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SIMULATION AND SHOT-BY-SHOT MONITORING
OF LINAC BEAM HALO*
Alan S. Fisher†, Mei Bai, Thomas Frosio, Alessandro Ratti, John Smedley, Juhao Wu
SLAC National Accelerator Laboratory, Menlo Park, California, USA
Isar Mostafanezhad and Ben Rotter, Nalu Scientific, Honolulu, Hawaii, USA
Abstract
FELs require a reproducible distribution of the bunchcore at the undulator entrance for robust and reliable lasing.
However, various mechanisms drive particles from thecore to form a beam halo, which can scrape the beampipeof the undulator and damage its magnets. Collimators cantrim the halo, but at the 1-MHz repetition rate of SLAC’s
LCLS-II superconducting linac, the collimator jaws can be
activated and damaged. The Machine Protection System(MPS) can detect excessive radiation and halt the beam,
but repeated MPS trips lead to significant downtime. Halo
control begins by studying its structure, formation, andevolution, using a sensitive halo monitor. To that end, weare developing a pixellated diamond sensor. Diamond offers a dynamic range of up to 7 orders of magnitude, extending from the edge of the core to the faint halo expected
at greater distances. Nalu Scientific has developed fastelectronics for high-rate shot-by-shot readout. Initial tests
are starting with a prototype 16-pixel sensor at the beam
dump of SLAC’s FACET-II test facility. The tests and simulations will guide more elaborate sensor designs.

INTRODUCTION
Generating a beam with the desired particle distribution
in the core of each bunch and maintaining this distribution
as the beam travels is a critical requirement for accelerators. However, various mechanisms drive particles fromthe core into a beam halo. At x-ray free-electron laser(XFEL) facilities, the shape of the bunch core at the entrance of the undulator is crucial for lasing. Reproducibilityof this shape is necessary for robust and reliable performance. If a halo surrounds the core, it could scrape on theundulator beampipe, generating a radiation shower thatdamages the undulator’s permanent magnets. Our numerical simulation of LCLS-II with a start-to-end electron
beam-dynamics model finds that collective effects willcause an electron beam—4 GeV, repeating at 300 kHz,180 pC, plus both dark current and halo electrons—to
scrape 1 to 10 W at the undulator entrance. Collimators arecommonly used to trim the halo, but at the 1-MHz repetition rate of LCLS-II, the resulting radiation could activateand damage the collimator jaws, making this approach
problematic. Moreover, the Machine Protection System(MPS) detects excessive radiation and halts the beam, but
repeated MPS trips creates significant downtime for users.
Advanced techniques to detect and mitigate halo are necessary. Non-destructive characterization of the halo___________________________________________* SLAC is supported by the U.S. Department of Energy, Office of Science,
under contract DE-AC02-76SF00515.
† afisher@slac.stanford.edu

distribution is crucial for understanding in detail the mechanisms behind its formation and for identifying countermeasures. We are developing pixellated diamond sensors
surrounding the beam core, to monitor halo noninvasively(“parasitically”), shot by shot, with high spatial and temporal resolution. Diamond offers several orders of magnitude of dynamic range, allowing measurements from theedge of the core to the faint halo expected at greater distances. Below we describe an initial test using a diamondchip with 16 pixels, by the beam dump of FACET-II [1],the test-beam facility at SLAC. Fast electronics built by
Nalu Scientific [2] provide shot-by-shot pixel readout.

PIXELLATED DIAMOND DETECTORS
Diamond Sensors
The diamond sensor used in this monitor is based on
more than a decade of experience using diamond as an xray beam diagnostic [3-5]. In effect, these devices form asolid-state ionization chamber: a photon or electron excites
an energetic electron within the material, which then loses
energy via electron-electron scattering, producing one
electron-hole pair for each 13.3 eV [6] of deposited energy.
The FACET electron beam is near the minimum of thedE/dx curve, with an expected collisional energy loss of
35 keV/electron for a 50-μm-thick diamond. With the current electronics, we anticipate being sensitive to 10 electrons per pixel.
These devices have demonstrated a linear dynamic range
spanning 11 orders of magnitude with photons [3, 4], due
to the lack of recombination in the bulk, and are very radiation tolerant. We anticipate at least 7 orders of magnitude
when used with electrons. The device can be read in pulsed
or current mode. The goal is to resolve the halo in space—by dividing the chip’s surface into pixels using lithographic
patterning of the electrodes—and in time—by using pulsemode readout for shot-by-shot data. In addition to its radiation hardness, diamond is an attractive medium for halo
monitoring because its low Z makes it significantly lesssensitive than other semiconductors to bremsstrahlung
photons, which create background for direct halo detection.
Figure 1 shows the relatively simple patterned diamondin the FACET-II test. The chip is a square single crystal,
4 mm on each side and 50 μm-thick. Four parallel electrodestrips, each platinum with a width of 750 μm and a thickness of 25 nm, cross one face with a 1-mm period. The second face has four similar strips, but orthogonal to those on
the first. Their intersections form a 44 grid of pixels.
Figure 2 shows a microscope photograph of the diamond.
Future detectors are envisioned with many more pixels,
spread over several chips. Each detector will surround the
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bunch core while remaining outside the beam stay-clear radius, to measure the halo noninvasively. A close approachto this radius will require a mechanical adjustment of thesize of the central hole.

Figure 1: Pattern (in mm) of the platinum electrodes onboth faces of the diamond chip.

Shot-by-Shot Pixel Readout
The device tested at FACET-II evolved from the readout
designed for a Time-Resolved Beam-Halo Monitor (TRBHM) developed by Nalu Scientific in collaboration withanother of the authors (J. Smedley).
The eight strips are wire bonded to a small printed-circuit board (Fig. 3) supporting the chip and fans out to eight
coaxial cables running to a nearby amplifier box. Eachchannel is AC-coupled to the amplifier and DC coupled toa bias voltage. The DC supply is typically set to 30 V. The
amplifier outputs are digitised in a separate module further
from the beamline and transported over ethernet to a computer outside the tunnel. We use Nalu Scientific’s AARDVARC digitiser hardware [2] along with Nalu’s Naluscope
software for viewing and control.
Normally the strips on one side are grounded (for example, the horizontal strips, which correspond to the y coordinate) and those on the other are biased (vertical, giving thex coordinate). In a low-density halo, only one pixel may
receive charge. This pixel is identified by the horizonal andvertical strips that receive nonzero signals. With a higher
density, the bias is applied to only one strip of the four.
Then the x coordinate is determined by the bias and the yprofile comes from reading the charge on the grounded
face. The bias is moved from strip to strip over four shots,
giving the full pixel distribution. For an NN grid, thismethod is a trade-off: N shots rather than one, but 2N cables
and channels rather than N2. The advantage is less significant for our simple chip than for future devices.

HALO TEST AT FACET-II

Figure 2: A microscope photo of the diamond. The diamond is clear. The electrodes, which appear black, on theupper surface run parallel to the lower edge. Those on thelower face can be seen through the gaps in the upper electrodes. Old wires from a previous bonding to the circuitboard hang from the lower edge.

Figure 3: The diamond detector of Fig. 2, mounted on theprinted circuit for fan-out to the 8 coaxial cables.

FACET-II [1] is SLAC’s test facility for advanced accelerator research, primarily studies of plasma-wakefield acceleration. The vertical bend at the beam dump serves as aspectrometer for the resulting wide changes in beam energy
from an initial energy of typically 10 GeV. Electrons exit
the beamline vacuum through a flange with a 5-mm-thickaluminium window and pass through more than 2 m of air
while going over a diagnostics table and bending downward into the dump. This arrangement is convenient for a
proof-of-principle test. It provides a first look at halo without the need for in-vacuum motion of the diamond sensor.
After a long downtime, FACET-II is now being tuned,starting at the injector. The beam should be sent to thedump around the time of this conference. We anticipate beginning parasitic measurements shortly afterward.
The detector is mounted 185 cm from the window, atbeam height and on a 50-mm horizontal translation stage.
At one end of the travel, the beam core is roughly centredon the chip. We will begin with it fully retracted away fromthe beam. Then the signals from the chip will give an indication of the noise background, from the pulsed RF power
and from the beam itself. This noise floor sets one end of
the measurement’s dynamic range. The beam can be
stopped on a collimator just upstream to see the ambient
electronic noise. The additional noise when the beam next
goes to the dump will give the background due to the beam.

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We will then translate the diamond slowly toward thebeam while monitoring the signals. An increase should beseen once the beam enters the halo. This movement can becontinued until the beam is approaches within perhaps 2
of the centre (where  is the RMS beam size). The dump
bend can scan the halo vertically to vary the signals as the
dipole shifts beam above and below the detector. Small
controlled horizontal moves can be made with a horizontal
corrector. (With no readback other than a video image ofthe position, the translation stage is imprecise for horizontal scanning.) The signals must be carefully watched near
the core to avoid saturation. Damage to the simple FR4 circuit board is a concern if too much beam scatters from thediamond; a more expensive ceramic board will be neededin the next in-vacuum test (discussed below). The dynamic
range extends at least between the highest level that seems
safe and the noise floor.
Additional tests are needed to validate the halo measurement, since halo has an uncertain distribution that can extend to several times  from the beam centre. The detector
should appropriately register changes in the bunch size as
the spectrometer quadrupoles change the focus. A suitablephase advance can image the collimator about 50 m upstream of the dump onto the detector. As the jaws are
opened or closed, the moving image of the jaws should cut
into the halo.

Figure 4: A GEANT4 simulation in which 10 primary
10-GeV electrons pass through the air path and the diamond detector. Electrons are shown in red and photons ingreen.

SIMULATION OF DIAMOND RESPONSE
We conducted a simulation to find the spread of thebunch after travel through the window and the air path, andthe resulting charge deposition in the diamond. This study
used a GEANT4 [7] model replicating the real geometry of
the pixellated diamond detector (Fig. 4). The model integrates the intricate arrangement of strips and the diamonditself. Essential parameters such as particle position, direction, and energy entering the FACET-II exit flange are obtained from the ELEGANT beam-dynamics code [8]. Figure 5 (a) shows the transverse distribution at the exit window. The x and y projections, in Figs. 5(b) and (c) respectively, demonstrate that both distributions have long tailsof halo electrons. The much lower density of the tails calls
for a detector with a wide dynamic range, to record electrons in both the core and halo. The parameters are given
to the GEANT4 simulation, which includes the exit window, the 185-cm air path to the diamond, and the detector
itself.
Our simulation applies two distinct scoring approaches.
The first is based on transfer functions provided by
Cividec [9], a manufacturer of diamond detectors. These
provide the interaction probabilities of various particle
types within the diamond and the resulting charge generated, as a function of particle energy. The data accounts
only for electrons, photons, protons, and neutrons. In thesecond scoring method, GEANT4 directly computes theenergy deposition within the diamond, and an electron-hole
pair is created for every 13.3 eV deposited [6].

Figure 5: (a) Simulated transverse distribution of the electron bunch at the exit window, with projections along the(b) ‫ ݔ‬and (c) ‫ ݕ‬coordinates.
We find that these two methods agree both qualitatively
and quantitatively. The spatial distribution of the diamondsignals aligns well, while relative differences of 20% to60% (Fig. 6) are observed between the two approaches. To
ascertain whether these discrepancies are due to secondary
particles beyond electrons, photons neutrons, and protons,
we simulated the energy deposited by alpha particles, deuterons, pions, and ions produced by interactions with air.
We find a negligible contribution from these secondaries tothe total energy deposition within the diamond, less than
0.015%, with 0.01% originating from alpha particles. This
correspondence suggests that the simulation captures thediamond detector’s response, which we will benchmark
with our upcoming measurements.
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Figure 6: Comparison of the GEANT4 simulation resultsfor FACET-II using the two scoring methods, in fC per primary electron. Left: Cividec transfer functions. Centre: Energy deposition from GEANT4. Right: Relative difference.

We plan to develop start-to-end particle-in-cell (PIC) numerical simulations of halo formation and evolution along
the accelerator as extensions of various existing codes.
Several well-benchmarked numerical simulation codes—
including ASTRA, GPT, Elegant, Lucretia, and IMPACT—have been used successfully in designing and operating LCLS, LCLS-II, FACET and FACET-II, from electron-bunch generation through acceleration and bunch
compression. These codes are well suited to form the basis
for a model of beam halo.
However, due to the large extent of the beam’s distribution in 6-dimensional phase space, a high-order map forbeam dynamics must be carefully implemented to model
the generation and evolution of beam halo in the injector,
accelerator, and bunch compressors. In particular, the corebeam’s collective effects can drive the halo to execute betatron motion at large amplitude and with potentially significant chromatic effects. In the compressors, the model
must incorporate 3-dimensional coherent synchrotron radiation (CSR) [10] to properly study transverse effects.
Understanding the formation and evolution of halo along
the accelerator, through both measurement and simulation,
will ultimately provide techniques for mitigating halo at its
sources. These developments should prove valuable for
LCLS and other XFELs.

REFERENCES[1] FACET-II Conceptual Design Report, SLAC, 2015.

Figure 7: Sketch of a modified wire-scanner “harp”.

FUTURE WORKFACET-II Measurement in Vacuum
As we begin the in-air measurements at the FACET-IIbeam dump, we are designing the next phase, a test in vacuum. To translate the detector from outside the beam into
the halo, we will modify a surplus wire scanner. Minorchanges to the “wire card” or “harp” that holds the wires
will allow mounting the diamond on the moving stage(Fig. 7). The signals from the 8 strips will be connected
through the vacuum in coaxial cables to a feedthroughflange for processing by Nalu’s electronics outside. The
wire scanner allows us to measure the profile of the bunchcore and the coordinates of it centroid relative to the diamond’s pixels. As the wire scanner moves the diamondthrough the halo, we will be able to map the halo over a
region much larger than the size of the chip.

Halo Evolution
Effective halo mitigation must be guided by a better understanding of halo formation by interpreting measurements with simulations. These are also needed to guide thedesign of more advanced sensors, so that their extent andresolution can match the halo distribution at various locations along the FACET-II or LCLS-II beamlines. The complexities underlying beam halo are not yet well modelled
and involve high-order nonlinear beam dynamics.

[2] L. Macchiarulo et al., “Design improvements and first results for the revision 3 of AARDVARC Waveform Sampling System on Chip”, in Proc. IEEE NSS-MIC 2020, Boston, MA, USA (virtual), November 2020.
doi:10.1109/NSS/MIC42677.2020.9507950[3] J. Bohon, E. Muller, J. Smedley, “Development of diamondbased X-ray detection for high-flux beamline diagnostics”,
J. Synchrotron Radiat., vol. 17, pp. 711-718, 2010.
doi:10.1107/S0909049510031420[4] E. Muller et al., “Transmission-mode diamond white-beam
position monitor at NSLS”, J. Synchrotron Radiat.,vol. 19,
pp. 381-387, 2012. doi:10.1107/S0909049512005043[5] T Zhou et al., “Pixelated transmission-mode diamond X-ray
detector”, J. Synchrotron Radiat., vol. 22, pp. 1396-1402,
2015. doi:10.1107/S1600577515014824[6] J. Keister and J. Smedley, “Single Crystal Diamond Photodiode for Soft x-ray Radiometry”, Nucl. Instrum.Methods A,
vol. 606, 774-779, 2009.
doi:10.1016/j.nima.2009.04.044[7] S. Agostinelli et al., “GEANT4–a simulation toolkit”, Nucl.
Instrum.Methods A, vol. 506, pp. 250-303, 2003.
doi:10.1016/S0168-9002(03)01368-8[8] M. Borland, “ELEGANT: A Flexible SDDS-Compliant
Code for Accelerator Simulation”, Advanced Photon Source
LS-287, September 2000.
[9] E. Griesmayer, personal communication.
https://cividec.at[10] Y. Cai and Y. Ding, “Three-dimensional Effects of Coherent
Synchrotron Radiation by Electrons in a Bunch Compressor”, Phys. Rev. Accel. Beams, vol. 23, p. 014402, 2020.
doi:10.1103/PhysRevAccelBeams.23.014402

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