Even Better Mousetraps

First test with new ASACUSA trap show promise for future antihydrogen beam experiments.

From left to right: Yoshinori Enomoto, Yasuyuki Kanai, Hiroshi Imao, Yuugo Nagata, Yasunori Yamazaki, Naohumi Kuroda, Akihiro Mohri and Takuya Shimoyama.

At the traditional AD end-of-run party on 12 November there were good reasons to celebrate. Aside from the end of shift work and the possibility of taking a rest, the Japanese-European ASACUSA group have recently made encouraging first steps towards the production of a low velocity antihydrogen beam.

Most antiproton experiments require a swarm of these particles to be confined in an evacuated container, or trap, where they can be cooled and manipulated in various ways. To do this magnetic and/or electric restoring forces must be applied to stop the antiprotons drifting out of the swarm to the container walls, where they would annihilate.

The ASACUSA team has now introduced a new trap design based on the familiar Helmholtz coils often seen in school and teaching labs. In such demonstrations two Helmholtz coils with spacing equal to their radius are excited with parallel currents to produce a uniform magnetic field parallel to the coil axes. The ASACUSA trap differs in that it has having antiparallel excitation currents, a configuration that produces a magnetic quadrupole field rather than a constant one, and which is also symmetric about the coil axis. A suitable electrostatic multipole field superimposed on this so-called magnetic cusp field results in an overall configuration that produces all the restoring forces necessary to confine both positive and negative charges, so that positrons, antiprotons and electrons can all be stored in the same trap. The whole thing makes for a compact design since the Helmholtz quadrupole field falls off rapidly outside the coils. As a consequence, an iron return yoke is not needed.

In the recent tests, the cusp trap was shown to be capable of storing a record number of antiprotons. The team then added electrons to the trap; these absorbed heat continuously from the antiprotons and dissipated it quickly by synchrotron radiation. It should eventually be possible using this electron cooling process to produce the extremely low temperature antiprotons necessary to form antihydrogen atoms with positrons subsequently added to the trap.

One further advantage of making antihydrogen atoms in a cusp trap is that since they are initially highly excited, they have a large magnetic moment. The magnetic quadrupole field exerts a further restoring force on this so these neutral atoms are trapped too. As the antiatoms de-excite, their magnetic moment weakens, until they are finally able to escape along the longitudinal trap axis as a spin-polarised antihydrogen beam. In a later stage it is planned to use such a beam for a measurement of the ground state hyperfine splitting of antihydrogen, a highly precise test of the CPT theorem.