Positron injection and trapping for positron-electron pair plasma creation
Final Report Abstract
Positrons (the antiparticles of electrons) have a great variety of uses - from explorations of fundamental physics to materials testing - but unfortunately their availability is limited. The strongest e+ source in the world, the NEutron-induced POsitron source MUniCh [NEPOMUC], is capable of delivering up to 109 e+/s (monoenergetic). This is a very large rate of antimatter; for comparison, however, it is a tiny fraction of the number of electrons that can be emitted from a typical glowing light bulb filament. For the many different scientific studies that would benefit from large numbers and/or high densities of positrons, it would be highly attractive if the NEPOMUC beam could be "saved up" for several hours, then released in less than a millisecond. This is what motivates this project: development of a device capable of accumulating large numbers of positrons, then delivering them in the form of intense pulses to user experiments. Excellent progress has been made on that development, in two key ways: (1) "A place to keep all those positrons": The first and most ambitious task is to design, build, and operate a "multi-cell trap" (MCT). The MCT consists of an array of "non-neutral plasma traps" (Penning-Malmberg traps) that will enable storing a record number of positrons for the required long durations. Requirements include a specialized set of precisely aligned electrodes in a uniform, high (3-T) magnetic field and UHV (ultra-high vacuum). Additionally, state-of-the-art non-neutral plasma manipulation techniques are needed to load and unload the storage traps without major losses. A major achievement of the project is the successful development of our MCT prototype; it is only the second ever such device in the world and, by a wide margin, the best-performing. (2) "A way to get the positrons in there": The second biggest challenge of the project was to develop techniques/equipment to transport low-energy positrons into the very high magnetic field of the MCT. (This is generally challenging due to magnetic mirroring.) Two different approaches to this were successfully pursued: (a) "in situ remoderation", in which a beam of high-energy positrons is converted to low-energy positrons via implantation in a solid (e.g., SiC) (b) commissioning of a buffer-gas trap (BGT) system for the NEPOMUC beam line, which is the "go to" way to a beam of low-energy positrons into dense, cold, short positron pulses - which can then be transferred to and stored in the MCT. In addition to the above key experimental advances, this project also involved a scoping study about the interactions of positrons with atoms/nano-particles/clusters (specifically C60). This is an example of one of the compelling uses of e+ pulses from (or in) the MCT. Thanks to the successful work on the prototype MCT, in situ remoderation, and BGT commissioning, we are now much closer to the ultimate goal of producing large, cold, dense positron pulses for scientific users.
Publications
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A new frontier in laboratory physics: magnetized electron–positron plasmas. Journal of Plasma Physics, 86(6).
Stoneking, M. R.; Pedersen, T. Sunn; Helander, P.; Chen, H.; Hergenhahn, U.; Stenson, E. V.; Fiksel, G.; von der Linden, J.; Saitoh, H.; Surko, C. M.; Danielson, J. R.; Hugenschmidt, C.; Horn-Stanja, J.; Mishchenko, A.; Kennedy, D.; Deller, A.; Card, A.; Nißl, S.; Singer, M. ... & Chin, K.
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Non-neutral plasma manipulation techniques in development of a high-capacity positron trap. Review of Scientific Instruments, 92(12).
Singer, M.; König, S.; Stoneking, M. R.; Steinbrunner, P.; Danielson, J. R.; Schweikhard, L. & Pedersen, T. Sunn
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Combined remoderation-drift scheme for positron injection into a magnetic trap. Physical Review Research, 5(2).
Hergenhahn, U.; Horn-Stanja, J.; Nißl, S.; Saitoh, H.; Singer, M.; Sunn, Pedersen T.; Hugenschmidt, C. & Stenson, E. V.
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Magnetic field considerations for a multi-cell Penning–Malmberg trap for positrons. Journal of Plasma Physics, 89(4).
Witteman, D.R.; Singer, M.; Danielson, J.R. & Surko, C.M.
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Multi-cell trap developments towards the accumulation and confinement of large quantities of positrons. Journal of Plasma Physics, 89(5).
Singer, M.; Danielson, J.R.; König, S.; Pedersen, T. Sunn; Schweikhard, L. & Stenson, E.V.
