A. Mohri

A source of antihydrogen for in-flight hyperfine spectroscopy

Antihydrogen, a positron bound to an antiproton, is the simplest antiatom. Its counterpart—hydrogen—is one of the most precisely investigated and best understood systems in physics research. High-resolution comparisons of both systems provide sensitive tests of CPT symmetry, which is the most fundamental symmetry in the Standard Model of elementary particle physics. Any measured difference would point to CPT violation and thus to new physics. Here we report the development of an antihydrogen source using a cusp trap for in-flight spectroscopy. A total of 80 antihydrogen atoms are unambiguously detected 2.7u2009m downstream of the production region, where perturbing residual magnetic fields are small. This is a major step towards precision spectroscopy of the ground-state hyperfine splitting of antihydrogen using Rabi-like beam spectroscopy.

Transport beam line for ultraslow monoenergetic antiprotons

A beam line for the transportation of slow antiprotons from a multiring electrode trap to an experimental chamber is described. The beam line is equipped with a three-stage differential pumping system in order to maintain a pressure lower than 1×10−12u200aTorr in the trap region while simultaneously having a pressure of around 1×10−6u200aTorr in the chamber. Tests have shown that 105 positive ions per trapping cycle were successfully extracted at 250 eV from the trap positioned in a superconducting solenoid. The ions were then further transported through three small apertures to the target area located 3.5 m downstream of the trap. Results from the first delivery of a 250 eV antiproton beam are described.

Development of a cold HCI source for ultra-slow collisions

Abstract A system consisting of a superconducting solenoid, a multi-ring trap and a slow positron source has been built to prepare a cold highly charged ion (HCI) beam, which will be applied to study interactions with solids or gases, e.g. potential energy deposition scheme during interaction of slow HCIs with a surface. An electron plasma with density more than 1010 cm−3 has already been prepared to trap positrons.

Accelerated Merging of Electron Vortices in Background Vorticity

We report new features observed in two-dimensional interactions of discrete vortices either isolated in vacuum or immersed in a background vorticity. The vortices are strings of electron plasma which are produced with a newly developed cathode array and trapped in a Malmberg trap. We observe long-lasting orbital motion of discrete vortices in vacuum, consistent with kinetic equations of point vortices, while a rapid re-organization occurs in the spatial distribution of vorticity when discrete vortices are immersed in an extended distribution of the background vorticity. The main features of the new observation are accounted for by a recently-proposed theoretical model incorporating collective interaction between two vortices.

Compact cryogenic system with mechanical cryocoolers for antihydrogen synthesis

We have developed a compact cryogenic system which cools a vacuum chamber housing multi-ring trap electrodes (MRTs) of an antihydrogen synthesis trap using mechanical cryocoolers to achieve background pressure less than 10(-12) Torr. The vacuum chamber and the cryocoolers are thermally connected by copper strips of 99.9999% in purity. All components are installed within a diametric gap between the MRT of phi108 mm and a magnet bore of phi160 mm. An adjusting mechanism is prepared to align the MRT axis to the magnet axis. The vacuum chamber was successfully cooled down to 4.0 K after 14 h of cooling with heat load of 0.8 W.

Wall and Temperature Effects on Electrostatic Oscillations of Spheroidal Non-neutral Electron Plasmas in the Multi-Ring Electrode Trap

Electrostatic oscillations of spheroidal non-neutral electron plasmas are experimentally investigated with the multiring electrode trap which provides an electrostatic quadrupole potential with a long axial extent and enables confinement of plasmas with large aspect ratios. The frequencies of axial modes l = 2 and 3 of trapped plasmoids shifts upwards from those predicted by Dubins cold plasma theory for the free boundary case, mainly due to the image charges induced on the conducting electrodes. These frequencies continue to increase as the plasma temperature is increased from 0.03 eV to 1.5 eV by rf heating. This temperature dependence can be interpreted as a change in the dielectric tensor caused by the temperature increase.

Production of ultra‐slow antiproton beams

We have recently succeeded in decelerating and confining millions of antiprotons, 50 times more efficiently than conventional methods, in an electromagnetic trap. These antiprotons were cooled by preloaded electron plasma to an energy below an electronvolt. They were then extracted out of the magnetic field of 2.5 T and transported typically at 250 eV along a beamline, designed for efficient transport at 10–1000 eV. This unique beam from our apparatus named MUSASHI opens up a new field of atomic and nuclear physics probed by ultra‐slow antiprotons.In this paper, the whole experimental setup and procedure will be overviewed: deceleration, capture, cooling and extraction of antiprotons will be discussed in detail, including technical description of diagnostic devices.

Positron accumulation and manipulation for antihydrogen synthesis

Our group ASACUSA-MUSASHI has established an efficient way for accumulating antiprotons in the cusp trap, a combination of an anti-Helmholz superconducting coil and a multi-ring electrode trap. The last piece for synthesizing antihydrogens in the cusp trap is positron. We have developed a compact system to effectively accumulate positrons based on N2 gas-buffer scheme with a specially designed high precision cylindrical multi-ring electrode trap. Millions of positrons were accumulated in the pre-accumulator just using polycrystalline tungsten moderators. The accumulated positrons were transported as a pulsed beam via three guiding coils and caught in the cusp trap under cryogenic and ultra high vacuum conditions without serious loss. Confinement of two kinds of numerous antiparticles, e.g., 108 positrons and 107 antiprotons, in the cusp trap becomes feasible.

Atomic collision experiment using ultra-slow antiproton beams

The development of techniques to decelerate, cool and confine antiprotons in vacuo with an electromagnetic trap has opened a new research field of atomic physics of cold antiprotons, including synthesis of antihydrogen atoms. At the Antiproton Decelerator (AD) facility at CERN, we the MUSASHI group of ASACUSA collaboration have so far achieved efficient confinement of millions of antiprotons in a Multi-Ring electrode Trap (MRT) installed in a superconducting magnet of 2.5 T, by a sequential combination of the AD (down to 5.3 MeV), an RFQD (Radio-Frequency Quadrupole Decelerator; down to 50–120 keV) and the MRT. Antiprotons, cooled to energies less than an electronvolt by preloaded electrons in the trap, was then extracted out of the magnetic field and transported along a 3-m beamline as a monoenergetic beam of 10–500 eV. With this unique ultra-low-energy antiproton beam, we are now planning the first atomic collision experiments under single collision conditions, to measure ionization and atomic capture cross sections of antiprotons against helium atoms. A supersonic atomic gas-jet target is prepared and crossed with the antiproton beam. Antiprotons as well as electrons emitted during the reaction will be detected by a microchannel plate (MCP) while the antiproton annihilation will be recognized by detection of annihilation products—mostly pions—by surrounding scintillation counters. When the antiproton is captured, it forms a neutral antiprotonic helium atom, some in a metastable state whose level structures have been well studied with spectroscopic methods. Severe identification of particles and atoms plays an essential role in the design of the experiment, to distinguish the small number of reaction events out of a huge pile of background events. Our strategies for our near-future experiments are discussed.

Comparison of non-neutral electron plasma confinement in harmonic and rectangular potentials in a very dense regime

Confinement of high density electron plasmas in a strong uniform magnetic field was experimentally studied in a multi-ring trap (MRT). The trap was housed inside a bore tube and surrounded by a superconducting solenoid. A 5u2009T magnetic field was used to provide radial confinement while an electrostatic harmonic or rectangular potential well was used for axial confinement. For trapped electrons of Nu2009=u20091.2u2009×u20091010 in a harmonic potential well (HPW) with the trap length of LTu2009=u2009320u2009mm, the plasma lifetime was about 104u2009s and it became much longer at lower Nu2009=u20094.5u2009×u2009109. Such long holding times were achieved without application of rotating electric fields. Contrastingly, in a rectangular potential well (RPW), the plasma of Nu2009=u20091.2u2009×u20091010 under the same trap length expanded to cover the whole Faraday Cup within 200u2009s, where its radial expansion rate was ηu2009=u20093.2u2009×u200910−2 mm/s, which was 20 times faster than HPW. The lifetime for RPW became shorter with increasing LT and scaled as 1/[LT]2. This scaling found for hig...