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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. Outline. SNS overview Facility Parameters Status Example Diagnostics Low energy emittance High energy emittance (future) Wire scanners

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review of profile and emittance diagnostics for the sns linac

Review of Profile and Emittance Diagnostics for the SNS Linac

Tom Shea

ORNL

for the ORNL Research Accelerator Division

December 11, 2008

outline
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

T. J. Shea, SNS Target Imaging April 2008

the sns partnership during construction
The SNS Partnership During Construction

Partner lab obligation completed:

  • LBNL: Sept 2002*
  • LANL: Sept 2004
  • JLAB: April 2005
  • BNL: April 2005

Accelerator Systems Overview and Plans

January 22-24, 2008

probability of communication
Probability of Communication

T. Allen, Sloan WP 165-97, MIT, 1997

Controls Brown Bag, October 5, 2006

sns accelerator complex
Chopper system makes gaps

945 ns

mini-pulse

Current

Current

1 ms macropulse

1ms

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

DTL

RFQ

SRF,b=0.61

SRF,b=0.81

CCL

Liquid Hg Target

Accelerator Systems Overview and Plans

January 22-24, 2008

sns linac design parameters
SNS Linac Design Parameters

Accelerator Systems Overview and Plans

January 22-24, 2008

performance
Performance

Henderson, Purcell

T. J. Shea, SNS Target Imaging April 2008

emittance measurement issues for the sns low energy beam transport
RFQ

RFQ

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 >35 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

low energy emittance measurements
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.

sns lebt emittance discussion
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

laser based emittance monitor under construction
Laser-based Emittance Monitor under construction
  • 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

Laser: 20 mJ, 0.2 mm

H-

Ho

Scintillator

D. Jeon, J. Pogge, Y. Liu, A. Menshov, I. Nesterenko, W. Grice, A. Aleksandrov, S. Assadi

Presentation_name

expected beam distribution at laser and 17 7 m downstream at scintillator
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

Presentation_name

wire scanners
Wire Scanners

Original requirements: wire position resolution of 0.2 mm, amplitude resolution of 0.65 microamps (2.5 sigma)

T. Roseberry

T. J. Shea, SNS Target Imaging April 2008

matching with wire scanners
Matching with wire scanners

Before

After

From fit to Trace3D model,

Emittance ~0.34 mm mrad

D -O Jeon

T. J. Shea, SNS Target Imaging April 2008

initial laser wire development at bnl
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 Nd:YAG

laser for similar 2.5 MeV test at LBNL

“You’ve gotta be a believer” – Roger Connolly (BNL)

laserwire system operating at sns
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

Camera

Mirror

Power meter

Laser room

Laser wire station

Cryomodule number

LR

  • 4 LW from 200 MeV
  • 4 LW from 450 MeV
  • 1 LW at 1 GeV

LR

32

1

2

3

4

5

12

13

14

15

17

32

CCL

HEBT

250 m

227 m

160 m

25 m

Liu, Assadi, Blockland, et al

S. Assadi HIB2008

bunch shape monitor installations
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

effect of beam loading in the linac bunch shape monitor results
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

mode locked laser longitudinal measurements
Mode Locked LaserLongitudinal Measurements

2.5 MeV H-, 402.5 MHz bunching freq, Ti-Sapphire laser phase-locked @ 1/5th bunching frequency

collected electron signal plotted vs. phase

Measured and predicted bunch lengthvs. cavity phase setting

Grice, Assadi, et al

imaging near momentum dump
Imaging Near Momentum Dump

W. Blokland, S. Murray, M. Plum

T. J. Shea, SNS Target Imaging April 2008

scintillator coating development
Scintillator Coating Development

Beam test at LANL

Electron backscatterimage

McManamy, Kenik

T. J. Shea, SNS Target Imaging April 2008

scintillator analysis
Scintillator Analysis

X-ray diffraction results: flame spray samples are >85% alpha phase alumina

F. Montgomery

T. J. Shea, SNS Target Imaging April 2008

optical transition radiation studies
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 2.7·1013 protons, we should collect 3.1·108 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 @ UMER

Fiorito, Shkvarunets

Presentation_name

comments
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

T. J. Shea, SNS Target Imaging April 2008

http neutrons ornl gov
http://neutrons.ornl.gov

Controls Brown Bag, October 5, 2006

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