Enhancing RHIC luminosity capabilities with in-situ beam piple coating Page: 3 of 5
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ENHANCING IC LUMINOSITY CAPABILITIES WITH IN-SITU BEAM
PIPE COATING*
Ady Hershcovitch4, Michael Blaskiewicz, Wolfram Fischer,
Brookhaven National Laboratory, Upton, New York 11973, U.S.A;
H. Joe Poole, PVI, Oxnard, California 93031, USA.A bstract
Electron clouds have been observed in many
accelerators, including the Relativistic Heavy Ion Collider
(RIC) at the Brookhaven National Laboratory (BNL).
They can limit the machine performance through pressure
degradation, beam instabilities or incoherent emittance
growth. The formation of electron clouds can be
suppressed with beam pipe surfaces that have low
secondary electron yield. At the same time, high wall
resistivity in accelerators can result in levels of ohmic
heating unacceptably high for superconducting magnets.
This is a concern for the RHIC machine, as its vacuum
chamber in the superconducting dipoles is made from
relatively high resistivity 316LN stainless steel. The high
resistivity can be addressed with a copper (Cu) coating; a
reduction in the secondary electron yield can be achieved
with a titanium nitride (TiN) or amorphous carbon (a-C)
coating. Applying such coatings in an already constructed
machine is rather challenging. We started developing a
robotic plasma deposition technique for in-situ coating of
long, small diameter tubes. The technique entails
fabricating a device comprised of staged magnetrons and/
or cathodic arcs mounted on a mobile mole for deposition
of about 5 pm (a few skin depths) of Cu followed by
about 0.1 pm of TiN (or a-C).
INTRODUCTION
Electron clouds, which have been observed in many
accelerators, including the Relativistic Heavy Ion Collider
at the Brookhaven National Laboratory [1-3], can act to
limit machine performance through dynamical beam
instabilities and/or associated vacuum pressure
degradation. Formation of electron clouds is a result of
electrons bouncing back and forth between surfaces,
which can cause emission of secondary electrons resulting
in electron multipacting effect. One method to mitigate
these effects would be to provide a low secondary
electron yield surface within the accelerator vacuum
chamber.
At the same time, high wall resistivity in accelerators
can result in unacceptable levels of ohmic heating that in
turn can lead to resistive wall induced beam
instabilities[4]. This is a concern for the RHIC machine,
as its vacuum chamber in the cold arcs is made from
relatively high resistivity 316LN stainless steel. This
effect can be greatly reduced by coating the accelerator
vacuum chamber with oxygen high conductivity copper
(OFHC), which has conductivity that is three orders [5,6]*Work upp rted by Wark supported under Contnrct No. DE-
AC02-98CH 1-986 with he US Dpartment of Energy,
#hIrshcov tCh$bni.gov
of magnitude larger than 316LN stainless steel at 4 K.
And, walls coated with titanium nitride (TiN) or
amorphous carbon (a-C) have sbown to have minimal
secondary electron yields[7,8]. This coating also protects
the underlying OFHC coating from oxidation, which
would reduce its performance.
Consequently, any of the new machines with RHIC-like
intensity and bunch spacing are being built with internal
coatings, the large hadron collider (LHC) design [9] being
but one example. Applying such coatings to an already
constructed machine like RHIC without dismantling it is
rather challenging due to the small diameter bore and the
access points, which are about 500 meters apart.
DEPOSITION PROCESSES AND OPTIONS
Coating methods (at least with relevance to OFHC and
TiN coating) can be divided into two major categories:
chemical vapor deposition (CVD) and physical vapor
deposition (PVD). Reference [10] contains a
comprehensive description of the various deposition
processes; unless otherwise noted, information contained
the next two sections is referenced to reference [10].
Due to the nature of the RHIC configuration, only PVD
is viable for in-situ coating of the REIC vacuum pipes.
First, the temperature under which coating can be made
cannot be high (400'C is required for one conventional
CVD TiN deposition), since the RHIC vacuum tubes are
in contact with superconducting magnets, which would be
damaged at these temperatures. A second very severe
constraint is the long distance between access points.
Introduction of vapor from access points that are 500
meters apart into tubes with 7.1 centimeters ID would
necessarily be very non-uniform, which would make
resultant coating properties very non-uniform.
But these constraints also severely restrict PVD options.
Obviously, evaporation techniques (ovens, e-beams)
cannot be used in 7.1 centimeters 1D, 500-meter long
tubes for the same reasons. Therefore, evaporation must
be accomplished locally. One option is a plasma device
on a mole that generates and deposits the vapor locally.
Presently, there are a variety of PVD methods used to
deposit coatings on various substrates[10]. By definition,
physical vapor deposition entails purely physical
processes of evaporating materials. The vapor then
condenses on the desired substrate. There is a wide
variety of vapor generation techniques ranging from high
temperature evaporation to sputter bombardment by
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Herschcovitch,A.; Blaskiewicz, M.; Fischer, W. & Poole, H. J. Enhancing RHIC luminosity capabilities with in-situ beam piple coating, article, May 4, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc935389/m1/3/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.