SPEAR III: A brighter source at SSRL Page: 3 of 5
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4 COLLECTIVE EFFECTS AND LIFETIME [5]
The two SPEAR II 5-cell RF cavities will be used
initially for SPEAR III. These cavities have numerous
high order mode resonances, necessitating longitudinal
and transverse feedback systems to damp multibunch
beam instabilities. Computed instability thresholds are
below 10 mA, assuming zero chromaticity and the
overlapping of synchrotron sidebands with HOM
resonances. A future upgrade possibility, especially if
the stored current is increased beyond 200 mA, is to
install single-cell, mode-damped cavities to reduce or
eliminate the need for feedback.
The broadband impedance of the ESRF was scaled
to the SPEAR circumference to yield IZ/nl = -2.5 ohms
and a Q=1 resonance centered at 30 GHz. This
impedance will initiate a turbulent regime at 0.6 mA.
Bunch lengthening and widening coefficients will be
1.9 and 1.5 respectively for a 2.8 mA single bunch
current (200 mA in 70 out of 280 RF buckets).
A 70 h Touschek lifetime has been computed
assuming a 3% momentum acceptance, 2.7 MV RF
voltage, and 200 mA in 70 bunches. This value can be
increased by filling the same current in more buckets
(i.e. a factor of 4 increase for the maximum 280
bunches) or by using a bunch lengthening cavity.
A 100 h Coulomb scattering lifetime has been
calculated for an NZ-equivalent pressure of 0.25 nTorr
and a minimum vertical full aperture of 12 mm in one
ID chamber. The bremsstrahlung lifetime is 300 h
assuming a 3% momentum acceptance.
The total 200 mA lifetime is 36 h for 70 bunches
and 60 h for 280 bunches.
5 ACCELERATOR COMPONENTS
5.1 Vacuum System
The girder vacuum chambers will be designed to
accommodate smaller magnet gaps and higher SR power
loads. The chamber cross section has ~36x90 mm inner
dimensions. Many of the existing straight section
chambers will be kept, including those for the IDs,
kicker magnets, and some diagnostic components.
Tapered transitions from new to old chambers will be
required in some cases to reduce impedance. RF-
shielded bellows elements will be designed to minimize
parasitic mode losses. Beamline front end masks and
absorbers will also be upgraded for higher SR power.
To maximize beam lifetime, we seek an average
ring pressure of order 1 nTorr. An antechamber design
with discrete photon absorbers would achieve this goal
and maximize chamber stability under varying SR power
load. Since an antechamber design may be more costly
and may necessitate more expensive C-core or Collins-
type magnets, we are also considering a narrow chamber
design. Since absorbed SR power may cause thischamber to move, beam position monitor modules would
need decoupling bellows and stable supports to reduce
transverse motion to the 10 pm level as requred by the
orbit stabilizing system.
5.2 Magnets and Supports
The preliminary separated function SPEAR III lattice
requires 36 dipoles (50 mm gap), 94 quadrupoles (70
mm bore diameter), and 72 sextupoles (80 mm bore
diameter), and 54 pairs of horizontal and vertical
correctors. The operating field for the 1.5 m, 10.6
dipoles will be 1.19 T at 3 GeV. Quadrupole gradients
are -20 T/m at 3 GeV. Sextupole strengths are on the
order of 300 T/m2. These field designs will permit 3.5
GeV operation with acceptable core saturation. A C-
core dipole accommodates the SR beamline exit
chamber. It has not been determined if open-core
quadrupoles and sextupoles will be needed.-
The new magnets will be mounted on existing 10 m
concrete girders, each of which is now supported by
three piers sunk 1.5 m into the ground. These girders
have a rotational oscillation mode at -5 Hz about the
long axis that could be stabilized with a fourth support.
The magnets will be mounted on the girders using
kinematic struts. New girders will be installed for the
four repositioned magnet cells flanking the IRs.
5.3 Injection
The booster synchrotron and booster-to-SPEAR transport
lines will be upgraded for 3 GeV injection. A new
septum magnet will be needed for the higher injection
energy and the reduced displacement between incoming
and stored beams (-15 mm). The three existing
vacuum-core kicker magnets will be reused.
A possible future upgrade is to move the injection
point to a long straight section unsuitable for beamline
use and to install shorter ferrite-core kickers. This
would liberate two arc straight sections and the second
12 m IR straight section for IDs.
6 ACKNOWLEDGMENTS
The authors are indebted to A. Bienenstock and B.
Richter for supporting this work; to H. Winick for his
encouragement; to J. Arthur, R. Carr and R. Tatchyn;
and to the SSRL engineering and design groups.
REFERENCES
[1] A. Garren, M. Lee, P. Morton, "SPEAR Lattice Modifications to
Increase Synchrotron Light Brightness", SPEAR Pub. 193, 1976.
[2] L. Blumberg, J. Harris, R. Stege, J. Cerino, R. Hettel, A. Hofmann,
R. Liu, H. Wiedemann, H. Winick, Proc. of 1985 IEEE PAC, 3433.
[3] J. Safranek, Ph.D Thesis, Stanford Uiversity, 1991.
[4] W. Davies-White, H. Wiedemann, "SPEAR Upgrade Program",
SSRL internal report, Jan. 8, 1997.
[5] A. Hofmann and C. Limborg, "Beam Instabilities in SPEAR III",
SSRL internal report, April 11, 1997.
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Hettel, R.; Boyce, R. & Brennan, S. SPEAR III: A brighter source at SSRL, article, June 2, 1997; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc693183/m1/3/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.