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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 2 7

STATUS OF GAS SHEET MONITOR FOR
PROFILE MEASUREMENTS AT FRIB*A. Lokey†, S. Lidia
Facility for Rare Isotope Beams, Michigan State University, East Lansing, MI, USAAbstract
We report on the status of work on a non-invasive profile
monitor under development for use at the Facility for Rare
Isotope Beams (FRIB), a heavy-ion LINAC which produces high-intensity, multi-charge state beams. The measurement will be made by collecting photons generated at
the interaction point of the beam and a collimated molecular gas curtain. These photons will be collected with an intensified camera system, generating a two-dimensional image and allowing for measurements of profile, beam halo,
and other properties more prevalent at specific locations ofinterest, such as charge state spread after folding segment
bends. Included will be ongoing design specifications, simulation results, and discussion of measurement techniques
for acquiring signal from the device.

SYSTEM OVERVIEW AND REQUIREMENTS
The beamline device is divided into three sections:(i) thegas generating chamber; (ii) the main interaction chamber;and (iii) the optics system. Within the gas generating section, nitrogen gas is pumped into a reservoir which feeds
into a thin rectangular cavity. Molecules of nitrogen areemitted from a slit at the front of the cavity and are further
collimated into sheet by a slit placed between the generating and main chambers as shown in Figure 1.

INTRODUCTION
A non-invasive profile measurement device using nitrogen gas fluorescence is in development for use in the Facility for Rare Isotope Beams (FRIB), a high-intensity
heavy ion facility which can provide beams of a variety ofdifferent ion species and charge states.
By using a non-interceptive method, it is possible tosample the state of the beam during operation, providing
information about the spread of the charge states at pointsof interest, such as after bending magnets in the folding
LINAC sections of FRIB. This method also allows the stateof the beam to be measured in both transverse directions atonce [1].
One way to accomplish this is by creating an ultra-thin,rarefied gas curtain and collecting the photons released in
the interaction between the beam and the working gas. Thisis known as beam-induced fluorescence (BIF) and has theadvantage of being a simpler system design than similarionization-based monitors, which require an electrode tosweep the electrons or ions into the detector and a magnet
to steer them to the correct location to be collected. Additionally, BIF monitors have greater time resolution, since
the time for the interaction particles to travel to the detector
is negligible [2]. By introducing only a small amount of gas
to the system, both pumping requirements and interaction
effects like scattering and charge stripping are reduced [3].
Assuming the gas sheet is uniform and thin, photons collected can then be correlated to the profile of the beam withsome degree of accuracy. This technique has been usedsuccessfully with protons and lighter ions at other facilities[1,3-5].
___________________________________________* This material is based upon work supported by the U.S Department ofEnergy, Office of Science, Office of Nuclear Physics and used resources ofthe Facility for Rare Isotope Beams (FRIB), which is a DOE Office of Science User Facility, under Award Number DE-SC0000661† lokey@frib.msu.edu

Figure 1: Overview of device layout.

Considerations
Although rarefied, gas generated from the monitor will
require significant pumping to prevent contamination andprotect the rest of the high vacuum system and this must beaccounted for in appropriately sizing of the pumps.
For the measurement to correlate with the beam profile,the sheet must be thin and uniform, positioned at a 45-degree angle to the incident beam with the detector perpendicular to the point of interaction. Careful alignment mustbe performed to ensure that the angle of interaction is correct [6].
Signals generated by the interaction will be of relatively
low intensity. The fluorescence interaction generates asmall signal compared to ionization monitors due to fluorescence cross sections being lower than ionization crosssections. To assist with collection and reduce scatteringfrom reflection, the interior of the main chamber will beblackened and an intensified camera array will be used toamplify the incoming signal. Light is gathered by the camera system at a small solid angle, and relatively few photons, are released in the interaction. For example, using a
residual-type BIF, Tsang et al. noted a production of 7 photons per 3×108 ion Uranium bunch with hydrogen as theworking gas [7]. Photon production is expected to behigher with use of a collimated sheet, as well as nitrogenas the working gas since it has a greater fluorescence cross

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03 Transverse Profile and Emittance Monitors

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section. However, additional amplification and noise reduction techniques will still be needed the optics system[1,2].
The ratio of the mean free path of the nitrogen molecules
is significantly larger than the length scale of the generator,
which puts the gas in the desired free molecular flow regime. This is represented by the Knudsen number, characterized by the mean free path of the molecule, λ, over thecharacteristic length of the system, l, see Eq. (1). The system is in molecular flow at a Knudsen ratio greater than 10.
𝐾(1)

The mean free path of the molecules in the generator slit
is approximately 704 mm, making the Knudsen number
7040. To achieve this, the generating slit will need to bemachined to a depth of 0.1 mm, which could pose challenging due to tolerances at that scale.

gas sheet generating slit in place, for alignment with thecollimator and interaction point.
Three turbomolecular pumps will be used to prevent thebuildup of gas in the system, one placed in the gas cover
before the collimating slit to remove excess nitrogen emitted from the generator at undesirable angles, one in themain chamber of the device to maintain high vacuum, andone at the exit of the main chamber to collect the gas sheet.

SIMULATION
The device was initially modeled using simplified geometry as reported in [8], using MolFlow+, a Monte Carlo gas
dynamics simulation software [9].

DESIGN OF MECHANICAL SYSTEM
To facilitate manufacturing, the gas sheet generating slit
will be formed from a half-cylindrical slug machined on
one side to the required depth and capped with a flat top
connected to the reservoir and gas injection. The generating slit will be secured to the gas cover and attached to arotatable CF flange to allow for alignment of the cover slit
with the collimator and interaction point. A SOLIDWORKS model is shown in Figure 2.
The gas sheet generator has dimensions of length
100 mm by width 100 mm with a depth of 0.1 mm. The slit
widens to 110 mm at its exit to widen the area of the gas
curtain, shown in Figure 2.

Figure 2: SOLIDWORKS model of sheet generator prepared for MolFlow+ simulation.
The collimator is to be located 10 mm from the exit of
the gas sheet generator and sized slightly larger at 0.3 mm
and 160 mm. The width of the collimating slit is determined such that approximately 10% of the gas molecules
exiting the gas sheet generator enter the main chamber, ormolecules that deviate from a path about 0.4 radians fromthe x-z plane.
For both the main chamber and gas cover sheet, off-theshelf components will be used. The cover chamber will be
manufactured using a vacuum nipple modified to hold theFigure 3: Molflow+ simulation set up.
Simulations were updated to include more complex geometry, as well as consideration for distance of the gas
sheet generator opening to the collimating slit. Figure 3
shows the prepared volume of the model in MolFlow+,while Figure 4 shows a close-up of the pressure distribution
at the gas sheet exit point of the model.
The gas is introduced on one of the reservoir chamber
faces with a cosine desorption and outgassing rate of
0.001 mbar-liter/s.

Figure 4: Width of the sheet at removal as seen along thex-y plane.
The interaction point is placed at the center point of themain chamber, and the gas sheet generator is positioned
without the angle to facilitate measurement of the sheetwidth.
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411

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, CanadaJACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 2 7The width of the sheet at the center point of the mainchamber was found to be 0.14 mm (see Figure 4).

MEASUREMENT
The proposed design for signal acquisition involves positioning an intensified camera perpendicular to the interaction point. For bench testing to determine uniformity ofsheet thickness and density, a hot-cathode gauge on a movable platform will be positioned in place of the camera system.
Photons will be produced in the range of 390 to
470 nm [1]. After validation of the gas sheet, the hot-cathode gauge will be replaced with an intensified camera withsensitivity in this range. Future work will include design ofthe optics platform and system specifications.

Use in Beam Halo Measurements
Additional functionality can be added to this device toallow for monitoring beam halo. Since profile image generated is two-dimensional, a coronagraph modification tothe optics could be made to block the core of the beam
when halo measurements are desired. This technique isadapted from optical coronograph devices used to view thehalo of the sun and would be done by adding a movableocculting disk, an additional field lens and Lyot stop to mitigate diffraction effects caused by masking the core. This
technique has been previous deployed in beam halo monitors with good results [10].

CONCLUSIONWork continues on the design of a molecular dynamicsbased gas-sheet monitor to be used with high-intensity
heavy ions beams generated at FRIB. This paper presented
an update on the design work performed. Future work will
include further specifications on the optics system andbench testing of the initial mechanical design.

ACKOWLEDGEMENTS

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[8] A. Lokey and S. M. Lidia, “Design Study for Non-Intercepting Gas-Sheet Profile Monitor at FRIB”, in Proc. NAPAC'22, Albuquerque, NM, USA, Aug. 2022, pp. 229-231.
doi:10.18429/JACoW-NAPAC2022-MOPA80[9] Molflow+, https://molflow.web.cern.ch/[10] A. Goldblatt, E. Bravin, T.M. Mitsuhashi, F. Roncarolo, andG. Trad, “Design and Performance of Coronagraph for
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Special thanks to S. Rodriguez, B. Ewert, and S.
Szustkowski for assistance and guidance with this work.

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