- Email: [email protected]
A Bragg-Fresnel multilayer electron beam monitor for third generation storage rings
__ Ifi
CQSJ
Nuclear Instruments and Methods in Physics Research A 36.5(1995) 40-45
mm
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH Sectron A
ELSEVIER
A Bragg-Fresnel multilayer electron beam monitor for third generation storage rings K.Holldack Berliner Elektronenspeicherring
*,
A. Erko, T. Noll, W.B. Peatman
Gesellschaj? fir Synchrotronstrahlung (BESSI? m.b.H, Lentzeallee 100, I4195 Berlin, Germany Received 27 March 1995
Abstract In anticipation of the small emittance of the third generation light source, BESSY II, we have developed a new approach to beam diagnostics based on the use of Bragg-Fresnel multilayer (BFM) optical components which will be able to withstand the high heat load expected. We report here on the successful performance of such a system as tested on a BESSY I dipole source. Because BESSY II will provide significantly smaller electron beam dimensions than BESSY I, the system was designed for operation at 6.53 nm (190 eV>, well below the diffraction limit of BESSY II. The optical system, designed with a reflecting lens and plane mirror, provides 1: 0.8 image transfer with a spatial resolution of N 7 p,m. The source shape for different ring currents and operation modes of the storage ring has been measured and is represented here.
1. Introduction BESSY II will be a third generation storage ring providing a very small and high brightness VUV and soft X-ray source with dimensions considerably less than 100 pm. Therefore, the diffraction limited optical imaging as usually applied to beam diagnostics at second generation sources will be not feasible to perform imaging with sufficient resolution of less than 10 pm as needed to obtain size information of the BESSY II source as determined from diffractive optical calculations [l]. Hence, one has to image the source in the X-ray range. At the advanced light source (ALS) a grazing incidence Kirkpatrick-Baez imaging system has been developed [2] using soft X-rays. In the hard X-ray energy range (more than 8 KeV) a crystal-based Bragg-Fresnel system has been recently tested at the ESRF [3]. For BESSY II, we have developed a new approach to beam diagnostics based on the use of Bragg-Fresnel multilayer (BFM) optical components [4] developed in IMT RAS [5] which will be able to withstand the high heat load expected. Compared with a previously reported BFM microfocus optical system [6], in this work we have designed a system based on a BFM lens in back-scattering geometry which provides a wide acceptance angle which is the main disadvantage of grazing incidence systems.
* Corresponding author. Tel. +49 30 82004 157, fax +49 30 82004 149.
We report here on the successful performance system as tested on a BESSY I dipole source.
of such a
2. X-ray optical scheme
The general set up is shown in Fig. 1. The diagnostic test system consists of two imaging systems which are installed outside of the optical path of a monochromator beamline. In the middle of the beamline a pinhole array camera [7] having a rather moderate image resolution is being tested for its application to BESSY II dipole front end monitors. Because its images can be directly compared to the image of the Bragg-Fresnel-system it acts a useful reference. On the right side of the beamline a small horizontal part (1 mrad) of the beam is accepted by the BFM lens. The BFM lens combines the energy dispersion of a multilayer and the focusing properties of a Fresnel zone plate as shown in Fig. 2. Hence, we expect a monochromatic image of the source. The beam is reflected under the Bragg angle of 2.8” onto a second multilayer of the same period as the first one, which reflects the beam again under 2.8” onto an Al-coated phosphor screen. To obtain 2-D focusing the BFM lens with circular zones was made using: electron beam lithography, optical lithography and ion beam etching processes. First, a Fe,O, mask on a glass substrate was obtained by electron beam lithography and chemical etching. Then, this mask was used for ‘JV lithography on the W/C multilayer coated with photoresist.
0168.9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00407-6
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res.
365 (1995)
-Apertures Multilayer Mirkor
Bragg-Fresnel Lens 7.1 m
12.7 m
3
= phosphorscreens (AL coated)
Ill
7
JJJL-heam size, beam position 50 Hz WritMNlt Fig.
’ to VLS-monochmmator
I. Diagnostic test beamline on a BESSY I bending magnet.
/I
Multilayer
Fig. 2. Optical image and cross section scheme of the BFM lens.
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (I 995) 40-45
42
Fig. 3. Aberrations and coma of a circular BFM lens.
Finally, an RF-excited ion-beam source was used to etch the multilayer substrate through the photoresist mask. In this work a W/C multilayer mirror prepared by magnetron sputtering at the IMT RAS by A. Yakshin has been used as
1 .5
1
(a)
-1.5 I -1.5
a substrate for the BFM lens. This material was chosen for technological reasons but is not the best for this wavelength region. The period spacing of the W/C bilayers was 3.3 nm and 130 layers were coated. The zone plate consists of 589 zones and has a focal length of 3.17 m. The calculated Bragg angle at 6.53 nm was found to be of 87.2”. The aperture of the lens was 7.1 mm and the minimum zone width 3 Fm. The main limitations of the resolution of the BFM lens arise from technological parameters (minimum zone width) and optical aberrations. The technology as used in this work limits the minimum zone size to 3 p,rn. Aberrations are more serious at off-axis angles of incidence. To analyse the off-axis aberrations, one can use an optical path difference assuming a rectangular groove profile. Thus, it is possible to find an analytical expression for the main off-axis aberrations: coma, astigmatism and field
1 .5 1.0
-1 .5 -1.0
-0.5
0 .o
0.5
1 .o
1 .5
-1.5
.o 8
0
.o a
E
E > x
0
0
1 -1.0
I. -0.5
I. 0.0
I. 0.5
x(mm)
X(mm)
0.1
W
-
I
0.10
0 08
0 -0 4
0 .02
0
.o
I
0
5780
5800
5820 7.
5840
(mm)
Fig. 4. Comparison of the imaging properties of a spherical mirror (a) and an elliptical BFM lens (b).
I. 1 .o
1 .5
43
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 40-45
curvature. According to the Rayleigh criterion, the angle of view, limited by coma, astigmatism and field curvature is
2.5
(a)
Peak
energy
FWHM
189.84eV 1.8eV
: 175
180
185
190 Energy
195
200
205
10
15
210
(eV)
where Armi, is the minimum zone size, F the focal length and 6n the Bragg angle. As an example, the field of view of the BFM lens characterised above is shown in the Fig. 3. Those parameters were chosen to demonstrate that astigmation and field curvature dominate close to normal incidence. Since we use a circular zone profile, the diffraction aberration characteristics of the optical scheme, shown in Fig. 1 can be calculated using aberration formulas for transmission zone plates [8]. Astigmatism is the main aberration for the reflecting BFM lens at normal incidence. The angular field of view was calculated using the formula for astigmatism and field curvature aberration:
where 6 is the desired spatial resolution in the horizontal plane, F is the focal distance, A is the wavelength. Thus, using the above mentioned lens parameters, the accepted opening angle of the source will be equal to N 56 mrad with 7 pm resolution. The field of view for our optical scheme covers our off-axis angle and is equal to 49 mrad (2.8”). For the beam reflected at 189.84 eV photon energy the image resolution of the BFM lens in the vertical direction is given by the width of the outer zone 1.22 Armin - 3.6 p.m and in the horizontal direction limited by coma to - 7 pm. To estimate the advantage of an elliptical BFM lens compared with spherical imaging optics the ray-tracing of the optical scheme, shown in Fig. 4, has been performed. Two possible optical elements, a multilayer coated spherical mirror and elliptical-shaped BFM lens have been compared. The ray-tracing code RAY [9] has been applied using a four-pixel array source. The stigmatic images were plotted for both cases in Fig. 4. One can see, that by the strong astigmatism of the spherical optics the test pattern in the left plot cannot be resolved. In case of the BFM
-10 Angle
-5 of
0 incidence
5
a0
(deg.)
Fig. 5. Energy dependence of the efficiency (a) and rocking curve (b) of the second mirror with first mirror settled at 2.8”.
lens, the stigmatically corrected image can be reproduced with 3 pm resolution. The energy dependence of the reflection efficiency as well as the rocking curve of the double-mirror system is shown in Fig. 5. Because the rocking curve is rather wide, the angular sensitivity of the adjustment of the Bragg-angle and thus, the sensitivity of the image intensity on the emission angle of the radiation is negligibly small. The large angular aperture leads to an insensitivity against emission angle changes and only direct beam position changes are detected. The depth of focus as defined by the energy resolution of the multilayer is 6.6 cm. However, the aperture of the lens defines a smaller depth of focus of 1.7 mm as given by the curvature of the electron beam in the storage ring.
3. Mechanical
and thermal performance
The BFM lens can be horizontally tilted and vertically moved and tilted, respectively, to fit the center of the lens to the angular maximum of the beam in a special UHV optical bench. The BFM lens is mounted on a lapped copper base which is cooled by means of a short, flexible copper braid which, in turn, is connected to a water cooled block. Both horizontal and vertical rotation as well as vertical translation of both elements with a minimum
44
K. Holldack et al. /Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 40-45
accuracy of 0.1 mrad were performed to adjust the image with respect to the phosphor screen, which is mounted 5.3 m further along (see Fig. 1). Several apertures in the system prevent heat loading of the mechanical components as well as stray light, which has to be blocked for a background-free detection of the image.
4. Image detection and analysis The image was detected by a phosphor screen with a 50 nm Al cap layer on the backside in order to make it solar blind. Different phosphors have been tested in a preliminary study. The best performance was achieved using a terbium doped gadolinium oxisulfide phosphor screen of 2 Frn grain size and 3 pm thickness. The phosphor in use has a sufficiently small grain size and is roughly a factor of 10 better in fluorescence yield at 189 eV than a Bi,Ge,O,,-single crystalline phosphor and 30% more efficient than a ZnS(Cd)-based screen (P20). The Al cap layer of 50 nm thickness gives optimum performance as determined by three effects: i) Visible photons from the phosphor become reflected from the cap layer on the backside which leads to an intensity enhancement by a factor of 2 (solid angle). ii) Visible photons which are divergently reflected by the multilayer mirrors are absorbed. iii) X-rays with 189 eV are efficiently transmitted. The optimisation was done using the REFLEC code [8]. It was shown that even with its relatively poor transmission at 189 eV (just above Al 2p edge) aluminium of 50 nm thickness is better than other cap layers as, for in-
stance, carbon, which does not reflect in the visible although it is capable of transmitting more X-rays. The magnification of the system at the test beamline is 0.8. The demagnified image of the source on the phosphor is then magnified (0.8 . . . 2) by a special objective lens on the air side of the beamline onto a l/2 in. chip of a CCD Camera with 12 pm pixel size. The latter is read out by a PC based frame grabber and image processor. The camera image is usually digitised to 512 X 512 pixels. The fastest method to extract the position and the beam size from the image is performed as follows: Firstly, the pixel intensities are horizontally averaged using a standard procedure of the image processor which writes the result into one line of an intermediate image into the framestore memory. The resulting intensity distribution, I, contains the horizontal projection of the image. The vertical centre of the image is then calculated by: 512
iZi
C
y=+,
(4) Czi 1
while the rms value of the projection from:
a,,,
is calculated
512
Czi %
= z(Y;fi
(5)
’
assuming a Gaussian shape for a faster read out. The values calculated this way are the relevant source parame-
is!y+(+f~ gJ)+(y< 2000
3000
2000
4000
3000
I
A
iii~~~
1000
BESSY I
5000
vettical
3000
2000
position [urn]
Fig. 6. Monochromatic images of the
4000
position [pm]
bending magnet source using the BFM monitor for normal operation of the storage ring (a) and small source operation (b).
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 40-45
500ym
700mA
621mA
587mA
367mA
280mA Fig. 7. Ring current dependency of the source size at BESSY I during a single fill.
ters for the users. However, a more complicated but slower image analysis can correct for the a non-Gaussian shape and a possible tilt of the source. After that the same procedure is performed vertically to determine the horizontal centre and size. First results of the source imaging are shown in Fig. 6, where one can see the BESSY I source in a bending magnet at different operation modes of the storage ring. The ring current dependence is represented in Fig. 7’. The read out rate of the source parameters lies between 50 Hz (position only) and 5 Hz (full information). However, using W/C multilayers a fast readout leads to a noise limited resolution at ring currents lower than 100 mA. In this case an analysis of single video frames is not feasible so that an averaging of a number of frames up to a number of 2.56 must be made.
5. Conclusions and further developments As shown above the use of Bragg-Fresnel optics permits beam monitoring of sufficient resolution as required for X-ray beam monitoring at third generation storage
45
rings. Monochromatic images at 190 eV and a spatial resolution of about 7 pm have been registered in video real time and analysed down to ring currents of 50 mA. A more sensitive camera using an image intensifier will be obtained to extend the dynamic range. Finally, we should mention, that a factor of 50 to 100 in intensity can be won using a Mo/Si multilayer. Both improvements will extend the dynamic range of the system down to 1 mA ring current and the image resolution to 0.5 pm. The main disadvantage of the system described above arises from the fact that the detector has to be about 5 m behind the optics which makes the adjustment and the fit to a BESSY I1 front end complicated. Therefore, a new design in preparation will be tested for use at BESSY II. This system will employ two Bragg-Fresnel lenses in a telescope geometry, where the plane mirror in Fig. 1 is simply substituted by a second BFM lens of short focal length. This way, a 1 : 1 magnification will be realised within a distance of about 60 cm. The very compact system can be used to image the source at bending magnets as well as at undulators at 130 eV photon energy regardless of its position in the front end or the beamline as long as it is in the range between 7 and 20 m.
Acknowledgement We gratefully acknowledge the co-operation with the IMT RAS especially with N. Garnakova, Yu. Koval and S. Piatkin for the lens preparation. Furthermore we are indebted to F. Schafers for fruitful discussions on raytracing and P. Kuske for the source size variation in the storage ring.
References
ill
A. Hoffmann and F. Meot, Nucl. lnstr. and Meth. 203 (1982) 483. I21 R.C.C. Perera, M.E. Melczer, A. Warwick, A. Jackson and B.M. Kincaid, Rev. Sci. lnstr. 63(l) (1992) 541. [31 E. Tarasona, P. Elleaume, J. Chavanne, Ya.M. Hartmann, A.A. Snigiriev and 1.1. Snigirieva, Rev. Sci. lnstr. 65(6) (1994) 1959. 141A.I. Erko, J. X-Ray Science and Technology 2 (1990) 297. Technology of the Russian PI Institute of Microelectronics Academy of Sciences. 161A.I. Erko, Yu. Agafonov, L.A. Panchenko, A. Yuakshin, P. Chevallier, P. Dhez and F. Legrand, Optics Commun. 106 (1994) 146. [71 K. Holldack and W.B. Peatman, A pinhole array camera X-ray electron beam monitor, BESSY Annual Report 1994, in press. 181M. Young, J. Opt. Sot. Am. 62 (1972) 972. [91 The RAY and the REFLEC code are developed by F. Schafers (BESSYJ.
CQSJ
Nuclear Instruments and Methods in Physics Research A 36.5(1995) 40-45
mm
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH Sectron A
ELSEVIER
A Bragg-Fresnel multilayer electron beam monitor for third generation storage rings K.Holldack Berliner Elektronenspeicherring
*,
A. Erko, T. Noll, W.B. Peatman
Gesellschaj? fir Synchrotronstrahlung (BESSI? m.b.H, Lentzeallee 100, I4195 Berlin, Germany Received 27 March 1995
Abstract In anticipation of the small emittance of the third generation light source, BESSY II, we have developed a new approach to beam diagnostics based on the use of Bragg-Fresnel multilayer (BFM) optical components which will be able to withstand the high heat load expected. We report here on the successful performance of such a system as tested on a BESSY I dipole source. Because BESSY II will provide significantly smaller electron beam dimensions than BESSY I, the system was designed for operation at 6.53 nm (190 eV>, well below the diffraction limit of BESSY II. The optical system, designed with a reflecting lens and plane mirror, provides 1: 0.8 image transfer with a spatial resolution of N 7 p,m. The source shape for different ring currents and operation modes of the storage ring has been measured and is represented here.
1. Introduction BESSY II will be a third generation storage ring providing a very small and high brightness VUV and soft X-ray source with dimensions considerably less than 100 pm. Therefore, the diffraction limited optical imaging as usually applied to beam diagnostics at second generation sources will be not feasible to perform imaging with sufficient resolution of less than 10 pm as needed to obtain size information of the BESSY II source as determined from diffractive optical calculations [l]. Hence, one has to image the source in the X-ray range. At the advanced light source (ALS) a grazing incidence Kirkpatrick-Baez imaging system has been developed [2] using soft X-rays. In the hard X-ray energy range (more than 8 KeV) a crystal-based Bragg-Fresnel system has been recently tested at the ESRF [3]. For BESSY II, we have developed a new approach to beam diagnostics based on the use of Bragg-Fresnel multilayer (BFM) optical components [4] developed in IMT RAS [5] which will be able to withstand the high heat load expected. Compared with a previously reported BFM microfocus optical system [6], in this work we have designed a system based on a BFM lens in back-scattering geometry which provides a wide acceptance angle which is the main disadvantage of grazing incidence systems.
* Corresponding author. Tel. +49 30 82004 157, fax +49 30 82004 149.
We report here on the successful performance system as tested on a BESSY I dipole source.
of such a
2. X-ray optical scheme
The general set up is shown in Fig. 1. The diagnostic test system consists of two imaging systems which are installed outside of the optical path of a monochromator beamline. In the middle of the beamline a pinhole array camera [7] having a rather moderate image resolution is being tested for its application to BESSY II dipole front end monitors. Because its images can be directly compared to the image of the Bragg-Fresnel-system it acts a useful reference. On the right side of the beamline a small horizontal part (1 mrad) of the beam is accepted by the BFM lens. The BFM lens combines the energy dispersion of a multilayer and the focusing properties of a Fresnel zone plate as shown in Fig. 2. Hence, we expect a monochromatic image of the source. The beam is reflected under the Bragg angle of 2.8” onto a second multilayer of the same period as the first one, which reflects the beam again under 2.8” onto an Al-coated phosphor screen. To obtain 2-D focusing the BFM lens with circular zones was made using: electron beam lithography, optical lithography and ion beam etching processes. First, a Fe,O, mask on a glass substrate was obtained by electron beam lithography and chemical etching. Then, this mask was used for ‘JV lithography on the W/C multilayer coated with photoresist.
0168.9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00407-6
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res.
365 (1995)
-Apertures Multilayer Mirkor
Bragg-Fresnel Lens 7.1 m
12.7 m
3
= phosphorscreens (AL coated)
Ill
7
JJJL-heam size, beam position 50 Hz WritMNlt Fig.
’ to VLS-monochmmator
I. Diagnostic test beamline on a BESSY I bending magnet.
/I
Multilayer
Fig. 2. Optical image and cross section scheme of the BFM lens.
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (I 995) 40-45
42
Fig. 3. Aberrations and coma of a circular BFM lens.
Finally, an RF-excited ion-beam source was used to etch the multilayer substrate through the photoresist mask. In this work a W/C multilayer mirror prepared by magnetron sputtering at the IMT RAS by A. Yakshin has been used as
1 .5
1
(a)
-1.5 I -1.5
a substrate for the BFM lens. This material was chosen for technological reasons but is not the best for this wavelength region. The period spacing of the W/C bilayers was 3.3 nm and 130 layers were coated. The zone plate consists of 589 zones and has a focal length of 3.17 m. The calculated Bragg angle at 6.53 nm was found to be of 87.2”. The aperture of the lens was 7.1 mm and the minimum zone width 3 Fm. The main limitations of the resolution of the BFM lens arise from technological parameters (minimum zone width) and optical aberrations. The technology as used in this work limits the minimum zone size to 3 p,rn. Aberrations are more serious at off-axis angles of incidence. To analyse the off-axis aberrations, one can use an optical path difference assuming a rectangular groove profile. Thus, it is possible to find an analytical expression for the main off-axis aberrations: coma, astigmatism and field
1 .5 1.0
-1 .5 -1.0
-0.5
0 .o
0.5
1 .o
1 .5
-1.5
.o 8
0
.o a
E
E > x
0
0
1 -1.0
I. -0.5
I. 0.0
I. 0.5
x(mm)
X(mm)
0.1
W
-
I
0.10
0 08
0 -0 4
0 .02
0
.o
I
0
5780
5800
5820 7.
5840
(mm)
Fig. 4. Comparison of the imaging properties of a spherical mirror (a) and an elliptical BFM lens (b).
I. 1 .o
1 .5
43
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 40-45
curvature. According to the Rayleigh criterion, the angle of view, limited by coma, astigmatism and field curvature is
2.5
(a)
Peak
energy
FWHM
189.84eV 1.8eV
: 175
180
185
190 Energy
195
200
205
10
15
210
(eV)
where Armi, is the minimum zone size, F the focal length and 6n the Bragg angle. As an example, the field of view of the BFM lens characterised above is shown in the Fig. 3. Those parameters were chosen to demonstrate that astigmation and field curvature dominate close to normal incidence. Since we use a circular zone profile, the diffraction aberration characteristics of the optical scheme, shown in Fig. 1 can be calculated using aberration formulas for transmission zone plates [8]. Astigmatism is the main aberration for the reflecting BFM lens at normal incidence. The angular field of view was calculated using the formula for astigmatism and field curvature aberration:
where 6 is the desired spatial resolution in the horizontal plane, F is the focal distance, A is the wavelength. Thus, using the above mentioned lens parameters, the accepted opening angle of the source will be equal to N 56 mrad with 7 pm resolution. The field of view for our optical scheme covers our off-axis angle and is equal to 49 mrad (2.8”). For the beam reflected at 189.84 eV photon energy the image resolution of the BFM lens in the vertical direction is given by the width of the outer zone 1.22 Armin - 3.6 p.m and in the horizontal direction limited by coma to - 7 pm. To estimate the advantage of an elliptical BFM lens compared with spherical imaging optics the ray-tracing of the optical scheme, shown in Fig. 4, has been performed. Two possible optical elements, a multilayer coated spherical mirror and elliptical-shaped BFM lens have been compared. The ray-tracing code RAY [9] has been applied using a four-pixel array source. The stigmatic images were plotted for both cases in Fig. 4. One can see, that by the strong astigmatism of the spherical optics the test pattern in the left plot cannot be resolved. In case of the BFM
-10 Angle
-5 of
0 incidence
5
a0
(deg.)
Fig. 5. Energy dependence of the efficiency (a) and rocking curve (b) of the second mirror with first mirror settled at 2.8”.
lens, the stigmatically corrected image can be reproduced with 3 pm resolution. The energy dependence of the reflection efficiency as well as the rocking curve of the double-mirror system is shown in Fig. 5. Because the rocking curve is rather wide, the angular sensitivity of the adjustment of the Bragg-angle and thus, the sensitivity of the image intensity on the emission angle of the radiation is negligibly small. The large angular aperture leads to an insensitivity against emission angle changes and only direct beam position changes are detected. The depth of focus as defined by the energy resolution of the multilayer is 6.6 cm. However, the aperture of the lens defines a smaller depth of focus of 1.7 mm as given by the curvature of the electron beam in the storage ring.
3. Mechanical
and thermal performance
The BFM lens can be horizontally tilted and vertically moved and tilted, respectively, to fit the center of the lens to the angular maximum of the beam in a special UHV optical bench. The BFM lens is mounted on a lapped copper base which is cooled by means of a short, flexible copper braid which, in turn, is connected to a water cooled block. Both horizontal and vertical rotation as well as vertical translation of both elements with a minimum
44
K. Holldack et al. /Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 40-45
accuracy of 0.1 mrad were performed to adjust the image with respect to the phosphor screen, which is mounted 5.3 m further along (see Fig. 1). Several apertures in the system prevent heat loading of the mechanical components as well as stray light, which has to be blocked for a background-free detection of the image.
4. Image detection and analysis The image was detected by a phosphor screen with a 50 nm Al cap layer on the backside in order to make it solar blind. Different phosphors have been tested in a preliminary study. The best performance was achieved using a terbium doped gadolinium oxisulfide phosphor screen of 2 Frn grain size and 3 pm thickness. The phosphor in use has a sufficiently small grain size and is roughly a factor of 10 better in fluorescence yield at 189 eV than a Bi,Ge,O,,-single crystalline phosphor and 30% more efficient than a ZnS(Cd)-based screen (P20). The Al cap layer of 50 nm thickness gives optimum performance as determined by three effects: i) Visible photons from the phosphor become reflected from the cap layer on the backside which leads to an intensity enhancement by a factor of 2 (solid angle). ii) Visible photons which are divergently reflected by the multilayer mirrors are absorbed. iii) X-rays with 189 eV are efficiently transmitted. The optimisation was done using the REFLEC code [8]. It was shown that even with its relatively poor transmission at 189 eV (just above Al 2p edge) aluminium of 50 nm thickness is better than other cap layers as, for in-
stance, carbon, which does not reflect in the visible although it is capable of transmitting more X-rays. The magnification of the system at the test beamline is 0.8. The demagnified image of the source on the phosphor is then magnified (0.8 . . . 2) by a special objective lens on the air side of the beamline onto a l/2 in. chip of a CCD Camera with 12 pm pixel size. The latter is read out by a PC based frame grabber and image processor. The camera image is usually digitised to 512 X 512 pixels. The fastest method to extract the position and the beam size from the image is performed as follows: Firstly, the pixel intensities are horizontally averaged using a standard procedure of the image processor which writes the result into one line of an intermediate image into the framestore memory. The resulting intensity distribution, I, contains the horizontal projection of the image. The vertical centre of the image is then calculated by: 512
iZi
C
y=+,
(4) Czi 1
while the rms value of the projection from:
a,,,
is calculated
512
Czi %
= z(Y;fi
(5)
’
assuming a Gaussian shape for a faster read out. The values calculated this way are the relevant source parame-
is!y+(+f~ gJ)+(y< 2000
3000
2000
4000
3000
I
A
iii~~~
1000
BESSY I
5000
vettical
3000
2000
position [urn]
Fig. 6. Monochromatic images of the
4000
position [pm]
bending magnet source using the BFM monitor for normal operation of the storage ring (a) and small source operation (b).
K. Holldack et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 40-45
500ym
700mA
621mA
587mA
367mA
280mA Fig. 7. Ring current dependency of the source size at BESSY I during a single fill.
ters for the users. However, a more complicated but slower image analysis can correct for the a non-Gaussian shape and a possible tilt of the source. After that the same procedure is performed vertically to determine the horizontal centre and size. First results of the source imaging are shown in Fig. 6, where one can see the BESSY I source in a bending magnet at different operation modes of the storage ring. The ring current dependence is represented in Fig. 7’. The read out rate of the source parameters lies between 50 Hz (position only) and 5 Hz (full information). However, using W/C multilayers a fast readout leads to a noise limited resolution at ring currents lower than 100 mA. In this case an analysis of single video frames is not feasible so that an averaging of a number of frames up to a number of 2.56 must be made.
5. Conclusions and further developments As shown above the use of Bragg-Fresnel optics permits beam monitoring of sufficient resolution as required for X-ray beam monitoring at third generation storage
45
rings. Monochromatic images at 190 eV and a spatial resolution of about 7 pm have been registered in video real time and analysed down to ring currents of 50 mA. A more sensitive camera using an image intensifier will be obtained to extend the dynamic range. Finally, we should mention, that a factor of 50 to 100 in intensity can be won using a Mo/Si multilayer. Both improvements will extend the dynamic range of the system down to 1 mA ring current and the image resolution to 0.5 pm. The main disadvantage of the system described above arises from the fact that the detector has to be about 5 m behind the optics which makes the adjustment and the fit to a BESSY I1 front end complicated. Therefore, a new design in preparation will be tested for use at BESSY II. This system will employ two Bragg-Fresnel lenses in a telescope geometry, where the plane mirror in Fig. 1 is simply substituted by a second BFM lens of short focal length. This way, a 1 : 1 magnification will be realised within a distance of about 60 cm. The very compact system can be used to image the source at bending magnets as well as at undulators at 130 eV photon energy regardless of its position in the front end or the beamline as long as it is in the range between 7 and 20 m.
Acknowledgement We gratefully acknowledge the co-operation with the IMT RAS especially with N. Garnakova, Yu. Koval and S. Piatkin for the lens preparation. Furthermore we are indebted to F. Schafers for fruitful discussions on raytracing and P. Kuske for the source size variation in the storage ring.
References
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A. Hoffmann and F. Meot, Nucl. lnstr. and Meth. 203 (1982) 483. I21 R.C.C. Perera, M.E. Melczer, A. Warwick, A. Jackson and B.M. Kincaid, Rev. Sci. lnstr. 63(l) (1992) 541. [31 E. Tarasona, P. Elleaume, J. Chavanne, Ya.M. Hartmann, A.A. Snigiriev and 1.1. Snigirieva, Rev. Sci. lnstr. 65(6) (1994) 1959. 141A.I. Erko, J. X-Ray Science and Technology 2 (1990) 297. Technology of the Russian PI Institute of Microelectronics Academy of Sciences. 161A.I. Erko, Yu. Agafonov, L.A. Panchenko, A. Yuakshin, P. Chevallier, P. Dhez and F. Legrand, Optics Commun. 106 (1994) 146. [71 K. Holldack and W.B. Peatman, A pinhole array camera X-ray electron beam monitor, BESSY Annual Report 1994, in press. 181M. Young, J. Opt. Sot. Am. 62 (1972) 972. [91 The RAY and the REFLEC code are developed by F. Schafers (BESSYJ.