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PC BASED SYSTEM FOR BEAM DIAGNOSTIC USING OTR
F. Nedeoglo, A. Ermakov Department of Physics, Moscow State University, A. Chepurnov, V. Shvedunov Institute of Nuclear Physics, Moscow State University, 119899, Moscow, Russia
length - 6 µs, frequency of electron pulse - 50 Hz, maximum pulse current - 50 mA.
Accelerating Structure Q1 Q2 Al foil CF

Abstract
During the startup of injector of 70 MeV pulsed Race Track Microtron the OTR beam visualization and diagnostic system based on PC was used. The system consists of black and white CCD camera with embedded ADC of CCD output and dedicated PCI add-on board installed into PC compatible computer. The CCD camera allow to detect source of light with weak luminosity up to 0.001 Lux due to hardware support of image accumulating directly on the CCD matrix. Fast parallel digital bus is used to acquire image and to control CCD camera. The software working under DOS operating system allows to observe OTR spot in real time and save image in BMP format on hard disk for further off-line image processing. The beam parameters such as beam size, profile and emittance have been measured with the help of the system.

mirror

CCD camera Pb shield

Figure 1: Layout of experiment: Q1, Q2 ­ quadrupoles, FC ­ Faradey cup.

3 VISION SYSTEM
To detect OTR we use black and white digital camera based on the Sony CCB-M27B/CE CCD matrix. The size of CCD matrix is 741x576 cells, and sensitivity is 0.25 Lux. Camera has embedded 10 bits analogue to digital converter, that is working on the shift frequency of CCD matrix. The camera has capability to accumulate image directly on the CCD matrix during the time up to 4 seconds without cooling. Due to this feature the camera can recognise the light sources with luminosity of 0.001 Lux. This camera with accumulating feature allows to solve problem of synchronisation between RF system of accelerator and CCD camera. Because of beam has impulse structure we should turn on the process of image digitising right at the moment of beam pulse crossing the vacuum-metal boundary. Otherwise we may get the image of noise between beam impulses. But if we turn on the accumulating process during the time longer than time interval between the beam impulses, we will exactly get the right image of OTR spot and the synchronisation between the camera and RF system of accelerator is not necessary. The digitizing image of OTR spot is transmitted via digital parallel bus to dedicated PCI add-on board installed into PC compatible computer. The board is intended to input image into RAM of computer and to control the camera remotely. The maximum input speed of the board is 25 full frames per second. So the vision system could be used to visualise electron beam in real time. The software running on PC under DOS operating system was used to output beam image on the screen of

1 INTRODUCTION
Under the construction of 70 MeV pulsed Race Track Microtron the tuning and testing of Rectangular Cavity Biperiodic Accelerating Structure were carried out [1]. The accelerating structure should provide energy gain of 5 MeV for electron beam per passing. During the testing the main beam parameters were measured. Optical Transition Radiation (OTR) method was used to perform beam visualisation and measurements of the size, profile and emittance of the beam [2-4]. The measurements of the emittance allow us to refine beam dynamics simulation later.

2 EXPERIMENTAL SETUP
The layout of experiment is shown at figure 1. Two quadrupoles (Q1, Q2) located at 1.2 meter from the exit of accelerating structure focus the electron beam on the surface of thin (12 µm) aluminium foil. The foil was installed at the angle of 45 degrees with respect to the beam direction. In that case transition radiation is emitted in the perpendicular direction with respect to beam axis, where electrons cross vacuum-metal boundary. Then radiation reflects from the mirror and falls on the optics of CCD camera. Using the mirror the camera could be placed out from the gamma radiation emitted from experimental chamber. The electron beam at the exit of accelerating structure has the following parameters: energy ­ 5.3 MeV, pulse


computer, to control operating of CCD camera remotely and to save image into the bitmap file on hard disk for further processing in off-line.

u

2 RMS

= ? 1?

RMS

2 = (m11? 0 - 2m11m12 ? 0 + m12 ? 0 )?

RMS

, (2)

3 METHODS AND RESULTS
The resulting images of the OTR spot of the beam contain high frequency spatial noise that produced by gamma rays passing through lead shields. To remove the noise we use digital filtering technique. Before any processing the image was convoluted by digital low pass filter with finite impulse response (FIR) (Figure 2). The coefficients of FIR is given by formula:

where m11 , m 12 are the two of four coefficients of transformation matrix between the exit of accelerating structure and the foil. Varying quadrupoles gradient (focal lengths) we can measure RMS beam dimension and approximate their dependence on gradient by formula (2) using the least square method.

h(x, y) = ?

J1 ( 2 ? ? x 2 + y 2 ) x2 + y
2

(1)
1

where is sampling frequency, and J function of first order [5].

is the Bessel

a)

Figure 2. Finite impulse response function used to filter the beam images. To estimate effective beam emittance we use the threegradient method. In this method it is supposed that phase space is enclosed in ellipse. To determine the three coefficients of the beam emittance ellipse equation, the three RMS dimensions of the beam at corresponding gradients of quadrupoles are needed.

b)

c) Figure 4. The processed beam image: a) curves of equal intensity, b) beam distribution after filtering, c) beam distribution without filtering. Scale of image: 1 point = 0.125 mm. The many different images of the beam were obtained during the experiments. The results of data processing are shown at Figure 4. The image of levels with equal intencity and beam distribution after FIR filtering is shown at figure 4a) and 4b) correspondently. Figure 4c) shows the initial beam distribution without filtering.

Figure 3. OTR image of the beam on the computer screen. To calculate more precise coefficients it' necessary to s process many beam profiles. Let ? 0 , ? 0 , ? 0 are ellipse parameters of a beam at the exit of accelerating structure, and ? 1 , ? 1 , ? 1 - ones at the point of observation (Al foil). The square of RMS beam dimensions at the foil can be written as:


To estimate beam emittance the equal and opposite sign excitation current was provided to quadrupoles coils, so that first quadrupole was focusing in vertical plane and the second in horizontal plane. Focal length of quadrupole douplet could be changed from infinity up to 0.2 m at the beam energy of 5.3 MeV. The figure 5 shows the dependence of RMS beam size in horizontal plane on the quadrupole doublet focal length at 90% of the maximum of beam current.

REFERENCES
[1] V.I. Shvedunov, A.N. Ermakov, D.I. Ermakov, F.N. Nedeoglo, N.P. Sobenin; W.P. Trower, "Rectangular Accelerating-Focusing Structure High Power Tests", Proc. of EPAC-2000 Conference. [2] L.Wartski, S.Roland, J.Lasalle, M.Bolore, G.Filippi, "Detection of Optical Transition Radiation and Its Application to Beam Diagnostics", Jour. Appl. Phys. 46 (1975) 3644. [3] R.Chehap, M.Taurigna, G.Bienvenu, "Beam Emittance Determination Using Optical Transition Radiation", Proc. EPAC92, Berlin (1992) 1139. [4] S.Dobert et al.. "Transverse and Longitudal Beam Diagnostics using Transition Radiation." Proc. EPAC96, Barselona (1996) 1648. [5] V. Cappellini, A. G. Constantinides, P. Emilani "Digital Filters and Their Applications", Academic Press, London, 1978

Figure 5. Dependence of RMS beam size in horizontal plane on the quadrupole doublet at 90% of beam current, dashed curve is results of fitting with formula (2). Table 1. Plane Horizontal Vertical 0,98 0,68 , mm*mrad

Results of beam emittance measurements are summarized in Table1. Emittance given in Table 1 refers to the point of accelerating structure exit.

CONCLUSIONS
Suggested system based on digital camera combined with PC is used for monitoring of the beam online. Due to wide range of sensitivity this system could be used as during initial tuning and testing of accelerator as at usual operating of linac. After developing of additional software it will be possible to calculate the emittance of the beam in real time mode.

ACKNOWLEDGEMENTS
The authors acknowledge the professor Peter Trower for financial support of this work. The authors acknowledge the Organising Committee of PCaPAC for providing the ability to participate the conference.