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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963
orga D-16ou-
Conf-650922-8
OCT6 1965
Paper to be presented at and published in the Proceedings of the Magnet
Technology Symposium, Stanford University, Sept. 8-10, 1965
EXTRACTION MAGNETS FOR CYCLOTRONS*

E, D, Hudson and R, S, Lord
Oak Ridge National Laboratory
Oak Ridge, Tennessee
TOUCEMENT
RELEASED 10?
IN NUCLEO? SCILICE ABSTRACTS
Summary
Three types of magnetic channels
meeting the varying requirements for beam ex-
traction from cyclotrons are described. They
produce field reductions of up to 9,000 gauss for
the deflected beam, but only a few gauss of first
harmonic in the region of the circulating beam.
The power levels are on the order of 200 kW.
5. The channel must either be enclosed
in a vacuum tight container, or designed to oper-
ate with the insulation and other components
exposed to high vacuum.
Introduction
Systems for extracting the circulating
beam from the strong magnetic fields used in
cyclic accelerators usually include an electro-
static element to separate the beam in the outer
orbit from the circulating beam. The rest of the
extraction system may be electrostatic or mag-
netic. The magnetic elements, to be discussed
here, may be composed of iron only, iron with
compensation coils, or coils only. The mag-
netic channel in the early cyclotrons usually
consisted of a simple iron box or iron plates
along the path of the extracted beam. In the 86-
In. Cyclotron at Oak Ridge, an extraction
channel with both a simple iron section and a
compensated-iron section has been used for 10 .
years. The Oak Ridge Isochronous Cyclotron
(ORIC) uses a coil-only channel section and an
iron section compensated with two independently
controlled coils.
For a fixed energy cyclotron, the first
choice that comes to mind is an iron box or iron
plates to shunt the magnetic field from the path
of the extracted beam. This simple iron-only
channel is often inadequate and usually produces
too large a disturbance in the circulating beam
region. A typical reduction in field that can be
obtained by using iron plates half as wide as the
gap is shown by the first curve in Fig. 1. Note
that at a gap field of about two kilogauss the iron
plates become saturated; any further increase in
gap field gives a corresponding increase in the
field between the iron plates. Since the amount
of field reduction remains constant in gap fields
above which the iron saturates, such a system is
not suitable for a variable energy cyclotron.
In the compensated-iron channel, in which
the iron' is usually saturated, the reduction in
field can be varied by energizing a coil wrapped
around the plates. From the third curve in Fig.
I we see that if the coil around the iron is driven
at 20,000 A-turns/in. the field between the plates
does not start to rise until the gap field nears
10,000 gauss. When the gap field is reduced
below about 6,000 gauss the coil field overrides
the gap field and the field between the plates
reverses,
Channel Requirements
The requirements of a magnetic channel
are usually quite severe:
A Fixed-Field Channel
...
.
1. It must provide an escape path for the
deflected beam along which the magnetic field of
the cyclotron is reduced by as much as 9000
gauss.
2. While reducing the field for the de-
flected beam, the channel must not unduly
disturb the circulating beam that may be a frac-
tion of inch away; that is, the first harmonic in
this region must be small.
3. For a variable energy cyclotron the
channel must be capable of producing a wide
range of magnetic field reduction,
4. Because of space limitations the
current density is usually very high; this leads to
short path lengths for cooling water and many
parallel water circuits,
The channel of the ORNL 86-In.
Cyclotron, shown in Fig. 2, is a practical exam-
ple of a system for fixed-field operation. The
individual conductors of 1/4-in, by l-in, copper
can be seen at the left, surrounding a set of iron
plates. The massive iron channel at the right
has iron plates 2 in, thick and about 12 in. wide.
The mechanical system used to position the
channel was designed to withstand a magnetic
force of 10 tons.
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The magnetic field in and adjacent to this
channel, with and without coil current, is shown
in Fig. 3. The iron alone reduces the field as
much as 3000 gauss; unfortunately this reduction
extends out into the circulating beam region,
With a current of 5200 amperes in the coils,
about 20,000 A-turns/in., the magnetic field
between the plates is reduced to less than 100
gauss, and the effect of the iron in the circulating
beam region outside the channel is completely
compensated.
..
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*Research sponsored by the U, S, Atomic
Energy Commission under contract with the
Union Carbide Corporation,
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Hudson, Lord - Page 1 (of 3 pages)
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The magnetic field along the deflected
beam path, with and without the compensating
channel, is shown in Fig. 4. The field rapidly
decreases to less than 100 gauss and romains
there to the end of the compensating coil. The
field then rises to about 1000 gauss before the
iron section takes over,
The field intensity along the deflected
beam path for ORIC is shown in Fig. 8. The
general shape of the channel contribution is very
similar to that for the 86-Inch Cyclotron channel;
the ORIC field intensity in the channel region is,
however, about 10,000 gauss larger.
Compensated-Iron Channel
Control of the gradient in the channel is
possible by the proper distribution of the con-
ductors. For example, at Michigan State
University a gradient of 600 G/in, is provided in
the cyclotron channel for focusing the beam,'
The Coaxial Channel
The ORIC Cyclotron uses essentially a
half-scale 86-In. Cyclotron compensated-iron
channel, with an additional inner coil, as shown
in Fig. 5 and 6. The water lines, water headers,
and the electrical leads are at the right. The
light colored objects near the center of the photo-
graph are insulating nylon tubes which supply
cooling water to each turn of the conductor.
These insulators are 1/2-in, OD and 1/4 in. ID.
About 1/4-in. from each end is a groove for an
O-ring of 1/16-in. cross section. The insulators
are exposed to the vacuum and carry water at
about 300 psi. The ORIC channel coils are
wound from 1/4-in, square copper conductor
with an 1/8-in. square water passage. The
inside channel coil can operate at currents up to
3500 amperes. This gives a current density of
about 64,000 A/in. in the copper. The power to
the coils, up to 200 kW, is supplied with water-
cooled flexible cable. Bare welding cable is
inounted in rubber hose like the hose that supply
water to the water headers. This method of
cooling the flexible leads at currents up to 5000
amperes in a vacuum tank was developed for the
86-In. Cyclotron, where the extraction channel
must be moved about two feet to provide space
for internal targets.
The ORIC deflected beam is separated
about 1/2 in. from the circulating beam by an
electrostatic deflector. This requires a special
magnetic channel to fit the very small separation
between deflected and circulating beams. An
iron-free channel with a very thin lip or
"septum," radial thickness of only 1/8 in., was
developed. The basic configuration of the con-
ductors is that of a coaxial transmission line.
In the ideal coaxial conductor there is no leakage
field outside the conductors (in the circulating
beam region), and there is a strong circular
magnetic field in the region between the conduc-
tors (in the deflected beam space). A
modification of this basic design produces a
region of nearly uniform field for the deflected
beam with very little disturbance outside this
region, regardless of magnet excitation or
channel current.
The use of two independently controlled
coils along with iron provides a system that can
be operated in the wide range of magnetic fields
used in variable energy cyclotrons, such as
ORIC, and at the same time provides the proper
field inside the channel for the deflected beam.
A section through the channel, along with the
related fields, is shown in Fig. 6. The inner
coil, between the iron plates, controls chiefly
the field level in the deflected beam region. The
main function of the outside coil is to compensate
for the effect of the iron in the circulating beam
region. Note that in the circulating beam region
the coil field contribution is very nearly equal to
but opposite the effect of the iron, giving essen-
tially complete compensation. In the deflected
beam region, inside the coils, the effect of both
the coils and the iron combine to reduce the mag-
netic field about 9000 gauss in less than 1.5 in. ,
as shown by the dashed lines.
A section through the ORIC "coax" chan-
nel, Fig. 9, shows the arrangement of the
coaxial conductors. The inner and outer conduc-
tors are formed by 48 wedge-shaped copper
elements connected in series so that the current
makes multiple traversals, yielding about
220,000 ampere turns. If the inner and outer
series of conductors were symmetrical in
azimuth, the magnetic field outside the channel
would be zero. It is necessary, however, to
provide space for the deflected beam. This was
done by leaving out a sector of both the inner and
outer conductors, producing the field gradient
shown by the dashed line. The missing sector
was replaced by two pairs of especially shaped
conductors, inserted as shown in Fig. 9. With
current in the insert, most of the symmetry is
re-established so that the entire channel pro-
duces a field gradient as shown by the solid line.
The insert is expendable and can be replaced in
case of damage; also, it can be shaped to provide.
radial field gradient in either direction. The
channel decreases the field 3600 gauss in the
deflected beam region but introduces a first har-
monic of only 20 gauss in the circulating beam
region, The insert design was not optimized for
minimum disturbance; harmonic coils already
built into the cyclotron can readily compensate
for a field harmonic of this amplitude. The
partially assembled concentric conductors are
shown in Fig. 10, and the completed assembly
The field conditions in the circulating
beam region that result from various combina-
tions of inside and outside coil currents are
shown in Fig. 7. When the channel is operating
as shown for curve 4 the first harmonic ampli-
tude is only 5 gauss or about 0.03%. The first
harmonic for the 86-Inch Cyclotron channel is
also 5 gauss.
,
Hudson, Lord - Page 2 (of 3 pages)
in Fig. 11. The vacuum tight container encloses
the wedge-shaped conductors but not the insert.
Some of the characteristics of magnetic
channels for beam extraction are summarized in
Table I. The Michigan State Cyclotron at East
Lansing also has an iron-free magnetic channel,
but of a different design from ORIC. The cyclo-
trons at the Naval Research Laboratory in
Washington, DC, and at the University of
California at Davis, California, will have extrac-
tion systems identical with ORIC.
References
1.
R. E. Berg and H. G. Blosser, IEEE
Trans. on Nucl. Science NS 12 No. 3, 392-
394 (1965).
Table 1
Characteristics of Some Magnetic Extraction Channels
lot Harmonic, Entrance
Current
1/2 in. from Thickness,
Channel
Powar Density AH conductor radial
Location Type (kW) (A/in. 2) (gauss) (gauss) (in.)
ORNL
86 In.
Cyclotron
iron
with
coils
180
20,000
8700
<5
190
64,000
9000
3/4
ORIC
iron
Iron
with
compensated coils
200
31,000
4500
ORIC
coax
1/8
coils
only
20
100
M. S. U.
Cyclotron
coils
only
26,000
3500
*Harmonic < 1 gauss at v=1.
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Hudson, Lord - Page 3 (of 3 pages)
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ORAL-DWG 65-8355

All
8 in
-COIL
IRON
3 in.
by!
ZERO amp-turn /in.
10,000 amp-turns/in
FIELD INSIDE CHANNEL (KG)
20,000 amp-turns/in.
O
2
.
4
6 8 10
GAP FIELD (KG)
12
14
16
EN
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Fig. 1. Magnetic field in a magnetic channel as a function of the gap field of the
cyclotron magnet, with the current in compensation coil, as a parameter,

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Fig. 2. Magnetic channel for the ORNL 86-In, Cyclotron, partially assembled.
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COPPER
IRON
IRON
COPPER
IRON
-
FIELD WITH 5200 AMPERES
COIL CURRENT
9000
Ti
8000
--FIELD WITH 'NO COIL CURRENT
MAGNETIC FIELO (OERSTEDS)
4000
3000
2000
FIELD INSIDE CHANNEL
1000
6
4
2
0
20
2 4 6 8 10 12 14 16 18
DISTANCE FROM CENTER OF CHANNEL (IN.)
Fig. 3. Radial distribution of the magnetic field inside and outside the 86-In, Cyclotron
magnetic channel, with and without compensution.
9000
ORNL-LR-DWG 13726

8000
.
-
MAGNETIC FIELD (oersteds)
-WITHOUT
CHANNEL
-WITH
CHANNEL
1000
12
0 12 24 36 48 60 72 84
DISTANCE FROM ENTRANCE OF MAGNETIC CHANNEL (in.)
96
Fig. 4, Magnetic fiold along the path of the deſlocted beam of the 86-In. Cyclotron, with and
without the magnetic channol,

Fig. 5. Compensated-iron channel for ORIC. The tungsten shielded entrance aperture
can be seen at the bottom of tho picture. The test section (upper) is no longer
used. The channel is exposed to the vacuum in the cyclotron.
$-2-02-016-1710

IRON
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MAIN FIELD
COIL FIELD
IRON CONTRIBUTION
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MAIN FIELD
COIL FIELD
IRON_CONTRIBUTION
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CIRCULATING
BEAM REGION
DEFLECTED
BEAM
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RESULTING FIELD
IN CHANNEL AREA
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IRON
CONTRIBUTION
INCHES
Fig. 6. Cross section of ORIC compensated iron channel, showing associated
magnetic field offect..
ORAL-DWG 64.4682
240

O
FIELD CHANGE (gouss)
CHANNEL IRON ONLY
3000 omo INSIDE COIL, Oomp OUTSIDE COIL
O omp INSIDE COIL, 1350 omp OUTSIDE COIL
-480 LO 3000 omp INSIDE COIL, 1350 omp OUTSIDE COIL
REF. – 18,500 gouss
AT R. 30 in.
- 1200
o
20
40
60
80
100 120 140
AZIMUTH (dog)
160
180
200
220
240
Fiz. 7. Effect of inside and outside coils of the ORIC compensated-iron channel on the field
in the region of the circulating bean.

ORNL-OWG 64-4693
CHANNEL REMOVED
WITH CHANNEL
3000 amp INSIDE COIL
1500 amp OUTSIDE COIL
MAGNETIC FIELD (kilogouss)
5
10
15
20
55
60
65
70
75
25 30 35 40 45 50
DISTANCE BEYOND AZIMUTH 120° (in.)
Fig. 8. Mugnetic field along the path of the dollected beam in ORIC, with and without
camponsated iron channel,
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DRAL-LA-OWO 79446R
-OUTER CONDUCTORS
INNER CONDUCTORS

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INSERT
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CIRCULATING BEAM
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ELD WITH INSERT
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FIELD (KG)
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FIELD WITHOUT INSERT
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4
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Fig. 9. Cross section and field of the ORIC coaxial magnotic channel.


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Fig. 10, Partial assembly of the 48 wedge-shape conductors for the coaxial magnetic channel.
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Fig. 1). The ORIC coaxial channel assenibly with insert.
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END
DATE FILMED
10/29/65
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