Wakefield and Beam Centering Measurements of a Damped and Detuned X-Band Accelerator Structure Page: 2 of 4
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WAKEFIELD AND BEAM CENTERING MEASUREMENTS OF A
DAMPED AND DETUNED X-BAND ACCELERATOR STRUCTURE *
C. Adolphsen#, K. Bane, R. Jones, N. Kroll, D. McCormick, R. Miller, M. Ross, T. Slaton,
J.W. Wang, SLAC, Stanford, CA, and T. Higo, KEK, Tsukuba, Japan
We present wakefield measurements of a prototype
Next Linear Collider (NLC) accelerator structure that
was built with dipole mode damping and detuning to
suppress the long-range transverse wakefield induced
by a beam. In addition, we describe beam centering
tests that use as a guide the dipole power coupled out
of the structure for damping purposes.
Nearly all designs being considered for future linear
colliders employ multibunch operation to improve
efficiency. A major concern in these designs is the
potentially strong coupling of the bunch motions from
the long-range transverse wakefields generated as the
beams traverse the linac accelerator structures. For the
NLC X-band (11.4 GHz) structures, two methods have
been developed to reduce the wakefields to a
manageable level. One method is mode detuning
whereby the frequencies of the lowest band of dipole
modes (near 15 GHz) are systematically varied along
the structure to produce a Gaussian distribution in the
mode density . This detuning results in the
destructive interference of the mode contributions,
yielding an approximately Gaussian falloff in the net
wakefield generated after each bunch. The structure
design parameters are chosen to produce about a two
orders of magnitude reduction by 1.4 ns, the minimum
bunch spacing being considered for the NLC.
This detuning works well to suppress the wakefield
for about the first 30 ns, after which its amplitude
increases due to a partial recoherence of the mode
excitations. This has led to the introduction of weak
mode damping to offset this rise . The damping is
achieved through the addition of four single-moded
waveguides (manifolds) that run parallel to the struc-
ture and couple to the cells through slots (see Fig. 1).
When terminated into matched loads, the manifolds
reduce the mode Q's from about 6000 to 1000, enough
to keep the wakefield from significantly increasing.
To date, three of these Damped and Detuned
Structures (DDS) have been fabricated . The cells
for the most recent version, DDS3, were manufactured
at LLNL  using single-diamond turning and then
assembled in Japan using diffusion bonding . This
structure is 1.8 m long and contains 206 cells. Relative
. Work Supported by DOE Contract DE-AC03-76F00515.
n Email: email@example.com
to its predecessors, the rf match through the output
ports of its manifolds is better. This improvement came
in part from using vacuum waveguide couplers on the
downstream ports instead of windows. Also, the dipole
frequency profile was changed slightly to enhance the
effect of the detuning. Instead of a Gaussian profile in
the mode density, it was made in the product of the
mode density and the mode coupling strengths to the
beam. In addition, the sigma of the frequency distribu-
tion was decreased (2.5% to 2.13% of 15.1 GHz), while
the number of sigma was increased (4.0 to 4.78) to
keep the frequency spread of the cells nearly the same.
Manifolds (Two of Four) Windows
Drift Tubendamental Mode
Drift Tube Input Couplerod
Fig. 1: Cutaway view of the upstream end of DDS3.
2 WAKEFIELD MEASUREMENTS
To measure the effect of the DDS3 changes on the
wakefield, the structure was installed in the Accel-
erator Structure SETup (ASSET) facility, which is
located in the upstream end of the main SLAC Linac.
Here a positron beam induced wakefields in the
structure that were 'witnessed' by an electron beam.
Just downstream of the structure, a chicane of magnets
steered the positrons (drive beam) to a dump and the
electrons (witness beam) into the linac where their
trajectory was measured. During the test, the eight
manifold ports were connected via Heliax cables to
processing electronics in the Klystron Gallery above
the linac tunnel. The signals were either measured with
a spectrum analyzer or downmixed to 310 MHz and
digitized to determine their amplitude and phase. A
signal from a nearby stripline BPM served as the beam
phase reference. Details of the wakefield and signal
measurements can be found in references  and .
In brief, each measurement of the wakefield ampli-
tude began by setting the relative beam timing to some
multiple of the linac rf period (350 ps). The relative
timing was then stepped in 3 ps increments to map the
wakefield over a few oscillation periods (- 200 ps). At
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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/m1/2/: accessed January 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.