 \documentclass{JAC2003}

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\begin{document}
\title{DISCUSSION SUMMARY OF SESSION~5: BEAM LOSSES}

\author{Chairman: Ralph W. A\ss mann \\Scientific secretary: Stefano Redaelli
\\ CERN, Geneva, Switzerland}

\maketitle
\begin{abstract}
This paper summarizes the discussions that followed the presentations of 
the ``Beam Losses'' session of the the LHC Beam Operation Workshop, EVIAN2010. 
%A summary with the critical points and open actions that arose in this sessions 
%is also given.
\end{abstract}

%+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\section{Introduction}
The fifth session of LHC Beam Commissioning Workshop was dedicated to the 
analysis of beam losses and included four talks: 
\begin{itemize}
  \item[1)] {\bf Multi-turn losses and cleaning}, 
    by Daniel Wollmann (BE-ABP);
  \item[2)] {\bf Injection and extraction losses}, by Wolfgang Bartmann (TE-ABT);
  \item[3)] {\bf Losses away from collimators: statistics and extrapolation}, 
    by Barbara Eva Holzer (BE-BI);
  \item[4)] {\bf BLM thresholds: limiting locations}, by Annika Nordt (BE-BI).
\end{itemize}
For each presentation of the session, summaries of the discussion that 
followed the presentations are given. 
%A summary of the critical points and open actions is also given.

\section{Multi-turn losses and cleaning (D.~Wollmann)}
{\it B.~Goddard} asked if the beam loss maps used for inefficiency calculations
weight in the same way the BLM readings at the collimators and at the magnets.
{\it D.~Wollmann} reply that for the final intensity reach estimate, 
appropriate BLM factors are taken into account for the different element 
types.

{\it F.~Zimmermann} asked if the ratio of loss peaks in IP7 and IP3 are 
correctly predicted by simulations in the case of betatron losses. 
{\it R.~Assmann} replied that they agree within a factor $\approx$~2.

{\it O.~Br\"{u}ning} asked if the hierarchy violation in IP3 was caused by a 
positioning error of the collimators. {\it D.~Wollmann} replied that this is 
not the case: the collimators were correctly sent back to the same positions
within the mechanical accuracy. He also pointed out that the machine was never 
at risk because the provided efficiency with hierarchy violated was still 
acceptable. {\it R.~Assmann} commented that the radiation resistance of the 
warm magnets in IP3 could have been compromised in case of larger stored 
energies because only the magnets downstream of the primary collimators are 
protected with passive absorbers.

{\it S.~Fartoukh} asked if the hierarchy violation was caused by an error 
at the primary or at the secondary collimator. {\it D.~Wollmann} replied that 
this is not clear because the source of the problem was not identified.

As the estimated intensity reach from the collimation system is well above the 
goal for 2011, {\it M.~Ferro-Luzzi} asked if there are other immediate 
limitations on the luminosity reach. {\it R.~Assmann} stated that the 2011 
goals should not be compromised. We have collimators dedicated to the 
absorption of the physics debris that have not yet been used but are fully 
operational and will be used if the debris from the IP will become an issue.

Having seem the excellent performance of the system, {\it O.~Br\"{u}ning} asked
if we really need an upgrade of the system. {\it R.~Assmann} replied that at 
7~TeV the margins will be reviewed and also reminded that the losses for 
ion beams are worst. 

{\it B.~Goddard} suggested to review offline the issue of non reproducibility 
of vertical losses in IP6 because there we only have horizontal collimators. 
Why should we have vertical losses? 
{\it S.~Redaelli} stated that for vertical losses at the primary collimators 
of IP7 there is also a leakage for the horizontal plane that induced losses 
in the collimators of IP6 as well.

{\it R.~Schmidt} suggested to fold the latest updates on the quench estimates 
for different integration times onto the cleaning results presented by 
D.~Wollmann. {\it B.~Dehning} reminded that the new calculations will only 
affect the long running sums whereas the short one should be correct.

{\it J.~Uythoven} asked if one should expect a different behaviour for smaller 
$\beta^*$ values. {\it D.~Wollmann} replied that the betatron cleaning from IP7 
should remain unaffected so the conclusions should not change (provided that 
the triplet magnets are locally protected in an appropriate way by the tertiary 
collimators).

\section{Injection and extraction losses (W.~Bartmann)}
{\it R.~Assmann} asked why the injection loss projections as a function of 
the number of bunches is not linear. {\it W.~Bartmann} replied that the 
measurement with more than 24 bunches were taken without re-optimization of 
the injection collimator settings. {\it M.~Sapinski} warned that the BLM 
thresholds were not the same for the various data taking and strongly 
recommended to make sure that this is properly taken into account. 

{\it P.~Collier} asked if it is clear why uncaptured beam at the SPS is seen 
in the LHC. This could only be explained by a mis-match between SPS extraction 
kicker and LHC injection kicker. To be checked.

{\it B.~Dehning} asked if the factor 3 improvement of TDI losses from the 
shielding will be enough. {\it W.~Bartmann} replied that this will not be the 
case. {\it B.~Goddard} warned that the simulations that predicted this value 
are preliminary and the final figures should be considered.
{\it B.~Dehning} stated that a beam-based validation of the improvement 
factor from shielding predicted by simulations should be addressed as soon 
as possible. Dedicated beam time should be foreseen for tests with beam. 

{\it G.~Arduini} commented that the scaling of losses versus bunch number 
must take into account the fact that so far we used smaller emittances than 
nominal. This will not be possible anymore with many more bunches per train. 
 
{\it G.~Arduini} also asked how we will make sure that the abort gap cleaner, 
proposed as a way to clean the space of the next injection train, will not 
act also on the already filled bunch slots. {\it W.~Hofle} replied that he has 
established a procedure for that. This method will have to be stamped by the 
machine protection panel.

{\it J.~Wenninger} pointed out that the ``BLM sun glass'' has not been discussed
yet at the machine protection meeting and therefore it should be considered 
just as a proposal at this stage. {\it W.~Bartmann} confirmed that this is the
case.

{\it R.~Jones} asked how much should one open the injection collimators in order
to reduce the loss spikes. {\it W.~Bartmann} replied that clearly opening 
collimators is an effective way to reduce losses: 0.5~$\sigma$ reduces the 
losses by about a factor 3. 
{\it R.~Assmann} commented that one could also increase significantly the 
thresholds of the TCTV collimators for the short integration times. 
{\it R.~Schmidt} suggested to consider also the possibility to increase the 
thresholds of the superconducting magnets in the injection regions. We should
not be too afraid of the quenches!

{\it W.~Hofle} reminded that, as he stated in his presentation, the present 
mechanism to clean the injection slot has a problems that require follow-up if
the area to clean is 8~$\mu$s.

\section{Losses away from collimators: statistics and extrapolation 
  (B.E.~Holzer)}
{\it Bernhard Holzer} asked if the degradation of the signal experienced with 
some of the 
ionization chambers was caused by radiation damage. B.E.~Holzer replied that 
the problem was identified as a bad soldering and is therefore independent 
of the beam.

{\it P.~Collier} pointed out that, as no UFO was observed at injection energy, 
one should think of a mechanism that only produces them at top energy. The 
theory of the dust might not be appropriate. Clearly, we do not have yet a 
satisfactory physics model. {\it O.~Br\"{u}ning} suggested that the dependence
on the synchrotron radiation should be taken into account. {\it T.~Camporesi} 
proposed to consider the possibility that single halo protons hit directly 
the aperture. 
Other ideas were proposed: looking in detail in the ramp data to see when 
UFOs appear ({\it R.~Assmann}), study the correlation with primary collimator
settings ({\it S.~Fartoukh}). 

\section{BLM thresholds: limiting locations (A.~Nordt)}
{\it B.~Goddard} asked if the pressure rise seen in case of losses at the 
collimators is real of could come from noise in the electronics. {\it A.~Nordt}
replied that she does not have this information.  

{\it J.~Jowett} stated that it would be interesting to perform a similar 
analysis of beam losses and pressure levels also for the ions.

{\it P.~Collier} asked if there is a rationale for the change of BLM 
thresholds. {\it R.~Assmann} commented that the purpose of this talk 
was indeed to identify locations for which a safe change of thresholds makes
sense. {\it A.~Nordt} pointed out that all the changes applied in the 
machine are subjected to a strict EDMS approval procedure.

{\it F.~Zimmermann} suggested to check if there is a correlation between
critical loss locations and $\beta^*$ values.

{\it H.~Burkhard} stated that the interaction of proton beams with the rest
gas is well known and the losses could be compared with simulations.

% \section{Open actions}
% \begin{itemize}
% \item \underline{Energy ramp}:\\
%   - The incorporation of injection settings into the ramp functions must 
%   be made available in the sequencer.\\
%   - Improve the FiDeL corrections with updated estimates of snapback (see 
%   also action list for the magnetic model).\\
%   - Improve - if possible - the understanding of the beam parameter evolution
%   during the ramp. \\
%   - The logging of data relevant for the ramps could be optimized by linking 
%   the data logging to the operational modes.
% \end{itemize}
%+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\end{document}

