ANL-HEP-CP-97-53
Co!Fv -1'705&3 -
HIGH GRADIENT DIELECTRIC WAKEFIELD DEVICE MEASUREMENTS
AT THE ARGONNE WAKEFIELD ACCELERATOR            I The submitted manuscript has been author.

P. Schoessow, M. Conde, W. Gai, R. Konecny, J. Power, and J. Simpson
Argonne National Laboratory, 9700 S. Cass Ave. Argonne IL 60439

Abstract
The Argonne Wakefield Accelerator (AWA) is a
facility  designed  to investigate  high  gradient
wakefield acceleration techniques. Wakefields are
excited using a drive beam produced by a 14 MeV
high current photoinjector-based linac. A second
photocathode gun generates a 4 MeV witness beam
which is used as a probe of the wakefields in the
device under test. The delay of the witness bunch with
respect to the drive bunch can be continuously varied
from -100 ps to >1 ns. The drive and witness bunches
propagate along collinear or parallel trajectories
through the test section. A dipole spectrometer is then
used to measure the energy change of the witness
beam. The complete wakefield measurement system
has been commissioned and wakefield experiments
using dielectric structures are underway. Initial
experiments have focused on collinear wakefield
device geometries where the drive and witness
bunches traverse the same structure. For attaining
very high gradients we will construct and study step-
up transformer structures in which the rf pulse
generated  by  the  drive  beam  is compressed
transversely and longitudinally.
1 INTRODUCTION
Research on dielectric loaded structures as high
energy wakefield accelerators has been proceeding for
some time [1]. These devices possess some obvious
advantages:
"   Simplicity of fabrication.
"   Parasitic wake control and suppression. The
HEM~ mode is in general lower in frequency
than the TM, accelerating mode [2], providing
greater tolerance to the beam breakup instability
than conventional structure based accelerators.
Multibunch beam breakup effects can also be
controlled with a simple mode suppression
scheme [3].
"   Adaptability  to  a  two-beam  (transformer)
configuration [4]. In this technique the wake
generated by multiple drive bunches in a low
impedance dielectric structure is coupled into a
high impedance structure. This provides a
transformer ratio  enhancement as well as
Submitted to the proceedings of the 1997
Vancouver, BC, May 12-16, 1997.
ST ER

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simplifying the staging of the drive and witness
beams.
There  are  also  potential difficulties with
dielectric devices:
" Breakdown limits at high fields.
" Charging from intercepted beam halo.
" Radiation damage effects on dielectric properties.
Initial experiments at AATF [5] concentrated on
understanding the low-field regime. One of the
primary goals of the AWA program is to study the
physics of dielectric wakefield structures at high
gradients with the emphasis on developing techniques
useful for high energy accelerators. We report here
the results of our initial experiments on collinear
drive-witness beam geometry, and describe planned
experiments with transformer structures.
2 WAKEFIELD MEASUREMENT SYSTEM
The AWA has been described in detail elsewhere
[6,7]. In brief, the facility consists of a unique high
current photoinjector and linac which generates the
drive beam, a second high brightness photoinjector
which produces the witness beam, beamlines to
transport drive and witness beams through the
wakefield device under test,   and a magnetic
spectrometer to measure the change in energy of the
witness beam from the wakefield of the drive beam.
The drive beam intensity is monitored using an
integrating current transformer immediately upstream
of the test section. The length of the drive beam can
be measured using an aerogel Cherenkov radiator
which can be inserted into the beamline. Light
produced in the radiator is then transported to a streak
camera.
The drive and witness guns share a common
laser system. A portion of the laser pulse is split off
and directed through an optical trombone before being
transported to the witness gun to adjust the delay
between the two beams. At the same time the phase of
the rf driving the witness gun is varied to maintain a
constant laser injection phase.
The witness beam is detected at the 600 port of
the spectrometer using a phosphor screen and
intensified CCTV camera. The energy acceptance of
the spectrometer is  165 keV; for all but the lowest
gradient device measurements the   spectrometer
Particle Accelerator Conference,
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