[MAP] First draft of a mucool note (566) on bunch coalescing

[Cary Yoshikawa]

Hi,

As a follow-up to a MAP Friday meeting talk on bunch coalescing I gave
about a month ago (6/24), I've uploaded mucool note 566 (actually
NFMCC-doc-566) at http://nfmcc-docdb.fnal.gov.   It is an expanded
version containing results of a simplified system (less number of RF
cavities).  A very lengthy abstract is at the end of this email for
those interested.  The note is a first draft, but the reason for the
haste is to have a chance that some of its content will make it into
the ICFA Beam Dynamics Newsletter muon collider article that is
currently being written with an internal deadline of Aug 1.  Both doc
and pdf versions have been uploaded.

Thanks,
Cary

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Abstract:
A high-luminosity Muon Collider requires bunch recombination for
optimal luminosity.  In this report, we take advantage of the large
slip factor in a helical transport channel (HTC) to coalesce bunches
of muons into a single one over a shorter distance than can be
achieved over a straight channel.  To reduce complications associated
with matching out of upstream and into downstream subsystems, we
designed the bunch coalescing subsystem with a value for the slip
factor that is representative of an existing helical cooling channel
(HCC) design.  Alternate designs have been developed to investigate
the tradeoffs between simplifications in reducing the number of RF
cavities versus performance.  All key components of the bunch
coalescing subsystem have been simulated in 3-D.  Excluding the
initial acceleration of all bunches to the desired operating energy
(not simulated), the coalescing subsystems designed have a horizontal
length of ~105m  and are able to achieve efficiencies of 99.7%, 98.4%,
and 94.2% for 9, 11, and 13 bunches, respectively, where each bunch
has emittances expected at the end of a HCC.  The simplified designs
incorporating fill factors for RF cavities of ~25% and ~50% obtained
efficiencies of 96%, 94-95%, and 90-91% for 9, 11, and 13 bunches,
respectively.  The efficiencies above do not include decay losses,
which would be ~8% for muons with kinetic energy of 200 MeV. Following
the RF capture into a single bunch, a series of radial wedges in the
helical channel may be needed to reduce the longitudinal emittance
(via emittance exchange afforded by allowable growth of transverse
emittance) as well as reduce the operating energy to that of the
second HCC.  The amount of longitudinal cooling and energy reduction
for the single bunch is dependent on the acceptance of the downstream
HCC, which is yet to be designed.  Results of the coalesced single
bunch provide a starting point for an iterative process between the
bunch coalescer and HCC with aim to create the shortest and most
efficient integrated design that accepts a hot string of muon bunches
and produces a single cooled bunch that is ready for extreme cooling
(emittance exchange), acceleration, and collision with its particle
counterpart at the energy frontier.