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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
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ORNA P-3091
70307-566
JUN 2 2 1969
To be submitted for publication in the June Issue of IEEE Transactions on Nuclear Science,
Proceedings of the 1967 U.S. National Particle Accelerator Conference, Washington, D. C. ,
March 1-3, 1967.
CFSTI PRICES
A SECTOR MAGNET FOR A 10-50 MeV SOC*.
Text
E, D, Hudson, R, S, Lord, and F, E, McDaniel
Oak Ridge National Laboratory
Oak Ridge, Tennessee
HQ 13.00MN 65
lip twist
Summary
A full-scale prototype magnet sector of a
12-sector, 10-50 MeV separated orbit cyclotron
has been built and tested. Eleven pairs of
alternating-gradient pole tips are mounted on a
yoka structure which provides a common mag-
netic return path. The pole tips are all excited
by a single pair of coils which surround the pole
tip mounting bases at the upper and lower yoke
faces. The pole tips combine both the negative-
and positive-gradient sections in a single unit;
the magnetic field gradients are about 800
gauss/in. The pole tips are 4.5 in. wide and
have a mean gap of 1.75 in. They were ma-
chined by the numeric controlled milling
process. Special edge contours were developed
to achieve a useful radial aperture of 3 inches,
Experience with the prototype magnet demon-
strates that magnets for separated orbit
cyclotrons present no special difficulty in
design, construction, or alignment,
sectors and 11 radio frequency cavities. One
space between sectors is left vacant to provide
room for the injection and extraction magnets,
and for beam diagnostic equipment. The energy
of the beam can be varied in steps of about:
3.6 MeV by moving the extraction magnet in a
radial direction to intercept the beam at the
desired turn. The accelerator will also provide
deuterona in the energy range 5-25 MeV and
alpha particles from 10 to 50 MeV. The main
features and dimensions of the accelerator are
listed in Table I.
-
Table 1 - Specifications for a 10-50 MeV SOC*
-
-
-
-
--
-
-
12
10
Energy range, MeV
Protons
10.50
Deuterons
5-25
Alpha particles
Turns
Sectors
Turn separation, min, in.
Turn separation, max, in,
20
Beam radius, min, in.
132
Beam radius, max, in,
282
Beam aperture, in.
1.5 x 3.0
Magnetic field, G
7000
Gradient, G/in.
800
Magnet copper, tons
15
Magnet steel, tons
Magnet power, kW
Harmonic number (protons)
24
Cavity frequency, MHZ
Cavity power losses, kW
~900
*This is the design on which the prototype mag-
net is based. The specification of an improved
design with 14 turns and an rf power require-
ment of 600 kW are given in reference 1,
550
240
eb.
We wise
introduction
The separated orbit cyclotron accelerates
ions along a spiral-like path with a pitch (turn-
to-turn spacing) of several inches. The guiding
magnetic field provides beam bending and
focusing forces along each turn independently.
In the usual arrangement of the accelerator a
number of sector-shaped magneta are arranged
radially at equal angular ir tervals ahout a circle
and radio frequency cavities ior bean accel-
eration are placed in the intervening spaces,
A sector magnet consists of a long yoke struc-
ture in which are mounted several pole tip pairs,
one pair for each of the separate turns.
The purpose of this work was to evaluate the
fabrication, alignment, and magnetic field
adjustment required for this unusual type of
magnet structure. This report reviews in par-
ticular the construction and testing of one sector
magnet of an 11-turn, 12-sector separated orbit
cyclotron which would accelerate protons from
an injection energy of 10 MeV to a final energy
of 50 MeV.
A model of the 10-50 MeV SOC was
constructed to illustrate the design of the accel-
erator, see Fig. i. There are 12 magnet
50
www.
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The Magnet Assembly
The structural features of the prototype
magnet are shown in Figs. 2 and 3. The 11 sets
of pole tips are mounted from the precision
faces of the upper and lower yoke pieces. The
magnet excitation coils surround upper and
lowor yoke faces. Trimming coils are provided
on each pole-tip pair adjusting the magnetic
field levels for the acceleration of alpha parti-
cles and deuterons. The required adjustment
varies smoothly from injection to final radius.
The maximum change from the nominal 7000-
gau88 level is about 5% (350 gauss); the
maximum trimming coil power is only 15 watts
per pole tip. The pair of hexagonal headed
.
.
*Research sponsored by the U, S. Atomic
Energy Commission under contract with Union
Carbide Corporation.
LEGAL NOTICE
.
..
DISTRIBUTION OF THIS, DOCUMENT S UNLIMTIEN
This report was prepared as an account of Government sponsored work. Neither the United
States, 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 thar the use
of any information, apparatus, method, or process disclosed in the report may not infringe
privately ownod righto; or
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,
Ao used in the above, "person acting on behalf of the Commission" includes any em-
ployee or contractor of the Commission, or employee of such contractor, to the extent that
such employee or contractor of the Commission, or employee of such contractor prepares,
disseminates, or provides access to, any information pursuant to his employment or contract
with the Commission, or his employment with such contractor,

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Fig. 4. At the center of each ridge a "T" slot
was machined to capture the bolt clamp assem-
blies (4 for each pole tip). This method of
mounting gives maximum flexibility for adjust-
ment of the radial and azimuthal position of the
pole tip. The pole tips are precisely positioned
by optical alignment methods.
In procuring the yoke, separate bids were
requested for flatness and parallelism toler-
ances of +0,002, +0,003, and 40.005 in. Over
this range of precision the price varied by only
7%. The 46-ton yoke assembly was procured
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Fig. 1. Model of the 10-50-MeV SOC Sirowing
Magnet Sectors Alternating with
Accelerating Cavities. The beam is
extracted at various energies with a
moveable 90° magnet.
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Fig. 3. Details of Pole Tip Pairs 3 and 4.
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VOIDS, SIMILAR SIZE AND SHAPE
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Fig. 2. Sector Magnet with Eleven Pairs of
Pole Tips.
CONTINUOUS
SLOT
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POLE TIP
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A
.
TRIMMING
adjustment screws shown in Fig. 3 protruding
from the base of each pole tip control the
coupling shims which are used to adjust pre-
cisely the field under each pole tip to the 7000
gauss specified at the ion path. The coupling
shims act as variable air gaps to compensate for
the magnetic potential drop in the long slender
yoke,
COIL-
-
...
-YOKE
.
Yoke
The yoke is of hot-forged low carbon steel.
A maximum carbon content of 0. 15% was
specified; the actual carbon analysis was 0.06%.
The pole-tip mounting surfaces of the upper and
lower yoke are siightly raised planes which run
the full length of the pole mounting area, see
Fig. 4. A Pole- Tip Assembly Mounted on the
Yoke.
IVO
.
(8-BREF) (gouss)
-
-
"-42
800 jours/in. REFERENCE
-200
.......
-2.0
-15
-1.0
-0.5
0.5
DISTANCE FROM Ro II.)
1.0
1.5
20
with tolerances of +0,002 in, at a price of
$31,250 (~$0, 34/1b). The additional cost of the
more precise yoke was more than offset by
subsequent simplification of the pole-tip
mounting and alignment systema.
The lower yoke was leveled at ORNL to
within <.001 in. of absolute flatness by optical
methods. The yoke is supported by six jacks,
two at each end and two at the center of the span
The 23-in, deep lower yoke is flexible enough to
be adjusted a few thousandths of an inch at each
support point with negligible changes at other
support points.
After the upper yoke was installed
(supported by a stanchion betweon pole tips near
center span), the upper pole mounting surface
was also flat within +0.001. Subsequent gap
measurements gave a variation of £0.0005 in.
about a mean value of 16.0025 in. for the 12
points measured.
These results illustrate that sufficiently
accurate median plane alignment can be achieved
by using the precision surfaces of the yoke itself
as the primary alignment surface.

-HYPERBOLIC REFERENCE
DISTANCE FROM MEDIAN PLANE (In)

".
- HYPERBOLKC REFERENCE
-2.25 -2.00 -1.75 -1.50 4.50 1.75 2.00 2.25
DISTANCE FROM R. (in.)
Fig. 5. Field Changes Produced by Changes in
Pole Edge Geometry. '
.

MAGNETIC FIELD (XG)
GRADIENT (gouss/in.)
MEBEAM APERTURE-
-2.5 -2.0 -1.5 -1.0 -0.
5 0 0.5 1.0 1.5 2.0 2.5
INCHES
Fig. 6. Magnetic Field and Gradient vs Radius.
Pole Tips
The eleven sets of pole tips for the separate
turns are designed to operate at a central orbit
field of 7000 gauss, hence each unit has a
different radius of curvature according to the
particle momentum. The pole-tip radil vary
from 25.8 in, for the first pole tip (injection
energy) to 57. S in, for pole tip eleven (final
energy). For constant axial and radial focusing
frequencies the pole tips would all have different
gradients; it has been shown, however, that the
machine is less sensitive to magnet errors if
constant gradients are used. The optimum
gradient is that which gives maximum accep-
tance at final energy, in this case 800 gauss/in.
The pole tip faces are basically hyperbolic
with edge shims to minimize field falloff near
the inner and outer radii, and thus shaped to
give the largest possible useful aperture. The
several stages in shaping the pole-tip section
are illustrated in Fig. 5. The initial shape,
curve 1, purposely over-compensated the edge
effect. The remaining curves show the results
of subsequent refinements. The pole-tip sur-
faces were cut with a numerically controlled
milling machine. Thus, it was practical to make
very small contour change between measure-
ments. The magnetic field and gradient for the
final pole-tip contour are plotted in Fig. 6. The
useful aperture (+1% AG/G) is about 3 in, wide;
without edge corrections it would be less than
2 inches. i
Most of the details of production of the tapes
were handled by the machine tool programmers.
Except for the first few sets of pole tips which
were replete with programming errors and ma.
chine operator mistakes, a dimensional
accuracy of +0.001 was achieved.
The fabrication of the pole tips including
programming and tape preparation required
about 4000 man-hours, of which about 1000 man-
hours should be allocated to development. The
unit cost (based on 3000 man-hours) was $750
per linear foot or $5/lb. It is believed that:
these costs could be reduced by about a factor of
two by simplifications in design and fabrication
methods.
Magnet Tests
The first measurements of the magnetic
field at the center of the several pole-tip gaps is
as shown in Fig. 7(a); the variation is a result of
magnetic potential drop in the long yoke. The
pole tips nearest the yoke ends have shorter
return flux paths, hence higher fields. In this
design all gap fields are made identical by
reducing the higher fields to the value of the
lowest. This is accomplished by providing an
appropriate magnetic potential drop at the base
of each pole tip by means of variable "coupling
ghims, "4 These consist of a pair of plates with
alternate recessed and solid areas, see Fig. 4,
In addition continuous slots of appropriate depth
were machined in the several coupler plates to
approximately equalize the magnetic fields; the
results obtained are shown in curve (b) of Fig. 7.
Fine adjustment of the couplers resulted in the
field shown in curve (c); all fields are within
ül gauns of the desired 7000 gauss level. The
coupling shims have a field adjustment resolution
of 50 gauss/in, or 2.5 gauss per turn of the
positioning screws. The magnet power for a
field of 7000 gauss is 18 kW; the magnet effi-
ciency at that field is about 80%.
A radial scan of the magnet along the center
of the positive gradient section of the pole tips,
Fig. 8, shows that the field between pole tip sets
varies from about 1 kilogauss at the low energy
end to 2 kilogauss at the high energy end. This
suggests that somewhat smaller turn spacings
(more turns) could be used which would reduce
the rf power requirements."
Detailed measurements of the magnetic field
distribution under the pole tips will be made in
1° steps azimuthally and 1/4 in. radially with a
precision Hall probe system to elucidate the
details of the fringing field in the region of beam
entrance and exit and the magnetic field distri.
bution in the transition region between the
positive and negative sections of the pole tip.

GAP FIELD (KG)
11
1 2 3 4 5 6 7 8 9 10
GAP NUMBER
Fig. 7. Magnetic Field Measured at R for
Each Gap (for reference only)."
(a) Solid Coupling Shims
(b) Continuous Gap on One Side of a
Coupling Shim
(c) After Final Adjustment of
Completed Shima
Conclusion
The design, construction, alignment, and
preliminary testing of the prototype sector
magnet for a 50-MeV SOC have demonstrated
that this type of magnet presents no special.
difficulty. Further work will include detailed
measurements of the pole-tip fields to establish
the effective magnetic length of the elements,
the effects on focusing of the fringing fields at
the magnet ends, and the magnetic field transi-
tion region between the positive and negative
gradient sections of the pole tip. The knowladge
gained in the construction, alignment tests of
the prototype will be directly applicable to mag-
net designs for separated orbit cyclotrons of
much higher energies.
Attttt
FIELD (KG)
References
1. R. S, Livingston, et al, Proceedings of V
International Conference on High Energy
Accelerators, Frascati 1965, pp 431-439.
2. R. S. Lord and E. D. Hudson, Proceedings
of International Symposium on Magnet
Technology, Stanford, California, 1965,
pp 709-714.
3. Ji A, Martin, IEEE Transactions on
Nuclear Science, NS-13, No. 4, 288-299
(1966).

19T10
90
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110
120
130
140
150
160
170
180
6o No Oct No

FIELD (KG)
23
10
20
30
60
70
80
90
40 50
RADIUS (in.)
Fig. 8, Magnetic Field of the Prototype Sector
with 11 Pairs of Pole Tips,
END
DATE FILMED
7 / 26 /67






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