CERN Accelerating science

ALPHA produces antihydrogen by slowly merging clouds of positrons and antiprotons inside specially designed Penning traps, found at the centres of the ALPHA 2 and ALPHA-g experiments. During each experimental cycle, positrons and antiprotons must be transferred into these parts of the machine from the positron accumulator and catching trap at opposite ends of the apparatus. Charged particles can be ejected from these traps as a “pulsed beam” by quickly changing the voltages that are applied to the Penning trap electrodes, allowing the particles to escape together in one direction. The various Penning traps are connected together by a beamline, which uses magnets to continually steer and focus these beams as they make their way around the experiment.

In most beamlines, particles are transported at relatively high energies of millions (MeV) or even billions (GeV) of electron-volts. In contrast, ALPHA’s Penning traps can only produce slow-moving beams with energies of 10 – 100 eV, which are easily affected by the strong magnetic fields of the traps themselves. To prevent this, the ALPHA beamlines are designed so that particles always move along the direction of the magnetic field. In strong magnetic fields, the beam is focused down to a small point, and in weaker fields the beam can expand to a larger diameter.

A special part of the beamline, known as the 'interconnect', is used to steer positrons and antiprotons between ALPHA’s spectroscopy and gravity experiments. The interconnect uses an arrangement of seven independent magnets to create a curved magnetic field. Beams that pass through the interconnect will follow the direction of this magnetic field, allowing us to steer them in various directions. We use diagnostic tools such as Micro-Channel Plates (MCP) to image the beam at different points along the beamline, and check that it is following the correct path.