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Review of Scientific Instruments

Cryogenic linear Paul trap for cold highly charged ion experiments

M. Schwarz, Oscar O. Versolato, A. Windberger, F. Brunner, Tim Ballance, S. N. Eberle, J. Ullrich, Piet O. Schmidt, Anders Kragh Hansen, Alexander D. Gingell, Michael Drewsen, J. R. Crespo López-Urrutia

Published Aug 1, 2012• 76 citations
Publisher

Abstract

Storage and cooling of highly charged ions require ultra-high vacuum levels obtainable by means of cryogenic methods. We have developed a linear Paul trap operating at 4 K capable of very long ion storage times of about 30 h. A conservative upper bound of the H(2) partial pressure of about 10(-15) mbar (at 4 K) is obtained from this. External ion injection is possible and optimized optical access for lasers is provided, while exposure to black body radiation is minimized. First results of its operation with atomic and molecular ions are presented. An all-solid state laser system at 313 nm has been set up to provide cold Be(+) ions for sympathetic cooling of highly charged ions.

DOI
10.1063/1.4742770

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•Jan 1, 2018•0 citations
Storage and cooling of highly charged ions require ultra-high vacuum levels obtainable by means of cryogenic methods. We have developed a linear Paul trap operating at 4 K capable of very long ion storage times of about 30 h. A conservative upper bound of the H(2) partial pressure of about 10(-15) mbar (at 4 K) is obtained from this. External ion injection is possible and optimized optical access for lasers is provided, while exposure to black body radiation is minimized. First results of its operation with atomic and molecular ions are presented. An all-solid state laser system at 313 nm has been set up to provide cold Be(+) ions for sympathetic cooling of highly charged ions.
Electron beam ion traps used for spectroscopy of highly charged ions (HCI) produce a deep trapping potential leading to high temperatures of the stored ions, and thus limiting the achievable spectral resolution. A novel device at the Max-Planck-Institut für Kernphysik, the Cryogenic linear Paul Trap Experiment (CryPTEx), attached to an electron beam ion trap, provides a new experimental platform to overcome these limitations. The trap assembly operates at a temperature of 4 K and offers optical access for quantum manipulation and imaging of the trapped ions. Since forbidden optical transitions in HCI do not support direct laser cooling, sympathetic cooling with Coulomb crystals of singly charged ions such as Be+ or Mg+ will be applied in order to reach the natural linewidth of optical forbidden transitions in HCI of interest. With the added advantage of long ion trapping times resulting from residual gas pressures of H2 at 4 K below 10−15 mbar, CryPTEx has been commissioned in collaboration with the Ion Trap Group in Århus using rovibrationally cooled MgH+ ions. Strong suppression of the black body radiation at the trap center, ion storage times of about 28 hours, and largely enhanced population of the rovibrational ground state were achieved.
Ultra-high vacuum conditions are ideal for the study of trapped ions. They offer an almost perturbation-free environment, where ions confined in traps can be studied for extended periods of time-facilitating precision measurements and allowing infrequent events to be observed. However, if one wishes to study processes involving molecular ions, it is important to consider the effect of blackbody radiation (BBR). The vast majority of molecular ions interact with BBR. At 300 K, state selection in trapped molecular ions can be rapidly lost (in a matter of seconds). To address this issue, and to maintain state selectivity in trapped molecular ions, a cryogenic ion trap chamber has been constructed and characterized. At the center of the apparatus is a linear Paul ion trap, where Coulomb crystals can be formed for ion-neutral reaction studies. Optical access is provided, for lasers and for imaging of the crystals, alongside ion optics and a flight tube for recording time-of-flight mass spectra. The ion trap region, encased within two nested temperature stages, reaches temperatures below 9 K. To avoid vibrations from the cryocooler impeding laser cooling or imaging of the ions, vibration-damping elements are explicitly included. These components successfully inhibit the coupling of vibrations from the cold head to the ion trap-confirmed by accelerometer measurements and by the resolution of images recorded at the trap center (at 9 and 295 K). These results confirm that the cryogenic ion trap apparatus meets all requirements for studying ion-neutral reactions under cold, controlled conditions.
A new cryogenic linear Paul trap for atomic, molecular and highly charged ions was designed and constructed at MPIK. Experiments with laser-cooled Mg+ ions in the mK-regime and with sympathetically cooled MgH+ molecular ions provided evidence for an excellent vacuum as well as a strongly reduced inuence of the black body radiation at the trap center. Measurements proved that more than 90% of the population of the MgH+ ions can be found in the lowest three rotational states. By laser excitation it was possible to transfer the main part of the ions into the absolute ground state. The population transfer rates for the lowest four rotational states were investigated, additionally to measuring for the first time the decay constant of a deterministically prepared rovibronic state of the MgH+ ion. A decay constant Γ = (7,02 +/- 1,26) Hz was determined for the radiative decay of the |ν = 1, J = 1> state to the ground state.
Coulomb crystals, containing up to a few hundred ions of which more than 50% were cooled sympathetically by the Coulomb interaction with laser cooled Mg+ ions, have been produced in a linear Paul trap. By controlling the balance of the radiation pressure from the two cooling lasers, the Coulomb crystals could be segregated according to ion species. Previous studies of ion crystals and molecular dynamics simulations suggest that the temperature may be around 10 mK or lower. The obtained results indicate that a wide range of atomic and molecular ions, which due to their internal structures are not amenable to direct laser cooling, can be effectively cooled and localized (crystallized) in linear Paul traps. For high resolution spectroscopy of such ions this may turn out to be very useful.
Recent advancements allow for sympathetic cooling of highly charged ions (HCI) to mK temperatures in a cryogenic linear Paul trap. This makes high precision spectroscopy of HCI possible, paving the way to the search for new physics and novel optical clocks. For this, a highly stable and well-understood Paul trap is required. In particular, excess micromotion needs to be characterized and reduced, since it can limit the lowest achievable ion temperature and therefore spectroscopic precision. Within this thesis, a new cryogenic supply system was partly assembled, which allows for vibrational decoupling of the trap, increasing storage stability. The first cool down test achieved a temperature of 4:23K in the trap environment, which is sufficiently low for superconductive operation of the intended RF-resonator made from niobium. Additionally, a photon-correlation technique was used on the existing cryogenic linear Paul trap using 9Be+ ions for determination of the trap frequency by measuring the phase shift between the driven ions motion and the driving field. From the trap frequency the magnification of the imaging system is deduced. The same photon-correlation technique is used to measure excess micromotion and to demonstrate its reduction by a factor of 100 compared to a previous measurement by using new electronics.