Since the LHC running season 2010, the TOTEM Roman Pots (RPs) are fully
operational and serve for collecting elastic and diffractive proton-proton
scattering data. Like for other moveable devices approaching the high intensity
LHC beams, a reliable and precise control of the RP position is critical to
machine protection. After a review of the RP movement control and position
interlock system, the crucial task of alignment will be discussed.Comment: 3 pages, 6 figures; 2nd International Particle Accelerator Conference
  (IPAC 2011), San Sebastian, Spain; contribution MOPO01

B Farnham

CERN D Wollmann

CERN W Snoeys

E Radermacher

F Lucas Rodriguez

F Ravotti

G Antchev

G Ruggiero

G Valentino

Geneva Switzerland

H Niewiadomski

H Sabba

I Atanassov

J Baechler

J Kašpar

J Morant

K Eggert

M Deile

M Dupont

M Dutour

M Sapinski

R Assmann

R B Appleby

R Bruce

S Ravat

S Redaelli

V Avati

X Pons

English

arXiv

Abstract Since the LHC running season 2010, the TOTEM Roman Pots (RPs) are fully operational and serve for collecting elastic and diffractive proton-proton scattering data. Like for other moveable devices approaching the high intensity LHC beams, a reliable and precise control of the RP position is critical to machine protection. After a review of the RP movement control and position interlock system, the crucial task of alignment will be discussed

CiteSeerX

THE FIRST 1 1/2 YEARS OF TOTEM ROMAN POT OPERATION AT LHC
M. Deile, G. Antchev, I. Atanassov, V. Avati, J. Baechler, K. Eggert, J. Kašpar, F. Lucas Rodriguez,
J. Morant, H. Niewiadomski, E. Radermacher, F. Ravotti, G. Ruggiero, H. Sabba, W. Snoeys,
CERN, Geneva, Switzerland, on behalf of the TOTEM Collaboration
R.B. Appleby, R. Assmann, R. Bruce, M. Dupont, M. Dutour, B. Farnham, X. Pons, S. Ravat,
S. Redaelli, M. Sapinski, G. Valentino, D. Wollmann, CERN, Geneva, Switzerland
Abstract
Since the LHC running season 2010, the TOTEM Ro-
man Pots (RPs) are fully operational and serve for col-
lecting elastic and diffractive proton-proton scattering data.
Like for other moveable devices approaching the high in-
tensity LHC beams, a reliable and precise control of the RP
position is critical to machine protection. After a review of
the RP movement control and position interlock system,
the crucial task of alignment will be discussed.
INTRODUCTION
The TOTEM Roman Pot system [1] has been designed
for measuring very forward protons originating from elas-
tic or diffractive scattering processes. With different beam
optics, four-momentum transfers down to ∼ 10−3 GeV2
can be reached, corresponding to scattering angles of a few
μrad. The proton detection is accomplished by stacks of
10 silicon sensors placed in movable beam-pipe insertions
– so-called “Roman Pots” (RPs) – that can approach the
beam. In some special runs the distance from the beam
centre has been smaller than 1 mm. The system consists of
24 RPs arranged in 4 stations at 147 m and 220 m on either
side of LHC’s IP5. Each station is composed of 2 units sep-
arated by a distance of 2 – 5 m. A unit consists of 3 RPs,
two approaching the outgoing beam vertically and one hor-
izontally, allowing for a partial overlap between horizontal
and vertical detectors, which is useful for the alignment.
THE ROMAN POT MOVEMENT
CONTROL SYSTEM
System Architecture
The RP movement control system [2] has been derived
from the one of the LHC collimators. The top level user in-
terface, a modified version of the Central Collimation Ap-
plication [3], is operated from the CCC. It allows the oper-
ator to send RP position requests and critical position limits
to the middle level control (see below) and to read back the
measured positions. A graphical display shows the time
evolution of requested and measured RP positions, the po-
sition limits, and the dose rates measured by Beam Loss
Monitors downstream of the RPs (see example screenshot
in Fig. 2). The middle level is the FESA-based [4] Colli-
mator Supervisor System [5], providing the link between
the Collimation Application and the low level motor con-
trol running on a PXI [6] crate. The PXI CPU controls
the stepping motors of the individual RPs and manages the
position read-outs from the stepping motor encoder (step
counter) and from the independent LVDT system (see next
subsection). The PXI computer also interfaces to FPGA
cards which independently from the CPU accomplish the
interlock-relevant comparison between actual RP positions
and their critical limits (see later subsection). The FPGA
cards have direct connections from the microswitches and
the LVDTs and to the beam interlock logic card.
Note that from the TOTEM control room no RP move-
ments can be triggered, except emergency retractions via
mechanical springs. This policy enforces centralisation of
controls in the CCC and enhances communication between
TOTEM and the LHC operators. However, the TOTEM
Detector Control System monitors passively all operations.
Microswitches and Position Measurements
Figure 1: Schematic overview of
the RP microswitches tagging some
particular pot positions and defining
the travel range.
The RP mechan-
ics has a series of
microswitches [7]
(Fig. 1) tagging
certain positions in
the RPs’ movement
range and trigger-
ing appropriate
responses. The IN
and OUT electrical
stoppers define the
range of motion
and stop the pot
at its first point of
electrical contact
with their spring
test probes. These
probes have an
elastic travel of
3.18 mm beyond
the first point of
contact. The OUT microswitch beyond the electrical
stopper acts as a redundant motor stopper. The OUT stop-
per also serves as absolute position reference; its on-off
transition point has been surveyed by laser calibration.
The HOME switch is relevant for interlock purposes.
The HOME flag is active in all retracted pot positions
beyond the first contact point with the HOME switch. The
ANTICOLLISION switch stops the motors if a top and a
bottom pot come too close to each other.
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The RP system has two redundant position measure-
ments: 1. The motor encoders, i.e. step counters, are used
for active movement control. Each step of the stepping mo-
tor corresponds to about 5μm. Due to a small position de-
pendence of the step size, step calibration curves are mea-
sured for each RP at least once per year. The encoder mea-
sures the position relative to the OUT electrical stopper; it
has no absolute position memory. In a fast extraction of a
pot with the springs, the mechanical coupling between the
pot and its motor is removed, leading to a loss of the abso-
lute position information. Hence, to ensure a correct cali-
bration, each RP insertion is preceded by an encoder reset
at the OUT stopper. 2. The LVDTs [8] provide an absolute
position measurement but are subject to small drifts and
need to be periodically recalibrated. These measurements
are used for the position interlock and as an independent
cross-check of the encoders.
TOTEM Beam Interlock Logic
Like all LHC experiments and many machine compo-
nents, the TOTEM RP system has an input to the LHC
beam interlock system to avoid beam operation when a RP
is in a position where it would represent a danger.
This logic is implemented in the programmable TOTEM
interlock card that receives beam mode information (SMP
parameters) from the General Machine Timing (GMT), RP
position validity information from the FPGA cards compar-
ing each RP’s LVDT position with its beam-mode depen-
dent critical limits, and simple electrical contact informa-
tion from the RP Home microswitches. The output signals,
INJECTION PERMIT and USER PERMIT are transmit-
ted by hardware links to the beam user interfaces (“CIBF”
and “CIBU”); copies are sent to the Detector Control Sys-
tem. The key functions of the interlock logic [2, 9] are
the following: 1. Withdrawal of INJECTION PERMIT:
Prevention of any beam injection when at least one RP
does not touch its HOME microswitch. 2. Withdrawal of
USER PERMIT: Beam dump and automatic RP extraction
if either (a) at least one RP is away from its HOME switch
in a beam mode where RP operation is not allowed, or (b) at
least one RP is outside the position range defined by beam-
mode dependent critical limits.
An example sequence of an interlock test performed with
the Collimation Application in the CCC is shown in Fig. 2.
ROMAN POT ALIGNMENT
Beam-Based Alignment
The beam-based alignment of the RPs is performed with
a technique established for the collimators [10]: a reference
beam edge is produced by closing the horizontal and verti-
cal primary collimators (TCPs) until they scrape the beam
halo at an aperture nTCPσTCP, where σTCP is the beam
size at the collimator, calculated for nominal optics func-
tions and for a given normalised emittance. This beam edge
is then used as a reference for the RPs: the pot approaches
the circulating beam in steps as small as 10μm. When it
Figure 2: RP movement sequence during the interlock tests
2011: black = motor step counter position, blue = LVDT
position, red = outer and inner dump limits, yellow = outer
and inner warning limits. At 19:26:40 the inner limits are
changed such that the RP position becomes illegal. Con-
sequently the pot is automatically retracted. The LVDT
correctly indicates the new position (∼39.7 mm). The step
counter, however, stays at 37 mm because in the emergency
extraction with the springs the motor coupling is lost. A
reset at the mechanical reference point (OUT stopper) is
needed.
reaches the edge, a beam loss is measured in a Beam Loss
Monitor (BLM) downstream (Fig. 3). In this condition, the
RP and the primary collimator are at the same normalised
distance nTCP from the beam orbit with an accuracy given
by the step size. At the contact position, the RP scrapes
slightly into the beam. To refine the scraped beam edge,
the primary collimator in the appropriate plane is moved
further towards the beam until it becomes again the aper-
ture bottleneck. The average of the two normalised refer-
ence distances nTCP of the primary collimators is used to
compute the normalised distance nRP of the RP.
−10
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5.5 5.25 5.0 4.5
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top RP
bottom RP
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45:00 46:40 48:20 50:00
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BLM at +221 m BLM at +227 m
BLM at +229 m
Figure 3: Beam-based alignment sequence for a vertical RP
pair in 2010. In this early alignment exercise the step size
was large (250μm). Nowadays, 10μm are used for better
precision. The integrated dose in the BLM spikes was also
used to calibrate the BLM response [11].
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Note that this procedure only establishes the distance be-
tween the thin window of the RP and the beam centre. The
alignment of the different silicon detector planes, both rel-
ative to each other and to the beam, is determined by soft-
ware, using real particle data.
Software Alignment
package of 10 detectors
Figure 4: Schematic view
of a RP station with a track
traversing the overlap zone
between top and horizontal
detectors.
A detailed discussion
of the software alignment
methods is given in [12].
Most degrees of freedom
of the relative alignment
between the RP sensors
can be obtained from the
reconstructed tracks. The
underlying idea is that
sensor misalignments give
raise to residuals, i.e. the
distances of the measured
hit positions from the fit-
ted tracks. This technique
is sensitive to shifts (along
the sensor read-out direc-
tion, i.e. perpendicular to
the microstrips) and rotations of individual detector planes
relative to each other, but not to global shifts or rotations.
The transverse overlap between vertical and horizontal de-
tectors (Fig. 4) establishes the alignment between the me-
chanically independent 3 pots of a RP unit. Fig. 5 shows
a comparison of the track-based alignment results for an
example RP with the optical metrology data.
V sensors U sensors
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optical
H8 tests (all)
H8 tests (even)
H8 tests (odd)
LHC 24 Aug
LHC 21 Sep
LHC 07 Oct
LHC 29-30 Oct
fixed planes
56-220-far-hor (DP 2)
Figure 5: Comparison of different alignment result for one
example detector package. “H8” refers to a test beam align-
ment before installation, “LHC” to an alignment with LHC
physics data in 2010, “optical” to the metrology measure-
ment during assembly.
The global misalignment modes (e.g. common shifts or
rotations of entire stations w.r.t. the beam) are inaccessi-
ble to the track-based techniques but can be constrained by
exploiting known symmetries of certain physics processes.
A prominent example is elastic scattering with its clean hit
distribution in the vertical detectors (Fig. 6). The horizon-
tal and vertical symmetries of the distribution lead to an
alignment precision of better than 10μm horizontally and
about 20μm vertically.
Figure 6: Illustration of RP station alignment based on the
symmetry of the elastic scattering hit distribution. Right:
distribution of track intercepts in a scoring plane between
the near and far units of a RP station. Note that the tilt of
the vertical band is mainly caused by optics imperfections.
Left: projection of (right) onto y. The symmetry line of
the vertical distribution (indicating the position of the beam
centre) is found by inverting the sign of the y < 0 part
(excluding the region near the acceptance cut) and shifting
it until it coincides with the y > 0 part.
REFERENCES
[1] TOTEM Collaboration: Technical Design Report, CERN-
LHCC-2004-002, addendum CERN-LHCC-2004-020. G.
Anelli et al.: The TOTEM Experiment at the CERN Large
Hadron Collider, JINST 3 (2008) S08007.
[2] M. Dutour et al.: TOTEM Roman Pots Control System Use
Cases Specification, EDMS 937276r9.0, 2010.
[3] R. Assmann et al.: Application Software for the LHC Colli-
mators and Movable Elements, EDMS 826861, 2007.
[4] FESA: www.cern.ch/project-fesa.
[5] S. Redaelli, A. Masi: Middle-Level Interface to Control
Movable Devices like LHC Collimators, EDMS 934341,
2008.
[6] Pci eXtensions for Instrumentation, www.pxisa.org.
[7] M. Deile et al.: Revision of the Movement Control for
the TOTEM and ALFA Roman Pots, EDMS 1141700v1.3,
2011.
[8] Macro Sensors: LVDT Basics, Techn. Bull. 0103, 2003.
[9] M. Deile et al.: The TOTEM Interlock Logic in 2011: Spec-
ification and Test Results, EDMS 1141699, 2011.
[10] C. Bracco et al., “Collimators and Beam Cleaning:
First Results and Future Plans”, Proceedings from the
LHC beam commissioning workshop, Evian 2010,
https://espace.cern.ch/acc-tec-sector/Evian/
Proceedings.aspx.
[11] R. Appleby et al.: Calibration of the BLM Response at
450 GeV by Beam Scraping with Roman Pots, CERN-
TOTEM-Note-2010-001, and references therein.
[12] J. Kašpar: Elastic Scattering at the LHC, PhD thesis,
Charles University Prague, being submitted.
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The First 1 1/2 Years of TOTEM Roman Pot Operation at LHC

Since the LHC running season 2010, the TOTEM Roman Pots (RPs) are fully operational and serve for collecting elastic and diffractive proton-proton scattering data. Like for other moveable devices approaching the high intensity LHC beams, a reliable and precise control of the RP position is critical to machine protection. After a review of the RP movement control and position interlock system, the crucial task of alignment will be discussed.Since the LHC running season 2010, the TOTEM Roman Pots (RPs) are fully operational and serve for collecting elastic and diffractive proton-proton scattering data. Like for other moveable devices approaching the high intensity LHC beams, a reliable and precise control of the RP position is critical to machine protection. After a review of the RP movement control and position interlock system, the crucial task of alignment will be discussed

Deile, M.

Antchev, G.H.

Appleby, R.B.

Assmann, R.W.

Atanassov, I.

Avati, V.

Baechler, J.

Bruce, R.

Dupont, M.

Eggert, K.

Farnham, B.

Kaspar, J.

Lucas Rodriguez, F.

Morant, J.

Niewiadomski, H.

Pons, X.

Radermacher, E.

Ravat, S.

Ravotti, F.

Redaelli, S.

Ruggiero, G.

Sabba, H.

Sapinski, M.

Snoeys, W.

Valentino, G.

Wollmann, D.

CERN Document Server

Since the LHC running season 2010, the TOTEM Roman Pots (RPs) are fully operational and serve for collecting elastic and diffractive proton-proton scattering data. Like for other moveable devices approaching the high intensity LHC beams, a reliable and precise control of the RP position is critical to machine protection. After a review of the RP movement control and position interlock system, the crucial task of alignment will be discussed. Copyright © 2011 by IPAC'11/EPS-AG

The First 1 1/2 Years of TOTEM Roman Pot Operation at LHC

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