Review of Profile and Emittance Diagnostics for the SNS Linac

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Review of Profile and Emittance Diagnostics for
the SNS Linac

• Tom Shea
• ORNL
• for the ORNL Research Accelerator Division
• December 11, 2008

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Outline

• SNS overview
• Facility
• Parameters
• Status
• Example Diagnostics
• Low energy emittance
• High energy emittance (future)
• Wire scanners
• Laser profile
• Bunch shape
• Longitudinal laser measurements
• Momentum scrapers
• Imaging
• Closing Comments

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The SNS Partnership During Construction

• Partner lab obligation completed
• LBNL Sept 2002
• LANL Sept 2004
• JLAB April 2005
• BNL April 2005

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Probability of Communication
T. Allen, Sloan WP 165-97, MIT, 1997
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SNS Accelerator Complex
Accumulator Ring
Collimators
Accumulator Ring Compress 1 msec long pulse to
700 nsec
1 GeV LINAC
Front-End Produce a 1-msec long, chopped, H-
beam
Injection
Extraction
RF
RTBT
1000 MeV
2.5 MeV
87 MeV
186 MeV
387 MeV
Ion Source
HEBT
SRF, b0.61
DTL
RFQ
CCL
SRF, b0.81
Liquid Hg Target
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SNS Linac Design Parameters
Linac Output Energy 1.0 GeV
Beam Power 1.4 MW
Linac Beam Duty Factor 6
Peak Linac Current 38 mA
Average Linac Current 1.6 mA
Linac pulse length 1.0 msec
Repetition Rate 60 Hz
Linac Bunching Frequency 402.5 MHz
LEBT emittance (65 keV) 0.2 p mm mrad
RFQ output emittance (2.5 MeV) 0.21 p mm mrad
SCL output emittance (1 Gev) 0.41 p mm mrad
RFQ output Longitudinal Emittance 0.1 p MeV deg
SCL output Longitudinal Emittance 0.6 p MeV deg
Transverse Halo Beam in Gap 1x10-4
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Performance
Henderson, Purcell
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Emittance Measurement Issues for the SNS
Low-Energy Beam Transport

• SNS Baseline System
• 12 cm long LEBT no diagnostics, no beam stop
• 1st online beam measurement after RFQ, but RFQ
  transmission unknown
• RFQ output routinely gt35 mA, 56 mA demonstrated
• Ion sources and LEBTs are characterized offline
  with SNS Allison Emittance scanner no mass
  separation (no magnetic analysis)
• Large inconsistencies between test stand and
  Front end

• Under development
• 1.2 m long, 2-solenoid LEBT
• SNS Allison scanners more

chopper
Stockli
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Low Energy Emittance Measurements
Original requirements for slit-collector devices
- 10 accuracy, measure only in low energy
sections (65 keV, 2.5 MeV, 7.5 MeV)
Source 0.2 ms
Source 0.6 ms
Allison scanner on source test stand 65 keV
MEBT 2.5 MeV Slit and harp system Expect 0.3 ?
mm mrad, rms, norm Results (? mm mrad, rms,
norm) ?X 0.29 ?Y 0.26
Stockli, Long, Penissi, Murray, Blokland, et. al.
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SNS LEBT Emittance Discussion

• Low-energy, high-current, non-neutral beams
  suffer rms emittance growth.
• Low-energy beams are large and suffer from
  aberration causing S-shaped emittance
  distributions.
• Important to characterize the distribution of
  the beam core, which is normally transmitted
  through the RFQ.
• Important to understand the tails of the
  distribution because they are transmitted through
  the RFQ when beam core is chopped.
• Requires reliable, artifact-free scanners
  2-slit system with suppressed and shielded
  Faraday cup.
• Allison scanners are compact and fast (electric
  angle scans)
• Ion Source and LEBT normally characterized on
  test stand by measuring the highly convergent
  beam that would be injected into the RFQ
  represented by a small beam spot.
• Characterization of low-divergence, large beam
  desired for 2-solenoid LEBT.
• Adjustable, water-cooled slits are in planning.

• SNS Allison scanners are adapted from the LBNL
  88 cyclotron scanner designs. Added features
  scatter-free slits, scatter-free deflector
  surfaces, external tilt/position adjustment.

Stockli
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Laser-based Emittance Monitor under construction
Laser 20 mJ, 0.2 mm
H-
Ho
Scintillator

• Technique proposed by R. Shafer as part of
  beam-in-gap system
• System under construction is located upstream
  HEBT Bending dipole deflects H- beam and
  remaining electrons while Ho beam will travel
  free from the influence of dipoles, quads etc
• Gas stripping background measured, appears low
  enough

D. Jeon, J. Pogge, Y. Liu, A. Menshov, I.
Nesterenko, W. Grice, A. Aleksandrov, S. Assadi
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Expected beam distribution at laser and 17.7 m
downstream at scintillator
X
Y
rad
rad
cm
X
Y
cm
X
Y
rad
rad
Y
cm
cm
X
D. Jeon
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Wire Scanners
Original requirements wire position resolution
of 0.2 mm, amplitude resolution of 0.65 microamps
(2.5 sigma)
T. Roseberry
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Matching with wire scanners
Before
After
From fit to Trace3D model, Emittance 0.34 mm mrad
D -O Jeon
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Initial Laser Wire Development at BNL
Laser Wire Profile with 100uA 200MeV Polarized
Beam
Scope was set on infinite persistence for several
hundred beam pulses. This is difference signal
at 200 MHz from upstream and downstream BPMs.
Small Q-switched NdYAG laser for similar 2.5 MeV
test at LBNL
Youve gotta be a believer Roger Connolly
(BNL)
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Laserwire System Operating at SNS
Original requirements decision to deploy laser
wire was based on goal of meeting requirements
for the displaced SCL carbon wire scanners
Liu, Assadi, Blockland, et al
S. Assadi HIB2008
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Bunch Shape Monitor Installations
Installed in CCL (July 2004)
BSM installed in D-plate (August 2003)
Before installation in D-plate

• In addition, BSMs recently installed HEBT at 1 GeV

Feschenko, Aleksandrov, et al
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Effect of beam loading in the LinacBunch Shape
Monitor results
Cavity field and phase droop with feedback alone
(left) and feedback feedforward (right) beam
loading compensation.
Phase width of the bunch along the pulse with
feedback alone (left) and feedback feedforward
(right). Phase width in CCL is larger than design
value.
Feschenko
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Mode Locked LaserLongitudinal Measurements
2.5 MeV H-, 402.5 MHz bunching freq, Ti-Sapphire
laser phase-locked _at_ 1/5th bunching frequency
collected electron signal plotted vs. phase
Measured and predicted bunch lengthvs. cavity
phase setting
Grice, Assadi, et al
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Imaging Near Momentum Dump
W. Blokland, S. Murray, M. Plum
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Scintillator Coating Development
Beam test at LANL
Electron backscatterimage
McManamy, Kenik
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Scintillator Analysis
X-ray diffraction results flame spray samples
are gt85 alpha phase alumina
F. Montgomery
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Optical Transition Radiation Studies
Broad angular distribution of OTR from a
aluminized screen 30 degrees from normal to an
800 MeV proton beam

• For GeV proton beams, photon yield is low
• For a single WNR pulse with ? 1.85 and 2.71013
  protons, we should collect 3.1108 visible
  photons in the proposed optical acceptance cone
• Utilize camera with high quantum efficiency and
  slow readout to enhance signal to noise without
  increasing susceptibility to background radiation
• However, initial test at LANL did not produce a
  discernable beam image we are eager to perform a
  follow-up experiment

Example of OTR from 5keV electrons _at_ UMER
Fiorito, Shkvarunets
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Comments

• Original diagnostics suite focused on
  commissioning and setup for ops. Outstanding
  readiness and performance in this role.
• Approaching full power, SNS essentially is loss
  limited machine (1W/m)
• Current Linac Tune-up
• Restore settings
• Check using baseline diagnostics
• Tune on loss measurement transport halo through
  linac to collimators
• As illustrated during Montauk workshop,
  significant opportunity in halo diagnostics

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