CERN-ACC-2018-0001 BE Department Annual Report 2016 G. Arduini, R. Billen, P. Collier, E

105 Pages·2017·8.47 MB·English

CERN-ACC-2018-0001

BE Department Annual Report 2016

G. Arduini, R. Billen, P. Collier, E. Hatziangeli,
E. Jensen, R. Jones, M. Lamont, A. Mackney, P. Sollander, R. Steerenberg

Abstract

The Beams Department hosts the Groups responsible for the beam generation, acceleration,
diagnostics, controls and performance optimization for the whole CERN accelerator complex.
This Report describes the 2016 highlights for the BE Department.

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LHC:

BE-ABP Group

The decision of setting the * target to 40 cm for the 2016 physics run, i.e., below the nominal
value of 55 cm, imposed an additional challenge to, at least, the task of optics measurement and
control. As a result of the beam commissioning campaign, beta-beating was corrected down to
1.3%-1.8% (rms value) and, to minimise the luminosity imbalance between the two high-
luminosity experiments, K-modulation had been routinely used to measure and correct *. Note
that the waist position had been also included in the list of variables controlled, and the average
waist shift went down from 19 cm, in the 2015 proton run, to only 2 cm.
Dynamic aperture simulation studies with beam-beam guided the performance of the machine at
flat top and proposed optimized tunes that increased lifetime during collisions, in particular when
the crossing angle was reduced from 185 to 140 μrad.
A special effort was devoted in 2016 to the development and the validation of a completely new
version of the LHC optics based on the so-called Achromatic Telescopic Squeezing (ATS) scheme,
a keystone of the HL-LHC project. Since the first ATS optics versions drafted on paper half a
decade ago, the latest version is now fully compatible with the existing LHC, preserving or even
improving in some cases the functionality of all its sub-systems, thus aiming at a possible
operational use in 2017.
2016 was a very fruitful year for the ABP MDs carried out in the LHC. Strong progress was done
in the LHC performance for 2017 operation and in demonstrating or exploring HL-LHC
parameters. The feasibility of β* = 30 cm was demonstrated for LHC operation as well as the use
of ATS optics. Crossing angle leveling was successfully tested for the first time in 2016 MDs.
These are likely configurations to operate LHC in 2017. The octupolar IR corrections where
finalized and demonstrated via the measurement of amplitude detuning, resonance driving terms,
feed-down from crossing angle scans and lifetime.
Looking further in the future, HL-LHC assumes about a factor 2 larger total and bunch intensities.
Beam-beam MDs demonstrated a beam-beam head-on tuneshift of 0.018 in single bunch mode,
not far from the 0.02-0.03 required for HL-LHC. The associated event pile-up of about 130 was
observed in the detectors, paving the future to learn how to deal with this unprecedented pile-up
level. A mixed filling scheme alternating BCMS and 8b4e trains (8 bunches, 4 empty) was put in
the LHC demonstrating a 40% reduction of heat-load with smaller impact on luminosity. This
filling scheme could be used in the HL-LHC if the e-cloud heat-load for nominal beams surpasses
the cooling capacity. Studies of the dependence of e-cloud heat-load versus bunch intensity were
also performed. Extrapolations to HL-LHC parameters make e-cloud a serious concern for nominal
parameters.
Crystal channeling was demonstrated for record-high energy Lead ions, in view of the option to
use crystals for collimation in HL-LHC.
Concerning more general purpose MDs, Beam Transfer Function measurements were explored
and the required developments were identified. The use of the ADT as a safer AC dipole (called
ADT-AC dipole) was demonstrated in 2016 MDs that allows for non-invasive coupling
[2]
measurements without dedicated pilot cycles. The measurement of the forced Dynamic Aperture
with the AC dipole was demonstrated for the first time in 2016 MDs. First promising steps were
taken towards using second order chromaticity as an extra source of Landau damping.
The 2016 heavy-ion run was devoted to colliding beams of protons and lead nuclei. The LHC
experiments had requested a variety of apparently incompatible operating conditions, according to
their diverse capabilities and physics programmes. Careful analysis of the beam physics and
operational requirements nevertheless led to an ambitious schedule comprising three different
beam modes that could potentially fulfil all requests.
The first set-up for p-Pb collisions at a centre-of-mass energy for colliding nucleon pairs of
5.02 TeV started on 5 November and physics data-taking started on 10 November. This run was
mainly dedicated to ALICE to increase an earlier collected sample of minimum-bias events. The
other experiments also participated with LHCb studying proton collisions with a target of helium
gas. As foreseen in the plan, beam lifetimes were extremely long, allowing seven days of nearly
uninterrupted running at a constant levelled luminosity of 0.8×1028 cm-2s-1. A total of 660 million
minimum-bias events were collected - increasing by a factor six the data set from 2013. One of the
first fills turned out to be the longest LHC fill ever, lasting almost 38 hours.
Just one day after the 5.02 TeV run ended, the second set-up, with a new high luminosity beam
optics, was completed and the LHC delivered p-Pb collisions at 8.16 TeV. This is the highest
energy ever produced by a collider for such an asymmetric system, and included a short run for
the LHCf experiment and also a third run in which the direction of the Pb and p beams was
reversed. Thanks to the superb performance of the injectors and numerous improvements in the
LHC, the luminosity soared to 9×1029 cm-2s-1, 7.8 times the design value set some years ago. It
could have gone even further had the intense flux of lead beam fragments from the collisions not
risked quenching nearby magnets.
On 4 December, the LHC was switched back to 5.02 TeV for a last 20 hours of p-Pb data taking
to top up the minimum-bias events for ALICE to 780 million.

BE-BI Group
Schottky Monitors
The LHC is equipped with four transverse Schottky monitors (horizontal and vertical plane for
each beam). After an overhaul of the Schottky beam pickups in LS1, the 4.8 GHz RF front-end
electronics received major improvements in YETS15-16 to enhance the quality of the
measurements. With these improvements, the transverse Schottky signals can be used to extract
betatron tune, synchrotron tune and chromaticity for both proton and ion bunches at injection
energy. During the ramp and at top energy the Schottky signals degrade (due to RF blow-up and
shrinking emittance) and the monitoring becomes much more challenging. Improvements on
fitting procedures and operational usability are ongoing with the aim of giving continuous, on-line
chromaticity measurements at injection for full intensity beams in 2017.

[3]
Figure 1. Schottky spectra during acceleration of Pb82+ ions (left). Fitting methods of the transverse Schottky
sideband in the presence of coherent tune lines (right).

Figure 2. Chromaticity measurements using the Schottky system at injection energy.

Transverse Beam Instability Network (LIST)
Measuring the properties of transverse beam instabilities is important, both for experimentally
qualifying the LHC impedance model and for understanding problems that occur during day-to-
day operation. In order to connect and synchronise the instruments that are capable of measuring
beam instabilities, an LHC Instability Trigger Network (LIST) has been developed through
collaboration between the BE-BI, BE-CO and BE-RF groups. The network, based on White Rabbit
technology, enables bidirectional trigger distribution with nanosecond resolution between
instruments located around the LHC. The installation at LHC Point 4 is shown in Fig. 3.

[4]
Figure 3. The LHC Instability Trigger network connects the various systems from BI and RF around LHC
point 4.

After deployment in mid-2015, the first major use of the network was to trigger the LHC Head-
Tail Monitor when the tune measurement system (BBQ) detects that an instability is occurring.
The system operated continually during the 2016 run and generated a large amount of Head-Tail
data for offline analysis. Software post-processing of the Head-Tail data was implemented and this
significantly reduced the amount of data that needs to be stored and manually analysed.
To improve the reliability of the trigger system, new algorithms were developed for the BBQ after
analysis of the signals observed under real beam conditions. Some examples of data acquired can
be seen in Fig. 4.

Figure 4. Two examples of instabilities seen by the BBQ Trigger algorithm and the Head-Tail Monitor
comparing the performance of two trigger algorithms.

Halo Measurement using a Coronagraph)
The development of a non-interceptive beam halo measurement is motivated by the HL-LHC
project, to monitor beam halo populations in the presence of strong beam-beam effects, with a crab
cavity malfunction, and with the possible use of a hollow-electron lens. After carrying out
extensive simulations, successful tests on a prototype coronagraph beam halo monitor were
performed in the laboratory in 2015. This system was installed on LHC beam 2 during YETS15-
[5]
16, on the optical table hosting the standard synchrotron light beam imaging monitor. The
coronagraph layout is shown in Fig. 5.

Figure 5. Layout of the new coronagraph system installed on Beam 2 in Point 4 of the LHC.

After parasitically setting-up and optimizing the system, two different experiments were
performed during a dedicated MD at injection energy, both requiring beam excitation since the
present LHC beams do not generate any measurable halo. During the first experiment the
transverse damper was used to blow-up the emittance of a bunch train. The coronagraph was used
to compare the beam halo before and after the blow-up. A result with emittance blow-up in the
horizontal plane is shown in Fig. 6. Although the raw image before and after the blow-up look
similar, the difference between images clearly shows the increase in halo population as the
emittance increases.

Figure 6. Beam halo measurement examples during horizontal emittance blow-up.

The second experiment started with a large emittance beam to simulate a high beam halo
population. The primary collimators were then closed in steps and the coronagraph system was
able to detect the halo reduction (i.e. cleaning by the collimators). An example from the vertical
plane is shown in Fig. 7.

[6]
Figure 7. Beam halo measurements while closing the primary collimators.

This world first for coronagraph measurements with a proton beam demonstrated the system’s
ability to resolve a beam halo with an intensity of about 2×10-3 of the beam core. A better contrast,
to achieve a halo to core intensity ratio of 10-5, is required for HL-LHC, with simulations already
underway for the design of an optimised, second prototype to be installed in LS2.

Fast Beam Current Transformer Upgrade
During 2016 a new fast acquisition system was installed and intensively tested on the LHC
development Fast Beam Current Transformer (FBCT) system. This is intended to overcome the
limitations of the existing system which is based on fast, 40MHz integrators and the old BE-BI
Digital Acquisition Board (DAB). The new system uses a fast, commercial ADC (1GS/s, 14bits),
in an FPGA Mezzanine Card (FMC) format sitting on the new BE-BI VME acquisition
motherboard (VFC-HD). The analogue part of the acquisition chain was also upgraded.
For the last 3 months of the year, the system, including the new FESA server, was running
flawlessly, giving reliable and accurate total intensity and bunch intensity data, see example in Fig.
8. During EYETS16-17 the old DAB-based systems in both rings as well the dump line will
therefore be dismantled and replaced with this new digital acquisition system.

[7]
Figure 8. LHC Total beam intensity. Comparison between DCCTs and VFC based system showing an
agreement much better than 1%.

BE-ICS Group
Access Control
A new and improved interface for the LHC access control was designed and deployed in 2016.
This will allow a smooth upgrade of the LHC Access Control System (LACS) operator interface
that is due for complete replacement during LS2. The change is necessary due to the programmed
obsolescence of the software package that is used for the operator interface, as well as to
accommodate the new and more performance Video camera interface.

LHC Cryogenics control system
Due to the issues with the heat loads influence on the cryogenics system, we deployed, in
collaboration with TE-CRG, a new beam screen control based on a feed-forward technique. The
results were excellent and gave a more robust regulation, which compensates the fast dynamic
beam heat loads and allows new injection schemas. The regulation schema was validated offline
with extensive dynamic simulations.
During the YETS-2016, we deployed a new firmware version in about 80 Schneider PLCs to
increase their robustness. The firmware was specially designed by Schneider Electric to be applied
[8]
to the CERN PLCs. During the year we still suffered 2 PLC crashes, which were not identified
and/or prevented by the new firmware. In parallel, we continue with the action of replacing those
PLCs by the new Modicon M580 model. 12 implemented in YETS-16 and 5 more during the TS1
in 2016 (including one complete refrigerator process: compressor and cold box). This effort will
extend until LS2 when all PLCs will have been replaced.

BE-OP Group
The 2016 LHC schedule
The 2016 run was eventful and, despite the numerous problems, successful for LHC operation.
Figure 9 shows the official LHC schedule as actually executed in 2016, which includes all the
changes made following the various events and decisions.

[9]
Figure 9: 2016 LHC schedule, as executed.

The summary of the schedule in numbers is given in Figure 10.

[10]