The Stanford Linear Collider Page: 2 of 7
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laser system. The source intensity is 7-8x1010 e per bunch
(3.5x1010 at the IP). During the second half of the run, the
cathode quantum efficiency was held below its maximum value
in order to yield the highest possible polarization. Periodic
cathode recesiations are performed every -5 days through a
simple computer automated process which requires -20
minutes to complete. The system has been remarkably reliable
with <2% unscheduled downtime. The success of the high
energy colliding beam physics program at the SLC is due in
large part to the success of the polarized electron source.
III. DAMPING RINGS AND BUNCH COMPRESSORS
One of two major SLC upgrades for 1994 was the design
and construction of new low impedance vacuum chambers for
the damping ring arc-sections . Measurements made in 1992
showed the onset of a bunch length 'sawtooth' instability at
beam currents of -3x 1010 ppb . This high current
instability also appeared as variations (jitter) in the extracted
beam phase which produced errant flyer pulses and associated
linac collimator losses and detector backgrounds. The net result
was to limit the pre-upgrade SLC beam currents to <3x1010
ppb. The cause of this instability was the high impedance
damping ring vacuum chamber which, prior to 1994, had a
computed inductance of 37.5 nH . An interim solution used
in 1993 was to ramp the rf voltage down just after injection
thereby lengthening the bunch and holding the peak current
below the instability threshold . The voltage was ramped up
again just before extraction. This procedure necessitated the use
of direct rf feedback to compensate increased beam loading at
The new vacuum chamber has many fewer flexible
bellows. Electro-discharge machining (EDM) methods were
used to produce smoother transition pieces. The resultant
impedance is seven times smaller than that of the old chamber
. Measurements show a significantly shorter bunch length
and a reduced high intensity lengthening. Fig. 2 shows the
measured bunch length at extraction versus beam intensity
both for the old and the new vacuum chambers. A single
bunch instability is still observed, but it is less severe and no
longer limits the SLC operating intensity .
0 1 2 3 4
Fig 2. Damping ring extracted hunch length vs. e- intensity for
old and the new vacuum chamber. Data points represent
measurements performed on the new chamber in 1994.
At the nominal machine repetition frequency of 120 Hz the
electron store time (-8 msec) is half that of the positron ring
(-16 msec). Consequently, the electron damping time is more
critical. In 1993 a reduction in transverse partition numbers
was achieved by stretching the ring circumference in order to
shorten the transverse damping time by -15% . Recent
measurements show damping times of 3.3-3.6 msec
horizontally and 4.1-4.2 msec vertically . With an 8.3 msec
store the typical extracted electron vertical emittance is 2-3
mm-mrad while it is possible to achieve <1 mm-mrad with a
16 msec store at a repetition rate of 60 Hz.
In the past, effort has been devoted to correcting transverse
emittance dilution in the SLC bunch compressors [10-11].
Skew quadrupoles, skew sextupoles and octupole magnets were
installed in previous years to correct first, second and even
third order anomalous dispersion. The large energy spread
(-1%) and the strong bending necessary for a potential ten-fold
bunch length compression present severe alignment,
construction and multipole field error tolerances. These efforts
have been, for the most part, successful. However a 10-30%
emittance dilution remains (partially due to an increased
compressor voltage-see below). Efforts need to continue here.
The form of bunch compression was changed in 1994.
Prior to this, the bunch was 'under-compressed' to 1.3 mm
with a 29 MV rf voltage which initiates a <90' longitudinal
phase rotation. Starting in 1994 the bunch is now 'over-
compressed', also to 1.3 mm, but by using an rf voltage of
41 MV for a phase rotation of >90'. The motivation is to
reduce the end-of-linac energy spread by partial cancellation of
energy spread due to the longitudinal wakefield in the linac and
that due to rf curvature . This technique successfully
reduced the end-of-linac energy spread from -0.25% prior to
1994, to -0.12% rms. In addition, long low-energy tails in the
bunch distribution are no longer generated. A small
compromise is made in beam transmission through the
compressor beamline where large dispersion and increased
energy spread (-1% at 29 MV and -1.4% at 41 MV) produce a
5-10% beam loss.
IV. MAIN LINAC
The main linac challenge is in high current emittance
preservation and stabilization of both the e- and e+ bunches in
the presence of the inevitable quadrupole and accelerating
structure misalignments. The requirements for vertical linac
emittance control have become even more challenging with the
advent of flat beam operation in 1993 where the linac entrance
emittance at 1.2 GeV is now: ysy = 2-3 mm-mrad, ySx = 30-
40 mm-mrad . Beam-based alignment techniques have been
used successfully in the past to control transverse quadrupole
alignment to -80 pm rms  and new ideas are under
investigation to align the disk-loaded wave guides using beam
generated dipole wakefields of the accelerating structures as an
error signal . Under normal operation, empirical linac
emittance correction is accomplished by introducing feedback
controlled trajectory oscillations  to minimize the
measured emittance of wire-scanner phase space monitors 
--e- - en Rm8
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Emma, P. The Stanford Linear Collider, article, June 1, 1995; Menlo Park, California. (digital.library.unt.edu/ark:/67531/metadc693368/m1/2/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.