A Study of the Gain of Microchannel Plates in the
Ionization Profile Monitors at Fermilab*

FERMILAB-POSTER-23-296-AD

R. Thurman-Keup#, D. Slimmer, C. Lundberg, J.R. Zagel,Fermi National Accelerator Laboratory, Batavia, USA

WEP025
Abstract
One of the on-going issues with the use of microchannel plates(MCP) in the ionization profile monitors (IPM) at Fermilab is thesignificant 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 poresurface changes 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.

Linear Stages

Experimental Devices

MCP
Schematic of MCP functional behavior showing amplification byelectron avalanche.
Incident
Electron

Anode
(+HV)

Electric Field(aka Clearing Field)

SecondaryElectrons

Electrons
Field-Shaping
Electrodes

Ionization
Happens

Beam(into page)Ions

RF Shield Grid

Introduction
Ionization profile monitors (IPM) are used in many accelerator
laboratories around the world [1-7]. They have been used in nearly
all the synchrotrons built at Fermilab, and presently are used in theMain 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 found that the gain of
the MCP decreases over time and have attributed it to the wellknown fact that they age with current extracted from them [9].
Thus, we have periodically replaced them.
Recently, more in-depth investigations have revealed that thedecrease in the gain is much more consistent with conditioning, or‘scrubbing’, of the MCPs, and not aging. Literature searches haverediscovered the fact that a decrease in gain with conditioning is aknown property of 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.
Scan of gain over the MCP showing the drop in
gain where the beam is normally positioned

Outgoing Electron Shower

Microchannel
Plate (MCP)
Anode Strips
1.5 mm Spacing

BoosterMagnet with
Vertical B Field
Cathode (–HV)

Electric Field

MCPs

SecondaryEmission Electron
Suppression Grid

Ions

Field-Shaping
Electrodes
Wire Mesh Gate
RF Shield Grid
Two Microchannel
Plates (MCP)
Anode Strips
500 mm Spacing

Beam(into page)

Ionization
Happens
Electrons

Magnetic
Field

MI / RR

Gain Depression

Main Injector StudyHistorical Booster Data
Booster IPM signal from injection (top) to extraction (bottom).

The MCP was moved to a new
region of the plate, and then run
repeatedly for a period of ~5 days.
MI IPM signal from injection (top) to
extraction (bottom).

Previous MCP Literature
Measured gain of an MCP (taken from [11]) at various stages in the preparation: initial,
after vacuum bake, after scrub, and after final seal-in. The gain decrease from after
vacuum bake to after scrub is a factor of 3. Scrubbing refers to the initial phase ofoperation of the MCP, where the electrons act to ionize adsorbed gases and remove
them from the surfaces of the pores.
MCPs can be conditioned byillumination with ultraviolet
radiation until the gainstabilizes [10]. Whether or not
the MCP can be exposed toatmosphere after conditioning
without losing the benefits ofthe conditioning is not yet
entirely clear to us [15,16].

Reproduced from Ref. [11]The raw IPM integrated
signals and the beam
intensities. During the 5days, the voltage wasperiodically adjusted to keep
the signals at similar levels.
This is indicated by thevertical magenta lines. The
processing corrected for the
beam intensity andaccounted for the changes involtage by scaling the data ineach voltage region to matchat the boundaries.

Gain vs. MCP voltage.

The lifetime of a MCP isgenerally > 1 C/cm2.
Conditioning of the MCP
happens much faster thanthat, typically less than
0.1 C/cm2 [9,19,21].

Reproduced from Ref. [17]Pre-flight flat field
The use of MCPs in satellites encounterssimilar gain issues as we do with the IPMs
[17-19]. When used in a spectrometer,
spectral lines are placed at defined
positions on the MCP. As it is apparently
impractical [16] to keep a satellite MCP
under vacuum, the initial operation of it inspace causes the brighter lines to scrub
faster resulting in a non-uniform gain, justas the beam in an accelerator is always
located at the same location and produces
a dip in the gain.

SUMER SatelliteLine structure fromscrubbing is visible

The distribution of Booster IPM acquisitions as a function of time. We
calculated the MCP gain from this data by scaling it by the beam intensity andby the voltage gain curve above. Red arrows indicate maintenance periods.

2018

2019

2020

2021

2022

Processing of the data corrected for the beam intensity and accounted for the changes in voltage byscaling the data in each voltage region to match at the boundaries. shows this gain as a function of
acquisition number, time, and integrated charge out of the MCP. Here as well, the change in gain isconsistent with scrubbing, not with aging. One additional thing to note is that since the MI/RR IPMs havea pair of MCPs, the scrubbing is mostly affecting the second MCP which has much higher current draw.

In-flight flat field

Gain relatively flat over 4 years

Historical MCP gain
resulting from the scaling
by beam intensities and thevoltage gain curve. It would
appear that the MCP gain
decreased after initialinstallation but has been
relatively flat after that.
This is a behavior consistent
with initial conditioning,
and not aging.

MCP gain vs. beam
intensity after scaling forbeam intensity. It is notflat and there are various
isolated regions (someindicated by green
arrows) that need to beunderstood.

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

Since the MCP should age with extracted charge, we calculate the extracted charge foran 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%). Shown here is the integral of the extracted charge vs. time which is a little
over 4 mC/cm2, not yet near the level where aging would be a concern.

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