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\begin{document}
\title{THE NEW LAYOUT OF THE LHC
CLEANING INSERTIONS}

\author{R. A\ss mann, O. Aberle, O.
Br{\"u}ning, S. Chemli, D. Gasser, J.B. Jeanneret, J.M. Jimenez,
\\
V. Kain, E. M{\'e}tral, G. Peon, S. Ramberger, C. Rathjen, T.
Risselada, F.
Ruggiero, \\
L. Vos, CERN, Geneva, Switzerland,  D. Kaltchev,
TRIUMF, Vancouver, Canada}

\maketitle

\begin{abstract}
The improved LHC collimation system required significant changes
in the layout and design of the warm insertions. Requirements for
collimation, optics, impedance, vacuum, and additional
infrastructure are described and the adopted layout is presented.
Various design principles have been explored during the re-design.
Magnet positions and collimators were moved significantly, such
that a good cleaning efficiency was maintained while impedance was
reduced by almost a factor of two. Space for upgrades of the
collimation system was reserved (metallic phase~2 collimators
would allow for a better efficiency than originally achievable).
Scrapers were allocated for beam shaping and diagnostics. The
required infrastructure was specified, including a powerful
cooling system for the collimators.
\end{abstract}

\section{LAYOUT CHANGES WITH THE IMPROVED COLLIMATION SYSTEM}

The decisions on an improved LHC collimation
system~\cite{epac04-improved-system} imposed a number of layout
changes for the LHC cleaning insertions in points 3 and
7~\cite{lhc-design-report} and provided an opportunity to
implement additional layout improvements:
\begin{itemize}

\item The two-stage cleaning with its special optics requirements
was kept, however introducing additional impedance
constraints~\cite{jbj-collim1}.

\item The space requirements per collimator were significantly
increased due to longer, low Z jaws and a detailed engineering
design. For most collimators and movable absorbers a space of
2.0~m is reserved~\cite{epac04-aless}, with
the exception of the scrapers which have a
space reservation of 1.2~m. Equal space reservations allow to
economize in the number of different components, minimizing the
overall design and production cost.

\begin{figure}[htb]
\centering
\includegraphics*[width=80mm]{moplt006-fig1.eps}
\caption{Design view of an LHC secondary collimator.}
\label{fig:design-view}
\end{figure}

\item The adoption of a phased approach effectively doubled the
number of space reservations for secondary collimators in the
cleaning insertions IR3 and IR7. Initially only phase~1 robust
collimators will be installed. Later it is foreseen to complement
the existing phase 1 collimators with advanced phase~2 designs.

\item The number of primary and secondary collimators were reduced
by about 30\%. The liberated space in IR7 is kept free for an
eventual upgrade of the collimation system. The space in IR3 has
been given up.

\item The collimators were moved to locations of higher beta
functions while keeping good cleaning efficiency. As a result the
collimator-induced impedance was almost halved for the same
normalized settings of collimators~\cite{lhc-design-report2}. This
was also supported by a slightly modified optics in IR7, providing
for less phase advance and slightly higher beta functions.

\item Collimator were relocated with the constraint that no
longitudinal overlap between collimators for beam~1 and beam~2
occur.

\item The new layout and optics was verified at each step to not
reduce the available aperture in IR3 and IR7. The aperture was
already tight for the old layout and could not be
improved~\cite{lhc-design-report}.

\item The inter-module space in the warm quadrupoles was increased
from 30~cm to 40~cm, allowing for good positioning of ion pumps
and reduction of intervention time.

\item The dipole correctors were moved such that they face the IP
side of the collimators. They are then much less exposed to beam
induced showers from the collimators.

\item The quadrupoles were turned such that their connection sides
are protected against excessive radiation.

\end{itemize}

\begin{figure*}[t]
\centering
\includegraphics*[width=160mm]{moplt006-fig2.eps}
\caption{The final layout of the momentum cleaning insertion in
IR3. Shown is the active length of components, e.g. a 1.0~m
flat-top jaw length instead of the full 2.0~m space reservation
for secondary collimators.} \label{fig:ir3}
\end{figure*}

\begin{figure*}[t]
\centering
\includegraphics*[width=160mm]{moplt006-fig3.eps}
\caption{The final layout of the betatron cleaning insertion in
IR7. Shown is the active length of components, e.g. a 1.0~m
flat-top jaw length instead of the full 2.0~m space reservation
for secondary collimators.} \label{fig:ir7}
\end{figure*}

The new layouts for momentum and betatron cleaning insertions were
finalized in close collaboration with the machine integration. The
final layout of the two insertions is shown in
Figures~\ref{fig:ir3} and~\ref{fig:ir7}. The "active" length of
components is shown: For example, a secondary collimator takes a
space of 1.0~m (the length of flat top for a Carbon
jaw~\cite{epac04-aless}) though a total space of 2.0~m is reserved
for the collimator jaw plus tapering plus vacuum tank plus
interconnection with quick-handling flanges and flexible bellows.

Both LHC beams are shown with beam~1 running from left to right
and from top to bottom. The direction of beam~2 is opposite. It is
seen that collimators act on only one beam and have no
longitudinal overlap with collimators in the other beam.

\section{PERFORMANCE}

The new layout was confirmed to provide cleaning efficiency
comparable to the old layout at a much reduced impedance. An
estimate of cleaning inefficiency is given in
Figure~\ref{fig:pred-ineff}, including hypothetical phase~2
collimators. Further results on cleaning efficiency are discussed
in~\cite{epac04-colleff}. Impedance of the LHC is reviewed
in~\cite{lhc-design-report}. The impedance of an LHC collimator
prototype will be measured both with beam and in the laboratory
with a vibrating wire.
%~\cite{epac04-impedance}.

\begin{figure}[t]
\centering
\includegraphics*[width=80mm]{moplt006-fig4.eps}
\caption{Predicted inefficiencies for betatron cleaning at 7~TeV
with 6/7~$\sigma$ settings. Phase~1 and phase~2 performance is
shown, assuming 1~m Cu jaws for phase~2.} \label{fig:pred-ineff}
\end{figure}

\section{INFRASTRUCTURE FOR COLLIMATORS}

The infrastructure for collimators was studied and included into
the machine integration. About 360 cables with a total length of
106 km must be installed for the IR3 collimators during phase 1
and phase 2. In the betatron cleaning insertion IR7 about 1008
cables with a total length of 148 km are required.

The cooling requirements for LHC collimators were specified to
provide cooling capacities of 160kW (IR3) and 420kW (IR7) with
total flow rates of 36~m$^3$/h (IR3) and 96~m$^3$/h (IR7). The
pressure drop per collimator can be 1.2 bars and a collimator
water inlet temperature of 27~$^\circ$C was accepted. This results
in slightly higher peak temperatures on the graphite block and a
slightly worse vacuum. However, then the demineralised water
circuit (27/42 degrees) can be used for collimator cooling. After
a vacuum intervention in a collimator region all collimators in
the vented area must be exposed to a bake-out. It is required to
empty the cooling water from the collimators beforehand.

\section{CONCLUSION AND OUTLOOK}

New layouts of the LHC cleaning insertions in IR3 and IR7 have
been worked out, supporting the phased collimation approach with
long collimators and reserve space allocations. At the same time a
thorough optimization of the layout was performed, resulting in
various design improvements. The collimator infrastructure has
been defined and included into the machine integration. Further
studies will aim at including various absorbers (both movable and
fixed) into the collimation layout.

\begin{thebibliography}{9}   % Use for  1-9  references
%\begin{thebibliography}{99} % Use for 10-99 references

\bibitem{epac04-improved-system} R. Assmann et al, ``An Improved
Collimation System for the LHC''. These proceedings.

\bibitem{lhc-design-report} The LHC Design Report, Vol. I, The
LHC Main Ring, CERN-2004-003 (2004).

\bibitem{jbj-collim1} J.B. Jeanneret, "Optics of a two-stage
collimation system". CERN-LHC-Project-Report-243 (1998).

\bibitem{epac04-aless} A. Bertarelli et al, ``The Mechanical Design for
the LHC Collimators''. These proceedings.

\bibitem{lhc-design-report2} The LHC Design Report, Vol. I, Chapter~5,
CERN-2004-003 (2004). See also A. Koschik et al, these
proceedings.

%\bibitem{epac04-impedance} F. Ruggiero et al, ``A Review of Collective
%Effects in the LHC''. These proceedings.

\bibitem{epac04-colleff} R. Assmann et al, ``Expected Performance
and Beam-Based Optimization of the LHC Collimation System''. These
proceedings.

\end{thebibliography}

\end{document}
