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An X-ray beam position monitor based on the photoluminescence of helium gas
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Nuclear Instruments and Methods in Physics Research A 540 (2005) 470–479 www.elsevier.com/locate/nima
An X-ray beam position monitor based on the photoluminescence of helium gas Peter Revesz, Jeffrey A. White Cornell High Energy Synchrotron Source (CHESS), 200L Wilson Laboratory, Cornell University, Ithaca, NY 14850-8001, USA Received 4 October 2004; received in revised form 17 November 2004; accepted 21 November 2004 Available online 1 January 2005
Abstract A new method for white beam position monitoring for both bend magnet and wiggler synchrotron X-ray radiation has been developed. This method utilizes visible light luminescence generated as a result of ionization by the intense X-ray flux. In video beam position monitors (VBPMs), the luminescence of helium gas at atmospheric pressure is observed through a view port using a CCD camera next to the beam line. The beam position, profile, integrated intensity and FWHM are calculated from the distribution of luminescence intensity in each captured image by custom software. Misalignment of upstream apertures changes the image profile making VBPMs helpful for initial alignment of upstream beam line components. VBPMs can thus provide more information about the X-ray beam than most beam position monitors (BPMs). A beam position calibration procedure, employing a tilted plane-parallel glass plate placed in front of the camera lens, has also been developed. The accuracy of the VBPM system was measured during a benchtop experiment to be better than 1 mm. The He-luminescence-based VBPM system has been operative on three CHESS beam lines (F hard-bend and wiggler, A-line wiggler and G-line wiggler) for about a year. The beam positions are converted to analog voltages and used as feedback signals for beam stabilization. In our paper we discuss details of VBPM construction and describe further results of its performance. r 2004 Elsevier B.V. All rights reserved. PACS: 07.85.Qe; 29.27.Fh; 78.55.m Keywords: Beam position monitor; Synchrotron X-rays; Helium gas photo luminescence
1. Overview of X-ray beam position monitors (BPMs)
Corresponding author.
E-mail address: [email protected] (P. Revesz).
The necessity of X-ray BPMs comes from the fact that experimental stations on beam lines are usually located far away (tens of meters) from the X-ray source and small source movements can
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have a great negative effect at the experimental stations. X-ray BPMs, therefore, are often installed along the beam line to monitor beam positions and often to serve as inputs for position feedback for the beam lines of the storage ring. CHESS utilizes a method of software feedback that makes use of corrector magnets to create a set of wave bumps 601 out of phase with each other. In this method the source points for the beam lines can be tuned relative to each other at 1/5 Hz to minimize the impact of machine instabilities such as temperature or regulation effects. This feedback is operated at the beginning of each experimental fill to increase reproducibility from one fill to the next. X-ray BPMs, therefore, are often installed along the beam line to monitor beam positions and often to serve as inputs for position feedback to the corrector magnets of the storage ring. A widely used type of BPM is based on the photoelectron effect. In its simplest form it [1] consists of two positively biased collector electrodes placed above and below the X-ray beam in a vacuum. These electrodes collect photoelectrons generated by the fringes of the X-ray beam from upstream beam aperture edges. In the first approximation the difference-over-sum of the top and bottom signals is proportional to the vertical beam position. In order to increase the linear range of the monitor, negatively biased guard electrodes are often placed downstream of the collectors to act as electron focusing elements. Several versions of the blade type of photoelectron BPMs [2,3] have been developed and used at a number of X-ray facilities. These monitors consist of four metal blades parallel to the X-ray beam direction, 901 apart from each other. There is a gap between the blades in the center where the X-ray beam passes through. The blades are well cooled, electrically isolated and they act as photo-electrodes. The photocurrent from the four electrodes gives both horizontal and vertical beam position information. In order to minimize problems of heat load on the metallic blades and the resulting mechanical deformations, these BPMs have also been created with CVD diamond blades [4] to be used in beam lines with high power insertion devices.
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A variation of photoelectron BPM uses a splitblade configuration [5]. This monitor consists of two pairs of water-cooled right triangle electrodes placed parallel and horizontal to the edges of the beam. A different approach to beam position monitoring using synthetic diamond is described in Ref. [6]. A thin synthetic diamond disk with four segment electrodes on both sides is placed in the X-ray beam. The X-ray passes through the diamond disk with negligible loss. This monitor can operate in two regimes: photoconductive and photoemission. The photoconductive or photoemission currents from the four segments can be combined to give horizontal and vertical position information. A further development of the photoconductive principle was the creation of a 16 16 pixel array on the same material to provide twodimensional (2D) beam profiles of the X-ray beam [6,7]. One problem of this type of BPMs is related to radiation damage as a result of direct exposure to high X-ray flux causing gradual degradation of its performance. Another class of BPMs uses lateral effect photodiodes [8]. These devices use commercially available VUV lateral photodiodes. In diode BPMs the lateral photodiode is usually covered with a water-cooled X-ray filter over the photosensitive area and it is inserted into the beam path to intercept a small portion of the beam. The photocurrent signals on the top and bottom anode electrodes are measured and the vertical position of the beam is proportional to the asymmetry of the two signals. An additional method to measure beam position and beam profile is described in Ref. [9]. A thin (o10 mm) carbon or beryllium foil is placed at 451 in the beam’s path. Scattered X-rays are detected by a linear array of X-ray photodiodes facing the foil. Soller slits are placed between the foil and the photodiode array to define the portion of the foil visible to each individual photodiode. The beam profile and position are determined from fitting a Gaussian to the photocurrent data. Another approach is the ionizing beam crosssection image detector (BCID) [10,11], capable of providing 2D profile and position of X-ray beams. In the BCID the passing X-ray beam ionizes the
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residual gas in the vacuum beam pipe. As a result of an applied perpendicular electric field, the energy of the electrons or ions will be proportional to the position of their origin. By using an electrostatic analyzer, the energy distribution of the electrons or ions is converted to spatial distribution along the input plane of an image converter consisting of a multi-channel plate and phosphor screen. A TV camera connected to a computer then picks up the image, from which one can calculate and display the beam position and profile.
2. Luminescence-based video BPM The use of luminescence to measure the particle beam profile in an accelerator has been previously demonstrated [12,13]. In Ref. [12] the luminescence monitor makes use of the excitation of the residual nitrogen gas (at 105–106 Torr). In case of Ref. [13], a CCD camera with a lens coupled multi-channel plate intensifier coupled with frame capture electronics produces the image of the luminescence from the ion beam. The authors of Ref. [13] measured proton beam profiles using intensified 16-bit CCD cameras by observing the luminescence in the presence of various residual gases (N, Ne, Ar, Kr and Xe) at 106 Torr pressure. These works showed that the beaminduced luminescence can be successfully used to characterize particle beam profiles. Images obtained from various sources have been widely used in machine vision applications. These applications include object identification, size and shape measurement, edge detection, object counting, motion tracking, etc. In all cases the images (image frames) are captured with a video device and the obtained pixel arrays are stored for analysis. The pixel arrays contain 2D digitized information of the intensity and/or color map of the image. Depending on the kind of information that one seeks to extract from the image map, various image enhancement methods may be applied. A meaningful way to determine the coordinate position of an object from its corresponding intensity map is to calculate its centroid (or center of gravity) [14]. The accuracy of calculated
centroid positions may well be determined to a fraction of the pixel size of the video camera [15]. The centroid of an intensity map is defined as the first moment(s): P P iI i;j jI i;j i;j i;j xc ¼ P and yc ¼ P ; I i;j I i;j i;j
i;j
where i and j are the horizontal and vertical pixel coordinates in the image intensity map, Ii,j is the 2D intensity map and xc and yc are the corresponding centroid coordinates. The extent of indices i and j is defined by selecting the regionof-interest (ROI) within the intensity map. In the case of X-ray beam position monitoring, the intensity map Ii,j is obtained by viewing the helium luminescence perpendicular to the direction of travel of the X-rays. The camera viewing this luminescence from the side shows a horizontal stripe of light and the yc centroid represents the beam position in the vertical direction. As the beam moves vertically, the vertical centroid coordinate yc, calculated from subsequent frame captures, changes correspondingly. The centroid coordinates obtained from the image map are expressed in pixel coordinates and they can be converted to spatial coordinates (such as microns) quite easily. Besides obtaining beam position via centroid calculations, there is additional information we can extract from the intensity map. Firstly, the P term I i;j is the total intensity of luminescence in the ROI. This intensity is proportional to the overall X-ray intensity and proportional to the beam current in the storage ring. Secondly, the intensity map integrated P along the horizontal pixel coordinate axis I y ¼ i I i;j yields the vertical beam intensity profile. Therefore, using video imaging of the luminescence we can visualize and evaluate the beam profile on a frame-by-frame basis.
3. CHESS’ video beam position monitor (VBPM) system hardware and software architecture All of the VBPMs at CHESS use Astrovid Stellacam EX analog CCD cameras. These
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cameras use high-sensitivity Sony HAD CCD chips, and provide gray scale images of the helium luminescence with S/N of 60 dB. In Fig. 1a, the schematics of a typical VBPM arrangement is shown. The two cameras, CV and CH for vertical and horizontal beam position measurement, are mounted next to the helium-filled beam line B (cross-sectional view is shown with two view ports). The cameras utilize the mirrors M1 and M2,3 to view the He luminescence to minimize scattered X-ray exposure which otherwise would create excessive noise and reduce the lifetime of the
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camera. The optical distance between the camera lens and the center of the beam is 25–50 cm, depending on physical constraints. The camera optics is set up to focus at the center of the X-ray beam. The depth-of-field of the optical system is typically narrower than the beam (horizontal) dimension, and therefore the resulting image of the beam is a sum of in-and-out of focus parts. As a result, the FWHM derived from the luminescence intensity profile is slightly larger than in reality. This, however, should not affect the centroid position. We have run
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F-Wiggler-VBPM Gaussian fit FWHM=2.84 mm
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Fig. 1. (a) Schematics of VBPM assembly for vertical and horizontal beam position measurement. CV and CH are the vertical and horizontal VBPM cameras viewing through mirrors M1 and M2,3 the luminescence in the He-filled beam pipe B. The beam shown in the center of the beam pipe is from the upstream direction. (b) Vertical image of the helium luminescence captured by the Centroid program. The white rectangle represents the region of interest (ROI) for centroid calculation (20 20 mm). The image shown here is a view of the luminescence as seen from the side at 901 with respect to the beam’s path. The beam shown in this image passes from right to left. (c) Vertical intensity profile of the helium luminescence with Gaussian fit to data points.
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experiments on an optical bench and established that defocusing does not adversely affect the position measurement. The camera assembly is enclosed in a heavymetal (tungsten) or lead enclosure to minimize degradation of the camera electronics and the lenses due to scattered X-ray radiation. The camera control box, the video multiplexer and the video server are located in the CHESS operations area. The video signal from the camera is carried at great lengths (hundreds of feet) without significant degradation. The video multiplexer allows working with images from four cameras with one video server. Separate video server computers with Hauppauge PCI video capture cards running on a Windows 2000 platform were dedicated to each beam line. Custombuilt units are connected via serial connection to each video server to provide digital output to control the video multiplexer and analog output signals corresponding to the beam positions for position feedback. We have developed a Windows-based program Centroid that provides the following functionality: 3.1. Image processing
Capture image frames Set ROI, threshold, frame averaging Calculate gray-scale image matrix Calculate centroid, integral, FWHM, skewness Set position ‘‘targets’’ (relative position origin coordinates) Retrieve position data from other BPMs for source position triangulation Calibrate (convert screen coordinates to microns at the object plane) Plot profile, position traces Control A/D and D/A via RS-232/485 Send data via LAN to signal logging instrumentation.
3.2. Hardware control
Provide analog position signal to position feedback Control camera aperture when needed
Activate camera calibrator when needed Provide a digital signal to control the multiplexer.
The Centroid program is a multi-threaded Win32 application with a user-friendly GUI-based interface. All setup, control and calibration data are saved in configuration files that make it easy to use. The heart of the program is the Frame Callback routine, which is called each time a new frame arrives from the frame grabber (15/s). Within this routine, the gray-scale intensity values (0–255) within a user-defined ROI are transferred into the intensity map array for further processing. The intensity map can be averaged over a variable number of frames (usually 10) depending on the noise levels in the video signal. The whole camera image is continuously displayed on the screen. The ROI is selected using mouse clicks. This portion of the screen is selected to incorporate the brightest area of the beam along with sufficient background portion. The camera lens aperture and the electronics shutter are set so that the intensity profile does not saturate within the ROI. A screen dump of the Centroid program (Fig. 1b) shows the image of the luminescence (F-line wiggler radiation) with the currently selected ROI and the beam profile plot. The ROI of screen corresponded to 20 20 mm in real space. The beam profile shown in Fig. 1c represents the portion of the data obtained by integration of the intensity data within the ROI along the vertical axis. The intensity profile data (dots) were fit with a Gaussian (shown as a line) with FWHM ¼ 2.74 mm. The conversion between image and the actual coordinates was 45 mm/pixel.
4. VBPM calibration Calibration of BPMs (photoelectron and diode types) is usually done by moving the BPM a known amount using jacks and simultaneously measuring the BPM’s response. This kind of solution would require precision jacks in case of VBPMs. Instead of moving the camera on a jack, our solution is to use a parallel glass plate placed
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in front of the camera that can be tilted a few degrees off the perpendicular position, as shown in Fig. 2a. By tilting the glass plate by angle a, the light path shifts parallel with respect to its original direction by distance h. This procedure thus executes a calibrated parallel shift of the light coming from the beam. The calibrator’s tilt is accurately measured on an optical bench. In Fig. 2b the dependence of the calculated image shift on the tilt angle is shown for a Corning 1736 glass plate with thickness of 1.1 mm. An actual beam position trace with an 88 mm step created by the calibration tilt mechanism is shown in Fig. 2c for the CHESS F-line wiggler VBPM. The calibration procedure using this method is implemented in the Centroid program. Typically, calibration constants are 50 mm/pixel. Another method to calibrate VBPMs is to place a mm grid at the focal plane of the camera for direct pixel-to-micron calibration. Naturally, this method of calibration is done at setup time without X-ray beam and the mm grid has to be removed before actual use. The results of two
calibration procedures are in agreement within 10%.
5. VBPM installations at CHESS There are a number of VBPMs installed on CHESS beam lines.
A-line (12 cm period 49 pole 0.8 T wiggler): measures vertical position of the white beam; F-line (hard-bend radius of 31.8 m): measures vertical position hard-bend beam; F-line (20 cm period 24-pole 1.2 T wiggler): measures both vertical and horizontal position of the wiggler beam; G-line wiggler (same as A-line): measures vertical position of the mirror reflected wiggler beam.
The beam line and instrumentation layout around the A-line is shown in Fig. 3a. The beam from the Aline wiggler first passes through the photoelectron 0.14
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Fig. 2. (a) Schematic of calibration method of VBPMs by tilting a glass plate in front of the video camera. (b) Calculated shift of image position as a function of tilt angle for 1-mm-thick glass (Corning 1736). (c) Measurement of beam position during run time while the position calibrator was activated. The position shift due to the tilting was 88 mm.
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Fig. 3. (a) Beam line and instrumentation layout for CHESS A-line. The VBPM is located 28 m from the X-ray source. (b) A-line VBPM (shield removed). The two cameras (vertical and horizontal VBPMs) are marked at CV and CH, The two cameras are mounted on a flange connected to the monochromator box M1000.
BMP and then enters the A1 monochromator box. In the helium-filled monochromator box the X-rays pass through the diode-BPM and further downstream part of the white beam is directed to the A1 station by the Si monochromator. The VBPM assembly is mounted on a nipple between the A2 station shutter and the A-1. The VBPM is shown in Fig. 3b with shielding removed. The two cameras, CV and CH next to the monochromator box M provide images of helium luminescence for vertical and horizontal position monitoring.
6. Accuracy of VBPMs The accuracy of the beam position measurement based upon centroid calculations of the image
produced by a video camera is especially interesting when we take into account that the beam typically has a vertical size (FWHM) of about 2 mm. The image is demagnified by the optical elements by a factor of 20 on the camera’s CCD chip (8 mm pixels). To determine VBPM accuracy we have tested our system on an optical bench, where an LED light source (1 mm wide) was mounted on an accurate piezo-driven stage. The LED was moved in progression with 0.4 mm steps to a total of 100 mm length. In Fig. 4a the measured centroid position vs. the actual LED position is shown. In Fig. 4b and c, the deviation between the measured and the actual light source position is shown for the full scale of travel and for a 20 mm portion of it, respectively. It is clear that the agreement
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Fig. 4. Measurement of VBPM accuracy on optical bench with an LED light source. (a) Centroid position vs. light source position. The light source is moved in 0.4 mm steps. (b) Deviation of centroid position from the actual position for a 20 mm portion of the full travel. (c) Simulation showing light distribution with a Gaussian profile converted to step-wise distribution with step-width equal to the CCD pixel size. The height of the steps is equal to the intensity integral over the CCD pixel area and it is rounded to the accuracy of the D/A conversion (1/255). (d) Simulation of relative centroid position, using a Gaussian with FWHM ¼ 2 mm de-magnified by 20 and converted to a step-wise distribution with 8 mm step-width. The heights of the steps were rounded to 8- and 12-bit accuracy.
between the measured and the actual positions is very good: the FWHM of the scatter is 0.4 mm! Shown in Fig. 4d is a computer simulation of the camera response to a Gaussian intensity profile with 2 mm FWHM de-magnified by 20 on a CCD pixel array with 8 mm pixels. In the simulation process the Gaussian profile was converted to a step-wise distribution with step-widths equal to the CCD pixel size where each height corresponds to the integral intensity over the pixel area. In addition, the intensity heights were truncated to 1/255 and 1/4096 accuracy to simulate 8- and 12-bit A/D conversions. Computer simulations show that the number of gray-scale levels, the linearity of the video A/D conversion and the
signal-to-noise ratio primarily determine the accuracy of the method. The effect of the CCD pixel size on the accuracy is directly linked with the number of gray-scale levels (or the number of bits of the A/D conversion) of the video system. When the image position shifts less than the pixel size, the integrated intensity in a pixel may or may not change depending on whether a rounding-off takes place or not. This will lead to a deviation of the centroid position from the expected position with a periodicity equal to the pixel size. The accuracy plots, both theoretical and experimental, show a periodicity equal to the actual pixel size. At CHESS we currently use three types of BPMs: photoelectron, lateral photodiode and luminescence-based video BPMs. In Table 1 we
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Table 1 Comparison of three types of BPMs used at CHESS: main features, advantages and disadvantages
Intercepting Beam use Linearity Profile info Position info Complexity Pro Con
0.2
Photo-electron BPM
Diode BPM
Video BPM
No Beam edges Small linear range No Derived from difference-over-sum of top and bottom signals Simple, reliable Relatively simple Small linear range
Yes Any portion Large linear range No Derived from difference-over-sum of top and bottom signals Simple, reliable Relatively simple Uses beam, radiation damage
No Actual beam Large linear range Yes Derived from the intensity map centroid More complex Gives complex info Complexity, sampling rate is limited
A-P.E. A-Wiggler VBPM
Beam position (mm)
0.1
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instability in different degree. The VBPM, which is located further downstream from the X-ray source, is thus more sensitive to particle beam movements than the photoelectron BPM. This figure also shows the usefulness of these position monitors, working in conjunction with orbital feedback, to stabilize the beam position. The position measurement method using image analysis of luminescence has proven to be a reliable tool and has been in regular use at CHESS during the last 2 years. Further studies are underway to further optimize this technique with various gases and pressures.
Run time (minutes)
Fig. 5. Vertical beam position traces for A-line photoelectron and VBPM. At 420 min into run time the beam position feedback was turned off. The plot shows that the two BPMs register the beam position instability to a different degree. The VBPM is located further downstream and thus is more sensitive to source movements than the photoelectron BPM.
summarize the most relevant properties of these devices.
Acknowledgements The authors thank Lee Shelp for help in designing experiment and Don Bilderback for encouragement and support. This work is based upon research at Cornell High Energy Synchrotron Source (CHESS) supported by the National Science Foundation and National Institute of Health/National Institute of General Medical Sciences under award DMR 0225180.
7. Results and discussion In Fig. 5 the position traces of A-line wiggler VBPM and photoelectron BPM are shown together. At 420 min into run time the beam position feedback was turned off. The plot shows that the two BPMs register the subsequent beam position
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Nuclear Instruments and Methods in Physics Research A 540 (2005) 470–479 www.elsevier.com/locate/nima
An X-ray beam position monitor based on the photoluminescence of helium gas Peter Revesz, Jeffrey A. White Cornell High Energy Synchrotron Source (CHESS), 200L Wilson Laboratory, Cornell University, Ithaca, NY 14850-8001, USA Received 4 October 2004; received in revised form 17 November 2004; accepted 21 November 2004 Available online 1 January 2005
Abstract A new method for white beam position monitoring for both bend magnet and wiggler synchrotron X-ray radiation has been developed. This method utilizes visible light luminescence generated as a result of ionization by the intense X-ray flux. In video beam position monitors (VBPMs), the luminescence of helium gas at atmospheric pressure is observed through a view port using a CCD camera next to the beam line. The beam position, profile, integrated intensity and FWHM are calculated from the distribution of luminescence intensity in each captured image by custom software. Misalignment of upstream apertures changes the image profile making VBPMs helpful for initial alignment of upstream beam line components. VBPMs can thus provide more information about the X-ray beam than most beam position monitors (BPMs). A beam position calibration procedure, employing a tilted plane-parallel glass plate placed in front of the camera lens, has also been developed. The accuracy of the VBPM system was measured during a benchtop experiment to be better than 1 mm. The He-luminescence-based VBPM system has been operative on three CHESS beam lines (F hard-bend and wiggler, A-line wiggler and G-line wiggler) for about a year. The beam positions are converted to analog voltages and used as feedback signals for beam stabilization. In our paper we discuss details of VBPM construction and describe further results of its performance. r 2004 Elsevier B.V. All rights reserved. PACS: 07.85.Qe; 29.27.Fh; 78.55.m Keywords: Beam position monitor; Synchrotron X-rays; Helium gas photo luminescence
1. Overview of X-ray beam position monitors (BPMs)
Corresponding author.
E-mail address: [email protected] (P. Revesz).
The necessity of X-ray BPMs comes from the fact that experimental stations on beam lines are usually located far away (tens of meters) from the X-ray source and small source movements can
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have a great negative effect at the experimental stations. X-ray BPMs, therefore, are often installed along the beam line to monitor beam positions and often to serve as inputs for position feedback for the beam lines of the storage ring. CHESS utilizes a method of software feedback that makes use of corrector magnets to create a set of wave bumps 601 out of phase with each other. In this method the source points for the beam lines can be tuned relative to each other at 1/5 Hz to minimize the impact of machine instabilities such as temperature or regulation effects. This feedback is operated at the beginning of each experimental fill to increase reproducibility from one fill to the next. X-ray BPMs, therefore, are often installed along the beam line to monitor beam positions and often to serve as inputs for position feedback to the corrector magnets of the storage ring. A widely used type of BPM is based on the photoelectron effect. In its simplest form it [1] consists of two positively biased collector electrodes placed above and below the X-ray beam in a vacuum. These electrodes collect photoelectrons generated by the fringes of the X-ray beam from upstream beam aperture edges. In the first approximation the difference-over-sum of the top and bottom signals is proportional to the vertical beam position. In order to increase the linear range of the monitor, negatively biased guard electrodes are often placed downstream of the collectors to act as electron focusing elements. Several versions of the blade type of photoelectron BPMs [2,3] have been developed and used at a number of X-ray facilities. These monitors consist of four metal blades parallel to the X-ray beam direction, 901 apart from each other. There is a gap between the blades in the center where the X-ray beam passes through. The blades are well cooled, electrically isolated and they act as photo-electrodes. The photocurrent from the four electrodes gives both horizontal and vertical beam position information. In order to minimize problems of heat load on the metallic blades and the resulting mechanical deformations, these BPMs have also been created with CVD diamond blades [4] to be used in beam lines with high power insertion devices.
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A variation of photoelectron BPM uses a splitblade configuration [5]. This monitor consists of two pairs of water-cooled right triangle electrodes placed parallel and horizontal to the edges of the beam. A different approach to beam position monitoring using synthetic diamond is described in Ref. [6]. A thin synthetic diamond disk with four segment electrodes on both sides is placed in the X-ray beam. The X-ray passes through the diamond disk with negligible loss. This monitor can operate in two regimes: photoconductive and photoemission. The photoconductive or photoemission currents from the four segments can be combined to give horizontal and vertical position information. A further development of the photoconductive principle was the creation of a 16 16 pixel array on the same material to provide twodimensional (2D) beam profiles of the X-ray beam [6,7]. One problem of this type of BPMs is related to radiation damage as a result of direct exposure to high X-ray flux causing gradual degradation of its performance. Another class of BPMs uses lateral effect photodiodes [8]. These devices use commercially available VUV lateral photodiodes. In diode BPMs the lateral photodiode is usually covered with a water-cooled X-ray filter over the photosensitive area and it is inserted into the beam path to intercept a small portion of the beam. The photocurrent signals on the top and bottom anode electrodes are measured and the vertical position of the beam is proportional to the asymmetry of the two signals. An additional method to measure beam position and beam profile is described in Ref. [9]. A thin (o10 mm) carbon or beryllium foil is placed at 451 in the beam’s path. Scattered X-rays are detected by a linear array of X-ray photodiodes facing the foil. Soller slits are placed between the foil and the photodiode array to define the portion of the foil visible to each individual photodiode. The beam profile and position are determined from fitting a Gaussian to the photocurrent data. Another approach is the ionizing beam crosssection image detector (BCID) [10,11], capable of providing 2D profile and position of X-ray beams. In the BCID the passing X-ray beam ionizes the
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residual gas in the vacuum beam pipe. As a result of an applied perpendicular electric field, the energy of the electrons or ions will be proportional to the position of their origin. By using an electrostatic analyzer, the energy distribution of the electrons or ions is converted to spatial distribution along the input plane of an image converter consisting of a multi-channel plate and phosphor screen. A TV camera connected to a computer then picks up the image, from which one can calculate and display the beam position and profile.
2. Luminescence-based video BPM The use of luminescence to measure the particle beam profile in an accelerator has been previously demonstrated [12,13]. In Ref. [12] the luminescence monitor makes use of the excitation of the residual nitrogen gas (at 105–106 Torr). In case of Ref. [13], a CCD camera with a lens coupled multi-channel plate intensifier coupled with frame capture electronics produces the image of the luminescence from the ion beam. The authors of Ref. [13] measured proton beam profiles using intensified 16-bit CCD cameras by observing the luminescence in the presence of various residual gases (N, Ne, Ar, Kr and Xe) at 106 Torr pressure. These works showed that the beaminduced luminescence can be successfully used to characterize particle beam profiles. Images obtained from various sources have been widely used in machine vision applications. These applications include object identification, size and shape measurement, edge detection, object counting, motion tracking, etc. In all cases the images (image frames) are captured with a video device and the obtained pixel arrays are stored for analysis. The pixel arrays contain 2D digitized information of the intensity and/or color map of the image. Depending on the kind of information that one seeks to extract from the image map, various image enhancement methods may be applied. A meaningful way to determine the coordinate position of an object from its corresponding intensity map is to calculate its centroid (or center of gravity) [14]. The accuracy of calculated
centroid positions may well be determined to a fraction of the pixel size of the video camera [15]. The centroid of an intensity map is defined as the first moment(s): P P iI i;j jI i;j i;j i;j xc ¼ P and yc ¼ P ; I i;j I i;j i;j
i;j
where i and j are the horizontal and vertical pixel coordinates in the image intensity map, Ii,j is the 2D intensity map and xc and yc are the corresponding centroid coordinates. The extent of indices i and j is defined by selecting the regionof-interest (ROI) within the intensity map. In the case of X-ray beam position monitoring, the intensity map Ii,j is obtained by viewing the helium luminescence perpendicular to the direction of travel of the X-rays. The camera viewing this luminescence from the side shows a horizontal stripe of light and the yc centroid represents the beam position in the vertical direction. As the beam moves vertically, the vertical centroid coordinate yc, calculated from subsequent frame captures, changes correspondingly. The centroid coordinates obtained from the image map are expressed in pixel coordinates and they can be converted to spatial coordinates (such as microns) quite easily. Besides obtaining beam position via centroid calculations, there is additional information we can extract from the intensity map. Firstly, the P term I i;j is the total intensity of luminescence in the ROI. This intensity is proportional to the overall X-ray intensity and proportional to the beam current in the storage ring. Secondly, the intensity map integrated P along the horizontal pixel coordinate axis I y ¼ i I i;j yields the vertical beam intensity profile. Therefore, using video imaging of the luminescence we can visualize and evaluate the beam profile on a frame-by-frame basis.
3. CHESS’ video beam position monitor (VBPM) system hardware and software architecture All of the VBPMs at CHESS use Astrovid Stellacam EX analog CCD cameras. These
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cameras use high-sensitivity Sony HAD CCD chips, and provide gray scale images of the helium luminescence with S/N of 60 dB. In Fig. 1a, the schematics of a typical VBPM arrangement is shown. The two cameras, CV and CH for vertical and horizontal beam position measurement, are mounted next to the helium-filled beam line B (cross-sectional view is shown with two view ports). The cameras utilize the mirrors M1 and M2,3 to view the He luminescence to minimize scattered X-ray exposure which otherwise would create excessive noise and reduce the lifetime of the
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camera. The optical distance between the camera lens and the center of the beam is 25–50 cm, depending on physical constraints. The camera optics is set up to focus at the center of the X-ray beam. The depth-of-field of the optical system is typically narrower than the beam (horizontal) dimension, and therefore the resulting image of the beam is a sum of in-and-out of focus parts. As a result, the FWHM derived from the luminescence intensity profile is slightly larger than in reality. This, however, should not affect the centroid position. We have run
(b)
F-Wiggler-VBPM Gaussian fit FWHM=2.84 mm
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Fig. 1. (a) Schematics of VBPM assembly for vertical and horizontal beam position measurement. CV and CH are the vertical and horizontal VBPM cameras viewing through mirrors M1 and M2,3 the luminescence in the He-filled beam pipe B. The beam shown in the center of the beam pipe is from the upstream direction. (b) Vertical image of the helium luminescence captured by the Centroid program. The white rectangle represents the region of interest (ROI) for centroid calculation (20 20 mm). The image shown here is a view of the luminescence as seen from the side at 901 with respect to the beam’s path. The beam shown in this image passes from right to left. (c) Vertical intensity profile of the helium luminescence with Gaussian fit to data points.
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experiments on an optical bench and established that defocusing does not adversely affect the position measurement. The camera assembly is enclosed in a heavymetal (tungsten) or lead enclosure to minimize degradation of the camera electronics and the lenses due to scattered X-ray radiation. The camera control box, the video multiplexer and the video server are located in the CHESS operations area. The video signal from the camera is carried at great lengths (hundreds of feet) without significant degradation. The video multiplexer allows working with images from four cameras with one video server. Separate video server computers with Hauppauge PCI video capture cards running on a Windows 2000 platform were dedicated to each beam line. Custombuilt units are connected via serial connection to each video server to provide digital output to control the video multiplexer and analog output signals corresponding to the beam positions for position feedback. We have developed a Windows-based program Centroid that provides the following functionality: 3.1. Image processing
Capture image frames Set ROI, threshold, frame averaging Calculate gray-scale image matrix Calculate centroid, integral, FWHM, skewness Set position ‘‘targets’’ (relative position origin coordinates) Retrieve position data from other BPMs for source position triangulation Calibrate (convert screen coordinates to microns at the object plane) Plot profile, position traces Control A/D and D/A via RS-232/485 Send data via LAN to signal logging instrumentation.
3.2. Hardware control
Provide analog position signal to position feedback Control camera aperture when needed
Activate camera calibrator when needed Provide a digital signal to control the multiplexer.
The Centroid program is a multi-threaded Win32 application with a user-friendly GUI-based interface. All setup, control and calibration data are saved in configuration files that make it easy to use. The heart of the program is the Frame Callback routine, which is called each time a new frame arrives from the frame grabber (15/s). Within this routine, the gray-scale intensity values (0–255) within a user-defined ROI are transferred into the intensity map array for further processing. The intensity map can be averaged over a variable number of frames (usually 10) depending on the noise levels in the video signal. The whole camera image is continuously displayed on the screen. The ROI is selected using mouse clicks. This portion of the screen is selected to incorporate the brightest area of the beam along with sufficient background portion. The camera lens aperture and the electronics shutter are set so that the intensity profile does not saturate within the ROI. A screen dump of the Centroid program (Fig. 1b) shows the image of the luminescence (F-line wiggler radiation) with the currently selected ROI and the beam profile plot. The ROI of screen corresponded to 20 20 mm in real space. The beam profile shown in Fig. 1c represents the portion of the data obtained by integration of the intensity data within the ROI along the vertical axis. The intensity profile data (dots) were fit with a Gaussian (shown as a line) with FWHM ¼ 2.74 mm. The conversion between image and the actual coordinates was 45 mm/pixel.
4. VBPM calibration Calibration of BPMs (photoelectron and diode types) is usually done by moving the BPM a known amount using jacks and simultaneously measuring the BPM’s response. This kind of solution would require precision jacks in case of VBPMs. Instead of moving the camera on a jack, our solution is to use a parallel glass plate placed
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in front of the camera that can be tilted a few degrees off the perpendicular position, as shown in Fig. 2a. By tilting the glass plate by angle a, the light path shifts parallel with respect to its original direction by distance h. This procedure thus executes a calibrated parallel shift of the light coming from the beam. The calibrator’s tilt is accurately measured on an optical bench. In Fig. 2b the dependence of the calculated image shift on the tilt angle is shown for a Corning 1736 glass plate with thickness of 1.1 mm. An actual beam position trace with an 88 mm step created by the calibration tilt mechanism is shown in Fig. 2c for the CHESS F-line wiggler VBPM. The calibration procedure using this method is implemented in the Centroid program. Typically, calibration constants are 50 mm/pixel. Another method to calibrate VBPMs is to place a mm grid at the focal plane of the camera for direct pixel-to-micron calibration. Naturally, this method of calibration is done at setup time without X-ray beam and the mm grid has to be removed before actual use. The results of two
calibration procedures are in agreement within 10%.
5. VBPM installations at CHESS There are a number of VBPMs installed on CHESS beam lines.
A-line (12 cm period 49 pole 0.8 T wiggler): measures vertical position of the white beam; F-line (hard-bend radius of 31.8 m): measures vertical position hard-bend beam; F-line (20 cm period 24-pole 1.2 T wiggler): measures both vertical and horizontal position of the wiggler beam; G-line wiggler (same as A-line): measures vertical position of the mirror reflected wiggler beam.
The beam line and instrumentation layout around the A-line is shown in Fig. 3a. The beam from the Aline wiggler first passes through the photoelectron 0.14
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Fig. 2. (a) Schematic of calibration method of VBPMs by tilting a glass plate in front of the video camera. (b) Calculated shift of image position as a function of tilt angle for 1-mm-thick glass (Corning 1736). (c) Measurement of beam position during run time while the position calibrator was activated. The position shift due to the tilting was 88 mm.
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Fig. 3. (a) Beam line and instrumentation layout for CHESS A-line. The VBPM is located 28 m from the X-ray source. (b) A-line VBPM (shield removed). The two cameras (vertical and horizontal VBPMs) are marked at CV and CH, The two cameras are mounted on a flange connected to the monochromator box M1000.
BMP and then enters the A1 monochromator box. In the helium-filled monochromator box the X-rays pass through the diode-BPM and further downstream part of the white beam is directed to the A1 station by the Si monochromator. The VBPM assembly is mounted on a nipple between the A2 station shutter and the A-1. The VBPM is shown in Fig. 3b with shielding removed. The two cameras, CV and CH next to the monochromator box M provide images of helium luminescence for vertical and horizontal position monitoring.
6. Accuracy of VBPMs The accuracy of the beam position measurement based upon centroid calculations of the image
produced by a video camera is especially interesting when we take into account that the beam typically has a vertical size (FWHM) of about 2 mm. The image is demagnified by the optical elements by a factor of 20 on the camera’s CCD chip (8 mm pixels). To determine VBPM accuracy we have tested our system on an optical bench, where an LED light source (1 mm wide) was mounted on an accurate piezo-driven stage. The LED was moved in progression with 0.4 mm steps to a total of 100 mm length. In Fig. 4a the measured centroid position vs. the actual LED position is shown. In Fig. 4b and c, the deviation between the measured and the actual light source position is shown for the full scale of travel and for a 20 mm portion of it, respectively. It is clear that the agreement
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Fig. 4. Measurement of VBPM accuracy on optical bench with an LED light source. (a) Centroid position vs. light source position. The light source is moved in 0.4 mm steps. (b) Deviation of centroid position from the actual position for a 20 mm portion of the full travel. (c) Simulation showing light distribution with a Gaussian profile converted to step-wise distribution with step-width equal to the CCD pixel size. The height of the steps is equal to the intensity integral over the CCD pixel area and it is rounded to the accuracy of the D/A conversion (1/255). (d) Simulation of relative centroid position, using a Gaussian with FWHM ¼ 2 mm de-magnified by 20 and converted to a step-wise distribution with 8 mm step-width. The heights of the steps were rounded to 8- and 12-bit accuracy.
between the measured and the actual positions is very good: the FWHM of the scatter is 0.4 mm! Shown in Fig. 4d is a computer simulation of the camera response to a Gaussian intensity profile with 2 mm FWHM de-magnified by 20 on a CCD pixel array with 8 mm pixels. In the simulation process the Gaussian profile was converted to a step-wise distribution with step-widths equal to the CCD pixel size where each height corresponds to the integral intensity over the pixel area. In addition, the intensity heights were truncated to 1/255 and 1/4096 accuracy to simulate 8- and 12-bit A/D conversions. Computer simulations show that the number of gray-scale levels, the linearity of the video A/D conversion and the
signal-to-noise ratio primarily determine the accuracy of the method. The effect of the CCD pixel size on the accuracy is directly linked with the number of gray-scale levels (or the number of bits of the A/D conversion) of the video system. When the image position shifts less than the pixel size, the integrated intensity in a pixel may or may not change depending on whether a rounding-off takes place or not. This will lead to a deviation of the centroid position from the expected position with a periodicity equal to the pixel size. The accuracy plots, both theoretical and experimental, show a periodicity equal to the actual pixel size. At CHESS we currently use three types of BPMs: photoelectron, lateral photodiode and luminescence-based video BPMs. In Table 1 we
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Table 1 Comparison of three types of BPMs used at CHESS: main features, advantages and disadvantages
Intercepting Beam use Linearity Profile info Position info Complexity Pro Con
0.2
Photo-electron BPM
Diode BPM
Video BPM
No Beam edges Small linear range No Derived from difference-over-sum of top and bottom signals Simple, reliable Relatively simple Small linear range
Yes Any portion Large linear range No Derived from difference-over-sum of top and bottom signals Simple, reliable Relatively simple Uses beam, radiation damage
No Actual beam Large linear range Yes Derived from the intensity map centroid More complex Gives complex info Complexity, sampling rate is limited
A-P.E. A-Wiggler VBPM
Beam position (mm)
0.1
0.0
-0.1
feedback on
feedback off
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-0.3
-0.4 0
200
400
600
800
1000
1200
instability in different degree. The VBPM, which is located further downstream from the X-ray source, is thus more sensitive to particle beam movements than the photoelectron BPM. This figure also shows the usefulness of these position monitors, working in conjunction with orbital feedback, to stabilize the beam position. The position measurement method using image analysis of luminescence has proven to be a reliable tool and has been in regular use at CHESS during the last 2 years. Further studies are underway to further optimize this technique with various gases and pressures.
Run time (minutes)
Fig. 5. Vertical beam position traces for A-line photoelectron and VBPM. At 420 min into run time the beam position feedback was turned off. The plot shows that the two BPMs register the beam position instability to a different degree. The VBPM is located further downstream and thus is more sensitive to source movements than the photoelectron BPM.
summarize the most relevant properties of these devices.
Acknowledgements The authors thank Lee Shelp for help in designing experiment and Don Bilderback for encouragement and support. This work is based upon research at Cornell High Energy Synchrotron Source (CHESS) supported by the National Science Foundation and National Institute of Health/National Institute of General Medical Sciences under award DMR 0225180.
7. Results and discussion In Fig. 5 the position traces of A-line wiggler VBPM and photoelectron BPM are shown together. At 420 min into run time the beam position feedback was turned off. The plot shows that the two BPMs register the subsequent beam position
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