Proceedings of IBIC2012, Tsukuba, JapanTUPB61

THE FIRST EXPERIENCE WITH
LHC BEAM GAS IONIZATION MONITOR
Mariusz Sapinski, William Andreazza, Bernd Dehning, Ana Guerrero,Marcin Patecki, Reine Versteegen, CERN, Geneva, Switzerland
Abstract
The Beam Gas Ionization Monitors (BGI) are used tomeasure beam emittance on LHC. This paper describes thedetectors and their operation and discusses the issues met
during the commissioning. It also discusses the various calibration procedures used to correct for non-uniformity ofMulti-Channel plates and to correct the beam size for effects affecting the electron trajectory after ionization.

The Beam Gas Ionization monitors (BGI), often called
Ionization Profile Monitors (IPM) on LHC are configured
to measure electrons produced in ionization of Neon gas,injected into LHC vacuum chamber. The pressure of injected gas reaches 10−8 mbar. The beam passes between
two ceramic electrodes with difference of potentials of
4 kV, over a distance of 85 mm. This potential brings theelectrons to Multi-Channel Plate (MCP, from Photonis),where the signal is amplified. A phosphor screen is located 2 mm behind the MCP. It is deposited on a right-angle
prism, which is the only optical element inside the vacuum. In order to minimize the transverse spread of the electrons, external magnetic field of 0.2 T, directed along electric field lines, is applied. Light produced by the phosphor
screen is directed through a vacuum window to an opticalsystem and a CID intensified camera (Thermo-Scientific
CID8712D1M-XD4). A schematics of the LHC BGI and apicture of the outside flange are shown in Fig. 1.
The image is amplified in tunnel electronics, which also
allows to control the gate of the camera and gain of theinternal camera intensifier. A cable of about 150 meter length connects the tunnel electronics with a framegrabber, which is a BTV card installed in a VME crate inthe underground gallery. There is no access to the gallery
during machine run.
Server programs are running on the crate CPU, controlling the HV and processing the image. The beam profiles
are fitted with gaussian assuming linear contribution fromthe background.

Figure 1: Up: a schematics of LHC BGI (from [1]). Bottom: a photo of the LHC BGI (flange with optical port) inthe tunnel, with magnet displaced.
done in a clean room, what could affect the durability of
the new equipment. A picture of an MCP in its holder isshown in Figure 2.

MCP USAGE
During the 2011 run the HV on the MCPs was kept on
for most of the time. As a result a local decrease of gain,mainly in the typical beam position, were visible. This nonhomogenity of the MCP response affects the beam profile
reconstruction.
Therefore, it was decided to exchange the MCPs during
the winter technical stop. Due to technical difficulties theoperation has been performed on beam one BGIs only (twoout of four installed on LHC). The exchange has not been

Figure 2: Multi-channel plate in its holder.
The newly installed MCPs had a much higher gain but, atthe same time, were more sensitive to the signals producedby high-intensity beams. One of them got broken during ascrubbing run in March 2012, when a large electron cloudISBN 978-3-95450-119-9
Transverse & Longitudinal Emittance Measurements

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○INTRODUCTIONTUPB61

Proceedings of IBIC2012, Tsukuba, Japansignal probably lead to high current through the MCP. The
second one started to show very unstable HV readings after a crash of HV server which has shut down the voltage
sharply. Therefore the remaining two detectors with oldMCPs are used along the 2012 run.
A calibration mechanism, which allows to compensate
for non-uniformity of the MCP response, has been foreseen[1]. It is based on Electron Generation Plate (EGP), whichemits a uniform distribution of electrons. Observation ofthe image of this emission on the MCP allows to measuregain distribution over surface.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The camera produces artifacts because of the way ittransforms the signal (CCIR format) and because of detector elements. It also picks-up electronic noise. The typicalimage after digitization is shown in Fig. 3.

Figure 3: 2D beam image after digitization. Transverse
strips are artefacts.
Various filters are tested in order to clean the images.
Simple Fourier filter on unfolded, 1D image, seems to provide basic removal of some of the artefacts, but the investigation continues.

CAMERA GAIN CONTROLThe width and amplitude of an ideal gaussian are independent parameters but in a presence of background andnon-uniform gain of the MCP the control of the amplitude is necessary to obtain correct measurement of σ. In
case of LHC BGI the intensifiers build-in into the cameras
provides the best way to control the signal amplitude withlarge precision and without regulating the sensitive HV.
The choice of the optimal amplitude range is a compromise
between a possible gas injection pressure, HV settings andcamera dynamic range.
An example of the BGI measirement evolution during aramp of beam energy is shown in Fig. 4.

CALIBRATION WITH ORBITAL BUMPThe scale calibration of the BGI can be performed using
orbital bump method. The orbital bump amplitude is regulated with large precision using Beam Position Monitors

Figure 4: Beam size behaviour during ramp of Pb beam.
Beam energy is given in ZTeV.
(BPM). A relation between BGI position and BPM position
allows to obtain pixel scaling factor.
The advantage of this method is that it takes into accountall possible scaling effects (optical system magnification,camera pixel size, etc.). The disadvantage can be the accuracy of the orbit determination: BGI is located in space inwhich the BPMs are about 60 meters away. This space contains magnetic elements, therefore the beam position interpolation has limited accuracy. During Long Shutdown 1(LS1) additional BPMs will be installed in vicinity of BGI.
In Fig.5 determination of the scaling factor is shown.
The pixel size is found to be 0.095 ± 0.003 mm. The measurement have been performed during Machine Development (MD) period in June 2012, using high intensity andlow emittance proton beam at 4 TeV.
BPM position [mm]

IMAGE PROCESSING

1

BGI B2V bump calibration

0.5
0-0.5

χ2 / ndf-1115

1.93 / 3

p0

12.04 ± 0.3819

p1-0.09504 ± 0.003008

120

125

130
135
BGI position [pixels]

Figure 5: Calibration of the BGI scale using orbital bump.

INTERCALIBRATION WITH WIRESCANNER
Wire Scanners (WS) provide a reference emittancemeasurements to other instruments on LHC. Because offragility of the carbon fibers from one side and low sensitivity of BGI from the other side the typical proton beam
does not allow to intercalibrate BGI with the WS. A unique
opportunity is presented by lead ion beam. On September 12, 2012 a beam of low intensity (15 bunches, about
1010 charges in total) has been in the machine. This beamISBN 978-3-95450-119-92Transverse & Longitudinal Emittance Measurements

Proceedings of IBIC2012, Tsukuba, Japan

1450 GeV

wire scanner

0.8

BGI

0.6
0.4

a.u.

0.2
0-0.4

-0.3

14 TeV-0.2

-0.1

00.1

• smearing due to dispersion of the electrons producedin MCP (about 32 μm);
• optical point spread function PSF (22 μm [3]);• cross-talk between pixels in the camera.
Two of the effects have been already estimated and they
are too small to explain the observed effect. Both of them
have the nature of PSF, and can be corrected in quadraturein order to obtain real beam width:2− σP2 SF(1)
σbeam = σBGIIn the following it is assumed that the other effects canbe corrected in the same way. In Fig. 7 the preliminary
results of cross-calibration between wire scanner and BGIare presented. The WS emittance during the ramp (calculated using relativistic gamma for protons) are represented with green dots. Red curve shows beam energy evolution and black line is BGI emittance obtained assumed
σPSF = 0.3 mm. Relatively low quality of the BGI signal
can be observed, as beam intensity was small.

0.2
0.3
0.4
position/ β [mm/ m]wire scanner

0.8

BGI

0.6
0.4
0.2
0-0.4

-0.3-0.2-0.1

00.1

0.2
0.3
0.4
position/ β [mm/ m]

Figure 6: Comparison of beam profiles in wire scanner andin BGI at injection and at 4 TeV.
At injection the BGI profile corresponds quite well to theWire Scanner one. On the other hand at 4 TeV, when beam
becomes quite small, the BGI profile is significantly larger.
Table 1: Optical β Functions for WS and BGI [2]
B2V [m]WS
BGI
injectionflat top

418.95451.04217.19225.35CORRECTION IN QUADRATURE
The broadening of the profile at flat top with respect tothe wire scanner one can have many reasons. The following
ones are investigated:• distortion of electron trajectories due to beam spacecharge;• contribution from electron-emitting elements;• smearing of electron position due to gyroradius;

Figure 7: Emittance behaviour during ramp.

CONCLUSIONS
Initial results of the BGI commissioning on LHC beams
are presented. Main aspects concerning the signal processing, scale calibration and correction of the MCP ageing arediscussed. A necessary quadratic correction to the beam
size measured by the BGI is shown. Preliminary results forion beam are promissing.

ACKNOWLEDGMENT
Authors want to thank to Michaela Schaumann for help
with analysis of ion-beam data and to Vincent Baglin, Francois Bellorini and Didier Calegari who build and serviced
the gas injection system.

REFERENCES[1] H. Refsum, ” Design, Simulation and Testing of a 2D Electron Source Based Calibrating System for a Proton Beam Ionization Profile Monitor”, CERN thesis 2004.
[2] http://beta-beating.web.cern.ch[3] D. Kramer et al., CERN-AB-2005-072

ISBN 978-3-95450-119-9
Transverse & Longitudinal Emittance Measurements

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○a.u.

have low intensity and a maximum gas injection can be
performed. At the same time the use of the Wire Scanners
was still allowed (no danger to the wire neither to magnetquench). All together the ion beams allowed to measure
the emittance with WS to compare it with BGI.
The BGI and WS are located in different locations
around the ring. In order to compare profiles obtained byboth instruments a scaling of the profiles with optical βfunction must be done. In Fig. 6 a comparison of beamprofiles is presented at injection and at flat top. The β functions used are shown in Table 1.

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