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2LC08
Passive Correction of the Persistent Current
Effect in Nb3Sn Accelerator Magnets
V.V. Kashikhin, E. Barzi, D. Chichili, J. DiMarco, M. Lamm, P. Schlabach, A.V. Zlobin
Abstract-Superconducting accelerator magnets must provide
a uniform field during operation. However, the field quality
significantly deteriorates due to persistent currents induced in
superconducting filaments. This effect is especially large for the
Nb3Sn conductor being implemented in the next generation of
accelerator magnets. A simple and inexpensive method of passive
correction of the persistent current effect was developed and
experimentally verified. This paper describes numerical
simulations of the passive correctors and reports the test results.
Index Terms-Hysteresis, magnetic fields, magnetic materials,
superconducting magnets.
I. INTRODUCTION
S UPERCONDUCTING accelerator magnets must meet the
field quality requirements. In most cases, the low order
harmonics at a reference radius must be less than 104 part of
the main field component. However, the field quality
significantly deteriorates at low fields due to magnetization of
superconducting filaments, caused by the persistent currents.
Magnetization of a superconducting filament is proportional
to the critical current density at a given field and the effective
filament size. Modern Nb3Sn strands used in accelerator
magnets have large critical current densities and effective
filament diameters that increases magnetization by an order of
magnitude with respect to the last generation of NbTi strands.
Apart from the type I superconductors, which are purely
diamagnetic, hard superconductors of type II exhibit non-
linear magnetic properties due to the field penetration inside
the filaments. It leads to non-linearity of the magnetic field
and generation of large low-order harmonics.
Passive correction of the persistent current effect has been
studied in the course of the development of NbTi accelerator
magnets. There were considered introduction of passive
superconducting strands [1] or nickel tapes [2] inside the coil
aperture, nickel powder inside the strands [3] and permanent
magnets [4] for the reduction of the persistent current effect.
However, due to their complexity and low efficiency none of
the proposed techniques found practical implementation in
existing accelerators.
A simple and effective method of passive correction of the
persistent current effect, based on thin iron strips has also
Manuscript received August 5, 2002. This work was supported by the U.S.
Department of Energy.
Authors are with Fermi National Accelerator Laboratory, P.O. BOX 500,
M.S. 316, Batavia, IL, 60510, USA. Corresponding author V.V. Kashikhin,
phone: +1 (630) 840-6546; fax: +1 (630) 840-2386; email: vadim@fnal.gov.
been proposed [5]. Implications of this method including
various corrector configurations, numerical simulations and
test results will be discussed.
II. SIMULATION OF THE PERSISTENT CURRENT EFFECT
Magnetic field in a magnet bore will be described in terms
of normalized harmonic coefficients according to:
B, (x, y)+ iB (x, y) =10-4 x Br (bn + ia, )Kx+y
where Bx(x,y) and By(x,y) - horizontal and vertical field
components; Bref - main field component at the reference
radius Rr; bn and an - normal and skew harmonic
coefficients. The reference radius used in this paper is 1 cm.
Simulations of the persistent current effect have been
performed using the finite-element code OPERA2D. The
simulation method, originally described and experimentally
verified in [6], retains high precision of the vector-potential
formulation and allows taking into account measured
hysteresis curve for any material, including superconductors.
The coil magnetization was characterized by the magnetic
properties of 1-mm Nb3Sn strand with a critical current
density of 1600 A/mm at 12 T and Cu/non-Cu ratio of 0.85,
measured at Fermilab [7]. The magnetization curve was
adjusted for the cable packing factor and transformed into
B(H) curve, suitable for OPERA 2D.
Simulation of the persistent current effect has been
performed for the shell type dipole and quadrupole magnets,
developed at Fermilab for VLHC. Fig. 1 shows the dipole and
quadrupole magnet cross-sections with the flux lines from the
persistent currents only (fields from the transport current and
iron magnetization were subtracted). Persistent current effect
in different block type magnets was analyzed in [5].
Y [mm] Y [mm]1
50-
40
40-
30 .3
10 10.
0 10 20 30 40 50X [mm]0 10 20 30 40 50 X [mm]
Fig. 1. Magnetization flux in dipole (left) and quadrupole (right) magnets.
Flux increment between adjacent lines is constant and equals to 5x105 Wb/m
in all similar plots of this paper.
1
