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Injection, extraction or splitting of a beam with high current septum magnets
NUCLEAR
INSTRUMENTS
AND METHODS
87 (I97O) I89-I96; © N O R T H - H O L L A N D
PUBLISHING
CO.
I N J E C T I O N , E X T R A C T I O N O R S P L I T T I N G OF A BEAM
WITH HIGH CURRENT S E P T U M MAGNETS A. P A U L I N
SIN-ETH, Zurich, Switzerland Received 4 May 1970 The technological possibility to build a high current septum magnet with a septum thickness of only 1 m m is shown. Through cooling with water, 14 W / m m a and more could be handled. The calculations show that with a proper septum form a broad region of magnetic fields needed for either beam splitter or inflector could be created. A proton beam of 0.3/~A/mm 2 can easily be split in any ratio between 0 and 100%.
1. Introduction To perform two or more experiments simultaneously, it is sometimes desirable to split an accelerated beam. The beam is usually switched from one target to another with the help of rf-hv devices, pulsed magnets or by slow switching of bending magnets. The resulting particle bunches of high beam intensity could overload the counters and are a waste if they are not necessary for the experiment. An electrostatic beam divider, as described by Jones and McWalters 1) is limited by the voltage that can be applied to the deflecting electrodes and, hence, the particle current which can be sustained by the beam cutting central (earthed) electrodes. Also the breakdown voltage is diminished by the particle energy lost on the electrodes. To overcome the above mentioned difficulties a magnetic beam cutting system is proposed which is, in fact, not new in principle. In our case, we should build a septum magnet where the required dividing septum is thin enough. This septum must be cooled well to handle the lost energy. That is the technological problem. A part of the 70 MeV proton beam should be directed to the low energy experiments, while the other part should be injected into the second stage of the 600 MeV cyclotron2). The bending magnetic field must be such that the emittance distortion is as small as possible. That is the problem of the particle optics. In the following, the solution of both problems will be shown. We are able to cut a high intensity beam in every ratio between 0 and 100% almost without distortion. The beam splitter length in the magnetostatic case would be much shorter than in the electrostatic one. The magnetostatic septum can also be used as an injector or extractor in particle accelerators. 2. Technology of the septum The most critical part of a beam splitting device is the thin septum sheet. This copper sheet must be cooled
well enough to carry away the heat due to the conduction current as well as the heat created by particles stopping in the copper. The higher the current intensity is, the higher is the magnetic field, and the shorter is the beam splitter; however, it is more expensive. In order to keep the particle losses low the septum should be as thin as possible. So we put as our technological goal a septum thickness of 1 ram. The arrangement of watercooling (with demineralised water) is seen schematically in fig. 1. We fabricated the experimental current septum in two ways. In the first case (fig. 2) Cu plates were castolin brazed together. An X-ray picture of this septum is shown in fig. 3. In the second case we made it by electro-forming. The negative was made from aluminium (fig. 4). In fig. 5 the X-ray picture is seen and in fig. 6 the microcut 75 times enlarged. Having succeeded in the fabrication of the septum we had to examine what thermal power we were able to carry off. The highest power source we had available was a rectifier giving 3.6 kA. The copper cross section was 5 x 1 m m 2 with 5 holes 0.5 x 0.5 m m 2. The current was fed in the same direction as the cooling water. The electric current, the voltage drop over the septum, the temperature rise on the septum, the water flow, and the pressure drop of the water in the cooling tubes were measured. By a tube length of 5 cm and diameter of 0.5 mm, a pressure drop of 7 atm and a water flow velocity of 10 ms -1 an overall power density of 14 kW/cm 3 could be handled, while the temperature in the hottest spot of the septum did not rise more than 80 ° C. We could feed the current sheet with 800 A / m m z. However, in most cases we are not limited by 80 ° C and, therefore, the current load could be higher. I f the septum is used as a beam splitter the absorption of the particle energy is local. Due to the high thermal conductivity of copper we would be allowed to let the temperature in this minispot rise still higher. Details
189
190
A. P A U L I N
!ill!¸ Fig. 4. Aluminium negative for the electro-formed septum.
Fig. 1. Schematic diagram of watercooling
~I p[((f((((([lllilllllllllill)lliiliTt
~i
~rungstenwire 0,l,8
Fig. 2. Cross-section of castolin brazed septum.
Fig. 5. X-ray picture of the electro-formed septum.
Fig. 3. X-ray picture of a brazed septum.
INJECTION,
EXTRACTION
OR
SPLITTING
OF
A BEAM
191
of the septum technology and measurement are available from the SIN-Report3). Having succeeded in the technology of the septum, we are starting with the optics which will lead to construction details. 3. Beam optics of the septum
Fig. 6. Cross-section
of the electro-formed
septum.
The septum- as beam splitter, beam injector or extractorshould not disturb the beam. We have to consider the purpose for which the device will be used. Let us consider first the beam splitter. In order that the beam loss and the heat loading of the septum are not too high the beam cross-section has to be expanded in the horizontal plane. In fig. 7 B, is shown4) as a function of the distance x from the septum in the symmetry plane (_Y= 0). Obviously the magnetic field to the left of the plane x = 0 has the same configuration for negative x but is opposite in polarity. The current has the z-direction and is proportional to the area inscribed in the copper cross-section of the septum as shown in fig. 7. If the magnetic field falls off like a
Fig. 7. The magnetic flux density By in the plane y = 0 for various forms of the copper septum cross-sections. measured in the same units as the septum drawn in the lower left corner.
The distance x is
192
A. PAULIN below 2% for B 0 = 0.72 Bm,x and g = 0.56 Bm,x/X. With further variations of the septum form, we could get nearer to the combination of a dipole and quadrupole magnetic field. To improve the beam deflection, a strong bending magnet should follow the beam splitter. The combination of both is shown schematically 5) in fig. 9. The iron part of the bending magnet is not shown on the drawing. The current circuit is closed by a coaxial cylinder in order not to disturb the field of the septum. The beam transport system of the SIN accelerator seems to be such that the beam which will travel from the injector cyclotron to the 600 MeV ring accelerator should not be distorted by the beam splitter at all6). For this reason, we have computed with the programme L I N D A 7) a septum magnet using the magnetization curve shown in fig. 10.
_a I B max
-Bx
_.t, [3 m a x
-Bx 0 13m a x
2T B
J
J
f,510 I
30 I
50 I
70 I
_.~1 x
Fig. 8. The magnetic flux density components Bu and Bx for various y-planes for the septum form drawn in the lower left corner (length units as in fig. 7). straight line the device will correspond to the combination of a bending magnet with Bo = ½(1 +b)Bm,x and a quadrupole with g = (l-b)Bm,x/X. Bm,x is the maximum magnetic flux density at the septum, x the distance from the septum which can be occupied by the beam (where g varies linearly with x), and b = B,,in/Bm,x. The By straight line dependence is pretty well satisfied up to x = 45 mm, as shown in fig. 8. The deviation lies
Fig. 9. A possible combination of beam splitter and magnet.
septum
1" 0
I 1
i 2
, ~
H
i P 3 Aim
Fig. 10. The permeability curve used in calculations. We can see the result on the following drawings. In fig. 11 we see the magnetic field of the components (a) current leading cooling tubes and (b) the beam dividing current septum. The septum will be cooled as mentioned in the technological part of this paper. In our calculation we have taken an effective thickness of 0.8 mm for the 1 mm thick septum. This effective thickness is estimated for the applicable form by an analogue model with the resistance paper method or by analytical calculation. The results of the magnetic field calculations are shown in fig. 12. By further iterations one might achieve results even closer to the ideal form. The same septum magnet form could be also used as a magnetic injector or extractor instead of an electrostatic oneS). However, in this case it would be set up in a fringing magnetic field. The remaining particle orbits would be changed. One has to calculate the decrease of the fringing magnetic field due to the yoke of the sep-
INJECTION, EXTRACTION OR SPLITTING OF A BEAM
0,10T
0.15 T-
o.1
0.I
193
°T 0,05
0.05
o,ol
0,01
G
I0
2O
30
40
GO
60
ram
-
0 -(],01T.
5
10
50
60 mm
50
60 mm
20
-O,OST
0.16T
0.15T
0J
0,I
7 0,05
0.05
0,01
0,01
: 5
; 10
. 2O
30
~0
__ 5()
: 60 mm
•
0
I
I
-0.(]I"
f -0.05
0.151
O.15T
i / (].0s
~ly/////S~y///~¢<
~ v_(._d (]0 mm I'
a.)
.... ~'{
bl
Fig. I 1. Magnetic field produced (a) by cooling tubes, (b) by the beam dividing septum. tum and compensate it with the magnetized septum magnet. In fig. 13, curve F, the field diminution by shielding is shown. It was estimated by the resistancepaper method (fig. 14). Curve C (fig. 13) gives the magnetic field of the C septum magnet. Curve B (fig. 13) shows the resulting field which is seen by the particle. There is a homogeneous magnetic field inside
the gap; the magnetic field outside the septum is not disturbed. 4. Septum and radiation According to Tripard 9) the 68 MeV protons impinging on copper would reach a maximum energy loss of approx. 380 MeV/cm in a distance of 6.6 mm.
-20
o ~Xd
,~>
I, lO
10
30
--
40 I
I2=-0.6 I3=-0,4
I1 = 1.0
40
SO J
50
e
60rnm
60ram
fR
,112
,113
,fK
3
2
1
!T
!
111)mT 117
,116
.
116 115
.117
_21
120mT1
'°
,o
30
"--I"
~
/gO
i
~
50
I
50
11= 1.0 12=-0.635 I3=-0.365
11-1.0 12._0, 6 •13 =-0.4
Fig. 12. Calculated magnetic field of a beam splitter. By varying the position of the septum (leading 12) and the ratio 12/la-in other words by the form of septum and cooling t u b e - a n approach is possible to the ideal form.
20
//////,/~
2O
11 - 1.0 I2--116 13--0.4
, 118mT
60mm
50,'rim
"1
11SmT 117 1115
l;
Is
I
t
t 118rnT .7 ,~6 115 t ,B
Z
> C
>
195
INJECTION~ E X T R A C T I O N OR S P L I T T I N G OF A BEAM
12°I ~_
~1
• .
0
i
I
-
-
. . . . .
-
I
I
I
20 30 ~o 50 60 mm Fig. 13. Calculated magnetic field of the inflector septum magnet. B represents the field resulting by the sum of C, produced by the magnet, and F, the field that remains after the shielding of the iron yoke. 10
----_____
Fig. 14. Magnetic field plot caused by shielding of the iron yoke put in the homogeneous magnetic field. From this the curve F in fig. 13 was estimated.
196
~.. PAULIN
A beam intensity o f 0.3 /~A/mm 2 corresponds to a thermal power density of 10 W / m m 3. According to our experimental results, we can handle this power. However, the protons could destroy the copper septurn or choke up the pipe with copper erosion. This problem seems to be solved at C E R N I ° ) . For other energies, it could be possible that other methods are preferable, especially when it is not important if the cooling fluid is an electrically conducting one or not. If the intensity o f the particles striking the septum would be too high, a beam shadow could be built to protect the septum. F o r that purpose tungsten wires could eventually be used. In same cases gas cooling could be of advantage.
References
1) W. B. Jones and P. F. McWalters, USA Pat. no. 3,072,786 (Jan. 8, 1963). 2) j. p. Blaser and H. A. Willax, Conf. Nuclear structure (Tokyo, 1967) p. 506. 3) A. Paulin, Th. Schaub and J. Ulrich, SIN-Report TM-06-02 (1968). a) A. Paulin, SIN-Report TM-06-03 (1968). ~) A. Paulin, SIN-Report TM-06-01 (1968). 6) j. Zichy, SIN-Report TM-09-07 (1969). 7) Programme by J. Dorst, L.R.L. Berkeley, Calif., adapted by H. Braun, SIN-ETH. s) A. Paulin, SIN-Report TM-06-05 (1969). 9) G. Tripard, SIN-Report TM-I 1-5 (1968). 10) F. Hoyer, Atompraxis (March/April 1969).
INSTRUMENTS
AND METHODS
87 (I97O) I89-I96; © N O R T H - H O L L A N D
PUBLISHING
CO.
I N J E C T I O N , E X T R A C T I O N O R S P L I T T I N G OF A BEAM
WITH HIGH CURRENT S E P T U M MAGNETS A. P A U L I N
SIN-ETH, Zurich, Switzerland Received 4 May 1970 The technological possibility to build a high current septum magnet with a septum thickness of only 1 m m is shown. Through cooling with water, 14 W / m m a and more could be handled. The calculations show that with a proper septum form a broad region of magnetic fields needed for either beam splitter or inflector could be created. A proton beam of 0.3/~A/mm 2 can easily be split in any ratio between 0 and 100%.
1. Introduction To perform two or more experiments simultaneously, it is sometimes desirable to split an accelerated beam. The beam is usually switched from one target to another with the help of rf-hv devices, pulsed magnets or by slow switching of bending magnets. The resulting particle bunches of high beam intensity could overload the counters and are a waste if they are not necessary for the experiment. An electrostatic beam divider, as described by Jones and McWalters 1) is limited by the voltage that can be applied to the deflecting electrodes and, hence, the particle current which can be sustained by the beam cutting central (earthed) electrodes. Also the breakdown voltage is diminished by the particle energy lost on the electrodes. To overcome the above mentioned difficulties a magnetic beam cutting system is proposed which is, in fact, not new in principle. In our case, we should build a septum magnet where the required dividing septum is thin enough. This septum must be cooled well to handle the lost energy. That is the technological problem. A part of the 70 MeV proton beam should be directed to the low energy experiments, while the other part should be injected into the second stage of the 600 MeV cyclotron2). The bending magnetic field must be such that the emittance distortion is as small as possible. That is the problem of the particle optics. In the following, the solution of both problems will be shown. We are able to cut a high intensity beam in every ratio between 0 and 100% almost without distortion. The beam splitter length in the magnetostatic case would be much shorter than in the electrostatic one. The magnetostatic septum can also be used as an injector or extractor in particle accelerators. 2. Technology of the septum The most critical part of a beam splitting device is the thin septum sheet. This copper sheet must be cooled
well enough to carry away the heat due to the conduction current as well as the heat created by particles stopping in the copper. The higher the current intensity is, the higher is the magnetic field, and the shorter is the beam splitter; however, it is more expensive. In order to keep the particle losses low the septum should be as thin as possible. So we put as our technological goal a septum thickness of 1 ram. The arrangement of watercooling (with demineralised water) is seen schematically in fig. 1. We fabricated the experimental current septum in two ways. In the first case (fig. 2) Cu plates were castolin brazed together. An X-ray picture of this septum is shown in fig. 3. In the second case we made it by electro-forming. The negative was made from aluminium (fig. 4). In fig. 5 the X-ray picture is seen and in fig. 6 the microcut 75 times enlarged. Having succeeded in the fabrication of the septum we had to examine what thermal power we were able to carry off. The highest power source we had available was a rectifier giving 3.6 kA. The copper cross section was 5 x 1 m m 2 with 5 holes 0.5 x 0.5 m m 2. The current was fed in the same direction as the cooling water. The electric current, the voltage drop over the septum, the temperature rise on the septum, the water flow, and the pressure drop of the water in the cooling tubes were measured. By a tube length of 5 cm and diameter of 0.5 mm, a pressure drop of 7 atm and a water flow velocity of 10 ms -1 an overall power density of 14 kW/cm 3 could be handled, while the temperature in the hottest spot of the septum did not rise more than 80 ° C. We could feed the current sheet with 800 A / m m z. However, in most cases we are not limited by 80 ° C and, therefore, the current load could be higher. I f the septum is used as a beam splitter the absorption of the particle energy is local. Due to the high thermal conductivity of copper we would be allowed to let the temperature in this minispot rise still higher. Details
189
190
A. P A U L I N
!ill!¸ Fig. 4. Aluminium negative for the electro-formed septum.
Fig. 1. Schematic diagram of watercooling
~I p[((f((((([lllilllllllllill)lliiliTt
~i
~rungstenwire 0,l,8
Fig. 2. Cross-section of castolin brazed septum.
Fig. 5. X-ray picture of the electro-formed septum.
Fig. 3. X-ray picture of a brazed septum.
INJECTION,
EXTRACTION
OR
SPLITTING
OF
A BEAM
191
of the septum technology and measurement are available from the SIN-Report3). Having succeeded in the technology of the septum, we are starting with the optics which will lead to construction details. 3. Beam optics of the septum
Fig. 6. Cross-section
of the electro-formed
septum.
The septum- as beam splitter, beam injector or extractorshould not disturb the beam. We have to consider the purpose for which the device will be used. Let us consider first the beam splitter. In order that the beam loss and the heat loading of the septum are not too high the beam cross-section has to be expanded in the horizontal plane. In fig. 7 B, is shown4) as a function of the distance x from the septum in the symmetry plane (_Y= 0). Obviously the magnetic field to the left of the plane x = 0 has the same configuration for negative x but is opposite in polarity. The current has the z-direction and is proportional to the area inscribed in the copper cross-section of the septum as shown in fig. 7. If the magnetic field falls off like a
Fig. 7. The magnetic flux density By in the plane y = 0 for various forms of the copper septum cross-sections. measured in the same units as the septum drawn in the lower left corner.
The distance x is
192
A. PAULIN below 2% for B 0 = 0.72 Bm,x and g = 0.56 Bm,x/X. With further variations of the septum form, we could get nearer to the combination of a dipole and quadrupole magnetic field. To improve the beam deflection, a strong bending magnet should follow the beam splitter. The combination of both is shown schematically 5) in fig. 9. The iron part of the bending magnet is not shown on the drawing. The current circuit is closed by a coaxial cylinder in order not to disturb the field of the septum. The beam transport system of the SIN accelerator seems to be such that the beam which will travel from the injector cyclotron to the 600 MeV ring accelerator should not be distorted by the beam splitter at all6). For this reason, we have computed with the programme L I N D A 7) a septum magnet using the magnetization curve shown in fig. 10.
_a I B max
-Bx
_.t, [3 m a x
-Bx 0 13m a x
2T B
J
J
f,510 I
30 I
50 I
70 I
_.~1 x
Fig. 8. The magnetic flux density components Bu and Bx for various y-planes for the septum form drawn in the lower left corner (length units as in fig. 7). straight line the device will correspond to the combination of a bending magnet with Bo = ½(1 +b)Bm,x and a quadrupole with g = (l-b)Bm,x/X. Bm,x is the maximum magnetic flux density at the septum, x the distance from the septum which can be occupied by the beam (where g varies linearly with x), and b = B,,in/Bm,x. The By straight line dependence is pretty well satisfied up to x = 45 mm, as shown in fig. 8. The deviation lies
Fig. 9. A possible combination of beam splitter and magnet.
septum
1" 0
I 1
i 2
, ~
H
i P 3 Aim
Fig. 10. The permeability curve used in calculations. We can see the result on the following drawings. In fig. 11 we see the magnetic field of the components (a) current leading cooling tubes and (b) the beam dividing current septum. The septum will be cooled as mentioned in the technological part of this paper. In our calculation we have taken an effective thickness of 0.8 mm for the 1 mm thick septum. This effective thickness is estimated for the applicable form by an analogue model with the resistance paper method or by analytical calculation. The results of the magnetic field calculations are shown in fig. 12. By further iterations one might achieve results even closer to the ideal form. The same septum magnet form could be also used as a magnetic injector or extractor instead of an electrostatic oneS). However, in this case it would be set up in a fringing magnetic field. The remaining particle orbits would be changed. One has to calculate the decrease of the fringing magnetic field due to the yoke of the sep-
INJECTION, EXTRACTION OR SPLITTING OF A BEAM
0,10T
0.15 T-
o.1
0.I
193
°T 0,05
0.05
o,ol
0,01
G
I0
2O
30
40
GO
60
ram
-
0 -(],01T.
5
10
50
60 mm
50
60 mm
20
-O,OST
0.16T
0.15T
0J
0,I
7 0,05
0.05
0,01
0,01
: 5
; 10
. 2O
30
~0
__ 5()
: 60 mm
•
0
I
I
-0.(]I"
f -0.05
0.151
O.15T
i / (].0s
~ly/////S~y///~¢<
~ v_(._d (]0 mm I'
a.)
.... ~'{
bl
Fig. I 1. Magnetic field produced (a) by cooling tubes, (b) by the beam dividing septum. tum and compensate it with the magnetized septum magnet. In fig. 13, curve F, the field diminution by shielding is shown. It was estimated by the resistancepaper method (fig. 14). Curve C (fig. 13) gives the magnetic field of the C septum magnet. Curve B (fig. 13) shows the resulting field which is seen by the particle. There is a homogeneous magnetic field inside
the gap; the magnetic field outside the septum is not disturbed. 4. Septum and radiation According to Tripard 9) the 68 MeV protons impinging on copper would reach a maximum energy loss of approx. 380 MeV/cm in a distance of 6.6 mm.
-20
o ~Xd
,~>
I, lO
10
30
--
40 I
I2=-0.6 I3=-0,4
I1 = 1.0
40
SO J
50
e
60rnm
60ram
fR
,112
,113
,fK
3
2
1
!T
!
111)mT 117
,116
.
116 115
.117
_21
120mT1
'°
,o
30
"--I"
~
/gO
i
~
50
I
50
11= 1.0 12=-0.635 I3=-0.365
11-1.0 12._0, 6 •13 =-0.4
Fig. 12. Calculated magnetic field of a beam splitter. By varying the position of the septum (leading 12) and the ratio 12/la-in other words by the form of septum and cooling t u b e - a n approach is possible to the ideal form.
20
//////,/~
2O
11 - 1.0 I2--116 13--0.4
, 118mT
60mm
50,'rim
"1
11SmT 117 1115
l;
Is
I
t
t 118rnT .7 ,~6 115 t ,B
Z
> C
>
195
INJECTION~ E X T R A C T I O N OR S P L I T T I N G OF A BEAM
12°I ~_
~1
• .
0
i
I
-
-
. . . . .
-
I
I
I
20 30 ~o 50 60 mm Fig. 13. Calculated magnetic field of the inflector septum magnet. B represents the field resulting by the sum of C, produced by the magnet, and F, the field that remains after the shielding of the iron yoke. 10
----_____
Fig. 14. Magnetic field plot caused by shielding of the iron yoke put in the homogeneous magnetic field. From this the curve F in fig. 13 was estimated.
196
~.. PAULIN
A beam intensity o f 0.3 /~A/mm 2 corresponds to a thermal power density of 10 W / m m 3. According to our experimental results, we can handle this power. However, the protons could destroy the copper septurn or choke up the pipe with copper erosion. This problem seems to be solved at C E R N I ° ) . For other energies, it could be possible that other methods are preferable, especially when it is not important if the cooling fluid is an electrically conducting one or not. If the intensity o f the particles striking the septum would be too high, a beam shadow could be built to protect the septum. F o r that purpose tungsten wires could eventually be used. In same cases gas cooling could be of advantage.
References
1) W. B. Jones and P. F. McWalters, USA Pat. no. 3,072,786 (Jan. 8, 1963). 2) j. p. Blaser and H. A. Willax, Conf. Nuclear structure (Tokyo, 1967) p. 506. 3) A. Paulin, Th. Schaub and J. Ulrich, SIN-Report TM-06-02 (1968). a) A. Paulin, SIN-Report TM-06-03 (1968). ~) A. Paulin, SIN-Report TM-06-01 (1968). 6) j. Zichy, SIN-Report TM-09-07 (1969). 7) Programme by J. Dorst, L.R.L. Berkeley, Calif., adapted by H. Braun, SIN-ETH. s) A. Paulin, SIN-Report TM-06-05 (1969). 9) G. Tripard, SIN-Report TM-I 1-5 (1968). 10) F. Hoyer, Atompraxis (March/April 1969).