The Stanford Linear Collider Page: 4 of 7
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or by observing a set of four off-axis screens
dilution in the main linac is usually contr
vertically and <30% horizontally at 3.5x101
temperature dependencies in the linac rf syst
day to night emittance variations which r
tuning. Improvements are presently under inve
A second challenge is pulse-to-pulse
trajectory stabilization of both the e and
Trajectory jitter not only degrades lumi
complicates and slows tuning schemes which
space monitors requiring many tens or hundr
large source of e trajectory jitter, identified i
to long range transverse wakefields. With equal
betatron phase advance, the jitter in the lead
resonantly amplified to the trailing e bunch.
vertical e+ betatron oscillation initiated in the
compressor, the trailing electron bunch is see
an oscillation due to the long range wakefield
This problem was significantly diminishe
a 10'/cell separation between the horizon
betatron tunes within the linac. Thus the reso
avoided. This linac lattice modification success
vertical trajectory jitter from -60% of the no
size (observed in the final focus) to -40%. So
is also observed in e- horizontal jitter. Fi
initiated e+ oscillation and its wake induced e
with and without a split tune lattice.
llable to <60%
0 ppb. However,
em can generate
and long term
size. Inspection of the supports has revealed a poorly supported
degree of freedom in magnet pitch angle which translates into a
significant vertical displacement component due to the
longitudinally biased pitch rotation axis of the support. In
response, magnet pitch wedges were installed for -2/3 of the
e+ beams . V. ARCS AND FINAL FOCUS SYSTEMS
nosity but also Prior to the 1994 run the optics of both final focus
ch rely on phase systems (FFS)-e- & e+-were upgraded in order to allow
eds of pulses. A reduction of the IP vertical beta function . One new
n 1994, was due quadrupole magnet per FFS was installed between the
l e+ and e linac chromatic correction section (CCS) and the final triplet. This
ing e+ bunch is quadrupole optimally adjusts the betatron phase advance
By introducing a between CCS sextupoles and triplet to reduce the dominant 3rd
positron bunch order aberration (U3466 coefficient in TRANSPORT notation
n to accumulate ). In addition, two more quadrupoles-one skew and one
. normal-were added to the upper transformer section (UT) to
d by introducing provide a full compliment of orthogonal tuning 'knobs' for
tal and vertical control of IP beta functions; cross-plane coupling and IP beam
nant condition is waist positions . Four new wire-scanners per FFS were
fully reduced e~ added for emittance and matching diagnostics within the FFS
minal rms beam and a fifth wire-scanner was installed at an IP image point in
me improvement the center of the first CCS bend magnet .
g. 3 shows the The new final focus beamlines were commissioned in April
oscillation both and May of 1994 using previously established techniques such
as quadrupole and sextupole beam-based alignment methods
[29-31]. The new orthogonal UT tuning knobs and image
(a) point wire-scanners were employed very successfully to
achieve the desired IP beta functions, coupling correction and
waist positions. Subsequent low current beam collisions (0.5-
1.0x1010 ppb) using a twice nominal e- damping ring store
(b) time to achieve ideal emittances produced vertical IP rms spot
sizes of 400 nm, clearly confirming the expected performance
W , of the upgrade. The horizontal spot sizes observed were also
within the expected value of 1.8-2.0 pm. Fig. 4 shows a
413 nm vertical beam-beam deflection scan  measured at
(C) low current and long damping ring store.
1 J. a,
Fig 3. Vertical e+ oscillation introduced before the linac (a) and
long range wakefield induced oscillation for c beam of -300 pm
before (b) and -100 pm after (c) installation of split tune lattice.
However, with the large mid-linac energy spread introduced
for BNS damping  and the new 'split-tune' quadrupole
settings, some increased chromatic emittance dilution within
the linac is expected (-10%). Efforts are underway to develop a
split-tune linac lattice with less chromatic dilution.
In light of previous successes , further efforts to
stabilize linac trajectories have centered around modifications
of quadrupole magnet support structures. Measurements of
quadrupole magnet vibrations using a geophone indicate
-300 nm rms vibrations for frequencies above 1 Hz .
Beam response modeling in these conditions predict trajectory
jitter which is -20% of the 10-50 pm nominal vertical beam
i I I I
i I , I I
-8 -4 0 4 8
Fig 4. Beam-beam deflection, 0, vs. separation, Ay, fitted with
Bassetti-Erskine formula showing 413 nm vertical rms beam size.
At higher beam intensities a significant emittance dilution
within the arc/FF systems appears which is not yet
e .,u ui14 .u
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Emma, P. The Stanford Linear Collider, article, June 1, 1995; Menlo Park, California. (digital.library.unt.edu/ark:/67531/metadc693368/m1/4/: accessed January 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.