Rapid crystallization of externally produced ions in a Penning trap

Abstract

We have studied the cooling dynamics, formation process, and geometric structure of mesoscopic crystals of externally produced magnesium ions in a Penning trap. We present a cooling model and measurements for a combination of buffer gas cooling and laser cooling which has been found to reduce the ion kinetic energy by eight orders of magnitude from several hundreds of eV to μeV and below within seconds. With ion numbers of the order of 1 × 10 3 to 1 × 10 5 , such cooling leads to the formation of ion Coulomb crystals which display a characteristic shell structure in agreement with the theory of non-neutral plasmas. We show the production and characterization of two-species ion crystals as a means of sympathetic cooling of ions lacking a suitable laser-cooling transition.

Sympathetic cooling in two-species ion crystals in a Penning trap

“…The case of single-species ion crystals in the present setup has been discussed in detail in (15 ), where we have achieved cooling of externally produced Mg + ions from energies of several hundreds of eV to energies of the crystalline state below µeV within seconds. Based on these results, two-species ion crystals have been produced by injection of singly charged admixture ions from the external ion source (on the right hand side of Fig.…”

“…Under the present experimental conditions, the confined Mg + ion crystals are subjected to laser cooling and to heating by interaction with residual gas (mainly helium at a pressure of the order of 10 −8 mbar and at the ambient temperature of about 4 K), with laser cooling dominating by far, hence the rapid crystallization previously observed for pure Mg + crystals (15 ). Note, that for Mg + at initially high energies (far above 4 K), the residual gas provides efficient buffer gas cooling, as discussed in (15 ), whereas for cold Mg + in the laser-cooling regime and close to the Doppler limit (far below 4 K) it acts as a heating mechanism.…”

“…Structures in the radial plane are not resolved since the global rotation of the crystal about the trap axis is much faster than the exposure time of imaging. Details of the imaging system have been given in (15 ). A typical image is shown in Fig.…”

“…As our radial projections of the magnesium ion cloud can reveal ordered structures only along the axial direction and not within each layer, we give limits on the actual Mg + ion temperature T before injection of the second species in the range of T D = 1 mK < T < 6 mK = T Γ , where T D is the Doppler-limited temperature (15 ) and T Γ is the temperature corresponding to a value of Γ p = 140 at the ion number density n = 3.4×10 7 cm −3 . This is in correspondence with detailed simulations of ion Coulomb crystals which show that as Γ p approaches a value of about 140, the ion cloud shows distinct density modulations along the axial direction as observed (with only a small amount of diffusion of ions from one layer to another), whereas in the radial direction the ions are more likely to change places (a liquid-like state) (44 ).…”

“…Under the present conditions, the magnesium ions form near-planar crystalline structures (15 ), and the second species can become embedded in the crystal, depending on the confinement details. Measurements have been performed for mesoscopic magnesium ion crystals injected with singly charged ions of mass-to-charge ratios m/q = 2 u/e (H …”