Computer System for Diagnostic of Electron Beam Using
Transition Radiation
A.S.Chepurnov, A.S.Alimov, B.S.Ishkhanov, V.I.Shvedunov, P.L. Tkatchev
Institute of Nuclear Physics, Moscow State University.
119899, Moscow, Russia,
chas@rtm-cs.npi.msu.su, (095) 939-5659
Abstract.
A beam diagnostic system based on transition radiation analysis is a high-sensitive method for studying physicalproperties of the beam and adjusting of an accelerator. Optical transition radiation converted to a standard TV signal isanalysed by the use of the unconventional computer based binary system of technical vision. It is possible to ensure beamdiagnostics in real time due to the special hardware algorithms of image processing implemented in a video processor.
This feature ensures cost effective solutions and simple software support. Data flows of digitised information are very
compact and full of information. A diagnostic system is connected to the computer control system of a racetrack
microtron injector - electron CW linac. Experimental results of the system application are described.

INTRODUCTION.
The optical transition radiation (OTR) effect [1-2] has gained general acceptance and can be used for many purposes
involving beam diagnostics. Using the method based on trhe OTR effect one can receive all the necessary characteristics
of a beam during the one cycle of OTR spot analysis:- beam energy spread [3];- beam positioning and beam profile [4];- beam emittance [3, 5, 6];- beam density and divergence [6].
The prime objective of our study was algorithms of image acquisition and processing, with the help of existing
hardware. All parts of our image acquisition system are based on the LSI-11 compatible minicomputer. This computer
type is widely used to-day for real time control of the injector of the Moscow CW racetrack microtron (RTM) [7, 8]. Thesystem is connected with an IBM PC compatible computer (PC) via a standard interface. Graphical abilities of a PC arevery suitable for man-machine interface support. The general scheme of the OTR analysis system is shown on figure 1.
The main feature of the computer control system is its control oriented architecture. Therefore, the presenting of a
3D image of the beam profile is an additional (not main) function of the image processing system. The main function ofthe system is "extraction" of digital indirect parameters corresponding to the beam properties. These parameters are used
for closing feedback loops in the main control system to adjust output characteristics of the accelerator beam in real time.
Of course, in some operational modes of an accelerator it is very useful to observe an image of the beam in a way
suitable for an operator. The operator could handle an overall image of the beam and "extract" simultaneously many
parameters from the beam image, but the time of processing an image by an operator is not less than few seconds. But thequestion of adequate interpretation is very difficult and depends on the qualifications of the operator and the quality of theimage restoring process.
The standard frequency of frames in normal TV signals is 50 Hz. Therefore, every 20 ms it is possible to get initial
data for control algorithms for accelerator adjustments. In special applications when the beam has a pulsed nature with afrequency of more than 50 Hz, non-standard (higher) frequencies of frames could be used.
Commercially available frame grabbers are image oriented; they are oriented on the functional transformation,recording or storing of the whole frame. The algorithm of image processing for control application should be different.
The main purpose of image processing for control is extracting a few digital parameters reflecting beam characteristics
that could be used for digital closing of feedback loops.
The most simple example is the application of adjusting the electron optic system on the basis of the XY position of
the beam OTR spot. XY coordinates of the beam spot center of gravity are only two numbers, and could be received as aresult of a direct image processing system operation.

IMAGE PROCESSING HARDWARE.

OTR is converted to standard TV signals with the help of a TV camera. The image with the spot of OTR is storedand processed in an image acquisition board during the time of the next half-frame. A binary digitising system is used. A
single bit sampling level could be set in a range of up to 256 levels of image brightness individually for every frame. The
video processor unit supports in silicon the functions of square measurement, contouring, search on a raster and otherspecial digital image processing functions. It is possible to realise beam diagnostics in real time due to features of theimage acquisition board. Only necessary and useful data are transmitted to the control system, presented to operator andstored. The output data could be received in concrete beam parameters (size and position for example).
The hardware features ensure cost effective solutions and simple software support.

Figure 1. General scheme of OTR analysis system.

FIRST EXPERIMENTAL RESULTS.
The system has been used for studying an effect of the coupling slots on beam dynamics in the accelerating structureof RTM. This effect is explained by the transverse magnetic field excited on the axis of coupling cells and providing
intercell coupling. Methods to compensate the coupling slot effect with external magnetic fields have been suggested [9].
For the exact measurements of the beam centre of gravity position and current distribution the method of OTR analysis
has been used. At the axis of the beam 9 micron Al foil was placed at 45o.
Standard TV signals from a black and white CCD camera with a 14:1 magnification objective were analysed by acomputer system. An example of a 20 serial frame 3D current distribution of the beam is presented in figure 2.

Figure 2. Restored 3D current distribution of the beam measured by means of OTR method.

IN THE FORESEEABLE FUTURE.
We are faced now with the problem of determining the accuracy of our measurement system carefully. The tasks ofanalog to digital channel linearization and increasing of the effective range and signal to noise ratio of the TV signal
should be studied.
The image acquisition and processing algorithms will be implemented in a special hardware device based on moderntechnologies - digital signal processor and, soon FPGA. After the complete check-out a standard fieldbus interface (MILSTD 1553B) will be added for fast connection with main computer control system of the accelerator, a system that isunder construction now [10].

REFERENCES.
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