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A STUDY OF THE GAIN OF MICROCHANNEL PLATESIN THE IONIZATION PROFILE MONITORS AT FERMILAB*
R. Thurman-Keup†, D. Slimmer, C. Lundberg, J.R. Zagel
Fermi National Accelerator Laboratory, Batavia, USA
Abstract
One of the on-going issues with the use of microchannel
plates (MCP) in the ionization profile monitors (IPM) at
Fermilab is the significant decrease in gain over time.
There are several possible issues that can cause this. Historically, the assumption has been that this is aging, where
the secondary emission yield (SEY) of the pore surfacechanges after some amount of extracted charge. Recent literature searches have brought to light the possibility that
this is an initial ‘scrubbing’ effect whereby adsorbed gasses
are removed from the MCP pores by the removal of charge
from the MCP. This paper discusses the results of studies
conducted on the IPMs in the Main Injector at Fermilab.

INTRODUCTION
Ionization profile monitors (IPM) are used in many accelerator laboratories around the world [1-7]. They havebeen used in nearly all the synchrotrons built at Fermilab,
and presently are used in the Main Injector (MI), Recycler
Ring (RR), and Booster synchrotron [8], with another one
being planned for the Integral Optics Test Accelerator(IOTA). All Fermilab IPMs, as well as many of those atother laboratories, utilize one or more microchannel plates(MCP) for signal amplification. Historically we have foundthat the gain of the MCP decreases over time (Fig. 1) andhave attributed it to the well-known fact that they age withcurrent extracted from them [9]. Thus, we have periodically replaced them.
Gain Depression

MCPs [10,11]. Our own historical IPM data and a recent
dedicated test show results which are much more consistent with what one expects from conditioning. In this paper, we summarize our current understanding of the behavior of the MCPs in our IPMs.

EXPERIMENTAL DEVICE
IPMs are devices which utilize the ionization of residual
gas to measure the transverse profile of the beam. Figure 2
is a diagram of the present Fermilab IPMs. When the beam
passes through the IPM, it ionizes the residual gas. The
IPM collects the ionization products using an electrostatic
field to accelerate them to the MCP.
Anode
(+HV)

Electric Field(aka Clearing Field)

Electrons
Field-Shaping
Electrodes

Ionization
Happens

Beam(into page)Ions

RF Shield Grid
Microchannel
Plate (MCP)
Anode Strips
1.5 mm Spacing

BoosterMagnet with
Vertical B Field
Cathode (–HV)
SecondaryEmission Electron
Suppression Grid
Field-Shaping
Electrodes
Wire Mesh Gate
RF Shield Grid
Two Microchannel
Plates (MCP)
Anode Strips
500 m Spacing

Electric Field

Ions
Beam
(into page)

Ionization
Happens
Electrons

Magnetic
Field

MI / RR

Figure 2: Diagrams of Booster and MI/RR IPMs.
The MCP is a thin plate with microscopic holes (akapores) through it, each of which acts as a charge multiplier,much like a photomultiplier tube (PMT) (Fig. 3) [12,13].
Figure 1: Scan of MCP plate across beam showing gain
depression where beam is normally positioned.
Recently, more in-depth investigations have revealed
that the decrease in the gain is much more consistent withconditioning, or ‘scrubbing’, of the MCPs, and not aging.
Literature searches have rediscovered the fact that a decrease in gain with conditioning is a known property ofIncident
Electron

SecondaryElectrons

Outgoing Electron Shower___________________________________________* This manuscript has been authored by Fermi Research Alliance, LLC
under Contract No. DE-AC02-07CH11359 with the U.S. Department ofEnergy, Office of Science, Office of High Energy Physics.
† keup@fnal.gov

Figure 3: Schematic of MCP functional behavior showing
amplification by electron avalanche.
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The output electrons from the MCP are collected via thin
conductive strips which function as the anode in the multiplication chain. This current is converted to a voltage by apreamp and digitized. The digitized signals from the IPM
are processed by a LabVIEW program that generates
gaussian profile fits to the data. The IPMs collect data on
request. The high voltage is turned on and data is collected
for a single injection-extraction cycle which is usually lessthan one second.
There are two IPM versions at Fermilab (as shown in
Fig. 2): magnetic and electrostatic. The magnetic version,used in MI and RR, collects electrons, and has both an electric field to accelerate the electrons and a magnetic field toconstrain their trajectories. Without the magnetic field, theelectrons would be moved transversely by the beam spacecharge and not maintain the position where the ionization
occurred. These IPMs are also installed on motorizedstages such that the MCP can be moved relative to thebeam. The electrostatic version, used in Booster, collects
ions, and has no magnetic field since the ions are heavy andnot dramatically affected by the beam space charge. These
are bolted to the beampipe and cannot be moved relative tothe beam.

PRIOR MCP LITERATURE
MCPs have been used since at least the early 1970’s forimage intensification [14]. A mathematical model of MCPs
was developed in reference [11]. In addition to the model,
the paper also showed the measured gain of an MCP at various stages in the preparation: initial, after vacuum bake,after scrub, and after final seal-in. The gain decrease fromafter vacuum bake to after scrub was a factor of 3. Scrubbing refers to the initial phase of operation of the MCP,
where the electrons act to ionize adsorbed gases and remove them from the surfaces of the pores. Since thesegases are additional contributors of secondary emission
electrons, the gain is higher with them than without them.
One approach to this conditioning is to illuminate the MCP
with ultraviolet radiation until the gain stabilizes [10].
Whether or not the MCP can be exposed to atmosphere after conditioning without losing the benefits of the conditioning is not yet entirely clear to us [15,16]. Ideally the
scrubbing would happen after installation of the MCP in
vacuum. The use of MCPs in satellites encounters similar
gain issues as we do with the IPMs [17-19]. When used ina spectrometer, spectral lines are placed at defined positions on the MCP. As it is apparently impractical [16] tokeep a satellite MCP under vacuum, the initial operation of
it in space causes the brighter lines to scrub faster resulting
in a non-uniform gain, just as the beam in an accelerator isalways located at the same location and produces a dip in
the gain. Reference [20] discusses gain issues impacting
MCP-based PMTs used in the PANDA experiment at
FAIR. They also consider the new atomic layer deposition
technique for MCPs. The lifetime of a MCP is generally
stated in units of C/cm2 of extracted charge and is usually
understood to be > 1 C/cm2. Conditioning of the MCP requires much less than that, typically less than
0.1 C/cm2 [9,19,21].

IPM DATAThe present versions of the IPMs were installed in the
MI and RR in 2014, and the Booster in 2017. All the datacollected since then is available for analysis and is what isused to assess the behavior of the MCPs.

Historical Booster IPM Data
The MCPs in the Booster IPMs were not changed until
2022, providing 5 years of MCP gain data. Two things simplify the analysis of Booster IPM data: each IPM has only
one MCP, and the Booster operation is always the same.
Figure 4 shows a typical IPM signal for a Booster cycle.

Figure 4: Booster cycle from injection (top) to extraction(bottom).
The goal of the analysis is to measure the relative gainof the MCP over time, ideally as a function of extracted
charge. To determine the gain, one must scale the signal tothe beam intensity, and adjust for the voltage setting of the
MCP. The beam intensity is recorded for each acquisition.
In addition, there were occasional measurements made at aseries of voltage settings, which are used to determine thegain vs. voltage curve (Figs. 5 and 6).

Figure 5: Collection of 32 acquisitions with varied MCP
voltage. From this we extract a voltage gain curve.
10 3
Data
Parabolic Fit

10 2

10 1
400

500

600

700

800

MCP Voltage (V)Figure 6: MCP voltage vs. gain curve with parabolic fit.
This fit is used to normalize the IPM data to extract the
MCP gain.

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Figure 7 shows the distribution of IPM acquisitions as afunction of time. We calculated the MCP gain from thisdata by scaling it by the beam intensity and by the voltage
gain of Fig. 6. The resulting MCP gain is shown in Fig. 8
as a function of time.
2018

2019

2020

2021

2022

Since the MCP should age with extracted charge, we calculate the extracted charge for an acquisition as 𝑄 = 𝐼𝑇𝐷,
where 𝐼 is the average current from the MCP during theacquisition, 𝑇 is the time the high voltage is on (~3 seconds), and 𝐷 is the Booster duty cycle (~50%). Figure 10
shows the integral of the extracted charge vs. time. At present, the total extracted charge is a little over 4 mC/cm2,
which is not yet near the level where aging would be a concern.

Figure 7: The red arrows indicate maintenance periods
which are usually July – October.

Gain relatively flat over 4 years

Figure 10: Integral of the extracted charge as a function oftime.

MI Dedicated Experiment

Figure 8: Relative MCP gain as a function of time. The
gain has been relatively flat for the last 4 years.
From this plot, it would appear that the MCP gain decreased after initial installation but has been relatively flat
after that. This is a behavior consistent with initial conditioning, and not aging. One thing that the reader might notice is the slight decrease over the last 600 days or so. It
was discovered late in the writing of this paper that scaling
the IPM signal by the measured beam intensity is not entirely accurate. As one can see in the Fig. 9, there is a clear
non-linear relationship even after scaling, and, in addition,
there are isolated regions, all of which must still be investigated. The non-linear relationship is in part responsible
for the decrease in the last 200 days where the beam intensity was much less for most of the acquisitions. Despite thisbehavior, we don’t see a dramatic decrease in the apparent
gain of the MCP over recent time.

Figure 9: Relationship between MCP response and beam
intensity after correcting the MCP response for the beam
intensity and voltage.

Recently, one of the MI IPMs was run continuously tomeasure the behavior of the gain. Since these IPMs can be
moved, the MCP was moved to a new region of the plate,
and then run repeatedly for a period of ~5 days. Figure 11
shows the typical MI IPM signals where one can see that
the MCP has been positioned to the side of the beam.

Figure 11: MI cycle from injection (top) to extraction (bottom). Some channels have obvious problems (blue).
Figure 12 shows the raw IPM integrated signals and thebeam intensities. During the 5 days, the voltage was periodically adjusted to keep the signals at similar levels. The
processing corrected for the beam intensity and accounted
for the changes in voltage by scaling the data in each voltage region to match at the boundaries. Figure 13 shows thisgain as a function of acquisition number, time, and integrated charge out of the MCP. Here as well, the change ingain is consistent with scrubbing, not with aging. One additional thing to note is that since the MI/RR IPMs have apair of MCPs, the scrubbing is mostly affecting the second
MCP which has much higher current draw. As such the
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gain will continue to decrease with time albeit much more
slowly.

charge and more uniformly cover the MCP. This may offer
a way to initially scrub the MCP.

REFERENCES[1] K. Satou, “Development of a gated IPM system for J-PARC
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(IBIC’19), Malmö, Sweden, Sep. 2019, pp. 343-346.
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beam gas ionisation profile monitors for the CERN Proton
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Figure 12: MI IPM signal (blue) and beam intensity (red).
The magenta vertical lines indicate when the MCP voltage
was changed.

doi:10.18429/JACoW-IBIC2021-TUOA05[3] A. Jansson et al., “IPM measurements in the Tevatron”, in
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Figure 13: Integral of the extracted charge as functions ofacquisition number, time, and extracted charge from the
MCP.

CONCLUSIONSBoth previous literature and the studies done with theIPMs indicate that aging is not the cause of the decrease ingain seen in our IPMs. The implication is that the MCPs do
not need to be replaced as often as they have been in thepast. Instead, our attention now focuses on the problem ofin situ conditioning of the MCP or periodic calibration of
the gain of the MCP.
As stated earlier, the MI and RR MCP devices aremounted on motorized stages and therefore can be moved.
They can be slowly moved across the beam to allow thescrubbing to uniformly condition the MCP and avoid uneven gains. The same technique could also be used to calibrate the gain as a function of position, which may be amore useful approach since the gain will continue to decrease with use.
In the Booster, there is no way to easily change the relative position of the beam on the MCP other than to movethe beam itself. However, if one lowers the electrostatic
field, the ions will spread out transversely due to space[8] J.R. Zagel et al., “Third generation residual gas ionization
profile monitors at Fermilab”, in Proc. 3rd International
Beam Instrumentation Conference (IBIC’14), Monterey,
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