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FYO7 SLAC S CIENCE AND T ECHNOLOGY S ELF E VALUATION
Objective 1.2 Leadership in Science and Technology
The US collaboration on High Gradient Research for future colliders was formed in 2006. The
specific goal is to establish the best frequency and accelerator structure design for a future multi-TeV
collider. The work plan assumes a period of 5 years. SLAC is the host of this collaboration and
research activities include theoretical and experimental studies of the rf breakdown phenomenon.
This work aims to establish a better understanding of the frequency scaling of the limiting gradient,
as well as its dependence on material, surface preparation, structure design, pulsed heating, etc. It
explores the high gradient barriers that result from choices made in current linear collider programs.
The experimental effort entails test facility upgrades and the development of new high-power rf
sources designed for high gradient testing. The goal is to produce and test at very high gradient an
accelerator structure suitable for use in a multi-TeV two-beam linear collider.
SLAC experimental facilities are made available to collaborators for experiments supported within
the collaboration.
The problems and potential solutions for high accelerating gradient include the following.
Frequency scaling
CERN and SLAC experiments have shown less frequency dependence than expected;
however, frequency links other parameters such as pulse length, filling time, power,
energy, and geometry.
Geometry dependence
There is a clear geometry dependence; e.g., accelerator structures with different a/λ,
and circuit dependence; e.g., standing wave vs. traveling wave; the origin of this
dependence is not clear.
Energy, power and pulse length
There is clear pulse length dependence; however, the laws that govern it are debatable
and differ from one experimental setup to another.
Materials
Very little is known about materials and DC voltage data does not seem to apply.
There may be opportunities for significant development here.
Surface processing technique (etching, baking, etc.)
There are known practices that have been proven to help; however, the basic physics is
still under debate, and the question of processing vs. initial condition of the surface is
in question.
Theory
There is no robust theory to date, although several attempts at particle tracking, scaling
with surface physics, and surface atom dynamics have been put forward.
Wake field damping
At the moment several ideas are being explored fro strong wake field damping. This is
important because it allows shortening the pulse length for the same train of bunches,
hence enable higher gradient. At the moment the most advanced technique was the one
developed for the NLC with moderate damping.
Accelerator Structure Research Program for the ILC
Accelerator structures for the ILC positron source
An improved alternative design for the L-Band normal conducting accelerating system for both the
ILC positron source and electron source is being developed. Results appear in the ILC RDR.
Due to the extremely high energy deposition from positrons, electrons, photons and neutrons behind
the positron target, normal conducting structures must be used up to an energy of 400 MeV. This
normal-conducting section must sustain high accelerator gradients during millisecond-long pulses in
a strong magnetic field, provide adequate cooling in spite of high rf and particle loss heating, and
produce a high positron yield with the required emittance. The proposed design contains both
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