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NANO-AMP BEAM CURRENT DIAGNOSTICFOR LINAC-TO-ESA (LESA) BEAMLINE*S.T. Littleton†, A.S. Fisher, T. Raubenheimer, SLAC, Stanford, CA, USA
C. Huang, Evergreen Valley High School, San Jose, CA, USA
Abstract
The LESA beamline is designed to transport dark current
from the LCLS-II and LCLS-II-HE superconducting linacs
to the End Station A for various fixed target experiments.
The primary experiment is expected to be the Light Dark
Matter eXperiment (LDMX) which required beam currentsof a few pA. The operation of the beam line must be parasitic to the LCLS-II / LCLS-II-HE FEL operation. The darkcurrent in the LCLS-II is expected to be at the nA-level
which will be below the resolution of most of the LCLS-IIdiagnostics (it will be degraded before the experiments asnecessary). This paper will describe a possible non-destructive diagnostic using synchrotron radiation that could
be applied at multiple locations along the LCLS-II and theLESA beamline.

INTRODUCTION
The SLAC Linac to End Station A (LESA) beamline isa staged concept to provide a near-CW beam at 186 MHzand sub-harmonics thereof to the SLAC End Station A(ESA) for experiments in particle physics requiring pA to25 nA electron beams with multi-GeV electron energy [1].
This capability is achieved parasitically by extracting unused bunches from the LCLS-II/LCLS-II-HE superconducting RF (SRF) linac [2, 3].

infrastructure, and (4) an optional laser oscillator that augments the dark current with a well-defined, low-current
beam at 46 MHz repetition rate within a ~500 ns macropulse between LCLS-II/LCLS-II-HE primary bunches asillustrated in Fig. 2.

Figure 2: LCLS-II pulse structure showing primary pulses
with 108 e- and LESA bunches from the gun with 30 e- perbunch. The LESA beam will control the bunch population
with an additional seed laser and/or a spoiler/collimation
system to deliver final current in the pA to μA range.
Table 1: LESA Electron Beam Parameters for an UltraLow-Current Beam (Baseline) and a Possible UpgradeExperiment Pa- Ultra-low-current Low-current
rameters(upgrade)
Energy4.0 GeV(upgrade to8.0 GeV in 2027)

4.0 GeV(upgrade to8.0 GeV in2027)

Bunch spacing

5.4 – 65 ns

5.4 nsBunch charge

0.04 – 10,000 e-

70,000 e(10 fC)

Macro
pulse 0.1 – 25 nA
beam current
Duty
cycle

Figure 1: Schematic of LESA beamline starting in SLAC
linac and connecting to the A-line.
The LESA beamline begins in Sector 28 of the SLAC
linac and continues into the Beam Switchyard, where it
connects with the A-line as illustrated in Fig. 1. The beamline consists of (1) a long-pulse kicker and septum magnet
to divert low-current bunches off the LCLS-II dump line,(2) a 250m long transfer line from the kicker in Sector 29
to the existing A-line, delivering beam to End Station A,(3) minor improvements in the existing End Station A___________________________________________* Work supported in part by DOE contract DE-AC02-76-SF00515.
† seantl@stanford.edu.

2 µA

55% (600 ns out of 55% (600 ns out1.1 µs)of 1.1 µs)

Norm. emittance ~100 µm;(rms)<1000 µm~1 µm

Bunchspread<1%energy <1%IP spot size w/ 4 cm x 4 cmrastering<250 µm including jitter

Electrons per year 1015 e- / year

1019 e- / year

LESA is being constructed in two stages: the first stage,
S30XL, is nearly complete and will demonstrate the darkcurrent extraction from the SRF linac while the 2nd stage,
on which construction is just starting, will transport the extracted current to ESA. S30XL will be commissioned inlate 2023 while the full LESA beamline is planned to becomplete at the end of 2024.
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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

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d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 4 2The presently envisioned program for LESA comprises
dark sector physics, electron-nuclear scattering measurements for the neutrino program, and a test beam program [4]. Beam parameters are listed in Table 1.
One of the many challenges for the LESA beamline is toquantify the very low current beam in a parasitic manner.
The expectation is that the incoming dark current will be at
the level of a few nA; if necessary, this will be attenuatedbefore the End Station using a spoiler in the A-line. Ongoing measurements at LCLS-II indicate this level of darkcurrent at the end of the SRF linac [5].

𝐹 𝜉9√38𝜋𝐾 𝜉 𝑑𝜉 ,where 𝐾 is a modified Bessel function. These photons will
be swept in angle as the electron traverses the dipole andwill have a vertical angle that scales as 1but depends on
the photon energy.

S30XL SYNCHROTRON RADIATIONThis paper will address one possible concept for a nondestructive diagnostic using synchrotron radiation and asensitive x-ray detector that could be applied at multiple
locations along the LCLS-II and the LESA/S30XL beamline. The goal is to routinely measure 100 nA to 10 pA
beams at the beginning of the LESA/S30XL beamline nondestructively so that we can tune the SRF linac and set thescattering foil correctly.
The LESA/S30XL beamline is illustrated schematicallyin Fig. 3. As shown, the S30XL kicker and septum extract
dark current from the SRF Dump beamline and transport it
across the CuRF linac before it is deflected to be parallelto the other beamlines and put into a low-power dump.
LESA will transport this into the A-line and ESA.

Figure 3: Schematic of the LESA/S30XL beamline (red)along with the SCRF beamline to the HXR FEL (green)
and the SCRF beamline to the SXR FEL (blue).
The last dipole in the S30XL line before the Dump is a1-meter bend and will deflect the beam by 19.1 mrad. Themagnetic field is 2.5486 kG at 4 GeV and twice that at8 GeV. The instantaneous radiated power and the photon
critical wavelength are given by the equations:𝑐𝐶 𝐸𝐸 GeV𝑃
2.22𝐸 𝑘𝑒𝑉,2𝜋 𝜌𝜌mwhere E is the electron energy,  is the bending radius, andC = 8.85e-5 [m/GeV3]. With a beamline of 4 GeV and abending radius of 52.4 meters, the instantaneous radiatedpower is 3.94 x 108 keV/s. The critical photon energy is
2.71 keV at 4 GeV and 21.68 keV at 8 GeV and the numberof photons emitted per second for a photon of energy u canbe determined using the equation [6]:𝑃𝑢𝐹,𝑛 𝑢𝑢𝑢
where uc is the photon critical energy, Ec, and F(u/uc) is afunction that can be written as:

Figure 4: In the diagnostic under consideration, bending
magnet radiation is intercepted by a sensitive detector.

Figure 5: Differential spectrum of the radiation emitted by
4 GeV electrons passing through the bend.

INTERCEPTED SPECTRUM
The spectrum intercepted by a detector of height h andwidth w a distance D from the location where the radiation
is emitted, as shown in Fig. 4, will differ from the full spectrum emitted by the electrons, since some low-energy photons will be radiated at vertical angles greater than 𝜃ℎ/2𝐷. To find the intercepted spectrum, we must begin
with the full differential spectrum [7], shown in Fig. 5.
𝑑 𝐼𝑑𝜔 𝑑Ω𝑒𝜔𝜌12𝜋𝜀 𝑐 𝑐1𝛾𝜃𝐺 𝜁 ,where𝐺 𝜁𝐾/ 𝜁𝜃
1/𝛾

𝜃𝐾/ 𝜁and𝜁𝜔𝜌 1𝑐 𝛾/𝜃.

TUP042286

06 Beam Charges and Current Monitors

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d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 4 2Integrating over vertical angles 𝜃 between −ℎ/2𝐷 andℎ/2𝐷, we find that the spectrum intercepted by a 4.5 mmx 4.5 mm detector at various distances is modified asshown in Fig. 6.

gain of 5.4 mV/fC, such a signal may provide usefulbunch-by-bunch data, though further amplification may berequired locally to prevent noise from contaminating thesignal on its way out of the tunnel.
Table 2: Expected Charge Produced by the Cividec Diamond Detector Per Electron in the LESA Beamline

Figure 6: The energy spectrum of the photons interceptedby a 4.5 mm x 4.5 mm detector at various distances fromthe point where the radiation is emitted.

DETECTOR RESPONSE
We have modeled the response of a 500 µm-thick
Cividec B1 CVD diamond detector to the incident synchrotron radiation spectrum, based on data provided byCividec [8] regarding the mean charge produced in the detector as a function of incident photon energy. Using aneighth-order polynomial fit to the Cividec data, with theconstraint that the response goes to zero near the ionizationenergy of carbon, we multiplied the intercepted photon
spectrum by the charge response curve, as shown in Fig. 7.
We then integrated over photon energies to estimate the total charge 𝑄 produced in the detector per beam electron for
detector distances of 1, 2, 5, 10, and 20 m, for both the 4
GeV beam and the 8 GeV LCLS-II-HE beam. The results
are shown in Table 2.

D [m]

𝑸 [fC/e-](4 GeV beam)

𝑸 [fC/e-](8 GeV beam)

13.1 × 10

3.0 × 1021.5 × 10

1.5 × 1056.1 × 10

6.0 × 10

10

3.0 × 10

3.0 × 10201.3 × 10

1.4 × 10

CONCLUSION
We have investigated the feasibility of using synchrotron
radiation emitted by dark current in LCLS-II and LESA tomeasure the associated nA-level currents. The detector wemodeled, a Cividec B1 CVD diamond detector, shows
promise as a potential detector for currents at the higher
end of the anticipated range of the dark current. To measure
lower currents, especially at the pA level, we will explore
alternative detectors, such as low-gain avalanche diodes(LGADs) and CCD arrays. The measurement may also besensitive to the radiation background in the accelerator tunnel. Further studies are needed to characterize the background in the proposed detector location and explore collimation and shielding strategies.

REFERENCES[1] T. Markiewicz et al., “The SLAC Linac to ESA (LESA)
Beamline for Dark Sector Searches and Test Beams”, arXiv.
doi:10.48550/arXiv.2205.13215

Figure 7: A side-by-side comparison of the photonspectrum and the diamond detector response curve on alog-log plot. The seemingly large region with no response
data is a negligible part of the integral.
At 186 MHz, a 10 nA dark current beam would containroughly 336 electrons per bunch. For 𝐷 = 1 m, the corresponding detector charge would be 1 fC/bunch for the
4 GeV beam or 10 fC/bunch for the 8 GeV beam. Whencoupled with a Cividec C6 fast charge amplifier [9], with a[2] T. Raubenheimer, ed., LCLS-II Final Design Report, internal
SLAC document, 2015.
[3] T. Raubenheimer, ed., LCLS-II-HE Conceptual Design
Report, LCLSII-HE-1.1- DR-0001 and SLAC-R-1098, Mar.
2018.
[4] A description of LESA and its science program can be foundin SLAC-R-1147. The LESA beamline has been known as
DASEL, S30XL, and YABBL.
[5] Y. Ding, private communication, 2023.
[6] M. Sands, “The physics of Electron Storage Rings”, SLAC121, 1971.
[7] J.D. Jackson, Classical Electrodynamics. Wiley. (1998).
[8] Cividec Instrumentation, “Particle interaction withdiamond”, internal working paper for Alan Fisher of SLAC,
2017.
[9] Cividec Instrumentation, Electronics Application Note.

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