VMI MCP, (2) extractor, (3) interaction region, (4) repeller, (5) flight tube. The repeller–extractor gap is 18 mm. The photograph is inverted with respect to the drawing and its orientation when mounted.

TOF mass spectrometer

         

TOF mass spectrum of singly and doubly ionized Xe and residual gas. Photon energy: 21.7 eV.

The TOF spectrometer is located opposite to the VMI spectrometer, and the first accelerating region is defined by the VMI geometry. It may be operated at the same time as the VMI to obtain ion and electron spectra simultaneously, but the resolution of the TOF depends on whether it operates independently or simultaneously with the VMI. For example, the mass resolution is 350 when the VMI is set to detect electrons with maximum kinetic energy 30 eV (mode 1), while it is 1400 in the independent regime (mode 2).


 














Cut-away view of the main chamber, showing the VMI and the 2D scattering detectors.

2D scattering detector


The 2D scattering detector is based on the gated MCP combining with the P-43 type phosphor screen and the 3 mm center hole for the FEL beam propagation through the detector [3]. The actual size of the MCP is 75 mm diameter with the 25 μm pore diameter. The MCP is covered by additional gold and magnesium fluoride coatings to decrease the total MCP resistance (usually used for the gating application) and increase the quantum efficiency of MCP (from 10% up to 50%) in the 40-100 nm wavelength range, correspondingly. The images from the phosphor screen are projected to a CCD camera outside vacuum by the plane mirror installed at the 45° angle with respect to the FEL beam. The LDM scattering detector has been recently commissioned and as a result on the figure 8 the images of the single-cluster scattering pattern from a He droplet of diameter 950 nm, taken with FERMI pulses at a wavelength of 64 nm are presented.




First single-cluster scattering pattern from a He droplet detected by 2D scattering detector at LDM.


B-TOF spectrometer

The B-TOF mass spectrometer consists of two acceleration regions, an adjustable slit (or iris diaphragm), two permanent magnets (or Helmholtz coils), a time-off-light tube and a position sensitive detector based on the combination of the MCP with a position sensitive delay-line anodes, which allows unambiguous determination of arrival time and position of at least eight simultaneously detected particles. The spectrometer will operate in two regimes: without and with a magnetic field. The magnetic field with maximum 800 G will be applied during the second regime. 

Supersonic beam diagnostics

The instrumentation for the atomic and cluster beam diagnostics consists of an EXTREL MAX-1000 quadrupole mass spectrometer equipped with 90◦ion deflection optics. The instrument has a mass range of 0–1000 amu and resolving power of 1800.A Langmuir–Taylor surface ionization detector of custom design is also installed in a dedicated chamber with separate pumping system, and is suitable for detecting clusters doped with alkali or alkaline earth atoms [4].

‘Magnetic-bottle’ electron spectrometer

Recently this type of spectrometer designed and built by Raimund Feifel and co-workers [http://www.physics.gu.se/english/about-the-department/staff?languageId=100001&userId=xfeira] has been commissioned at LDM. The name ‘magnetic-bottle’ arises from the shape of the magnetic field (see figure 9). Magnetic-bottle spectrometer can be employed for electron detection both normal time-of-flight methods as well as zero-kinetic energy electron detection with almost laser-limited resolution  The electrons are detected at the end of the flight tube by two microchannel plates (MCPs) chevronned together The magnetic-bottle electron spectrometer has the advantage of high detection efficiency for the detached electrons. The intrinsic energy resolution of the magnetic-bottle spectrometer depends on various parameters like the length of the drift region, the length of the laser pulse, the details of the magnetic fields, and the size of the laser focus.








[1] D. Pentlehner, R. Riechers, B. Dick, A. Slenczka, U. Even, N. Lavie, R. Brown, and K. Luria. Rapidly pulsed helium droplet source. Rev. Sci. Instr. 80, 043302 (2009).
[2] E Barborini, P Piseri and P Milani. A pulsed microplasma source of high intensity supersonic carbon cluster beams. J. Phys. D: Appl. Phys.32, L105–L109 (1999).
[3] C. Bostedt, M. Adolph, E. Eremina, M. Hoener, D. Rupp, S. Schorb, H. Thomas, A. R. B. de Castro, and T. Möller. Clusters in intense FLASH pulses: ultrafast ionization dynamics and electron emission studied with spectroscopic and scattering techniques. J. Phys. B 43, 194011 (2010).
[4] Stienkemeier F., Wewer M., Meier F. and Lutz H. O.,Langmuir–Taylor surface ionization of alkali (Li, Na, K) and alkaline earth (Ca, Sr, Ba) atoms attached to helium droplets. Rev.Sci. Instrum. 71, 3480 (2000).
[5] A.M. Rijs et al.,‘Magnetic bottle’ spectrometer as a versatile tool for laser photoelectron spectroscopy. Journal of Electron Spectroscopy and Related Phenomena 112, 151–162, (2000).

Laser emission source for LDM experimental station  

The seed pulse starts from a femtosecond Ti:Sapphire oscillator (Vitara, Coherent) which is repetition rate locked to the reference timing signal distributed over the facility. The main part of the oscillator output beam is amplified in a chirped-pulse regenerative amplifier-single pass amplifier tandem, which delivers pulses with an energy of up to 7 mJ and duration about 100 fs at wavelength 784nm (14 nm FWHM).
after leaving the Ti:Sapphire amplifier the IR pulse is split and about 70% of its energy is used to pump an OPA (optical parametric amplifier OPerA-Solo, Coherent) and for THG (third harmonic generation).  UV emission from OPA or THG is used as seed for FEL operation. In the other arm, the remaining IR pulse energy (typically about 1.5 mJ) propagates to the experimental stations along a total distance of about 150 m. The beam propagation to the beamlines preserves a very good beam quality, and introduces a slight positive chirp and pulse lengthening to about 120 fs due to material dispersion. The duration of the transported pulse can be varied in the range 80-250 fs range by a compact transmission grating pulse compressor installed at the end of the beam transport system.

 

LDM beamline optical setup description 

The optical setup used to prepare and deliver the external laser pulse for pump-probe experiments at the LDM end-station includes two optical breadboards which are used for laser beam manipulation, insertion into the chamber and diagnostics. The breadboards are covered with protective boxes and filled with nitrogen to prevent dust contamination if optical components. CCD1 controls the position and spatial parameters of the incoming beam after the vacuum window W1 and is incorporated in the beam transport system. The motorized mirror mount M1 and CCD2 allow to control the laser beam direction of propagation on the optical breadboard. An optical attenuator, made of half waveplate (l/2 @ 784 nm)WP1 and two thin film polarizers (PL1, PL2), allow adjustment of the exact pulse energy delivered to the region of interaction.  A translation stage TS1 (PI M403.8PD) provides a variable pulse delay relative to the FEL pulse in the range of +-570ps with minimum step delay of 1.6 fs. The setup includes a high-bandwidth copper coax cable based antenna. A directly at crossing position of laser and FEL beam which allows to estimate time delay between laser and FEL pulses with fast oscilloscope (typical resolution of pulse position less than 50 ps). The fast photodiode (PD) signal is used as a trigger source for the oscilloscope in this case.   A compact single-shot auto-correlator AC, build from Fresnel prism, thin BBO SHG crystal and CCD3 camera, is installed for input pulse duration diagnostics. It is possible to change laser wavelength to 392 nm or to 261 nm by generating second harmonic (SH) or third harmonic (TH)  in BBO crystals,. In the TH case there are additional calcite time delay plate TDP and double wavelength waveplate (l/2@780 nm,l@390 nm) WP2. Interference filter F1 (HT@390 nm, HR@780nm) is used to select for operation only second harmonic emission when required. Set of four mirror with HR@260 nm, HT@390 nm HT@780nm is used as filter F2 to select only the TH UV emission. Polarization state control of the laser light arriving in the chamber is performed by using rotation stage RS2 and flipper FL2, where there are installed waveplates. Half wave waveplates WP l/2 are installed on rotation stage RS2 andquarter wave waveplates WP l/4 are installed on flipper FL2. This design provides possibility to provide horizontal/vertical linear polarization or left/right circular polarization of the emission for any operating wavelength (changing set of l/2 and l/4 waveplates). Energy meter heads EM1 and EM2 allow to control pulse energy delivered in region of interaction. Optical system including three lenses L3, L4, L5 provide focusing laser beam to a spot diameter down to 80 μm (at level 1/e2). The lens L3 is mounted on motorized translation stage TS2 (Standa 8MT173-25), that allows to keep focus position at switching between the above mentioned of operation wavelengths or to increase beam diameter up to 500 μm (at level 1/e2). CCD cameras CCD4 and CCD5 are used as virtual focal points and allow controlling beam size and position in the interaction region. CCD4 is used for operation at fundamental (784 nm) and second harmonic (392 nm) wavelength. CCD5 and UV-visible converter UVC are used for operation at third harmonic (261 nm) wavelength. Motorized mount M2 with incorporated piezo Tip-Tilt mirror allow to precisely scan the beam position or used as pointing stabilization system with beam position stability (rms) less than 5μm.  IR pilot laser IRPS is installed on the breadboard to assist the preparation of the experiment when the SLU is used by other beam lines at Fermi. 
 

Main parameters (see also here)

Parameter Value

Laser emission wavelength (nm)

784 nm
392 nm
261 nm

Maximum pulse energy

750 μJ @ 784 nm
100-200 μJ @ 392 nm
20-50 μJ @ 261 nm

Pulse energy stability (rms)

<0.5 % @ 784 nm
<1.0 % @ 392 nm
<1.0 % @ 261 nm

Pulse duration, fs ( FWHM)

80-250 @ 784 nm
80-120 @ 392 nm
120-170 @ 261 nm

Pulse repetition rate

1-10 Hz, 50 Hz

Timing jitter relative to FEL (rms)

<10 fs

Scan range pulse delay relative to FEL 

-570…+570 ps

Minimal step delay

1.6 fs

Beam diameter at region of interaction (at level 1/e2)

80-500μm

Beam position stability (rms)

<3μm



The end-station has a data acquisition system developed by the FERMI controls group, in collaboration with the LDM staff. This system is integrated into the control system of the facility and is based on the TANGO control framework [1]. It features the facility to configure and control the acquisition hardware, which includes a CAEN digitizer, model VX1751, 1 GS s1(S: Sample, 10 bits) on eight channels, with up to 100 μs of acquisition per shot, and the camera for the VMI spectrometer.
Several trigger signals are necessary, for example to open the gas source valve or the shutter of the camera, to provide time zero for the TOF mass spectrometer, and for ancillary as well as future instrumentation. These signals are generated by the timing system that synchronizes the operation of the entire FEL and, in particular, arms the firing of a given FEL shot [2]. Trigger and pretrigger signals can be made available at dedicated distribution ports at each beamline: LDM has 12 such ports. An arbitrary amount of delay can be ADDED via user-accessible software panels to each channel separately.
Presently the acquired data include the VMI image, the digitized traces from the TOF mass spectrometer, all end-station parameters like pressure and voltages, as well as a few machine parameters like the photon spectrum and the shot intensity (both from the PADReS diagnostics). The user can select what data is to be stored for the specific experiment. The acquisition system is flexible and other instruments can be added as long as a real-time TANGO driver is available.
The acquired data can be analysed almost in real-time (with a delay of a few seconds, depending on acquisition settings) by a fully configurable pre-processing engine that can apply user-defined algorithms and return the results via the TANGO system, so that they can be displayed while the acquisition is still running. The acquired data is stored using the HDF format [3] in a central storage server and can be directly accessed through the local network for further analysis a few minutes after the acquisition is done.


[1] www.tango-controls.org
[2] Ferianis M. et al., Proc. 33rd Int. Free Electron Laser Conf.
[3] www.hdfgroup.org
Last Updated on Wednesday, 17 June 2015 16:06