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Equipment on ID01
 

Optics on ID01

last modified 10-01-2012 14:37

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The ID01 Optics

The optics consists of a double-crystal monochromator located between two mirrors. This arrangement provides a fixed-exit monochromatic beam and maintains the focal spot constant during energy tuning. The X-ray fan is vertically reflected by the first mirror which will make the beam almost parallel so as to match the vertical divergence to the acceptance angle of the first flat Si monochromator. The first mirror is water cooled. The full horizontal divergence of the source is accepted by the second Si crystal which can focus sagittally. A fixed beam-stop located after the monochromator vessel intercepts the white beam and the Bremsstrahlung radiation. Finally meridional focusing is achieved by the second mirror which will also be used to reject the harmonics. The mirror surfaces is composed of three tracks corresponding to (i) uncoated Si, (ii) Rh coated Si and (iii) Pt coated Si in order to cover the whole energy range by translating the mirror.

The beamline can be operated without mirrors too. In year 2000 both of the mirrors and their associated bending mechanisms have been installed and tested. The best focusing peformance resulted in a vertical dimension of 40 µm at 12 keV at sample position. It is also possible to focus the beam at a position different from the one of the sample, e.g. detector or CCD camera. Typical size of the focused beam in the whole 6-16 keV energy range is 100 x 100 microns2 in a standard setup. Note that the sagital (horizontal) focusing works only in the above mentionned energy range. Outside it, over- and under-focused wider beams are obtained at sample position.

If smaller (micron sized) beams are needed, there is the option of mounting micro-focussing optics before the sample. Depending on the energy and desired focal distance and spot size, different setups can be proposed like Kirckpatrik-Baez reflective optics, planar-, circular-lenses or Fresnel Zone Plates. They should be booked and their setup discussed well in advance of an experiment.

Si crystal monochromators are available with the (111) orientations. They allow for an energy resolution of 1 eV on an absolute scale, after calibration by appropiate absorption edges.

Here is an overview table with some general characteristics for the ID01 beamline's optics:

Optical elements: Mirror 1 Double monochromator Mirror 2
distance from source: 33.4 m  34.1 m  36.1 m 
distance to sample:  infinity 12 m  9.2 m 
surface: cylindrical  flat/bent
(see also next table and figure)		 cylindrical 
focusing type:  vertical  horizontal  vertical 
beam size at sample (focused, ideal mirror): 0.2 x 0.06 mm2 (HxV) FWHM at 12 keV measured (Sept. 2000) 
beam size at sample (focused):  typically 0.1 x 0.1 mm2 in 6-16 keV energy range (measured Nov.2003)
spectral range:  2 - 42 keV 3.2 - 24 keV
Si(111) crystal		 2 - 42 keV
resolution in E/E:    <10-4   
expected flux at sample: 1.1 x 1013 ph s-1/in E/E= 0.1 %/0.2A/0.1x0.1mm2, at 8 keV
measured flux at sample: 9.8 x 1012 ph s-1/in E/E= 0.1 %/0.2A/0.1x0.1mm2, at 7.2 keV (without capton-foil monitors)
3 x 1013 ph s-1/in E/E= 0.1 %/0.2A/0.1x0.1mm2, at 12 keV

 

 

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Focusing double bounce Monochromator

 

 

General Description

 

 

Two Si(111) crystals, the first one cryogenically cooled, the second one equipped with a bender for sagittal focusing. Both of them rotate around a horizontal axis located on the surface of the first crystal. The second crystal is equipped with a piezoelectric actuator able to modify its Bragg angle in order to maximize the transmission of the monochromator (pitch adjustment) (for more details see Fabio Comin in Rev. Sci. Instrum. 66 (2) February 1995 pf 2082). SPEC related motors are:

"Theta": defines the Bragg angle of the monochromator crystals.
"nrj": gives the monochromator energy in keV.
"hgt": defines the relative height of the second crystal with respect to the first.
"pz1": gives the piezo voltage for the second crystal tunning.
"roll": allows for a rotation of the second crystal around an axis parallel to the beam (horizontal movement of the beam).
"yo": rotation of second monochromator crystal around surface normal.
"sf": pneumatic actuator for bending of second crystal.
Distance monochromator-source: 34m; distance monochromator-diffractometer center: 14m
Some reference positions for the monochromator related motors at different energies, without mirrors (Nov. 2003). (*)Note: The value of pz1 when the mirrors are in is around 4.5; (**)Note: The value of roll when the mirrors are in is around 6
Energy(keV) pz1(*) boffset mohgt roll(**)
24.35
20
17
16
14
12
10
8
7
6		 2.508
2.534
2.476
2.426
2.38
2.466
2.32
2.27
2.22
2.18		 33.83
33.86
33.86
33.56
33.568
33.42
33.27
32.8
32.646
32.036		 -1.33
-1.33
-1.83
-1.83
-1.83
-3.83
-4.13
-5.83
-8.83
-8.83		 6.24
6.54
6.24
6.24
5.79
5.74
5.29
5.09
5.065
4.965

Vacuum

The rotary feedthrough has three stages of differential pumping. There are no seals between the stages but only a small gap. This means that venting of one of the parts puts the others (and the main vessel) also in air. The first two low vacuum stages are in practice merged in only one stage and they are pumped with a mechanical pump. Normally the pump is only used during the conditioning of the vacuum and it is stop during normal operation. The second stage is pumped first with a turbo pump (during conditioning) and later with a small ion pump. The main vessel is ion pumped.

Typical pressures during operation in the main vessel 10-8 mbar

Cryogenic loop

Highly efficient cooling is necessary for a silicon monochromator crystal to overcome high heat load problems with the third generation undulator radiation. According to simulations done by L. Zhang (Optimisation design of optics for BL17, 7 July 1995) liquid nitrogen cooling is mandatory to have an acceptable thermal slope error (25 mrad at I=200 mA and at gap=16mm). An integrated channel cooling has been designed for the first crystal. The main problem is the way to connect the crystal to the tubing:
        Si is very fragile and optical surface allows a very low level of stress.
        The connection must be UHV compatible.
        Low temperature with differential thermal expansion and insulation problems.
A search in the cryogenic field brought out some basic technology:
        The use of invar with silicon (same thermal expansion at LN2 temperature).
        The use of indium for gasket (still very soft at LN2 temperature).
Prototypes were realised to test different combinations. Many tests have been achieved with the help of the technical service (drawing office and vacuum group) and the BLPO (M. Rossat). Satisfactory results have been obtained using metallic C-Seals with Indium coating. It appeared that, when correctly installed (surface parallel and free of scratch), the assembly was absolutely tight, even at 5 or 6 bar, at room temperature . But we observed pressure instability in the vacuum vessel during cooling down and when running at low temperature . A good diagnostic was not able to be done because of the cryogenic pumping effect on the cold surface of the crystal. It was very difficult to evaluate the leak and quite impossible to localise it. Some new calculations (L. Zhang) and measurements were done that showed that the intrinsic elasticity of the metallic gaskets was not sufficient to compensate the differential contraction during the cooling phase. This is mainly due to the fact that the cooling time for the crystal ( about 1 mn) is very different from the one of the Invar parts (some hours). Big improvements have been obtained by the addition of a set of elastic washers. But although no leaks can be observed when running in a stable regime there remains still a vacuum problem during transitory phases.

Typical settings:

    Frequency 30 Hz,
    all the pressures around 3 bars,
    Temperatures of the different sections ~ -190oC
    Temperature of Heater 14: -176oC

 

Motors

 

 

The monochromator vessel is supported on three mechanical jackets that allows z, qx and qy displacements. The qx and qy displacements are under the control of a two axis high precision inclinometer (Sensorex), for positioning and also for a safe operation of the two bellows, upstream and downstream the vacuum vessel, which stand a very small angle in qx

.

 

SPEC related motors are:

"mohgt": corresponds to the relative height of the monochromator with respect to the gider.
"moxti": corresponds to the absolute x angle.
"moxti": corresponds to the absolute y angle.

Sagittal Focusing

 

 

The second crystal is bent with a pneumatic actuator (a bellow) installed between the legs of the fixation of the second crystal. The maximum allowed pressure is 3.1 bars. The spec related motor is:

"SF": corresponds to the voltage sent to the pneumatic controller. It goes from 0 up to 10V. 
 
"roll": allows for a rotation of the second crystal around an axis parallel to the beam (horizontal movement of the beam).
Sagital focussing (sf) values versus the Energy
Energy(keV) sf value
7
8
10
12
14
16
17		 0
1.05
3.1
5.9
7.1
8.9
9.9

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Channel Cut monochromator

Page in construction


Fresnel zone plate (optional)

A giant linear Fresnel zone plate to focus the beam at the sample position

Authors: A. Mazuelas(1), P. Boesecke(1), H. Djazouli(1), T.H. Metzger(1), A. Snigirev(1), I. Snigireva(1), and C. David(2)

(1) European Synchrotron Radiation Facility (ESRF), B.P. 220, 38043 Grenoble, France
(2) Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, 5232 Villigen-PSI, Switzerland

In January 2003, we have installed a "giant" Fresnel Zone Plate (FZP) (4 x 6 mm2) to focus the ID01 beam horizontally.

The FZP is made of Si and is defined by electron beam lithography followed by chemically wet etching. The central groove is 75 microns wide, the outermost lines have a width of 0.35 microns, and the structures have a height of 10 microns (Fig.1). These parameters result in a focal length of 9 m, a phase shift of p, and a diffraction efficiency of 30% when inserted in the beam at about 7.3 keV.


Fig.1. Scanning Electron Microscopy images of the linear FZP. In a) general view, b) central aperture, and c) pitchs of 10 μm height

Test results of the FZP at ID01

The FZP was inserted in the beam, 38.5 m from the source and 9 m from the sample. There the beam has a horizontal dimension of 4 mm. The beam size, intensity and local gain at the sample position were measured. We have studied the dependence of beam size and local gain as a function of energy in the range 7 to 8.5 keV (Fig. 2 and 3). We found the optimum energy range is 7.2 to 7.4 keV, where the focused beam width was 0.1 mm. The experimental local gain was 9.2, for an expected theoretical value of 9.6. Besides the performance test, the focused beam has successfully been used in real experiments.

We find that this FZP is an attractive alternative to curvable sagital focussing monochromators.

Fig.2. Plot of the local gain as a function of lateral position (y) for different energies.

Fig.3. Graph of the beam size (FWHM) (squares) and local gain (circles) versus energy



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Other components

Front End Valve (Pneumatic)

It is installed inmediately after the tunnel wall at 27 m from the source.

This valve can isolate the front end vacuum section from the beam line. It is vacuum interlocked with a PLC that is in the technical gallery.

If the valve is not open and one tries to open the beamshutter with IDAPPLI an error message (Experiment Disabled ) is written in the Status window. To open or close the valve use Vacuum Aplication ID1, then select Front End, Options and Control.

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Primary slits

They consist of four (4) independent blades made of copper and water cooled. Temperature, pressure and water flow interlocks are installed. They are moved by stepper motors and they are equipped with absolute encoders.

See beamline layout figure to identify the names of the blades and their positions relative to the beam. The motor names associated are:
psu- Primary Slits Up
psd- Primary Slits Down
psf- Primary Slits Front (external side of the OH)
psb- Primary Slits Back (internal side OH, close to the wall)

The four blade positions are controlled by SPEC and also the verical and horizontal gaps. The related names are:
pshg- Primary Slits Horizontal Gap
psvg- Primary Slits Vertical Gap
psho- Primary Slits Horizontal Offset
psvo- Primary Slits Vertical Offset

The standard settings of the slits are 4mmH and 1mmV centered in the beam, with an accuracy of about 50 micrometers. The vacuum vessel is ion pumped, having a typical pressure in the low 10-8 mbar.

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Attenuator

There are three axis with 5 positions each that are used to attenuate the beam if necessary. The filters have the dimensions of 20x10 mm2 (Horizontal and Vertical) and they are water cooled.

To operate the attenuators run the Attenuator application in the Beamline Control Applications in the palette of the workstation. In the application program are written which kind of attenuator corresponds to each position.



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Secondary slits I

A pair of uncooled slits clean the white beam. They are step motor driven, equipped with limit switches and an incremental encoder. They are pumped with a 220 l/s ion pump. The power supply of the pump is in the Electronics Rack 01.
Typical peressure is 10-8 mbar.

The motors are controlled with SPEC. the names are
ss1u (secondary slit up)
ss1d (secondary slit down)
ss1f (secondary slit front)
ss1b (secondary slit back)
ss1vo (secondary slit vertical offset to define the center of the gap)
ss1vg (secondary slit vertical gap to define the aperture).
ss1ho (secondary slit horizontal offset to define the center of the gap)
ss1hg (secondary slit horizontal gap to define the aperture).


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Optic guider

All the optics elements are supported on an optical bank. The vertical position and angles are made by means for three step motor drivers, equipped with limit switches and incremental encoders. The vertical position of the "gider" with respect to the beam is given by a XBPM placed in its head. The angular position in x-y directions are given by a two axes inclinometer.

Spec related motors are:
gihgt which controls the gider height.
gixtil which controls the gider angle in the x-direction.
giytil which controls the gider angle in the y-direction.

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White beam position monitor

A Wire X-ray Beam position monitor is placed in the front part of the gider in order to repear the position of the white beam. The working principle is simple: the beam induces a current when the wire is in the x-ray beam. The WXBPM has two wires in parallel at 30mm of distance between both.
The WXBPM is moved by means of an step motor.
To operate the WXBPM run the WXBPM1 application in the Beamline Control Applications in the palette of the workstation. In the application program are written which the conditions of scans that are going to be used.

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Monochromator stabilizer (Mostab)

The Mostab works maintaining the transmission of the monochromator constant. This is done by adjusting the Bragg angle (by means of the piezo "pz1" command) of the second crystal in order to have the ratio between the output and input flux of the monochromator constant by means of a feedback loop. In most cases the Bragg angle of the second crystal is slightly detuned to reduce harmonics (80% of rejection is a normal value).
The electronic device is under development and in the meantime this function is sofware controlled. We use the ring current as the reference incident intensity and the diode-down as outgoing intensity.
The mostab program runs in the beam monitor SPEC version.

Optimization of the horizontal and vertical beam position at the sample:
Between refills the heat load on the monochromator crystals decreases with the synchrotron current. Due to differences in thermal expansions in the crystals the position of the beam can change slightly. The fine tuning of the monochromator is done in spec session 'monitor'. The vertical beam position is fine-tuned with the piezo motor ('pz1') that inclines the second crystal. The horizontal beam position is fine-tuned with the motor 'roll' that sweeps the second crystal about its longitudinal axis.
To reoptimize the beam intensity at the sample position manually, stop mstab by pressing {control-c}. Remove all detectors out of the direct beam, switch off the high voltage of the gasfilled detector and remove the fast shutter ('fsout' in spec session sixc) if it is not already done. Remove all filters ('filteroff;f0' in sixc). Then tweek or dscan the motors roll and pz1 manually to optimize the intensity of monitor 2 ('mon2') that records the beam intensity after the last slit in front of the sample:

e.g. 'twct roll 0.005'
'twct pz1 0.005'

If this is done restart the spec macro 'mstab' that stabilizes the ratio between the intensity of the monochromatic beam and the synchrotron current:
'mstab {diode-down intensity} {SR-current}'
Type 'ct 1' to get the current values of diode-down and the ring current. This stabilizes the vertical position of the beam at the sample position.
Put the filters back ('fm;filteron' in sixc). If you use the gas-filled detector do not forget to insert again the fast shutter ('fsin' in sixc) before you turn on the high voltage.

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Monochromatic beam position monitor

A combination of diagnostic elements are used to visualise the monochromatic beam (see overview of optical hutch).
Wire X-ray Beam position monitor gives the vertical position of the monochromatic beam. The working principle is simple: the beam induces a current when the wire is in the x-ray beam. The WXBPM has two Tungsten wires in parallel at 30mm of distance between both. The WXBPM is moved by means of step motor. To operate the WXBPM run the WXBPM2 application in the Beamline Control Applications in the palette of the workstation. In this application program are written which the conditions of scans that are going to be used.
A camera used as beam monitor . A pneumatic actuator drives in or out a phosporous coated foil at 45 degrees from the beam. The reflected visible light at approximately two theta 90 degrees is collected with a camera mounted in a suitable support. The pneumatic actuator is controlled with the command fluor In/Out in SPEC.
Two diodes placed in vertical . A pneumatic actuator drives in or out a kapton foil at 45 degrees from the beam. The diffused intensity at approximately two theta 90 degrees is collected with a two diodes placed up and down to the foil. The pneumatic actuator is controlled with the command kap In/Out in SPEC. And intensity is reported in the diode-up and diode-down counters.

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Secondary slits II

A pair of uncooled slits define the monochromatic beam. They are step motor driven, equiped with limit switches and an incremental encoder. They are vacuum pumped with a 220 l/s ion pump. The power supply of the pump is in Electronics Rack 1. Typical pressure 10-8 mbar.

The motors are controlled with SPEC. The names are:

ss2u (secondary slit up)
ss2d (secondary slit down)
ss2f (secondary slit front)
ss2b (secondary slit back)
ss2vo (secondary slit vertical offset to define the center of the gap)
ss2vg (secondary slit vertical gap to define the aperture).
ss2ho (secondary slit horizontal offset to define the center of the gap)
ss2hg (secondary slit horizontal gap to define the aperture).


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