Transcript The Development of the Argonne Positron Source

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
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 positronproduction 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
Acknowledgements
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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
Characteristics of Argonne Linac
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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)
Positron Source layout
Installed equipment
Microprobe
Existing equipment, not installed
PAES
Penning trap
Proposed equipment
PALS and DB
linac
Diagram of positron transport
Microchannel plate
Shield
Converter/moderator
Up and Down
30 degree solenoid
Aperture
R 6”
Lead
shield
Vacuum
valve
Radiation
Detector
Present condition of positron production line at CSE
division linac
Front end
bends to
separate
electrons
from
positrons
shielding
Output end
detector
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
Signal from microchannel plate detector
Band holding
Moderator in
bright spot
from thick
part of mesh
Sharp focus shows little space-charge effect
 counting
Positron
(moderator +)
0.511 MeV (positron-annihilation 
Radiation
(moderator -)
Background
(beam off)
Na22
Microchannel plate current as a function of voltage
pulse
50 volts
22 volts full current
22 volts (shortened pulse)
10 volts
The higher the voltage, the sooner the positrons come out
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
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
New Moderator-converter
New
converter/moderator
chamber
Existing
setup
e-
Table 1
e-
Table 2
e-
e-
Beam stop
e+
e-
e+
Positron flux
Technique
Positrons per second
Measured with 100A
beam,
1 Amp peak
1.5 x 107
As is with 200A beam
1A peak current
3.0 x 107
Modify
converter/moderator
3.0 x 108
Couple directly to linac
Use reflection geometry/
increase linac power
6 x 108
3.0 x 109
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
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.
Positron moderation efficiency calculations
50 m
1 m
Fast e+
Slow e+
transmission
Slow e+
reflection
Fraction of positrons stopped
1 m
10-1
Reflection
Transmission
10-2
10-3
10-4
10-5
10
W foil
Geometry used for positron yield calculation
5 67
0.1
2
3 4 5 67
1
Energy (MeV)
2
3 4 5 67
10
Fraction of the positron stopped in 1 m layer of
the moderator
Positrons stopped as a function of energy
100 keV shift
200
14
Positron count
150
12
Yield from shifting spectrum by 100 keV
Yield is relative to transmission = 1
10
100
8
6
50
4
2
0
0
0
2
4
6
Energy, (MeV)
8
Energy spectrum of the positrons produced in 2 mm
W target bombarded with 15 MeV electrons
original
reflection
shifted
reflection
original
shifted
transmission transmission
Comparison of the slow positron yield for
original and shifted by 100 kev energy
distribution for transmission and reflection
Advanced techniques for better positron moderation
I
Drift positrons to achieve
spatial separation
E
E
Use RF cavity or electrostatic
potential for deceleration
t
Use RF cavity to “uniformize”
the energy of the positrons
E
E
t
t
Calculations
a
b
Schematic of the slow-positron beam-line design,
cavity gap=5cm, 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.
Bz 
10000
(1)
1  0.129  z
(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.
Compression and translation of positron spectrum
600
No.1-Originla Spectrum
500
No.2- Result from case 1
400
No.3-Result from case 2
300
200
100
0
0.01
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)
0.1
1
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.
10
Increase in yield expected
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.
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