Design Considerations
for a Prototype Beam Position Monitor
for the X-Ray Microscopy Beamline ID21 at the ESRF
G. S. Dermody1, C. J. Buckley1, J. Susini2, R. Barrett2
1
Department of Physics, King's College London, WCR 2LS, United Kingdom
2
European Synchrotron Radiation Facility, Grenoble Cedex, France
Abstract. The beam position monitor will be located after the focusing optics
at the monochromator exit slit, 50 m from the undulator source and is intended
to operate over the energy range 0.2–6.0 keV. Ray-tracing is used to evaluate
the size, location and photon density of the monochromatic beam at the exit
slits. This has been performed for the beamline in the two basic operation
modes and at several undulator K values. The results of this work provides the
information necessary to design detectors suitable for measuring the beam
position and intensity. The results are provided in this paper together with an
outline of prototype detection schemes.
1 Introduction
The distance from the undulator to the monochromator to exit slit on the X-ray
microscopy beamline at the ESRF is approximately 50 m. The undulator is constructed in three sections to provide both soft X-rays for imaging in the water
window and harder X-rays up to 6 keV. This energy range is too large to be covered
by a single monochromator, therefore the beamline has been designed to operate
with either a double crystal monochromator (DCM) or a plane grating
monochromator (PGM) without movement of the exit slit [1]. A beam position
monitor (BPM) will be installed at the exit slit to ease the initial alignment of the
optics and the interchange between imaging modes and to provide information on
the temporal and spatial stability of the probe during image acquisition.
Unlike second generation sources, the spatial and temporal stability of the probe
cannot readily be achieved by overfilling the apertures, slits and zone-plates of the
beamline [2]. Overfilling would result in the loss of photons exhibiting useful
coherence properties because the undulator beams are highly collimated and have a
much larger fraction of coherent flux within the central cone. The BPM will ensure
the stability of the probe by providing real time correction with a feedback loop
linked to a piezo-electric stage on the last mirror mount if needed.
Various designs for beam position monitors, including the use of impinging and
transmitting blades, have been described previously [3−8]. Here the detection scheme
is limited by the non-uniformity of the beam at the exit slit and the microscope itself.
The beamline layout is described briefly in section 2. The calculation of the flux
available to the monitor is described in section 3 and in section 4 a number of
detection schemes are outlined.
I - 182
G. S. Dermody et al.
2 The Beamline Layout
The beamline has been designed to enable the formation of a microprobe over the
energy range 0.2−6 keV. The optical layout is shown in figure 1. In both imaging
modes two horizontally deflecting plane mirrors stop bremsstrahlung radiation and
act as a low pass filter. The X-ray beam is then collimated by three 5×5 mm
apertures, the first two within the machine hutch and the third 27 m from the
undulator to remove scattered and diverging photons.
M od e 1
E x it S lit
P lan vie w
DCM
M od e 2
E x it S lit
P la n e m ir ro r
PGM
C ollim a to r
A p e r tu re
H o r iz o n ta l F o c u sin g M irro r
V e r tic a l F o c u sin g M irr o r
Fig. 1. Schematic diagram of the beamline optical layout.
The beam is focused onto the monochromator exit slit by a two component
Kirkpatric-Baez mirror system. In the harder X-ray region (1.6−6keV) both mirrors
precede the monochromator, whereas in the softer X-ray region (0.2−1.6keV) the
monochromator precedes the second focusing mirror.
3 The Beamline Simulation
The beamline was modeled to calculate the flux available to a BPM immediately
before the monochromator exit slit. The efficiency, geometrical transmission,
bandpass and focusing properties of each optic were used to calculate the energy
density distribution at the monochromator exit slit.
The intensity reflectivities Rp and Rs of a plane wave reflected from a smooth
surface were calculated from the Fresnel equations. The reflectivity from a coating
with a residual r.m.s. surface roughness σrms is then given by
[
Rσ = Rp exp −(4 πσ rms sin θ / λ )
2
]
(3.1)
with an analogous expression for s-polarised radiation.
The beamline was simulated using the geometrical ray tracing software
SHADOW [4]. Both beamline modes were traced at a number of wavelengths to
obtain photon density distributions at the exit slit, figure 2a. By assuming 100%
efficiency of all the optical surfaces the geometrical transmission factor, GTFλ , as a
function of wavelength was calculated. The simulation was also used to give the
Design Considerations for a Prototype Beam Position Monitor
I - 183
transmission and DCM bandpass from the traced rocking curves. The transmission is
given by the height of the curve after the second crystal and the bandpass BPDCM is
calculated from the differential form of the Bragg equation
∆λ ∆E
=
= ∆θ cot θ B
λ
E
where θB is the Bragg angle and ∆θ is the width of the rocking curve.
The total flux at the exit slit for both imaging modes is then given by
(3.2)
Fmod e1 = U B × U N × GTFλ × Tλ × TSi(111) × BPDCM
(3.3)
Fmode2 = U B × U N × GTFλ × Tλ × ξ PGM
(3.4)
and
2
2
where UB [photons/sec/mrad /mm into 0.1% bandpass] is the undulator brilliance,
UN normalises the fractional bandpass, Tλ is the combined transmission of the
mirrors, TSi(111) is the transmission of the DCM and ξPGM is the efficiency into the
PGM first order. From the flux curves, figure 2b and figure 2c), the photon density
distribution and the energy density for any wavelength can be calculated, figure 2c.
4 Detection Schemes
The monitor is required to measure accurately the position of the beam centroid with
high short and long term repeatability and to act as an incident flux monitor.
In the DCM mode the beam is small, has a clear centroid in both the horizontal
40
20
0
-20
-100
Flux [photons/sec into BPDCM
banpass]
Flux [photons/sec into 100%
bandpass]
60
a)
c)
7E+15
80
-50
0
50
100
Horizontal Position [µm]
1E+12
5E+15
4E+15
3E+15
2E+15
1E+15
0E+00
0
500
1000
1500
2000
Energy [eV]
60
1E+11
1E+10
1E+09
1E+08
1E+07
1E+06
2500
750 l/mm
1200 l/mm
1800 l/mm
6E+15
b)
Vertical Position [µm]
Vertical Position [µm]
100
50
40
30
20
10
34
47
0
20
17
7
40 27
-10
3500
4500
Energy [eV]
5500
6500
-40
d)
-20
0
20
Horizontal Position [µm]
40
Fig. 2. a) The photon distribution at the exit slit with the DCM set at λ=0.3nm, b) the flux
reaching the exit slit for PGM modes c) the flux at the exit slit for the DCM mode and d) the
energy density [mW/mm2] with the DCM set at λ=0.3nm at the exit slit.
I - 184
G. S. Dermody et al.
and vertical directions and a highly non-uniform beam profile. The main problem in
this imaging mode, therefore, is to use this profile to obtain the position of the beam
centroid without degrading the imaging probe.
The beam centroid can be located with a pinhole mounted on an X-y stage. By
minimising the photoelectric signal from the foil around the pinhole the beam
centroid can be located, figure 3a. A quadrant photon detector upstream of the
pinhole will detect any changes in beam position.
In the PGM mode the beam is focused into a vertical strip with continuously
varying wavelength. If only a single wavelength is considered the profile is similar to
that of the DCM mode. The location of the beam in this mode poses a more
interesting problem. In the horizontal, the position can be obtained by a single pair
of blades in the normal way. In the vertical direction, however, no simple solution to
give the beam position exists because the beam lacks a vertical centroid.
We are aiming to build a detector that will provide vertical position and intensity
normalisation of the X-ray probe at specific wavelengths. Three detection schemes
have been considered. The first consists of a slit mounted upon a linear feedthrough
in front of two off axis coated blades, figure 3b. The difference between the
photoelectron signal from the two blades would provide the horizontal position. By
choosing a blade coating material with an absorption edge within the working energy
range the vertical position could be found. As the slit is tracked across the blade a
drop in intensity would be observed at the absorption edge hence providing the
position and intensity of a specific wavelength from the grating. A number of
interchangeable blades with a number of different coatings would provide vertical
position and intensity normalisation at a number of wavelengths.
The second solution is also based upon the utilisation of a spectral absorption
feature. In this solution two slits are mounted upon a linear feedthrough in front two
off axis coated detectors, figure 3c. The detector coating is chosen such that a
NEXAFS resonance peak is within the working energy range. The vertical position
would be obtained by tracking the slits and detectors across the off axis portion of the
beam. As the slits move the resonance peak signal would be observed in the signal
from first one then the other detector. By positioning the slits so that the energy
exciting the resonance is obscured by the central stop any beam motion would result
in a fluctuation of signal in one detector, indicating the direction of motion. When
balanced the total signal can be used to normalise the probe at a specific wavelength.
a)
b)
Signal
Detector Slit
Side elevation
End elevation
Signal
Exit Aperture
Coated
Blades
c)
Slits
Coated
Detectors
Exit Aperture
Fig. 3. A schematic diagram of a) BPM using a range of pinholes, b) a BPM using an
absorption edge for vertical position and c) a BPM using a ZANES peak for vertical position.
Design Considerations for a Prototype Beam Position Monitor
I - 185
Both solutions are only possible because a third generation undulator source is
used to produce soft X-rays. In the energy range of interest, 0.2−1.6keV, the
undulator will operate in near wiggler mode producing a broad spectrum. This will
increase the number of possible coating materials with useful absorption features and
allow the assumption that intensity across the absorption features is constant.
The third solution, for the PGM mode, would combine one of the above PGM
solutions with the use of a higher diffracted order.
5 Conclusion
The beam parameters have been calculated at a number of undulator K values
spanning the energy range of interest. This has enabled a number of prototype
detection schemes to be evaluated. The prototypes will be constructed at King’s
College London in the latter part of 1996 and tested in house in early 1997.
Initially the beamline will operate in the hard X-ray region with the DCM. The
first monitor scheme to be installed will be the pinhole BPM which will also act as
the entrance aperture to the microscope. At a later date the addition of up to three
PGMs will enable the microscope to cover the water window. By this time the inhouse testing will have been completed and the BPM will be ready for ESRF
beamline testing.
Acknowledgments
G.D would like to thank the EPSRC for supporting this work and Oxford
instruments for sponsorship.
References
1
J. Susini and R. Barret The X-ray microscopy project at the ESRF, these proceedings.
2
J. Kirtz, H. Ade, C. Jacobsen, C.H. Ko, S. Lindaas, I. McNulty, D. Sayre, S. Williams,
X. Zhang and M. Howells, Soft X-ray microscopy with coherent X-rays, Rev. Sci. Inst.
63 (1) 557 1992.
3
B.A. Karlin, P.L. Cowan and C. Woicik, X-Ray, soft X-ray and VUV beam position
Monitor, Rev. Sci. Inst. 63 (1) 526 1992.
4
T. Mitsuhashi, A.Ueda and T. Katsura, High-flux photon beam position monitor, Rev.
Sci. Inst. 63 (1) 534 1992.
5
T. Warwick, D. Shu, B. Rodricks and E.D. Johnson, Prototype photon position monitors
for undulator beamlines at the Advanced Light Source, Rev. Sci. Inst. 63 (1) 550 1992.
6
Johnson, UHV photoelectron beam position monitor Nuc. Inst. and Methods A291 427430 1990.
7
T. Miyahara and T. Mitsuhashi, Self-tracking optical beam monitor, Rev Sci. Inst. 63 (1)
538 1992.
8
S.H. Southworth et al, X-ray beam position monitor using a quadrant pin diode, Nuc.
Inst. and Methods Vol A319 Iss 1-3 515 Aug 1992.