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ORAL-P 1/35
- CONE-650306-53
MAGNET FOR AN 800-MEV SEPARATED ORBIT CYCLOTRON
APR 2ን 1965
R. S. Lord and E. D. Hudson
Oak Ridge National Laboratory
Oak Ridge, Tennessee
MASTER
Summary
tolerance and the ease of alignment of the pole
tips in setting up the machine are important,
along with magnet power. The overriding
criterion, of course, is that the cost of the
entire system be the minimum that is consistent
with good quality beam.
The Separated Orbit Cyclotron consists of
individual sector magnets and rf cavities placed
alternately in a circular arrangement. The
beam is bent and focused by alternating gradient
fields produced by pole tips with a minimum
radial spacing of 4.5 inches. Magnet design
and construction problems are minimized by
using the same pole-surface contour and the
same mean-field intensity throughout the entire
machine. All pole tips in one sector are
mounted on a common yoke and driven by a
common pair of coils, Shimming gaps are pro-
vided between the yoke and the pole tips so that
the same mean field intensity can be achieved
for all orbits. Only the azimuthal length of
the poles is changed to maintain the correct
focusing and bending properties of the magnets.
Our recent studies have dealt chiefly with
an SOC having a magnet configuration in which
the alternating gradient pole tips of a given
"sector" are all driven from a common yoke
structure by a common coil pair. This arrange-
ment yields certain pole-tip alignment advantages
and uses less magnet power than some other
designs. The configuration is best illustrated
by Fig. 1, which shows a magnet sector and an
adjacent cavity.
The Separated Orbit Cyclotron, as char-
acterized by F. M. Russell in his original
proposal', was a 'beehivell structure. That is,
its orbit formed a spiral helix with individual
focusing magncis periodically piaced along the
orbit path and with accelerating cavities. between
the magnets. In this paper, and in the others
describing SOC in this conference, 2, 3, 4 the
orbit path is in the form of a flat spiral as is :
normal in a cyclotron, but with much more
radial separation between turns. This transi-
tion was made because of the economies
: achieved with the flat SOC."
For simplicity of design and construction
the average magnetic field between a pair of
poles is rnaintained at a constant value of
7 kgauss throughout the machine and only the
azimuthal length of the pole tips is altered to
maintain the proper field strength as the parti-
cles gain energy. The magnitude of the field
gradient, and thus the pole face contour, is also
maintained constant at about 1 to 1.5 kgauss/ cm
throughout the machine. The length of the
gradient region of a magnet can be varied to
meet the criterion of optimum focusing force as
a function of particle energy. Zero gradient,
or flat, sections can then be added to each
sector to achieve the proper average field for
each orbit. Under these conditions, and when
a beam aperture of 1.5 in. is provided, the
minimum spacing between orbits can be as
small as 4, 5 in. (We have used 5 in. for most
of our studies.)
The beam makes only one traversal through
any given strong-focusing magnet element. The
necessary orbit spacing is obtained by using a
very high energy gain per revolution and by
using & relatively low average magnetic field,
that is, the diameter of the machine is made
large. Synchronism is maintained for a large
machine by "harmonic operation." The time
for a particle to make one revolution is an
integral number of rf periods, in most practical
designs between 10 and 20.
A number of criteria govern the choice of
a magnet design for SOC. A minimum orbit
spacing is desired so that the overall machine
size will be minimized. This, in conjunction
with the beam aperture requirement, limits the
achievable gradients to a narrow band. The
ease of fabrication to a tight dimensional
A cross section of four pole pairs with
their associated beam tubes and field shaping
shims is shown in Fig. 2. This back-to-back
arrangement of the poles 1.9 98 sential for
minimum orbit spacing. It can be achieved
only if there is an odd numbril: of magnet sectors
and a magnet configuration 3'equiring an even
number of sectors to complete the focusing
period, for example, alternating triplets (FDF,
DED, ...) or alternating singlets (F, D, ...).
The field-shaping shims extend the useful region
of the magnetic field by shunting some of the
fringe field away from the gap in the region
where the field does not decrease rapidly enough.
The fields obtained with and without these shime
are shown for comparison in Fig. 3. The useful
region is 1. 8 in. wide with the shims but only
1.7 in. without, but perhaps of more significance
is the reduction of the minimum orbit spacing
from 5.5 in. to 4.5 inches.
LEGAL NOTICE
This report was prepared sa an account of Government sponsored work. Neither the United
,
M
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A. Malo my warrat e teprontotou, pred or implied, moh oprot to the scop
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na Sloveton, prom, method of process dwelmoed do Hlo report mag mot tartaro
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Wayne er utrette the Columbidon, or empty w wote contractor, to the extent that
wol empler a una frustor the O nliston, « unplegua awal attractor propria,
diensten, ur monter som ko, may wormation perman Mouplesment o ornat
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*Research sponsored by the U. S. Atomic
Energy Commission under contract with the
Union Carbide Corporation,
PATENT CLEARANCE OBTAINED: REBASE TO
THE PUBLIC IS APPROVED. PROCEDURES,
ARBAN EILE THE AFORIVING SCHON,
NO
Figure Captions
Fig. 1-Sector magnet and rf accelerating cavity
for the 200-800 MeV stage of an SOC.
The magnet coils, omitted here, are
shown in Fig. 5.
Fig. 2Cross section of four pole pairs of an
SOC magnet.
A gap has been provided between the pole
and the yoke to take care of variations in yoke
saturation with radius so that all poles in a
sector can be driven to the same field level
with a singie coil pair. The variation of field
with radius in a 1/8-scale model is shown in
Fig. 4, along with the appropriate gap required
to correct the variation and the results of a
preliminary test on a crude 1/8-scale model of
an SOC sector magnet. By careful adjustment
of the magnetic material in the pole-base gap
it should be possible to achieve the desired
field in the beam aperture i,o the required
tolerance. The 1/8-scale model fronr: which
the data were obtained is shown in Fig. 5. For
economy of construction the yoke was "burned
out of an obsolete model of another machine.
The pole tips are straight and do not have
contoured surfaces. The contour of the pole
tips has been modeled separately at 1/2 scale.
Fig. 3- Magnetic field from a pair of poles
without (left) and with the field-shaping
shims (right). The data are from a
1/2-scale model of the pole 8.
Fig. 4-Magnetic field in the beam aperture
(1/8-scale model) with and without a
corrective gap between the base of thie
poles and the yoke. The effect of
varying the reluctance in the magnetic
circuit for orbits of different radii, due
to saturation of the yoke, can be seen.
An SOC for accelerating protons to 800
MeV could be made in any one of many ways.
One design which has been studied in consider-
able detail and at present seems to be rather
attractive is composed of several stages.
Stage 1 is a 0 to 0.75 MeV Cockcroft Walton
and a 0.75-20 MeV linac, stage 2 is a 20-100
MeV SOC, stage 3 is a 100-350 MeV SOC, and
stage 4 is a 350-800 MeV SOC. The choice
of the energy division points is largely one of
er onomics. The magnets for all stages are
the same in principle. Some of the charac-
teristice of each stage are shown in Table I.
Fig. 5- An 1/8-scale model of a segtor magnet.
The pole gap is maintained by brass
spacers that would be omitted from the
final machine. This model is suffic-
ientiy accurate for obtaining only gross
properties of the magnetic circuit.
References
1.
F. M. Russell, Nucl. Inst. and Meth. 23,
229-230 (1963).
2.
N. F. Ziegler, "Accelerating Cavities
for an 800-MeV SOC," to be published in
the proceedings of this conference,
E. D. Hudson, R. S. Lord, and
R. E. Worsham, "A Low Energy Separated
Orbit Cyclotron," to be published in the
proceedings of this conference.
.
R. E. Worsham, The Beam Dynamics of
the SOC," to be published in the proceed.
ings of this conference,
Table 1: Design Characteristics of a Multi-Stage SOC
Stage
2
3
20-100
100-350
350-800
15
15
24
13
21
770
Energy span (MeV)
No. of sectors
No. of orbits
Harmonic no.
Weight of steel (tons)
Weight of copper (tons)
Magnet power (kw)
Orbit path length (ft)
Orbit radius, inner (in.)
outer (in. )
26
2100
35
335
3400
250
3800
56
540
7260
550
675
101
213
213
340

.
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;
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negres
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Cr
con
O
Fig. 1. Sector magnet and rf accelerating cavity for the 200-800 MeV
stage of an ŞOC. Tlie magnet coils, omitted here, aru shown in :
Fig. 5.
ORNL-DWG 65-498R

a
--
.
de
POLE-BASE
PLATE
FIELD SHAPING
SHIMS
ÓÓÓ
BEAM PIPE
POLE-BASE
GAP
0 1 2 3 4 5 6
... melalui
INCHES
LLLLLLLL
Fiy. 2. Cross section of four pole pairs of an SOC magnet.
ORNL-OWG 64-9972R
.

per 1, 70'
S
f
1.821
2. 25"
K . 2, 7511
0 NI = 63,000 amp-turn.
NI = 46, 200 amp-turn.
B . 7. 28 KG
= 8.1 KG
de 3.3 kG/la.
DE 3.5 kG/In.
Bmax = 11.4 kG
Bmax = 11.1
Orbit apucing = 4.5 in.
Orbit specing & 5.3 In.
Fig. 3. Magnetic field from a pair of polos without 'lost) and with the
field-shaping shimo (right). The data are from a 1!2-scale
model of the poles.
ORNL-DWG 65-1976
WITHOUT GAP
Souborno
MAGNETIC FIELD (kgauss)

WITH GAP

POLE BASE GAP (in.)
·
450 500 : 550 600 650
RADIUS (in.)
Fig. 4. Magnetic fiold in the beanı aperture (1/8-8cale model) with and
without a corrective gap between the base of the poles and tho
yoke. The effect of varying the reluctance in the magnetic circuit
for orbits of diffurent radii, due to saturation of the yoke, can be
suen,
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5. An 1/8-scale model of a sector magnet. The pole gap is maintained
by brass spacers that would be omitted from the final machine.
This model is sufficiently accurate for obtaining only gross properties
of the magnetic circuit.
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6 / 21 /65
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LEGAL NOTICE -mon
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UT
2
W
-
S
.
This report was prepared as an account of Government sponsored work. Neither the United
Statas, nor the Commission, nor any person acting on behalf of the Commission:
A. Makes any warranty or representation, expressed or implied, with respect to the accu-
racy, completeness, or usefulness of the information contained in this report, or that the use
of uny information, apparatus, method, or process disclosed in this report may not infringe
privately owned rights; er.
B. Assumes any liabilities with respect to the use of, or for damages resulting from the
use of any information, apparatus, method, or process disclosed in this report.
As used in the above, "person acting on behalf of the Commission" includes any em-
ployeo or contractor of the Commaission, or employee of such coatracior, to the extent that
such employee or contractor of the Commission, or employee of such contractor propares,
disseminates, or provides access to, any information pursuant to his employment or contract
with the Commission, or his employment with such contractor.
.
07
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END
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