Beam Loss Monitoring for Run 2 of the LHC

Abstract

The Beam Loss Monitoring (BLM) system of the LHC consists of over 3600 ionization chambers. The main task of the system is to prevent the superconducting magnets from quenching and protect the machine components from damage, as a result of critical beam losses. The BLM system therefore requests a beam abort when the measured dose in the chambers exceeds a threshold value. During Long Shutdown 1 (LS1) a series of modifications were made to the system. Based on the experience from Run 1 and from improved simulation models, all the threshold settings were revised, and modified where required. This was done to improve the machine safety at 7 TeV, and to reduce beam abort requests when neither a magnet quench nor damage to machine components is expected. In addition to the updates of the threshold values, about 800 monitors were relocated. This improves the response to unforeseen beam losses in the millisecond time scale due to micron size dust particles present in the vacuum chamber. This contribution will discuss all the changes made to the BLM system, with the reasoning behind them.

Full text

BEAM LOSS MONITORING FOR RUN 2 OF THE LHC
M. Kalliokoski, B. Auchmann, B. Dehning, E. Effinger, J. Emery, V. Grishin, E.B. Holzer, S.
Jackson, B. Kolad, E. Nebot Del Busto, O. Picha, C. Roderick, M. Sapinski, M. Sobieszek,
F.S. Domingues Sousa, C. Zamantzas, CERN, Geneva, Switzerland
Abstract
The Beam Loss Monitoring (BLM) system of the LHC
consists of over 3600 ionization chambers. The main task
of the system is to prevent the superconducting magnets
from quenching and protect the machine components
from damage, as a result of critical beam losses. The
BLM system therefore requests a beam abort when the
measured dose in the chambers exceeds a threshold value.
During Long Shutdown 1 (LS1) a series of modifications
were made to the system. Based on the experience from
Run 1 and from improved simulation models, all the
threshold settings were revised, and modified where
required. This was done to improve the machine safety at
7 TeV, and to reduce beam abort requests when neither a
magnet quench nor damage to machine components is
expected. In addition to the updates of the threshold
values, about 800 monitors were relocated. This improves
the response to unforeseen beam losses in the millisecond
time scale due to micron size dust particles present in the
vacuum chamber. This contribution will discuss all the
changes made to the BLM system, with the reasoning
behind them.
BEAM LOSS MONITORING SYSTEM
Energy deposition from beam losses can cause a
quench of the superconducting LHC magnets or even lead
to damage. The main protection against this is provided
by the Beam Loss Monitoring (BLM) system. The BLM
system consists of almost 4000 detectors spread around
the LHC ring. The main detector type is an Ionization
Chamber (IC), which are 50 cm long with an active
volume of 1.5 l, filled with N2 at 100 mbar overpressure.
The detectors are parallel plate chambers with 61 circular
aluminium electrodes of diameter of 7.5 cm, separated by
a drift gap of 0.5 cm [1].
To cover the full dynamic range in locations with high
losses, the ICs are installed in parallel to other less
sensitive monitor types: Secondary Emission Monitors
(SEM) or Little Ionization Chambers (LIC). Both are
based on the same geometry as the ICs, but consist of
only of three electrodes. The LICs have the same
properties as the ICs but due to the reduced volume are 60
times less sensitive, while the SEMs operate in a 10-7
mbar vacuum and are 70,000 times less sensitive than the
ICs.
For the start of Run 2, only the ICs are connected to the
Beam Interlock System (BIS) [2] and able to give beam
abort requests. The two other detector types are installed
for monitoring purposes only.
NEW INSTALLATIONS
Relocation of Monitors
The operation of the LHC during Run 1 was affected by
losses on the millisecond time scale. These losses are
suspected to be provoked by dust particles falling into the
beams, so-called Unidentified Falling Objects (UFOs) [3-
4]. UFO events are seen as the most likely loss scenario in
the LHC arcs during Run 2. Based on measurements
performed with secondary particles generated by the
beam wire scanner, it is calculated that the resulting signal
of a UFO event at 7 TeV will be about 3 times higher than
at 3.5 TeV [5].
To improve the response and the protection of the
magnets against UFO losses, 816 ionization chambers
were relocated from the quadrupole magnets (MQ) onto
the intersection of the bending magnets (MB) in the arcs
and dispersion suppressors (DS) of the LHC. Figure 1 shows
how the existing BLMs were relocated. Figure 2 shows a
monitor at the new location on top of a dipole-dipole
interconnection. In this new location, the detectors
monitor the losses from both beams.
Figure 1: Relocation of beam loss monitors in the LHC
arcs.
New Installations and Replacement of SEMs
with LICs
SEMs are installed in parallel with the ICs to extend the
dynamic range of the system towards higher dose rates to
avoid saturation of the detector or electronics [6]. During
Run 1 it was seen that in the events which surpassed the
dynamic range of the ICs, the signal from the SEMs was
still dominated by noise and no proper measurements
could be made. Thus the SEMs were replaced with LICs
in several locations. To further increase the dynamic
range of the LICs, they are installed with RC filters
connected. These filters reduce the peak amplitude for
short losses, stretching the length of the signal by a factor
depending on the values of the RC circuit.
Proceedings of IPAC2015, Richmond, VA, USA MOPTY055
6: Beam Instrumentation, Controls, Feedback, and Operational Aspects
T23 - Machine Protection
ISBN 978-3-95450-168-7
1057
Copyright ©2015 CC-BY-3.0 and by the respective authors

In addition to these replacements, new monitors were
installed in the locations which were seen to have been
missing monitors during Run 1, e.g. additional monitors
for ion losses, or where new equipment was added such as
the new collimators or Roman Pots installed during LS1.
A total of 50 new monitors were installed.
Figure 2: A relocated BLM on the transition between two
dipole magnets.
Changes to the Injection Regions
For the injection regions a set of RC filters were added
to the monitors to avoid saturation from the short losses
observed during the injection process. In certain locations
where the ICs would still be expected to saturate even
with filters, they were replaced by LICs to obtain a factor
10 increase in the dynamic range.
Some of the filtered ICs were connected to special
“blindable” crates. The “blindable” option allows the
selected BLMs to be blocked from giving a beam
interlock request during the injection process. After the
injection the monitors return to normal operation. This
was done as a result of issues observed with losses
coming from upstream in the transfer lines, which could
lead to a dump of the circulating beam in the LHC. The
need for this “blindable” option is being studied in
conjunction with the stability of the injection process
during the first part of Run 2 [7].
BEAM ABORT THRESHOLDS
The main goals of the BLM system are to avoid
quenching the superconducting magnets and to prevent
damage from beam losses [8]. This is done by requesting
a beam abort when the losses cross a predefined
threshold. The thresholds are optimised such that the
protection functionality does not reduce the LHC machine
availability.
The signal observed for a beam loss provoking a
magnet quench can be calculated as follows:
󰇛 󰇜󰇛󰇜󰇛 󰇜
󰇛 󰇜 , (1)
where BLMre sponse is the dose in the BLM per lost proton,
QuenchLevel is the energy required to quench a magnet
and EnergyDeposit is energy deposited in the magnet per
lost proton. The input values are based on FLUKA [9] and
QP3 [10] simulations which were fine-tuned through
quench test measurements [11-12].
BLMsignal can be used to set levels for protecting with
the BLMs. These values are called Master Threshold
values (MT) and are calculated as:
󰇛 󰇜   󰇛 󰇜  
, (2)
where N is a safety factor that ensures the threshold levels
are below the quench level. Currently the value is set to 3.
fcorr accounts for corrections that are applied to adjust for
effects from electronics, filters, injection losses etc., and
for adjustments based on dedicated tests and experience
from operation.
The final thresholds, the Applied Thresholds (AT), are
obtained by multiplying the master thresholds with a
factor that is specific to each monitor, the Monitor Factor
(MF):
  . (3) (3)
The monitor factor can be a value between 0 and 1 and
allows fast adjustments of the threshold values during
operation.
The BLM system integrates the signals produced by
beam losses in 12 different time intervals (running sums,
RS), spanning from 40 µs to 83.8 s. Furthermore, the
system takes into account 29 energy steps from 0 to 7
TeV.
The thresholds are grouped in families based on the
element they are protecting and the position of the
monitor with respect to the protected element. All the
monitors in each family have identical master thresholds.
THRESHOLDS FOR RUN 2
During LS1, all the threshold values that were used in
Run 1 were re-evaluated. Improvements in FLUKA and
QP3 simulations and experience from Run 1 showed that
new underlying models for threshold calculations were
needed. Due to this, most of the thresholds for Run 2 are
completely new.
Since the losses from Beam1 and Beam2 were found to
be identical, the division based on beam was removed. In
addition, the number of SEM and LIC families is initially
reduced to one, each with threshold limits set to the
maximum of the electronics limit. These settings might
change during Run 2 based on operational experience.
Based on the observations of the UFO losses during
Run 1, quench tests and new simulation models, it was
seen that the relocated monitors could all have the same
threshold settings and be placed in a single threshold
family. Since a UFO loss is now the most probable loss
scenario in the LHC arcs, all the quadrupole BLM
MOPTY055 Proceedings of IPAC2015, Richmond, VA, USA
ISBN 978-3-95450-168-7
1058
Copyright ©2015 CC-BY-3.0 and by the respective authors
6: Beam Instrumentation, Controls, Feedback, and Operational Aspects
T23 - Machine Protection

families in the arcs and dispersion suppressor region were
also modified to protect against UFO losses.
Figure 3 shows a comparison of the Run 1 and Run 2
thresholds for one of the BLM monitors on the main
quadrupoles in the arc. In general the new thresholds are
at the same level or higher than the old ones.
Figure 3: Comparison of Run 1 and Run 2 thresholds for
BLM monitors on the main quadrupoles in the arc. The
new thresholds are marked with a continuous line.
From the comparison with the old thresholds it can be
seen that the new thresholds are very similar to the old
ones, even though they are based on very different
models. This is due to the fact that the old thresholds were
corrected with data during Run 1 using the results of
various tests and operational experience. This similarity
can be seen as a first level of validation of the new
thresholds, which will be further improved during Run 2
with the adjustments based on operational data at higher
beam energies.
NEW BEAM ABORT THRESHOLD
CALCULATION TOOLS
During Run 1 the thresholds were produced by a set of
C++ scripts executed interactively in ROOT. To change
the thresholds, the scripts were modified and the output
then uploaded into the LHC Software Architecture (LSA)
database [13-14] using a Java API interface. This type of
modification required detailed bookkeeping of the
changes and was clearly a potential source of human
error. To reduce the possibility for mistakes, a new tool to
calculate the thresholds at the database level was designed
and introduced during LS1 [15].
With the new tool, new models and calculation
methods can be introduced into the database via specific
templates or in a table format. For instance the QP3
output can be written-in directly in a table format to be
used as quench level values in Equation 1. The user then
selects the set of parameters and methods via the
interface, adds corrections and launches the calculations.
Comparison of the calculated thresholds against the
previously calculated values or against other families can
also be done. In addition, the calculator can retrieve the
monitor factor parameters from the database to allow a
comparison to the applied values. This can also be done
for the historic values of all families. Figure 4 shows an
example of a comparison of applied threshold values for
the threshold families of Fig. 3.
Figure 4: Comparison of the difference between old and
new threshold settings. From the plot it is easy to see in
which running sums and for which energy level the main
changes have been applied.
CONCLUSIONS
The BLM system of the LHC has been updated during
LS1 to improve the protection of the elements for the
increased operation energy of Run 2. New monitors were
installed and existing monitors relocated to better respond
to the losses foreseen at this unprecedented energy. All
the threshold values that were used during Run 1 were
also reviewed and modified where required.
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Proceedings of IPAC2015, Richmond, VA, USA MOPTY055
6: Beam Instrumentation, Controls, Feedback, and Operational Aspects
T23 - Machine Protection
ISBN 978-3-95450-168-7
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MOPTY055 Proceedings of IPAC2015, Richmond, VA, USA
ISBN 978-3-95450-168-7
1060
Copyright ©2015 CC-BY-3.0 and by the respective authors
6: Beam Instrumentation, Controls, Feedback, and Operational Aspects
T23 - Machine Protection