Reduction of iron saturation in cosine theta dipoles Page: 1 of 4
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Presented at ICFA Workshop on Superconducting Magnets and Cryo'genics - Brookhaven National Laboratory, May 12-16, 1986
BNL--38974REDUCTION OF IRON SATURATION IN COSINE THETA DIPOLES
C. Morgan
Brookhaven National Laboratory
Upton, N.Y. 11973DE87 003487
INTRODUCTION
It is possible to reduce or eliminate iron
saturation effects in high field cosine theta
dipoles by putting a large gap between the coil
and the iron, but part or most of the field added
by the iron will be lost. With a small gap be-
tween cdil and iron and with sufficient irca on
the midplane, iron saturation always occurs first
at the poles. Thus to delay saturation onset, it
may be sufficient to remove iron near the poles,
The RHIC dipole as presently designed is an
ideal candidate for a study of possible improve-
ment, since it attains a quench field greater than
4.5 T and has a small gap, about 5 mm, between
coil and iron. The coil designed for it,(1) which
has 4 blocks of 16, 9, 6 and 3 turns, respec-
tively, is close to perfect at low field, as shown
in Table 1.
Table 1
Harmonic Content Af the RHIC Coil at m mu, 10-4 30radius
25 mm
32 mm
radius
25 mm
32 mmb2 b4 b6
.08 0. 0.
.14 0. 0.
-------------------b12 b17
-.05 -.11
-1.00 -3.62b0
0.
0.b10
.02
.22b16 b18
-.13 .08
6.60 6.93At infinite mu, the algebraic sum of the harmonics
at 32 mm given in Table I is ,3.78, which is close
to the desired value(2) of 2.
The iron contributes substantial sextupole
and decapole at both intermediate and high
fields(3i as shown in Table 2. The moderately
large b2 at 3.44 T can be nulled to first
approximation by external trim sextupoles, and the
b4 and b6 terms add algebraically to 0.1 at r = 32
MM. The possible benefits of a shaped iron
aperture may arise at higher fields, where b4 and
b don't cancel and b2 requires substantial correc-
tion.
Table 2
Harmonics at 25 mm, 10-4 30BO,T
I, kA
TF,T/kA
b2
b4
b60.805
0.09
0.0
0.00.141
0.177
0.798
-20.
-0.4
-0.10.395
0.496
0.798
0.
-0.4
-0.13.44
4.37
0.780
7.2
0.7
-0.44.10
5.39
0.749
20.
-1.2
-0.5The aperture chosen for study has a 3 mm gap
between coil and iron at the midplane rather than
5 in order to get a larger iron benefit.
Research carried out under the auspices of
DE-AC02-76CH00016.In a preliminary report,(4) it was shown that
a basic elliptical aperture with a half height
about 5 mm greater than the midplane half-width of
52.84 mm reduces the b2 shift due to iron satura-
tion at 4.1 T to essentially zero. The ellipse in-
troduces at low field, a large b2 and a smaller b4
term which must be compensated for by a matching
coil design. The combination of new coil and el-
liptical aperture has a b4 saturation shift larger
than the original circular design, and to compen-
sate for this shift, the ellipse is perturbed by
a bump at 54 degrees of maximum height -0.9 mm.
This perturbed ellipse plus a new matching coil
has essentially zero shift in both b2 and b4 due
to saturation, but has a b6 saturation shift
larger than in the original circular design.
The present paper shows that the b6 shift can
be controlled by an additional bump at 64.3 de-
gree. As might be anticipated, the resulting
aperture plus matching coil introduces a b8 shift,
but continuing the trend that each new higher har-
monic shift introduced was smaller than the
preceeding, the b8 shift introduced is small
enough to be tolerated. Another difference be-
tween the present work and the earlier is that
sharper bumps are used.
Procedure
It is a supposition that the perturbation
should be placed precisely at the harmonic pole po-
sitions nearest the fundamental pole, which :n a
dipole magnet are given by eq. 1.
n = a (1/2 - 1/(n+1)), n = 2, 4, 6, -- (1)
What one would like to achieve by choice of the
perturbations is complete separation of the satura-
tion effects, i.e., a bump at 90 degree (the el-
lipse elongation) would change only the sextupole
saturation shift, and the one at 54 degree would
change only the decapole saturation shift, etc.
An alternative method of looking at the prob-
lem is to assume that an aperture shape with
matching coil exists which has the property that
it provides a pure dipole field at low field and
at the highest field prior to midplane iron satura-
tion becoming the predominant influence on field
shape. With this viewpoint, the perturbations can
be regarded as a set of basis functions which can
be used in a variational process. Use of an arbi-
trary set of orthogonal functions for basis func-
tions may require evaluation of the effects of a
large number of perturbations and it appears that
the more general approach can be bypassed by the
use of the perturbations mentioned above.
the U.S .Department of Ene rac t No.
175 - DISTRIBUTION OF THIS nO'LMa IS UNLIMITED
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Morgan, G. Reduction of iron saturation in cosine theta dipoles, article, January 1, 1986; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc1191189/m1/1/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.