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INFLUENCE OF MAGNET MULTIPOLE FIELD COMPONENTS ON BEAM
DYNAMICS IN JLEIC ION COLLIDER RING
G.H. Wei†, V.S. Morozov, F. Lin, Y. Zhang, F. Pilat,
Jefferson Lab, Newport News, VA 23606, USA
Y.M. Nosochkov, M.-H. Wang, SLAC, Menlo Park, CA 94025, USA
Abstract Multipole field components of magnets and especially
those of the magnets in the interaction region are the
primary cause of dynamic aperture limitation in a collider.
For the Jefferson Lab Electron Ion Collider (JLEIC)
project, having a large enough dynamic aperture (at least
±10 of the rms beam size) is important to get a
luminosity level of a few 1033
cm-2
s-1
with low beam loss.
Depending on the dynamic aperture requirements,
limiting multipole field components of magnets are
surveyed to find a possible compromise between the
requirements and what can be realistically achieved by a
magnet manufacturer. Dependence of the dynamic
aperture on beam emittance and magnetic field
imperfections is also analysed by numerical simulations.
INTRODUCTION
To get a luminosity level above 1033
cm-2
s-1
at all
design points of the Jefferson Lab Electron Ion Collider
(JLEIC) project, small β* values in both horizontal and
vertical planes are necessary at the Interaction Point (IP)
in the ion collider ring. This also means large β in the final focus area, chromaticity correction sections, etc.
which sets a constraint on the field quality of magnets in
these large beta areas necessary to ensure a large enough
dynamic aperture (DA). In this context, limiting multipole
field components of the magnets are surveyed to find a
possible compromise between the requirements and what
can be realistically achieved by a magnet manufacturer.
This paper describes that work.
LATTICE AND EMITTANCE OF THE JLEIC ION COLLIDER RING
The JLEIC ion collider ring accelerates protons from 8 to up to 100 GeV/c or ions in the equivalent momentum range and is designed to provide luminosity above 1033 cm-2s-1 [1, 2]. The overall collision lattice of the ion collider ring is shown in Fig. 1. The ring consists of two 261.7 arcs connected by two straight sections intersecting at an 81.7 angle. The total circumference of the ion collider ring is 2153.89 m.
The beam emittance in ion ring is determined by a
balance of the intra beam scattering (IBS) and electron
cooling. With strong cooling, the normalized rms
emittances of 0.35/0.07 mm-mrad (H/V) are considered.
With initial weak cooling, larger values of 1.2/1.2 mm-
mrad (H/V) are assumed.
Figure 1: Linear optics of the JLEIC ion collider ring
starting from IP.
The JLEIC ion collider ring has 343 main magnets including 133 dipoles, 205 quadrupoles, and 75 sextupoles, which are shown in Table 1. In the interaction region (IR), there are 2 dipoles and 6 final focus quadrupoles whose multipole components are a common bottleneck for the dynamic aperture.
Table 1: Magnets in the JLEIC ion collider ring
Dipole Quadrupole Sextupole
All 133 205 75
IR 2 6 0
With β >200 m 21 19 8
REFERENCE RADIUS AND MAGNET FIELD QUALITY
The non-linear magnetic field of magnets can be
defined by the following expansion using the US
multipole convention [3].
n
Nn
nnNxyr
iyxiabBiBB )(10
0
4 (1)
where the an and bn coefficients are the relative values of the skew and normal multipole field determined at a reference radius r0 in units of 10-4, and BN is the main field at r0. Furthermore, each an and bn is composed of the systematic and random terms, where the random values are randomly generated based on their Gaussian distributions.
For a superconducting magnet, r0 is usually set at 1/3 of
the coil aperture, as an edge of the good field region of
the magnet. The multipole terms an and bn scale with the
reference radius r0 and the coil diameter dc by the
expressions in Eqs. (2) and (3) [3]. Furthermore, to keep