SNS Laser Wire Design Proposal Introduction

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SNS Laser Wire Design ProposalIntroduction

• Diagnostic Groups BNL, LANL,LBNL
• And ORNL
• Collaborators SLAC
• Presented by Saeed Assadi

May 20, 2002
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Multi-National Laboratory Diagnostic Collaborators
Tom Shea, Sasha Aleksandrov, Saeed Assadi, Willem
Blokland, Craig Deibele, Warren Grice, Dave
PurcellDavid Purcell
ORNL
BNL
Peter Cameron, Roger Connolly, John Cupolo,
Craig Dawson, Chris Degen, Sheng Peng, Marty
Kesselman, Joe Mead, Al Della Penna, Bob Sikora,
Mike Plum, John Power, Bob Shafer, Jim Stovall
LANL
LBL
Larry Doolittle, Darryl Oshatz, Alex Ratti
SLAC
Joe Frisch , Keith Jobe, Marc Ross
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Outline

• Progress Report.
• 2) Summary of the collaboration (mini Workshop at
  SLAC) report.
• 3) Measurement results from MEBT and BNL 200 MeV
  line.
• 2) Carbon Wire vs. Laser wire, radiation issues
• 5) Whats Next

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Approach
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Present Baseline Diagnostics
Dave Purcell
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Laser wire vs. conventional wire

• Laser Wire
• Minimal impact on normal operation
• Virtually no impact on SRF cavities or vacuum
• Low signal to noise ratio
• No parts inside the vacuum
• Radiation hardness unknown

• Conventional Wire
• Requires off-operation with 100 ms macro-pulses
  at low rep rate
• Ablation from the wire may contaminate the SRF
  cavity
• Signal to noise not a problem
• Maintenance requires vacuum access
• Very radiation hard

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Laser Wire Development Collaboration

• 1) Until Fall 2001, all Laser Wire development
  and RD was conducted at BNL as a one-of-a-kind
  SNS diagnostic.
• Since September 2001, LANL, ORNL and SLAC have
  joined BNL to study the feasibility of using
  Laser Wires as a potential alternative to carbon
  wire scanners or to supplement them.

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Collaboration Highlights

• 1) Warren Grice from the Main Laboratory has
  joined the diagnostic group on half time Term
  position.
• 2) We had a one and half day workshop at SLAC to
  discuss the Laser wire design, Choices of Lasers,
  transport line, optics, laser room and safety.
• We have carried out extensive studies of signal
  to noise ratio, 5 Tech
• notes. The results have lead us to concentrate
  on Q-switched lasers.
• 4) We have studied the effect of laser beam
  reflection from the laser beam dump (Ghost
  effect).
• 5) We are considering a number of detection
  techniques, including electron detectors.
• 6) We have conducted Laser studies on the MEBT
  and the BNL 200 MeV line.
• 7) We are at early stages of establishing
  collaboration with FNAL to test the Laser-wire
  and the electron detector at 400 MeV LINAC
  (Sept-2002).

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Proposed Laser Wire Installations
402.5 MHz
805 MHz
HEBT
To Ring and TGT
MEBT
DTL
CCL
SRF, ß0.61, 0.81
RFQ
Injector
1000 MeV
86.8 MeV
186 MeV
2.5 MeV
MEBT 5 WS (elec. only) 6 BPM (elec. only) 2
SlCol (act. only)
DTL 5 WS 10 BPM 6 CM (p/u only) 5 ED/FC
CCL 8 WS 12 BPM 2 CM (p/u only) 1 ED/FC
SCL 32 WS (16 elec.) 32 BPM
HEBT 3 WS (dumps) 22 BPM (elec. only)
RTBT 1 Harp
D-plate (7.5 MeV) 1 WS 3 BPM 1 CM (p/u only) 1
ED/FC 2 SlColl emit 1 Phosphor screen 1 8 seg.
halo scraper 1 Beam stop / F-Cup
Laser Wires
Key WS wire scanner BPM beam position
monitor SlCol slit and collector emittance
station CM current monitor ED/FC energy
degrader Faraday Cup
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SCL wire scanner locations
SCL
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SCL wire scanner assembly and laser wire ports
Laser viewing port
Original design
SCL wire scanner assembly
Laser viewing port
Vacuum box supports
Vacuum pump port
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Carbon Wire Motion with respect to the Laser wire
ports

In
OUT
Middle
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Carbon Wire crossing with respect to the Laser
wire line of sight
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Some Background about the carbon wire scanners

• SCL position accuracy is 0.13 mm.
• Minimum beam size (at a wire scanner) in SCL is
  12 mm.
• To get 10 width accuracy for this beam size need
  wire position accuracy of about 0.30 mm. Wire
  scanner thus meets requirements.
• To meet 2 mm beam position accuracy requirement
  need about 4 mm wire position accuracy. Easily
  meet this requirement.
• Note that simulations do not take beam jitter or
  beam intensity fluctuations into account. This
  will result in a increased error. We are working
  closely with the physics teams to quantify the
  beam jitter and to develop beam tuning
  algorithms.

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Wire temperatures

• 32 micron C wire
• Temperature at the center of beam

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Max. Wire Temperature In The Injection Line (2 MW
case)
Data provided by C.J. Liaw
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Wire scanner signal levels

• 32 micron dia. carbon wire.
• Electronics should measure down to 1 of weakest
  signal from 5 mA avg. beam current, and up to
  about 200 of strongest signal from 26 mA avg.
  beam current
• Total current range is 16 nA to 1.8 mA.

Signals at center of beam, 26 mA avg. beam current
Transition from neg. to pos. signal at about 107
MeV. Note Signal levels are accurate to about
a factor of two.
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LEDA carbon wire tests

• Broken wire shows evidence for ablation at the
  broken end, but nowhere else.
• SNS-Linac conventional carbon wire scanners are
  fitted with 32 micron C wires.

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Laser Wire Activities
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BNL(lead), LANL, ORNL and SLAC are Collaborating
in Development of the SNS Laser Beam Profile
Monitor

• First ionization potential for H- ions is 0.75eV.
  Photons with llt1500nm can remove an electron
  leaving neutral H plus electron.
• NdYAG laser can be used to place a current
  notch in selected portion of beam and the notch
  depth is measured with the BPMs.
• All laser hardware is exterior to beam-line. No
  parts are inserted into the beam (with the
  exception of the possible electron collector),
  thus eliminating risk of damage to
  super-conducting cavities or beam-pipe.
• Eliminates risk of damage to super-conducting
  cavities
• Measurements at 750 keV on BNL linac produced
  clean profiles past 2.5 s
• Method being refined with 200 MeV beam at BNL
• MEBT platform is built and some results obtained

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Collaboration Results
One laser per station 1) Simple 2) Quality
of laser severely limited by the budget
Single laser station 1) More resources
available for a higher quality laser 2)
Requires some sort of distribution system
Choice Single laser

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Collaboration Results (cont.)
Q-switched laser 1) Less expensive 2) Not
necessary to match the phase of the ion pulse
train 3) Only 1 of the laser energy overlaps
with the ion pulses
Mode-locked laser 1) All of the optical energy
available for neutralization 2) Because less
energy per pulse is needed, optical damage is
less likely 3) Longitudinal profiling also
possible 4) More expensive 5) Pulses must be
properly timed
Choice Q-switched laser
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Collaboration Results (cont.)
Distribution by fiber 1) Optical damage likely,
especially with longer, Q-switched pulses. 2)
Radiation darkening would be a serious
problem. 3) For mode-locked pulses, additional
dispersion compensation would be required
Distribution by direct beam propagation 1)
Optical damage less likely 2) System should
accommodate multiple laser systems 3) Active
stabilization may be necessary
Choice Direct beam propagation

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Collaboration Results (cont.)
Beamline optics in dry purge 1) Special stages
and mounts not required 2) Purge should be
sufficient to keep optics clean.
Beamline optics in vacuum 1) Requires
vacuum-safe optical stages and mirror
mounts Choice Dry purge
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Effects of Secondary Reflections

• Reflected beam can also neutralize H-

• The effect contributes lt 1 to the total
  neutralization because...
• the reflected beam is much weaker
• and the energy is distributed over a much
  larger area.

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Beam Box Considerations

Mechanical Engineer Danny Mangra
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Beam Box Considerations
Mechanical Engineer Danny Mangra
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Required Magnetic Field to Collect Electrons in
SCL
Choosing
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Laser Wire Data Acquisition Setup
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MEBT Laser Monitor on the Beam Box
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MEBT Laser Wire
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750 keV experiment, using BCT
Diagram of experiment installed on BNL linear
accelerator. The laser is on the platform at the
left. The top-center mirror switches between
vertical and horizontal scans. Mirror at
top-right scans horizontally and mirror at bottom
left scans vertically.
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Beam Current notch measures the local Point on
the Transverse profile (BNL studies)
Oscilloscope trace of output of
current transformer in 750 keV test showing
current notch created by laser. The signal is
filtered with a 50 MHz low pass filter to remove
the Linac 200 MHz rf. Profile measurements were
made by measuring the notch depth at each
mirror position. S/N at beam center of 25dB.
Signal to 2.5 s.
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Beam Current notch measures the local Point on
the Transverse profile (MEBT) May-16-2002

Beam is negative current (H-) and the notch makes
it positive
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SNS-MEBT First Laser Wire Measurement at LBNL
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SNS-MEBT Laser Wire Measurement at LBNL
May-18-2002 Lesson Learned

We will change our data acquisition and analysis
methods
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Transverse Beam Profile Measurement of SNS MEBT
using Laser Wire
Horizontal (top) and vertical beam profiles
measured on the SNS HEBT. RMS widths
are 1.600.04 mm horizontal 4.160.16 mm
vertical All laser and mirror control and all
data acquisition were accomplished with one
"push of the button".
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Carbon Wires Used in MEBT to Compare the
Transverse Profiles to the Laser
Wire Measurements at the same longitudinal
locations. Same beam conditions
SNS Wire systems, 3-wires per fork to measure
Horizontal, vertical and 45 Degrees., Data
provided will be raw, fit and some statistical
analysis such as standard deviation, FWHM, mean
position..
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Expected Beam size in the SCL
Slide from Sowmass 2001 talk by John Galambos
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Differential BPM measurement
Top figure shows the differential nulling that
was successful on April 17. The pickups were
single BPM striplines and the differential
measurement was made digitally on the
scope. Earlier difficulties in nulling
the upstream-downstream signal may have been
caused by small phase shifts introduced by a
variable attenuator or harmonic generation by
power combiners. When these were removed we
achieved CMR greater than 20 dB.
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Recent measurements with 200 MeV beam are very
promising
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Beam Profiles Measured on BNL LINAC
Profile measurements of the 200 MeV linac beam.
Top plot was made with 0.10 mA polarized beam.
Bottom plots were made on 10 mA unpolarized
beam. Left plot is LPM and right is wire scanner
measurement made one day earlier. s(wire) 3.5
mm s(LPM) 5.3 mm
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Radiation Issues

• To summarize, the maximum radiation dose
  equivalent to the silicon quality factor at the
  surface of beam pipe is listed here
• DTL 3 rads/hr
• CCL 14 rads/hr
• SCL medium beta 2.5 rads/hr
• SCL high beta 8 rads/hr
• SCL spare module section 30 rads/hr
• For a 30 year facility lifetime, 10 months
  operation per year, the maximum dose from normal
  operations is 30 x (10/12) x 365 x 24 x (30
  rads/hr) 7 Mrads. Since the tune up and
  off-normal loss rates occur for such a short time
  compared to the normal loss rates, the overall
  radiation exposure will be only slightly more
  than that for normal losses. All beam diagnostics
  components located by the beam line should
  therefore ideally be rated for 10 Mrads.

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Radiation hard components

• Radiation dose over 30 y lifetime of SNS has been
  calculated to be about 10 MRad.
• The Laser system is located outside of the
  tunnel.
• BNC connectors use rexolite insulator, rated to
  1000 Mrad.
• Windows are made from fused silica.
• Mirrors and lenses do not have coatings. No
  teflon, no PVC. Should be sufficiently rad hard.
• Stepper motors not rad hard due to expense, but
  designed to be easily replaced.

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Radiation Environment That the Optics Encounter
The allowable beam loss is about 1 Watt/m during
full-current operation (1.4 mA average
current).1 watt/m corresponds to about 100 nA,
10 nA, and 1 nA per meter at 10, 100, and 1000
MeV respectively.
Data is provided by Franz X Gallmeier.
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Status
1. Pulsed laser at 750 keV with BCT gave good
signals. S/N25dB at beam center with 10-pulse
averaging. 2. Profile measurements at 2.5 MeV
on MEBT beam worked first time. a. Average of
25 beam pulses/laser position gave 40dB S/N at
beam center with faulty BCM signal cables. b.
Most 'noise' was rf pickup which can be reduced
by filtering for a faster measurement. c. Both
horizontal and vertical profiles measured with
one "push of the button". 3.
Recent measurements with 200 MeV beam are very
promising. a. Studies on April 17, 2002
resulted in very clean laser notch from stripline
differential measurements. b. A few software
bugs are being worked out for measurements April
25.
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NEXT

• Converge on an acceptable design from cost and
  implementation
• Stand Point.
• Present the Plan to the diagnostic advisory
  committee and the ASD
• Management with a realistic schedule.
• Refine the data acquisition and analysis
  programs.
• Compare different collectors.
• Verification of methods on BNLs 200 MeV line.
• 6) Collaborative effort on testing laser wire
  at FNAL.

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