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Focusing of the synchroton scattered-out beam
NUCLEAR
INSTRUMENTS
1 (1957) 351-353; N O R T H - H O L L A N D
PUBLISHING
CO. - A M S T E R D A M
FOCUSSING OF THE SYNCHROTON SCATTERED-OUT BEAM H. B. V A N D E R R A A Y
Deparlment o/ Physics, B~rmingham University Received 23 A u g u s t 1957
A m e a n s of f o c u s s i n g t h e s c a t t e r e d - o u t b e a m of t h e B i r m i n g ham Proton Synchrotron by modifying the magnetic fringing field is outlined. T h e b e a m is b r o u g h t to a s p o t
focus of a b o u t 20 sq. c m a t a d i s t a n c e of 5 m e t e r s from t h e exit p o r t a n d h a s a m e a n i n t e n s i t y of 5 × 103 particles per sq. c m per pulse.
1. Introduction
spherical stainless steel window (see fig. 1). The area of this window defines the "external beam" available for experimental work. By altering the azimuthal or radial position of the target, the mean angle of scatter of the external beam may be varied over a limited range. Since many experimental techniques require the placing of apparatus at some distance from the machine, focussing of the scattered-out beam will enable experiments to be performed for which the necessary intensity would otherwise be lacking.
At the present time no means of extracting the circulating beam of the Birmingham proton synchroton 1) is available. Hence all experiments requiring an external proton beam rely on the scattering produced by a suitably positioned target. Some of those particles scattered in the forward direction pass through a step in the vacuum box, which is fitted with a thin hemi-
2. Vertical Focussing
The scattered beam passing through the thin window has a considerable path length in the fringing field of the synchrotron magnet. Due to the radial decrease of magnetic field intensity, the scattered particles experience a force directed toward the magnetic median plane. Also those particles which are moving at greater radii will be in a region of lower magnetic field and consequently their radius of curvature is greater than those protons in orbits closer to the machine. The result of these two factors is a net focussing of the beam in a vertical plane and a rapid defocussing in a horizontal plane. Consequently a horizontal line focus is formed in the median plane of the machine at a distance of approximately 150 cm from the exit port for a particular target position corresponding to a mean angle of scatter of 4 °. Fig.
1. Q u a d r u p o l e in position s h o w i n g h e m i s p h e r i c a l exit port.
1) Moon R i d d i f o r d a n d S y m o n d s , Proe. Roy. Soc. 230
(1955) 204. 351
352
H . B . VA.N D E R
Past this point the beam diverges in both directions. Thus if experimental equipment could be placed at or before this point, a fairly well defined beam of reasonable intensity could be obtained. Due to the limited space and the intense magnetic field in this region, only experiments using nuclear emulsions are practicable.
RAAY
plates are supported on a rigid stainless steel framework clamped to the body of the magnet, because, during a pulse, a force of several tons is exerted on the plates:
3. Quadrupoles The action of the fringing field is similar to that of the first section of a magnetic quadrupole lens systemS, s) with a superimposed constant magnetic field. As with such a system it is necessary to add a second lens of opposite polarity i.e. one producing horizontal focussing and vertical defocussing. Thus a magnetic field region in which the gradient is of opposite sign to that already existing is required. The existing gradient is effective from the point of scatter of the protons to some metres beyond the exit port where the field has fallen to a negligible value. For practical reasons the second lens will have to be made much shorter than this and to achieve the desired result the gradient will have to be far greater than the mean value of the first lens. As the beam rapidly diverges, the second lens must be placed as close as possible to the exit port, thus ensuring that all the protons passing through the port will be collected b y a lens of reasonable aperture. This necessitates placing the lens in a region where the fringing field of the synchrotron magnet is intense and the possibility exists of redistributing a part of this fringing flux so as to produce a region of opposite gradient. This system has been found very successful, as is described in the next section. As no external power is required to establish the desired field, the system has come to be known as a "Self Excited Quadrupole".
tO
~9
g8 .7 6
3 482
484
486 RADIUS
488
OF
MACfllNE.
490
tN
49a
CM
F i g . 2. R a d i a l field d e p e n d e n c e b e f o r e a n d a f t e r m o d i f i c a tion.
By constructing the quadrupoles of 9" steel plates 20" long, a very flexible arrangement is obtained and field gradients of various intensities may be had by simply transposing the plates. Hence various degrees of focussing are easily secured for the various beams to be used. The field existing with and without the quadrupole in position is shown in fig. 2. The
EXIT
60crn
4. Experimental Arrangement To obtain the desired field gradient, mild steel plates are placed as shown in figure 1. These 2) C o u r a n t , L i v i n g s t o n a n d S n y d e r , P h y s . R e v . 88 (1952) 1190. 3) S t e r n h e i m e r , R e v . Sci. I n s t . 24 (1953) 573.
t 60 ¢m:
- ""
' 260 cm
360 ¢m
F i g . 3. B e a m profile a t v a r i o u s d i s t a n c e s f r o m t i l e e x i t of q u adrupole.
FOCUSSING
OF T H E S Y N C H R O T O N
inset shows the relative positioning of the steel. It is evident that a linear of 1 100 gauss/cm is established over the region of interest. The correct steel arrangement to produce an almost parallel beam was determined by using X-ray film as a beam monitor. The beam profile found in this way at various distances from the exit port is shown in fig. 3. In fig. 4 the horizontal and vertical extents of the beam are plotted as functions of distance
t IOO DISTANCE
2O0 FROM
EXIT
OF
t
30O
400
OUADRUPOLE
IN
500 CM
£
tOO
DISTANCE
200 FROM
300 EXIT
OF
QUADRUPOLE
400 IN
EOO CM
Fig. 4. V e r t i c a l a n d h o r i z o n t a l e x t e n t of b e a m as a f u n c t i o n of d i s t a n c e from q u a d r u p o l e .
from the exit of the quadrupole. From these curves it is evident that an almost parallel
SCATTERED-OUT
BEAM
353
beam is obtainable. The intensity of the beam, determined with nuclear emulsions, is approximately 5 x 103 particles per s~. cm per pulse, over an area of about 20 sq. cm. The energy spread in the beam may be determined from the size of the image and a knowledge of the mean trajectory. Calculation shows that an energy spread of :2 20 MeV is consistent with the observed results when considering a beam of mean energy 950 MeV. In many experiments it is desired to undertake measurements over a range of energies. In a sychrotron this is simply effected by stopping the accelerating cycle at the desired instant. As the quadrupole is excited by the fringing field of the synchrotron magnet, its strength is automatically adjusted to the momentum of the scattered-out particles. Well focussed beams have been obtained at as low an energy as 350 MeV without altering the steel arrangement used to focus 950 l~{eV protons. However, to obtain optimum conditions, slight alterations may be necessary to allow for the saturation effects prevalent at peak field. The initiation of this work followed discussions with Dr. J. L. Symonds and Professor R. R. Wilson of Harvard University. The co-operation of members of the synchrotron staff is gratefully acknowledged. The assistance of Mr. D. Hoare is also greatly appreciated.
INSTRUMENTS
1 (1957) 351-353; N O R T H - H O L L A N D
PUBLISHING
CO. - A M S T E R D A M
FOCUSSING OF THE SYNCHROTON SCATTERED-OUT BEAM H. B. V A N D E R R A A Y
Deparlment o/ Physics, B~rmingham University Received 23 A u g u s t 1957
A m e a n s of f o c u s s i n g t h e s c a t t e r e d - o u t b e a m of t h e B i r m i n g ham Proton Synchrotron by modifying the magnetic fringing field is outlined. T h e b e a m is b r o u g h t to a s p o t
focus of a b o u t 20 sq. c m a t a d i s t a n c e of 5 m e t e r s from t h e exit p o r t a n d h a s a m e a n i n t e n s i t y of 5 × 103 particles per sq. c m per pulse.
1. Introduction
spherical stainless steel window (see fig. 1). The area of this window defines the "external beam" available for experimental work. By altering the azimuthal or radial position of the target, the mean angle of scatter of the external beam may be varied over a limited range. Since many experimental techniques require the placing of apparatus at some distance from the machine, focussing of the scattered-out beam will enable experiments to be performed for which the necessary intensity would otherwise be lacking.
At the present time no means of extracting the circulating beam of the Birmingham proton synchroton 1) is available. Hence all experiments requiring an external proton beam rely on the scattering produced by a suitably positioned target. Some of those particles scattered in the forward direction pass through a step in the vacuum box, which is fitted with a thin hemi-
2. Vertical Focussing
The scattered beam passing through the thin window has a considerable path length in the fringing field of the synchrotron magnet. Due to the radial decrease of magnetic field intensity, the scattered particles experience a force directed toward the magnetic median plane. Also those particles which are moving at greater radii will be in a region of lower magnetic field and consequently their radius of curvature is greater than those protons in orbits closer to the machine. The result of these two factors is a net focussing of the beam in a vertical plane and a rapid defocussing in a horizontal plane. Consequently a horizontal line focus is formed in the median plane of the machine at a distance of approximately 150 cm from the exit port for a particular target position corresponding to a mean angle of scatter of 4 °. Fig.
1. Q u a d r u p o l e in position s h o w i n g h e m i s p h e r i c a l exit port.
1) Moon R i d d i f o r d a n d S y m o n d s , Proe. Roy. Soc. 230
(1955) 204. 351
352
H . B . VA.N D E R
Past this point the beam diverges in both directions. Thus if experimental equipment could be placed at or before this point, a fairly well defined beam of reasonable intensity could be obtained. Due to the limited space and the intense magnetic field in this region, only experiments using nuclear emulsions are practicable.
RAAY
plates are supported on a rigid stainless steel framework clamped to the body of the magnet, because, during a pulse, a force of several tons is exerted on the plates:
3. Quadrupoles The action of the fringing field is similar to that of the first section of a magnetic quadrupole lens systemS, s) with a superimposed constant magnetic field. As with such a system it is necessary to add a second lens of opposite polarity i.e. one producing horizontal focussing and vertical defocussing. Thus a magnetic field region in which the gradient is of opposite sign to that already existing is required. The existing gradient is effective from the point of scatter of the protons to some metres beyond the exit port where the field has fallen to a negligible value. For practical reasons the second lens will have to be made much shorter than this and to achieve the desired result the gradient will have to be far greater than the mean value of the first lens. As the beam rapidly diverges, the second lens must be placed as close as possible to the exit port, thus ensuring that all the protons passing through the port will be collected b y a lens of reasonable aperture. This necessitates placing the lens in a region where the fringing field of the synchrotron magnet is intense and the possibility exists of redistributing a part of this fringing flux so as to produce a region of opposite gradient. This system has been found very successful, as is described in the next section. As no external power is required to establish the desired field, the system has come to be known as a "Self Excited Quadrupole".
tO
~9
g8 .7 6
3 482
484
486 RADIUS
488
OF
MACfllNE.
490
tN
49a
CM
F i g . 2. R a d i a l field d e p e n d e n c e b e f o r e a n d a f t e r m o d i f i c a tion.
By constructing the quadrupoles of 9" steel plates 20" long, a very flexible arrangement is obtained and field gradients of various intensities may be had by simply transposing the plates. Hence various degrees of focussing are easily secured for the various beams to be used. The field existing with and without the quadrupole in position is shown in fig. 2. The
EXIT
60crn
4. Experimental Arrangement To obtain the desired field gradient, mild steel plates are placed as shown in figure 1. These 2) C o u r a n t , L i v i n g s t o n a n d S n y d e r , P h y s . R e v . 88 (1952) 1190. 3) S t e r n h e i m e r , R e v . Sci. I n s t . 24 (1953) 573.
t 60 ¢m:
- ""
' 260 cm
360 ¢m
F i g . 3. B e a m profile a t v a r i o u s d i s t a n c e s f r o m t i l e e x i t of q u adrupole.
FOCUSSING
OF T H E S Y N C H R O T O N
inset shows the relative positioning of the steel. It is evident that a linear of 1 100 gauss/cm is established over the region of interest. The correct steel arrangement to produce an almost parallel beam was determined by using X-ray film as a beam monitor. The beam profile found in this way at various distances from the exit port is shown in fig. 3. In fig. 4 the horizontal and vertical extents of the beam are plotted as functions of distance
t IOO DISTANCE
2O0 FROM
EXIT
OF
t
30O
400
OUADRUPOLE
IN
500 CM
£
tOO
DISTANCE
200 FROM
300 EXIT
OF
QUADRUPOLE
400 IN
EOO CM
Fig. 4. V e r t i c a l a n d h o r i z o n t a l e x t e n t of b e a m as a f u n c t i o n of d i s t a n c e from q u a d r u p o l e .
from the exit of the quadrupole. From these curves it is evident that an almost parallel
SCATTERED-OUT
BEAM
353
beam is obtainable. The intensity of the beam, determined with nuclear emulsions, is approximately 5 x 103 particles per s~. cm per pulse, over an area of about 20 sq. cm. The energy spread in the beam may be determined from the size of the image and a knowledge of the mean trajectory. Calculation shows that an energy spread of :2 20 MeV is consistent with the observed results when considering a beam of mean energy 950 MeV. In many experiments it is desired to undertake measurements over a range of energies. In a sychrotron this is simply effected by stopping the accelerating cycle at the desired instant. As the quadrupole is excited by the fringing field of the synchrotron magnet, its strength is automatically adjusted to the momentum of the scattered-out particles. Well focussed beams have been obtained at as low an energy as 350 MeV without altering the steel arrangement used to focus 950 l~{eV protons. However, to obtain optimum conditions, slight alterations may be necessary to allow for the saturation effects prevalent at peak field. The initiation of this work followed discussions with Dr. J. L. Symonds and Professor R. R. Wilson of Harvard University. The co-operation of members of the synchrotron staff is gratefully acknowledged. The assistance of Mr. D. Hoare is also greatly appreciated.