Start-to-end Beam Optics Development and Multi-particle Tracking for the ILC Positron Source Page: 4 of 18
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2. Start-to-end optics developments
The positrons emerging from a thin Ti target are collected and accelerated in a
beamline named as Optical Matching Device (OMD) and pre-acceleration capture
system. The OMD is used to transform positrons characterized by small spot size and
large divergence at the target into small angular divergence and large size to match pre-
acceleration capture system consisting of normal conducting L-band RF structures
embedded in a solenoid. It is a tapered solenoid from 6 T on the target to 0.5 T on the L-
band structures. The optimum on-axis field distribution for the matching device needs to
obey: B(z) = B0 /(1 + g - z), where Bo is the magnetic field on the target, g is the taper
parameter, and z is the longitudinal coordinate. The condition for an adiabatic field
variation is given by gP /(eBO) <<1 (P is particle momentum); to fulfill the condition for
particles with higher energy the taper parameter g should be small. However, the small g
means that the matching section becomes long, and thus the bunch lengthening becomes
stronger [1]. Parameter optimizations in the OMD and the capture cavities are described
in detail in Ref. [2]. The positrons are accelerated to 125 MeV through the pre-
acceleration capture system.
2.1 Optics of the PCAP, PPA, and PPATEL
The positrons following the capture system are transported by the "Positron CAPture"
beamline (PCAP), which separates positrons from electrons and photons by using a
dogleg with a horizontal offset of 2.5 m. The collimators to scrape the positrons with
large incoming angles and large energy errors are installed. The optics details can be
found in [3].
The "Positron Pre-Accelerator" beamline (PPA) immediately downstream of the
PCAP is used to accelerate positrons from 125 MeV to 400 MeV. It consists of the
normal conducting L-band RF structures [4] embedded in a constant solenoid field of 0.5
T. The MAD code does not have an element containing RF structure embedded in a
solenoid, but one can use an approximate model in the MAD by longitudinally slicing
both the RF structures and the solenoid, and alternating these slices for a smooth effect of
both the RF and solenoid field. In this model, the solenoid field Bz in each slice is
constant, but the solenoid strength Ks= Bz /(Bp), where Bp is the magnetic rigidity, varies
from slice to slice with the beam energy. For an accurate model, the number of RF and
solenoid slices must be sufficiently large. The optics calculation with this model does
agree well with PARMELA calculation, which can directly model RF structures
embedded in a solenoid. The accelerating gradient of the L-band structure in the PPA
system is 8.0 MV/m, and about 34.6 m of the PPA length is required to accelerate the
beam to 400 MeV.
The beamline named as "Positron Pre-Accelerator To the Electron main Linac tunnel"
(PPATEL), is to transport the 400 MeV beam from the PPA to the electron main linac
tunnel. It uses a horizontal and vertical dogleg to deflect the beam by 5 m and 2 m in the
horizontal and vertical planes, respectively. As a result, the positron line at exit of the
PPATEL is positioned 2 m high and exactly on top of the electron main linac beamline.
The optics details can be found in Ref. [3].
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Zhou, F.; Batygin, Y.; Nosochkov, Y.; Sheppard, J. C. & Woodley, M. D. Start-to-end Beam Optics Development and Multi-particle Tracking for the ILC Positron Source, report, January 25, 2007; [Menlo Park, California]. (https://digital.library.unt.edu/ark:/67531/metadc891126/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.