23A02
Low Energy Anti-Hydrogen Atoms Produced with Non-Neutral Plasmas
非中性プラズマを用いた低エネルギー反水素生成
H.Higaki, Y.Enomoto1, N.Kuroda2, K.Michishio3, C.H.Kim2, Y.Nagata1, Y.Kanai1, H.A.Torii2,M.Corradini4, M.Leali4, E.Lodi-Rizzini4, V.Mascagna4, L.Venturelli4, N.Zurlo4, K.Fujii2,M.Otsuka2, K.Tanaka2, H.Imao5,Y.Nagashima3, Y.Matsuda2, B.Juhász6, A.Mohri1 andY.Yamazaki1,2
檜垣浩之, 榎本嘉範1, 黒田直史2, 満汐孝治3, 金燦鉉2, 永田祐吾1, 金井保之1, 鳥居寛之2,
M. Corradini4, M. Leali4, E. Lodi-Rizzini4, V. Mascagna4, L. Venturelli4, N. Zurlo4, 藤居甲基2,大塚未来2,田中香津生2,今尾浩士5, 長嶋泰之3, 松田恭幸2, B.Juhász 6, 毛利明博1, 山崎泰規1,2
Graduate School of Advanced Sciences of Matter, Hiroshima University,Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8530, Japan
1
RIKEN Advanced Science Institute, Hirosawa, Wako, Saitama 351-0198, Japan
2
Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan3
Department of Physics, Tokyo University of Science, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
4
Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali,Universitá di Brescia & Istituto Nazionale di Fisica Nucleare, Gruppo Collegato di Brescia, 25133 Brescia, Italy
5RIKEN Nishina Center for Accelerator Based Science, Hirosawa, Wako, Saitama 351-0198, Japan6
Stefan Meyer Institute for Subatomic Physics, Austrian Academy of Sciences, 1090 Wien, Austria
広島大学大学院先端物質科学研究科 〒739-8530 東広島市鏡山1-3-1
1
理化学研究所基幹研究所 〒739-8530 和光市広沢2-1
2
東京大学大学院総合文化研究科 〒153-8902 目黒区駒場3-8-1
2
東京理科大学物理学科 〒162-8601 新宿区神楽坂1-3
4Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali,Universitá di Brescia & Istituto Nazionale di Fisica Nucleare, Gruppo Collegato di Brescia, 25133 Brescia, Italy
5
理化学研究所仁科加速器センター 〒739-8530 和光市広沢2-1
6
Stefan Meyer Institute for Subatomic Physics, Austrian Academy of Sciences, 1090 Wien, Austria
Since the production of low energy anti-hydrogen atoms reported in 2002, it has been of great interest tohave those atoms available for physics experiments. At CERN Antiproton Decelerator, three groups arecompeting for the first physics result. Recently, ASACUSA-MUSASHI group made a step forward foranti-hydrogen beams by producing anti-hydrogen atoms in a magnetic field gradient (cusp field). In thecourse of experiments, non-neutral plasmas played crucial rolls. Here, the basic procedure for producing
low energy anti-hydrogen atoms and possible experiments are reviewed.

1. Introduction
Non-neutral plasmas have been studied in great
detail with Paul traps and Penning-Malmberg
traps in a uniform magnetic field. Physicalinterests included are laser cooled strongly
coupled plasmas, strongly magnetized plasmas ina high field, vortex structure formation, etc. They
are also studied in complex fields like torus-typemagnetic fields and magnetic mirror fields.
Here, synthesis of cold anti-hydrogen ( H )atoms in a cusp trap is reported [1]. Low energy
H atoms were first produced in 2002 by mixing a
low energy positron (e+) plasma and antiprotons( p ) in a uniform magnetic field [2], which wasmade possible with the knowledge acquired
through studies of cold non-neutral plasmas.
However, those H atoms were not suitable forphysics experiment. The next step was, with themagnetic moment of H atom, to confine H atoms
in a minimum B field or to produce H -beams with amagnetic field gradient. ASACUSA-MUSASHI
group utilizes a magnetic cusp field for thepurposes [3].
2. Experiment
Figure 1 shows the experimental setup, which iscomposed of a unique low energy antiproton beamsource named MUSASHI (which can provide 0.1~1keV p beams [4]), a compact low energy e +accumulator equipped with ~1GBq 22Na RI source[5], and a cusp trap, where H atoms are produced.
At first, 40 to 60 pulses of ~100 eV, 105 e+ fromthe accumulator are injected into the cusp trap at
the temperature of 15K [6]. About 4~6 x 106 e+

p ’s. After kicking out electrons and applying
rotating electric field to p ’s [9], about 3 x 105 p ’s

Fig.1. ASACUSA-MUSASHI experimental setup

are cooled through synchrotron radiation and arotating electric field is applied [7] to form a cold
high density e+ plasma (with density n andtemperature T ). This is necessary for the higher
production rate of H atoms, because it is expectedthat the cross sections for the three body andradiative recombinations are proportional
to n 2T −9 / 2 and nT −1 / 2 , respectively [8]. For theproduction of H atoms, three body recombinationis thought to be the dominant process.
Meanwhile, a pulsed antiproton beam of ~5
MeV provided from AD is decelerated to ~100keV
through Radio Frequency Quadrupole Decelerator,
which is available only for ASACUSA, andinjected into MUSASHI. Antiprotons with theenergy less than ~13 keV are caught in theMUSASHI trap and a pre-loaded non-neutralelectron plasma is used for the electron cooling ofwith the energy of 150 eV are injected into the e+plasma in the cusp trap.
Since the relative energy of injected p ’s
against e+ plasma is ~ 20 eV as shown inFig.2, p ’s are cooled by collisions with positrons
and highly excited H atoms (with the principle
quantum number ~ 45) are produced. Those highlyexcited H atoms coming into the field ionization
potential, 250 mm away from the production point,
are re-ionized by the electric field and p ’s aretrapped in the potential. The annihilation signals
synchronized with the destruction of the fieldionization potential is regarded as the evidence thatcold H atoms are created [10].
3. Future Plan
To measure the ground state hyperfine splittingof H atom through the Stern-Gerlach type
experiment, it is desirable to extract an intense
ground state H atomic beam into a field free region.
Thus, higher production rate of low energy
H atoms is mandatory.
The new RF cavity for inducing hyperfine
transition and sextapole magnet for selecting thestate of H atoms will be installed for the coming
physics experiment.
Acknowledgments
Authors acknowledge the support of AD operators.
This work is supported by the Grant-in-Aid for Specially
Promoted Research (19002004) of the Japanese Ministryof Education, Culture, Sports, Science and Technology(MonbuKagaku-sho), Special Research Projects for
Basic Science of RIKEN, Universitá di Brescia (Italy),and Istituto Nazionale di Fisica Nucleare (Italy).
References[1] Y. Enomoto et al., Phys.Rev.Lett.105 (2010) 243401.
[2] M. Amoretti et al., Nature 419 (2002) 456.
[3] A. Mohri and Y. Yamazaki, Europhys. Lett. 63(2003) 207.
[4] N. Kuroda et al., Phys. Rev. Lett. 94 (2005) 023401.
[5] H. Imao et al., Hyperfine Interact. 194 (2009) 71.
[6] M. Shibata et al., Rev.Sci.Instrum.79(2008)015112.
[7] H. Saito et al., Phys. Rev. A 77 (2008) 051403(R).
[8] M. E. Glinsky and T. M. O’Neil, Phys. Fluids B 3(1991) 1279.
[9] N. Kuroda et al., Phys.Rev.Lett.100 (2008) 203402.
[10] G. Gabrielse et al., Phys. Rev. Lett. 89 (2002)
213401.

Fig.2. (a) Magnetic field lines in the cusp trap, (b) Nested
potential and field ionization potential on the axis.