Preparation of a high density, low temperature positron plasma

The goal of the ASACUSA Collaboration is to measure the ground-state hyperfine splitting of antihydrogen [1]. Achieving a substantial ground-state fraction in an antihydrogen beam requires high-density, low-temperature positron plasmas [2]. In strong magnetic fields, positrons can cool efficiently via cyclotron radiation [3]. However, interactions with residual gas lead to ionization via positronium formation, introducing positive ions that drive plasma expansion. Reducing ion contamination is therefore crucial for controlling plasma expansion, and hence plasma temperature and density, which are key parameters for efficient production of antihydrogen in a beam [4]. Many of the plasma preparation techniques inherently produce ions, making it difficult to avoid ion formation entirely. Active purification of the positron plasma immediately before the mixing step is thus essential. The presence of antiprotons and electrons in the trap imposes further constraints on purification methods. This talk will discuss the primary sources of ion contamination, strategies to mitigate it, and the purification techniques employed in ASACUSA's Cusp trap.

[1] E. Widmann, R. S. Hayano, M. Hori, and T. Yamazaki, Measurement of the hyperfine structure of antihydrogen. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 214:31–34, January 2004.

[2] B. Radics, DJ. Murtagh, Y. Yamazaki, and F. Robicheaux, Scaling behavior of the ground-state antihydrogen yield as a function of positron density and temperature from classical-trajectory Monte Carlo simulations, Physical Review A 90 (2014). DOI: https://doi.org/10.1103/PhysRevA.90.032704

[3 C. Amsler et al., Reducing the background temperature for cyclotron cooling in a cryogenic Penning–Malmberg trap. Phys. Plasmas 1 August 2022; 29 (8): 083303. https://doi.org/10.1063/5.0093360

[4] S. Jonsell, and M. Charlton, Formation of antihydrogen beams from positron–antiproton interactions. New J. Phys. 21 073020 (2019) https://doi.org/10.1088/1367-2630/ab2bdc