7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Injection and trapping of pulsed positrons in dipole magnetic field toward
pair-plasma creation

Haruhiko Saitoh1, Jens von der Linden2, Adam Deller2, Hiroyuki Higaki3, Koji Michishio4,
Toshihiko Hori4, Nagayasu Oshima4, Stefan Nissl2, Eve V. Stenson2
1
Department of Advanced Energy, The University of Tokyo, 2 Max Planck Institute for Plasma
Physics, 3 Division of Advanced Science and Engineering, Hiroshima University,4National Institute of Advanced Industrial Science and Technology
e-mail (speaker): saito@ppl.k.u-tokyo.ac.jp
Plasmas composed of equal-mass charged particles arecalled pair plasmas and are theoretically predicted toexhibit unique properties of wave propagation andinstabilities. Pair plasmas consisting of electrons andtheir antiparticles, positrons, are believed to exist widelyaround high-energy celestial bodies such as pulsars.
Understanding electron-positron plasmas is crucial not
only for basic plasma physics but also for studying thefundamental processes of astrophysical phenomena.
However, the confinement of a large number of positrons
together with an equal number of electrons that satisfy
the plasma conditions is challenging, resulting in only afew experimental studies on electron-positron plasmas.
We utilize a magnetospheric dipole field geometry andslow positron beams for creating and investigating
electron-positron plasmas. The dipole field generated bya magnetically levitated superconducting coil is one of
the so-called innovative confinement concepts for fusion
plasmas. The RT-1 experiment [1] of the University ofTokyo has successfully achieved stable confinement ofhigh-beta plasmas suitable for advanced fusion and nonneutral (pure electron) plasmas, demonstrating thecapability to confine both positrons and electrons inprinciple. In the final plan of the electron-positronplasma project, a large number of positrons (>109) are
accumulated in a linear trap with strong field (5T), using
an intense positron source and a buffer gas trap [2].
These positrons are then extracted as a pulsed beam,injected, and trapped in a small-volume levitated dipole[3] with electrons that satisfy the pair-plasma state.

provide new insights into the behavior of positrons after
injected into the dipole magnetic field.
Orbit calculations showed that injected positrons of ~104
become uniformized in the toroidal direction in ~100us(Fig.1), due to the finite temperature spread of ~1eV.
This process, along with the predicted temporal variationof annihilation rate of positrons at different toroidal
positions (Fig.2), is consistent with the observations ofannihilation gamma rays. Once an almost uniform
toroidal positron cloud is generated, more than 96% of
positrons are trapped in a volume of ~170cm3. However,
the averaged number density and the Debye length ofpositrons in this region are ~108 m-3 and on the order ofmeters, respectively, indicating that the positron cloud
does not currently satisfy the plasma conditions. Further
increase of the trapped positron number using the highfield accumulator will be conducted in the future.

Fig.1 Temporal evolutions of the spatial profile ofpositrons after injected into permanent magnet dipole.

Efficient (~100%) injection [4] and relatively long (~1s)
trapping [5] of the positron beam have been achieved ina prototype dipole trap at the NEPIMUC positron facilityin the Technical University of Munich. In the DC beam
experiments at NEPOMUC so far, the number of
positrons stored in the trap has been less than 103.
In order to realize the plasma state of positrons andinduce collective phenomena, a significant increase in
the trapped particle number is needed, which is achieved
through the utilization of pulsed positron beams [2,6]. In
this study, prior to the accumulation of a large number of
positrons in the 5T trap, we conducted experiments using
pulsed positron beams supplied from a linac-based
system with a buffer gas trap [2] and a prototype
permanent magnet dipole. The use of pulsed positronsource allowed the separation of particle injection period
and stable confinement period of the dipole field trap,
leading to an improved number of positrons. This will

Fig.2 Expected annihilation rate (a.u.) of positrons near
three gamma-ray detectors at different toroidal positions.
References[1] 2022 N. Kenmochi+ Nucl. Fusion 62, 026041.
[2] 2020 H. Higaki+ Appl. Phys. Express 13, 066003.
[3] 2020 H. Saitoh+ Rev. Sci. Instrum. 91, 043507.
[4] 2020 J. Horn-Stanja+ Phys. Rev. Lett. 121, 235003.
[5] 2020 E.V. Stenson+, Phys. Rev. Lett. 121, 235005.
[6] 2020 M.R. Stoneking+, J. Plasma Phys. 86, 155860601.