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PROGRESS ON AN ELECTRON BEAM PROFILE MONITOR
AT THE FERMILAB MAIN INJECTOR*
R. Thurman-Keup†, T. Folan, M. Mwaniki, S. Sas-Pawlik
Fermi National Accelerator Laboratory, Batavia, IL, USA
Abstract
The current program at Fermilab involves the construction of a new superconducting linear accelerator (LINAC)
to replace the existing warm version. The new LINAC, together with other planned improvements, is in support ofproton beam intensities in the Main Injector (MI) that will
exceed 2 MW. Measuring the transverse profiles of thesehigh intensity beams in a ring requires non-invasive techniques. The MI uses ionization profile monitors as its onlyprofile system. An alternative technique involves measuring the deflection of a probe beam of electrons with a trajectory perpendicular to the proton beam. This type of device was installed in MI and initial studies of it have beenpreviously presented. This paper will present the status andrecent studies of the device utilizing different techniques.

INTRODUCTION𝐹⃗ 𝑟⃗ ∝𝑑 𝑟⃗′𝜌 𝑟⃗′𝑟⃗ − 𝑟⃗′|𝑟⃗ − 𝑟⃗′|(1)

and the change in momentum is∆𝑝⃗ =𝑑𝑡 𝐹⃗ 𝑟⃗ 𝑡

(2).

For small deflections, 𝑟⃗ ≈ 𝑏, 𝑣𝑡 , and the change in momentum is∆𝑝⃗ ∝𝑑𝑥∙𝑑𝑦 𝜌 𝑥 , 𝑦𝑑𝑡

𝑏 − 𝑥 , 𝑣𝑡 − 𝑦𝑏 − 𝑥 + 𝑣𝑡 − 𝑦(3),where {} indicates a vector. For small deflections,|∆ ⃗|𝑝⃗ ≈ 0, 𝑝 and the deflection is 𝜃 ≈ | ⃗| . The integral overMeasuring the transverse profile of circulating beam ina proton synchrotron is a challenging endeavor. Historically, thin wires were rapidly swept through the beam anddownstream losses recorded as a function of the position of
the wire to construct the profile. These were called flying
wires [1]. In the Main Injector (MI) at Fermilab, they wereremoved around 2012 due to wire breakage which was notunderstood. Since it was already known that wires in thecirculating beam might pose a problem at the even higher
planned intensities, work had begun on building an electron beam profiler (EBP) to complement the ionization
profile monitors that were already installed.
The concept of a probe beam of charged particles to determine a charge distribution has been around since at least
the early 1970’s [2-4]. Several conceptual and experimental devices have been associated with accelerators
around the world [5-9]. An operational device is presently
in the accumulator ring at the Spallation Neutron Source at
Oak Ridge National Lab [10]. The EBP was constructed
and installed in the Main Injector (MI) in 2014 and initial
results have been presented previously [11]. This paper
presents an update on the status of the EBP.

time can be written as sgn(𝑏 – 𝑥′) leading to an equation
for the deflection

THEORY

Figure 1: Probe beam deflection (red) for some impact parameter b.

The principle behind the EBP is electromagnetic deflection of the probe beam by the target beam under study(Fig. 1). If one assumes a target beam with 𝛾 ≫ 1, no magnetic field, and 𝜌 ≠ 𝑓 𝑧 , then the force on a probe particle
is [12]___________________________________________* 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

𝜃(𝑏) ∝𝑑𝑥𝑑𝑦 𝜌(𝑥 , 𝑦 ) sgn(𝑏 − 𝑥 ) ,(4)where sgn(𝑥) = −1 for 𝑥 < 0 and + 1 for 𝑥 ≥ 0.
If one takes the derivative of 𝜃(𝑏) with respect to 𝑏, thesgn function becomes 𝛿(𝑏 − 𝑥′) leading to𝑑𝜃(𝑏)∝𝑑𝑏𝑑𝑦 𝜌(𝑏, 𝑦 ) ,(5)which is the profile of the charge distribution of the beam.
Thus, for a Gaussian beam, this would be a gaussian distribution and the original deflection angle would be the error
function, erf(𝑏).
xTargetBeamb(b)yEXPERIMENTAL TECHNIQUE
To obtain 𝜃(𝑏), one needs to measure the deflection fora range of impact parameters. This can be accomplished bystepping the electron beam through the protons and recording the deflection at each step, or by rapidly sweeping theelectron beam through the proton beam (Fig. 2). The latterworks without issue provided the sweep time is muchsmaller than the rms bunch length of the proton beam toWEP023

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avoid coupling the longitudinal and transverse distributions. In the MI, this condition is not met since the bunch
length is only 1-2 ns.
b)a)

Problems such as these must be overcome through, forexample, timing shifts or interleaving across multiple camera frames. Before attempting this approach, we have performed a slow diagonal sweep as in Fig. 2 but covering
many bunches. The deflections are not as clear, but it issimpler to acquire them.

EXPERIMENTAL DEVICEFigure 2: a) Long bunch relative to streak time. b) Shortbunch relative to streak time (e.g. MI) leading to bunchlength distortion of deflections.
An alternative method that will be attempted for the MI
is a type of raster scan. The beam is swept rapidly along
the direction of the proton beam producing an approximate
longitudinal distribution (Fig. 3), and slowly transverse tothe proton beam. This technique has the potential to allow
longitudinal slicing of the transverse profile assuming thelongitudinal distribution either remains constant over theseries of impact parameter measurements or can be corrected for synchrotron motion.
Electron Sweep

Proton Beam

Electron deflection
replicates longitudinalbunch structure

Figure 3: The electron beam is swept along the direction of
the proton beam with a sweep time comparable to theproton bunch length. This records the deflection as afunction of longitudinal position. A series of these sweeps
is collected at different impact parameters to obtain 𝜃(𝑏).
Simulations of the deflections in this approach are
shown in Fig. 4. Here successive sweeps along the proton
direction at different impact parameters are displayed in
the same image. Each simulated electron produces aGaussian spot with a rms of 1 mm. The simulation was
done for injection parameters of 3 mm horizontal proton
beam size, and 2 ns bunch length. One can see that the central deflections may overlap each other.

The EBP is comprised of an EGH-6210 electron gun
from Kimball Physics, followed by a cylindrical, parallelplate electrostatic deflector, and finally a phosphor screenacting as the beam dump (Fig. 5).
OpticalBreadboard~ 60 cm x 150 cm

Main Injector
Beampipe

OpticalComponents
Box

Ion Gauge
Ion Pump
60 keV Electron Gun
Pneumatic
Beam Valve
Electrostatic
Deflector
Ion Gauge
Pneumatic
Insertion Device
with OTR StainlessSteel Mirror
Phosphor Screen

Figure 5: Model of the EBP showing the main components.
The gun is a 60 keV, 6 mA, thermionic gun with a LaB6
cathode, that can be gated from 2 s to DC at a 1 kHz rate.
The gun contains a focusing solenoid and four independent
magnet poles for steering/focusing. The minimum working
spot size is <100 m. The electrostatic deflector (Fig. 6)
contains 4 cylindrical plates that are 15 cm long and separated by ~2.5 cm. Following the electrostatic deflector is
the intersection with the proton beamline.

Figure 6: Electrostatic deflector showing the curved plates.

Figure 4: Simulated image of successive electron beam
sweeps along the proton beam direction. The sweeps near
the center are difficult to separate and may need to be split
across multiple camera frames.

After the proton beam intersection, there is a phosphor
screen from Beam Imaging Systems. It is composed of P47(Y2SiO5:Ce3+) with an emission wavelength of 400 nm, adecay time of ~60 ns and a quantum yield of 0.055 photons/eV/electron. The phosphor screen has a thin conductive coating with a drain wire attached. The phosphor
screen is surrounded by a stainless-steel absorber that is instrumented to function as a faraday cup (Fig. 7).

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Phosphor Screen

Figure 7: Phosphor screen and angled faraday cup. The faraday cup is intended to be the starting and ending locations
for the electron beam during a sweep.
The light from the phosphor traverses a two-lens system
plus optional neutral density filters or polarizers before entering a Hamamatsu V6887U-02 gated image intensifier.
The output of the intensifier is imaged by a 3710DX12charge injection device (CID) camera from Spectra-Physics (now Thermo Scientific) which is fiber-optically coupled to the intensifier through a fiber-optic taper to improvelight collection (Fig. 8).

Figure 9: Slow deflection waveforms. The two waveforms
are on opposite plates. A MI revolution is 11 μs.
Figure 10 shows images of the sweep both with andwithout MI beam. These are an average over 10 camera acquisitions. The individual spots are when the image intensifier is gated. A slight enhancement can be seen in thebeam image due to the deflections of the electron beam.
a) BeamDeflections

Deflection
Direction

CID Camerab) No Beam

Image Intensifier Housing

Fiberoptic
Taper

Figure 8: Combination of image intensifier and CID camera coupled together with a fiberoptic taper to map thelarger intensifier output to the camera.

Figure 10: Slow sweep of electron beam. The individual
spots are image intensifier gates. A slight enhancementfrom the electron deflection is visible in (a) where the proton beam is present.
To extract the deflection, we subtract the no-beam image
from the beam image (Fig. 11), where the deflections are
now more obvious.

Timing for the system is generated by accelerator clock
events and requires triggers for camera reset, image acquisition, image intensifier gating, electron gun trigger, fast
deflection trigger, and slow deflection trigger. The use andarrangement of the triggers depends on the measurement
approach.

MEASUREMENTS

Deflections

Slow Diagonal Sweep
As stated above, we have performed a slow diagonal
sweep utilizing adjacent pairs of deflecting plates to sweep
the electron beam diagonally across the proton beam. The
sweep is relatively slow such that there are several beam
revolutions between deflection measurements (Fig. 9).

Figure 11: Visible deflections after no-beam image subtraction.
From this image, we find the maximum pixel value ineach column. We also find the maximum pixel in each column of Fig. 10b. In Fig. 12, these two curves of maximum
pixels are overlayed on the image from Fig. 11.

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Faraday Cup

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Undeflected

80 nsDeflected

Figure 12: Dark lines show the maximum pixel location forboth the deflected electrons and the undeflected electrons.
The undeflected curve shows the step structure of thediscrete spots in the original image (Fig. 10b). The difference between these two curves is the deflection distance.
We evaluate the average deflection distance for each step
region, together with the impact parameter for each step toproduce curve (b) in Fig. 13.
a)Figure 14: Fast deflection waveforms. A single rf bucket is~18.8 ns.
a)Bunch Center
DeflectedDown

SweepDirectionb)

NotDeflectedc)
DeflectedUpb)

Figure 15: Fast sweep images for the electron beam: a)above the proton beam, b) through the proton beam, and c)
below the proton beam.

CONCLUSIONS

Figure 13: a) The two maximum pixel curves. b) The deflection vs. impact parameter curve generated from (a).
The resulting fit sigma is 2.21 mm.
The curve is fit with an error function to extract the underlying gaussian parameters of the proton beam. The
sigma of the fit is 2.21 ± 0.18(stat.) mm. This is in reasonable agreement with the same measurement from the ionization profile monitor, 1.71 mm.
It is not clear that using the peak pixel is the best approach, and we may refine this technique in the future.

Fast Sweep
The original goal of the EBP was to perform the fastsweep shown above in Fig. 3. Figure 14 shows a scope
trace of the fast deflection waveform. We have made one
test of the fast sweep (albeit without an actual measurement) and it is shown in Fig. 15. The size of the sweep line
is larger than naively expected. This will make the measurement more difficult as we may not be able to put morethan one sweep in a single image.

We have successfully measured the proton beam profile
with a single camera image using a slow sweep method. Itis in reasonable agreement with the same measurementfrom the ionization profile monitor. The fast sweep method
has been attempted and some initial traces were shown.
More work is required on this approach before we canmake a measurement.

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