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Generation of High Intensity Positron Beam Using 20 MeV linac

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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Generation of High Intensity Positron Beam Using 20 MeV linac

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  1. 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

  2. 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 Timeline of the positron source development at ANL

  3. 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

  4. 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)

  5. Installed equipment Microprobe Existing equipment, not installed Proposed equipment PAES Penning trap PALS and DB linac Positron Source layout

  6. Diagram of positron transport Microchannel plate Shield Converter/moderator Up and Down 30 degree solenoid Aperture R 6” Vacuum valve Radiation Detector Lead shield

  7. Present condition of positron production line at CSE division linac Front end bends to separate electrons from positrons shielding Output end detector

  8. 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

  9. Signal from microchannel plate detector Band holding Moderator in bright spot from thick part of mesh Sharp focus shows little space-charge effect

  10.  counting Positron (moderator +) Radiation (moderator -) Background (beam off) 0.511 MeV (positron-annihilation  Na22

  11. 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

  12. 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

  13. 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

  14. New converter/moderator chamber Existing setup e- Table 1 Table 2 e- e- e- Beam stop e+ e- e+ New Moderator-converter

  15. Positron flux

  16. 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

  17. 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.

  18. 50 m 1 m 1 m -1 10 Reflection Transmission -2 10 Fast e+ Fraction of positrons stopped -3 10 Slow e+ transmission Slow e+ reflection -4 10 -5 10 5 6 7 2 3 4 5 6 7 2 3 4 5 6 7 10 0.1 1 10 Energy (MeV) W foil Positron moderation efficiency calculations Fraction of the positron stopped in 1 mm layer of the moderator Geometry used for positron yield calculation

  19. Yield from shifting spectrum by 100 keV 200 14 Yield is relative to transmission = 1 12 150 10 Positron count 8 100 6 50 4 2 0 0 2 4 6 8 0 Energy, (MeV) original reflection shifted reflection original transmission shifted transmission 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

  20. I E E E t E t t 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

  21. 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. (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.

  22. 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)

  23. Increase in yield expected

  24. 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.

  25. 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

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