Non-interceptive Beam Diagnostics in a H- Linac
During Operations Using Laser Comb and Virtual Slit
Yun Liu

Spallation Neutron Source
Oak Ridge National Laboratory
ORNL is managed by UT-Battelle, LLC for the US Department of Energy

Outline• General Overview• Recent Progress– Virtual slit for short bunch measurement– Laser comb for time-resolved measurement– Recent measurement examples• Outlook• Challenges• Summary2Transverse Profile

What is laser wire?

Laser wire is a nonintrusive wire scanner in which a focused laser beam
replaces the carbon or metal wires.
LaserPosition
scan

Photodetachment

Laser
beamepe

H-FC

Laser
port

MagnetFCe

H-H0pPhase
scane-eMagnet3

HH0Longitudinal Profile

Laser wire based beam instrumentation inaccelerator facilities• Los Alamos Accelerator Test Stand– First demonstration of laser wire - IEEE Trans. Nucl. Sci. (1985).

• KEK-ATF– Developed laser-based beam profile/emittance monitors for electron beam.

• BNL – Laser wire beam energy/profile monitor• RAL front end test stand• CERN – Transverse emittance measurement at LINAC4 injector for LHC• Fermilab– Laser chopper at the PIP LEBT line
– Laser wire system in Fermilab PIP-II Injector Test beam line4

Laser wire measurement stations in the SNS
superconducting linac
Laser-ion interaction chamber• Non-intrusive, applicable to operational beam
• No moving parts in vacuum, less concern on cavity damage
5• Longitudinal profile scan using the same setup

Laser wire measurement stations in the SNS
superconducting linac

6

Y. Liu et al., Phys. Rev.
Accel Beams 16, 012801(2013).

Longitudinal profile scan using a laser wire
Faraday cup

Laserbeam

H-e-B

MagnetIn laser wire based longitudinal parameter measurements,
the beam transverse size affects the measurement.
The position of the electrons in FC can be controlled by themagnetic field.
7H-

Longitudinal profile scan using a laser wire
Faraday cup

Laser
beam

H-e-

B

MagnetIn laser wire based longitudinal parameter measurements,
the beam transverse size affects the measurement.
The position of the electrons in FC can be controlled by themagnetic field.
8H-

Longitudinal profile scan using a laser wire
Faraday cup

Laser
beam

H-e-

B

MagnetIn laser wire based longitudinal parameter measurements,
the beam transverse size affects the measurement.
The position of the electrons in FC can be controlled by themagnetic field.
9H-

Longitudinal profile scan using a laser wire
Faraday cup

Laser
beam

H-e-

B

MagnetIn laser wire based longitudinal parameter measurements,
the beam transverse size affects the measurement.
The position of the electrons in FC can be controlled by themagnetic field.
10H-

Longitudinal profile scan using a laser wire
Faraday cup

Laser
beam

H-e-

B

MagnetIn laser wire based longitudinal parameter measurements,
the beam transverse size affects the measurement.
The position of the electrons in FC can be controlled by themagnetic field.
11H-

Longitudinal profile scan using a laser wire

Measurement from SNS

Faraday cup

Laser
beam

H-e-

B

MagnetIn laser wire based longitudinal parameter measurements,
the beam transverse size affects the measurement.
The position of the electrons in FC can be controlled by themagnetic field.
12H-

Longitudinal profile scan using a laser wire

Measurement from SNS

Faraday cup

Laser
beam

H-H-e-B
Simulation from Fermilab
MagnetIn laser wire based longitudinal parameter measurements,
the beam transverse size affects the measurement.
The position of the electrons in FC can be controlled by themagnetic field.
13

Creation of a virtual slit
H-Y. Liu et al., Phys. Rev. Accel Beams 26, 042801 (2023).
H-Faraday cup
Laserbeame-B
yxz𝑢𝑢 𝐵𝐵
𝑠𝑠 2
𝑁𝑁𝑝𝑝𝑝𝑝 𝑠𝑠; 𝐵𝐵 ∝ exp − 2 �𝑃𝑃 𝑠𝑠; 𝑦𝑦 𝑑𝑑𝑑𝑑
2𝜎𝜎1 −∞
14𝑦𝑦 − 𝑎𝑎𝑎𝑎 2
𝑃𝑃 𝑠𝑠; 𝑦𝑦 = exp −2𝜎𝜎22

Npd: detection efficiency
s: phase to be measured
B: magnetic fieldu(B): cut-off position that is a linear function of B

Creation of a virtual slit
H-Y. Liu et al., Phys. Rev. Accel Beams 26, 042801 (2023).
H-Faraday cup
Laserbeame-B
yxz𝑢𝑢 𝐵𝐵𝑠𝑠 2
𝑁𝑁𝑝𝑝𝑝𝑝 𝑠𝑠; 𝐵𝐵 ∝ exp − 2 �𝑃𝑃 𝑠𝑠; 𝑦𝑦 𝑑𝑑𝑑𝑑
2𝜎𝜎1 −∞
15𝑦𝑦 − 𝑎𝑎𝑎𝑎 2𝑃𝑃 𝑠𝑠; 𝑦𝑦 = exp −2𝜎𝜎22

Npd: detection efficiency
s: phase to be measured
B: magnetic fieldu(B): cut-off position that is a linear function of B

Creation of a virtual slit
H-Y. Liu et al., Phys. Rev. Accel Beams 26, 042801 (2023).
H-

Faraday cup
Laserbeame-B
yxz𝑢𝑢 𝐵𝐵𝑠𝑠 2
𝑁𝑁𝑝𝑝𝑝𝑝 𝑠𝑠; 𝐵𝐵 ∝ exp − 2 �𝑃𝑃 𝑠𝑠; 𝑦𝑦 𝑑𝑑𝑑𝑑
2𝜎𝜎1 −∞
16𝑦𝑦 − 𝑎𝑎𝑎𝑎 2𝑃𝑃 𝑠𝑠; 𝑦𝑦 = exp −2𝜎𝜎22

Npd: detection efficiency
s: phase to be measured
B: magnetic fieldu(B): cut-off position that is a linear function of B

Creation of a virtual slit
H-Y. Liu et al., Phys. Rev. Accel Beams 26, 042801 (2023).
H-

Faraday cup
Laserbeame-Byxz𝑢𝑢 𝐵𝐵𝑠𝑠 2
𝑁𝑁𝑝𝑝𝑝𝑝 𝑠𝑠; 𝐵𝐵 ∝ exp − 2 �𝑃𝑃 𝑠𝑠; 𝑦𝑦 𝑑𝑑𝑑𝑑
2𝜎𝜎1 −∞
17𝑦𝑦 − 𝑎𝑎𝑎𝑎 2𝑃𝑃 𝑠𝑠; 𝑦𝑦 = exp −2𝜎𝜎22

Npd: detection efficiency
s: phase to be measured
B: magnetic fieldu(B): cut-off position that is a linear function of B

Creation of a virtual slit
H-Y. Liu et al., Phys. Rev. Accel Beams 26, 042801 (2023).
H-Faraday cup
Laserbeame-B
yxz𝑢𝑢 𝐵𝐵𝑠𝑠 2𝑁𝑁𝑝𝑝𝑝𝑝 𝑠𝑠; 𝐵𝐵 ∝ exp − 2 �𝑃𝑃 𝑠𝑠; 𝑦𝑦 𝑑𝑑𝑑𝑑
2𝜎𝜎1 −∞
18𝑦𝑦 − 𝑎𝑎𝑎𝑎 2𝑃𝑃 𝑠𝑠; 𝑦𝑦 = exp −2𝜎𝜎22Npd: detection efficiency
s: phase to be measured
B: magnetic fieldu(B): cut-off position that is a linear function of B

Creation of a virtual slit
H-Y. Liu et al., Phys. Rev. Accel Beams 26, 042801 (2023).
H-Faraday cup
Laserbeame-B
yxz𝑢𝑢 𝐵𝐵𝑠𝑠 2
𝑁𝑁𝑝𝑝𝑝𝑝 𝑠𝑠; 𝐵𝐵 ∝ exp − 2 �𝑃𝑃 𝑠𝑠; 𝑦𝑦 𝑑𝑑𝑑𝑑
2𝜎𝜎1 −∞
19𝑦𝑦 − 𝑎𝑎𝑎𝑎 2𝑃𝑃 𝑠𝑠; 𝑦𝑦 = exp −2𝜎𝜎22

Npd: detection efficiency
s: phase to be measured
B: magnetic fieldu(B): cut-off position that is a linear function of BMeasured transverse profile

Measured longitudinal profile

σv = 2.56 mm

Imag = 11 A
13.28 ± 0.27 ps

The value is close to thedesign value

Reconstructed longitudinal
profile from virtual slit

20

Time-resolved measurement using a laser comb
Macropulses:1 ms @ 60 Hz
H- beam

Laser comb
timeTturn ~ 940 ns

5Tturn21

2.48 ns

H- beam emittance measurement using laser wire

••••
22

Essentially a slit-detector emittance scanner
Laser wire creates H0 beam slit that preserves the angular distribution of the H- beam
Measurement of divergence of H0 beam leads to the determination of H- beam divergence
Emittance measurement is time consuming, normally taking 20-30 minutes on a 60-Hz beam

H- beam emittance measurement using laser wire••••23

Essentially a slit-detector emittance scanner
Laser wire creates H0 beam slit that preserves the angular distribution of the H- beam
Measurement of divergence of H0 beam leads to the determination of H- beam divergence
Emittance measurement is time consuming, normally taking 20-30 minutes on a 60-Hz beam

Multiple emittance slices over the ramp-up of theH- beam macropulse measured from a single scan
Horizontalx′x
Verticaly′y024510t (Turn)
We measured up to 30 emittance slices (would take 12 hours) from one scan (< 30 minutes)
Y. Liu et al., Phys. Rev. Accel Beams 23, 102806 (2020)).

Time-resolved measurements of longitudinal
profilesH- beam macropulse
16.7 ms~1 ms

H- beam turn (mini-pulse)
Tturn ~ 940 ns

25

Measured bunch width variations within a H- beammini-pulse and over different mini-pulses

26

Measured bunch width variations within a H- beammini-pulse and over different mini-pulses

27Outlook - Laser wire-based longitudinal phase spacemonitor
Step 3: H0 pulse
broadening measurementwith BSM

Step 2: H0
propagation andseparation
Laser wire
profile monitor
Electrondetector
Dipole28

Electrondetector

Step 1: Hneutralization fromlaser wireOutlook: laser-based proton beam extraction• At SNS, a study is being carried out to leverage

the existing accelerator serving new missions:Muon Spectroscopy (µSR) and Single Event
Effects (SEE).

• µSR - material characterization, especiallysensitive measurement of local magnetic field• SEE – using n/p+ to test equipment against

radiations for aerospace industry• Using a laser beam with sufficient pulse energy,

one can neutralize large portion of H- bunches
and extract them from the accelerator. The
laser-based beam extraction is non-interceptive
and has negligible impact on the primary
neutron scattering mission.

Y. Liu et al., Nucl. Instrum. Methods Phys. Res., A
962, 163706 (2020).
29

The optimal proton pulses for muon spectroscopy would
be 30 ns pulses spaced 20 µs apart (50 kHz), which canbe produced using photo-neutralization. The fraction of theextracted beam is negligible (~ 0.2%).

Challenges and limitations• Laser source• Optical transport line
• Position stabilization

Laser

1234

5

12 13 14 15

1732SCL250 m

30160 m

25 m

HEBT
60 m

Linac dump

R. Hardin, Y. Liu, C. Long, A. Aleksandrov, W. Blokland, Opt. Express 19 (2011) 2874-2885.
Y. Liu et al., SPIE Proc. 12399 (2023) 123990D1-6.

Challenges and limitations• Laser source• Optical transport line
• Position stabilizationTelescope

Telescope

Beam sampler
Laser

Motorized stage
GigE Camera
1234512 13 14 151732

SCL250 m

31160 m

25 m

HEBT
60 m

Linac dump

R. Hardin, Y. Liu, C. Long, A. Aleksandrov, W. Blokland, Opt. Express 19 (2011) 2874-2885.
Y. Liu et al., SPIE Proc. 12399 (2023) 123990D1-6.

Challenges and limitations
H-scanMitigation of reflected/scattered light in themeasurement chamber is critical to achieve
high dynamic range measurements.

V-scanIon
Beam32

Challenges and limitations
H-scanMitigation of reflected/scattered light in themeasurement chamber is critical to achieve
high dynamic range measurements.

V-scan
Ion
Beam

33

Challenges and limitations
H-scanMitigation of reflected/scattered light in themeasurement chamber is critical to achieve
high dynamic range measurements.

V-scanIon
Beam

Orbit compensation and control of beamloss from the neutralized hydrogens areimportant to reassure non-interceptiveoperation.

34

Summary• We have described a laser wire system for the diagnostics of high-power

H- beam at the SNS linac during neutron production.

• The virtual slit technique enables longitudinal profile measurement of H-

beam bunches with a few picoseconds bunch width.

• Laser combs provide time-resolved measurements at a much higher

speed, makes it possible to study beam parameter variations within a very
short time interval.

• We have demonstrated that the laser-wire-based beam instrumentation

can be made operational in a high-power accelerator facility and it
provided novel functions which are not possible with conventional wire
scanners or bunch shape monitors.

35

Acknowledgements
Technical contributions to the development of virtual slit and laser comb
• Cary Long
• Alexander Aleksandrov• Andy Webster• Syd Murray III• Andrei Shishlo• David Brown• Randy Thurman-keup (Fermilab)• Vic Scarpine (Fermilab)

Support from• BI Group members at RAD/ORNL• Sarah Cousineau• Fulvia Pilat

36