Wakefield and Beam Centering Measurements of a Damped and Detuned X-Band Accelerator Structure Page: 4 of 4
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-oo -o B Ps (1o0 150
Horizontal Bean Position (gin)
Fig. 3: 15 GHz dipole signal power (pluses) and phase
(circles) versus horizontal beam position. The fits to
the data (solid lines) are described in the text.
U .p -8-
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Distance Along Structure (m)
Fig. 4: Horizontal structure straightness measured with
a beam (diamonds) and a CMM (solid line).
signal power. These values provide a measure of the
structure straightness when plotted versus the corre-
sponding points of origin of the signals (on average)
along the structure. Figure 4 shows the horizontal
straightness results that where derived using the
frequency-to-cell mapping obtained from the equivalent
circuit model of the structure . Also shown are
straightness results from a pre-installation survey of the
outer surfaces of the cells (30.5 mm radii) that was
done with a Coordinate Measuring Machine (CMM).
The good agreement between these measurements, and
the micron-level local straightness of the structure,
attest to the high precision with which the cells were
fabricated and assembled. The vertical CMM and
beam-based results agree less well, although a post-run
survey of the structure indicates that it had vertically
distorted by about 30 gm since the initial survey.
From the straightness results, a beam orbit can be
derived that should zero the integrated short-range
transverse wakefield along the structure. To test how
well this works, the ASSET drive beam was steered to
an orbit based on the minimum dipole power results,
and then the short-range wakefield (T < 200 ps) was
measured with the witness bunch. In this case, an
absolute wakefield was derived instead of one
differential in the drive beam offset. For convenience,
it was normalized to the differential short-range wake-
+ + '-
. ' ++ .
0 50 100 150
Fig. 5: Vertical absolute wakefield measurements
made as part of the beam centering tests. The solid
lines are 15.1 GHz (top) and 21 GHz (bottom) sine
wave fits to the T > 50 ps data.
K. A. Thompson et al., SLAC-PUB-6032 (Nov. 1993).
N. M. Kroll, SLAC-PUB-7541 (May 1997).
J.W. Wang et al., FRA18, these proceedings.
J. Klingmann et al., MOP51, these proceedings.
T. Higo et al., FRA16, these proceedings.
C. Adolphsen et al., SLAC-PUB-7519 (May 1997).
M. Seidel, SLAC-PUB-7557 (June 1997).
R. Jones, private communication.
C. Adolphsen et al., SLAC-PUB-6629 (Aug. 1994).
K Bane, private communication.
T. Shintake, FRA14, these proceedings.
40 I 1
T1I I I
field amplitude (i.e., the T = 0 point in Fig. 2). Hence,
if the drive beam was offset by 20 gm, for example,
the absolute wakefield should essentially be a sine
wave with a 20 gm amplitude. In practice, additional
components of wakefield were observed that were inde-
pendent of the drive beam position. Figure 5 shows two
measurements in which these contributions are
apparent. In the top plot, the T < 50 ps values are
shifted negative for data that otherwise appear consis-
tent with a beam offset of about 40 gm. The shape of
the added wakefield is better seen in the bottom plot
where the beam was likely centered to < 10 gm. At
this level, a sine-like 21 GHz component also becomes
apparent. The T < 50 ps component is thought to
originate from wakefields generated upstream of
ASSET since it was also observed in measurements of
a C-Band structure . The origin of the 21 GHz
component is not known, but it would have little
impact on NLC operation. Despite these complications,
the attempts to center the drive beam generally yielded
offsets < 20 gm based on the size of the 15.1 GHz
component. Thus, these techniques appear promising
for meeting the NLC beam centering requirements.
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Adolphsen, Chris. Wakefield and Beam Centering Measurements of a Damped and Detuned X-Band Accelerator Structure, report, September 14, 1999; Menlo Park, California. (digital.library.unt.edu/ark:/67531/metadc624169/m1/4/: accessed January 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.