Generation of High Intensity Positron Beam Using 20 MeV linac

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Generation of High Intensity Positron Beam Using
20 MeV linac

• Sergey Chemerisov and Charles D. Jonah
• Chemistry Division, Argonne National Laboratory

March 25, 2009 Jefferson Lab Newport News, VA
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Timeline of the positron source development at ANL

• October 2003 ANL was approached about the
  possibilyty of setting up a positron- production
  facility at the CSE Division linac
• 19 and 20 August 2004 Invitational Workshop on
  Linac-based Positron Beams
• September 2004 Memorandum of understanding was
  sent to LLNL for the loan of of the
  positron-production equipment.
• May 2005 Positron front end arrived from LLNL
• September 2005 First slow positron beam was
  measured at ANL linac
• February 2006 Improvements to the positron
  transport system were implemented. Positron beam
  with conversion efficiency of 3.5 x 10-8 slow
  positrons per fast electron was measured
• June 2008 new positron converter/moderator
  assembly was installed and tested

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Acknowledgements

• Ashok -- Palakkal Asoka-Kumar (formerly LLNL)
• Hongmin Chen (University of Missouri, Kansas
  City)
• Ken Edwards (United States Air Force)
• Wei Gai (Argonne National Laboratory)
• Rich Howell (formerly LLNL)
• Alan Hunt (Idaho State University)
• Jerry Jean (University of Missouri, Kansas City)
• Charles Jonah (Argonne National Laboratory)
• Jidong Long (Argonne National Laboratory)
• David Schrader (Marquette University)
• Al Wagner (Argonne National Laboratory)
• Lawrence Livermore National Laboratory
• Funding
• DOE
• US Air Force Research Labs

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Characteristics of Argonne Linac

• L-band
• 20 MeV no-load energy
• Steady-state mode 15.5 MeV at 1-amp pulse
  current
• Steady-state mode 14 MeV at 2 amp pulse current
• Peak current at 30-ps pulse of 1000 A
• Repetition rate 0-60 Hz (can be increased by
  about a factor of 5)
• Pulse width 30 ps-5 ?sec
• Maximum average current 200 ?A due to windows
  thermal load limitations.
• 1/12 sub harmonic buncher (108 MHz)

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Positron Source layout
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Diagram of positron transport
Microchannel plate
Shield
Converter/moderator
Up and Down 30 degree solenoid
Aperture
Vacuum valve
R 6
Radiation Detector
Lead shield
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Present condition of positron production line at
CSE division linac
Front end
bends to separate electrons from positrons
shielding
Output end
detector
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Characteristics of Positron system

• First measurements were done using 1-cm thick
  tungsten target that was borrowed from LLNL --
  about a factor of 5 too thick for our energy
  range
• Moderator was either the original vaned LLNL
  moderator or that supplemented by 3 layers of
  tungsten mesh
• New converter is 2 mm thick. Converter holder is
  water cooled, but converter itself is not.
• New moderator is 10 layers of tungsten mesh
• Transport system uses 4-inch stainless-steel
  tubing
• Positrons are guided using both Helmoltz coils or
  a solenoid

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Signal from microchannel plate detector
Band holding Moderator in
bright spot from thick part of mesh
Sharp focus shows little space-charge effect
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? counting
Positron (moderator ) Radiation (moderator
-) Background (beam off)
0.511 MeV (positron-annihilation ??
Na22
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Microchannel plate current as a function of
voltage
50 volts 22 volts full current 22 volts
(shortened pulse) 10 volts
pulse
The higher the voltage, the sooner the positrons
come out
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Energy dependence for slow positron production
Difference between experimentally measured
positron yield and total number of positrons
leaving is due to the difference in the energy
spectrum of the positrons
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Improvements

• New converter and moderator configuration
  (installed)
• According to EGS calculation, using a converter
  optimized for our beam energy and a repositioned
  moderator will improve flux by factor of 10.
  Moderator thickness is not optimal judging from
  bright spots on the MCP image.
• Couple apparatus to linac and remove window
  limitation
• Window limits the electron current to 200 ?A
  without window we should be able to put out 600
  ?A (factor of 3 in positron intensity)
• Increase linac power by installing new power
  supplies.
• That will increase repetition rate from 60 Hz to
  300Hz or factor of 5 of the average current.
• Use single crystalline moderator in reflection
  mode.
• Apply electrostatic potential between converter
  and moderator (factor of 3).
• Total improvement is 450 times

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New Moderator-converter
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Positron flux
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How to increase yield of slow positrons?

• Increase moderation efficiency

It is known that moderation is much more
efficient if the positrons are at lower energies.
If we can lower the energy of the positrons
exiting a converter, we should be able to
moderate more efficiently.

• Avoid moderation entirely

If we can bunch the positrons into a narrow
energy range, we should be able to inject them
into a Penning-type trap and slow them via
natural processes
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How have we explored these options in silico?

• Yield of slow positrons as a function of positron
  energy

We have used the EGSnrc program to simulate the
yields of positrons as a function of energy. We
have used the yield of positrons stopped
(reduced to less than 2 keV) within 1 micron of
the surface as a proxy for the yield of slow
positrons.

• The slowing and bunching of positrons

We have simulated an RF cavity, drift space,
magnetic fields and phase of RF using the program
Parmela.
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Positron moderation efficiency calculations
Fraction of the positron stopped in 1 mm layer of
the moderator
Geometry used for positron yield calculation
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Positrons stopped as a function of energy
100 keV shift
Energy spectrum of the positrons produced in 2 mm
W target bombarded with 15 MeV electrons
Comparison of the slow positron yield for
original and shifted by 100 kev energy
distribution for transmission and reflection
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Advanced techniques for better positron moderation
Drift positrons to achieve spatial separation
Use RF cavity or electrostatic potential for
deceleration
Use RF cavity to uniformize the energy of the
positrons
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Calculations
a
b
Schematic of the slow-positron beam-line design,
cavity gap5cm, considering the fringe field, the
total length of affected region along z is set
25cm. In the AMD, magnetic field along z axis
decreases from 10000Gauss to 720 gauss from
entrance to exit (100 cm). The field in the AMD
satisfies optimized design equation.
(1)
(a) transverse phase ellipse of the beam at the
AMD entrance, (b) transverse phase ellipse of the
beam at the exit horizontal coordinator is x
axis in cm, vertical coordinator is x prime
(Px/Pz) in mrad.
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Compression and translation of positron spectrum
Energy spectrum comparison for cavity that
operates at 108 MHz and cavity that have 108 and
216 MHz frequencies. Case 1, the peak value is
around 873 positrons out of 59034 within
80keV,100keV or 1.47 . Case 2, the peak
location shifts to 40keV,60keV while value
raised to 1.6 of total positrons. In both
cases, the average axial electrical fields are
less than 5MV/m in the cavity.
Energy spectrum of the positrons before and after
one 108 MHz cavity optimized for the number of
positrons in the narrow band (60-80 keV) and wide
band (0-100 keV)
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Increase in yield expected
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Where is the sweet spot for slow positron
production?
Relative yield of positrons as a function of the
incident electron energy. The yield of total
positrons increases virtually continuously
(closed squares) while the number of thermalized
positrons appears to approach saturation at about
60 MeV both for reflected moderation (filled
circles) or transmitted moderation (open
circles). If one is going to design an
electron-linac-based positron source the optimal
electron energy for positron generation will be
in of 40-60 MeV range.
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Summary

• We have substantial yield of slow positrons at
  present (108 slow e/s)
• Simple techniques to increase the power on the
  converter target should enable a substantial
  increase in positron flux
• Accelerator-based techniques to alter the energy
  spectrum of positrons have potential to increase
  slow positron flux by 2 orders of magnitude.
• The ideal accelerator for slow positron
  production is in 40-60 MeV energy range