The simulation and design of the beamline, which will be used to measure the ground-state hyperfine splitting of antihydrogen, continued in 2006. This beamline will consist of two sextupole magnets, a resonance microwave cavity between them, and an antihydrogen detector. It was decided that superconducting magnets will be used for the sextupoles. Different designs were simulated, and we found that the best shape for the sextupoles is a half tapered (i.e. conical), half straight design (see Fig. 1). This provides relatively high efficiency in a compact size.

Fig. 2 shows a preliminary design of the spectrometer line, including the two sextupoles and the microwave cavity, connected to a superconducting two-frequency cylindrical Paul trap, which can produce the required antihydrogen atoms by simultaneously trapping antiprotons and positrons. This Paul trap is in turn connected to another, linear Paul trap, which will capture and cool antiprotons. The building of the latter trap and its cryostat (as shown in Fig. 2) has already started at CERN, Geneva, Switzerland.

Another option to produce antihydrogen atoms is a cusp trap (i.e anti-Helmholtz coils). The building of this trap is already finished at RIKEN, Japan, and now is awaiting extensive tests.

Outlook

Two antihydrogen sources are being built now: two superconducting Paul traps (which are connected to each other) at CERN, Switzerland, and a cusp trap (i.e. anti-Helmholtz coils) at RIKEN, Japan, both by Japanese collaborators. The building of the first Paul trap has started at CERN, and will be finished in 2007. The testing of this trap with antiprotons will most likely happen during the beamtime of 2008.
The cusp trap is already built, and will soon be tested with protons. Since this trap might be able to produce polarized and focussed antihydrogen atoms, the first sextupole might not be needed if this trap is used, since this sextupole essentially just polarizes the antihydrogen atoms. SMI will carry out simulations to determine the quality of the antihydrogen beam that will come out of the cusp trap. Based on the results, SMI will design the sextupole magnet(s) for this setup too.
SMI also took responsibility to design, build and commission the spectrometer line, which has to be ready and working by 2009, when it will be connected to the antihydrogen source. Thus the spectrometer line will be designed in 2007, and the construction will start at the end of that year. We will visit the Superconducting Magnet Group at the Brookhaven National Laboratory in the United States, who has a large experience in building similar magnets. Thus they might build the sextupoles for us, but we will also talk with other groups or companies as well. Previously, the time schedule of the antihydrogen production traps was not clear, and some delay was caused by the technical and conceptual complexity of these devices. With the cusp trap now being tested with protons and electrons, and the Paul trap being built, the time horizon has become well defined so that the construction of the spectrometer line has to start end of 2007 to be ready in time. Thus a fraction of the investment money will already be needed in 2007.

One important part of the experimental setup is the antihydrogen detector. Since the detection rate will be very low (1-2 atoms per minute), it is important to suppress the background coming from cosmic rays (mostly muons). This task is not simple, and different detector designs are currently investigated by SMI.