Improvement of Laser Cooling of Ions in a Penning Trap by use of the Axialisation Technique

We report a study of the axialisation and laser cooling of single ions and small clouds of ions in a Penning trap. A weak radiofrequency signal applied to a segmented ring electrode couples the magnetron motion to the cyclotron motion, which results in improved laser cooling of the magnetron motion. This allows us to approach the trapping conditions of a Paul trap, but without any micromotion. Using an ICCD camera we show that the motion of a single ion can be confined to dimensions of the order of 20 μm or less. We have measured increased magnetron cooling rates for clouds of a few ions, using an rf-photon correlation technique. For certain laser cooling conditions, the magnetron motion of the center of mass of the cloud grows and stabilises at a large value. This results in the ions orbiting the center of the trap together in a small cloud, as confirmed by photon-photon correlation measurements. PACS numbers: 32.80.Pj, 03.67.Lx, 41.20.-q, 42.50.Vk Submitted to: J. Phys. B: At. Mol. Opt. Phys.