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arXiv:1809.08606 · Physical Review A

Resistive cooling of highly charged ions in a Penning trap to a fluidlike state

Mehri Sadat Ebrahimi, Zhi Min Guo, Manuel Vogel, M. Wiesel, G. Birkl, W. Quint

Published Aug 27, 2018• 15 citations
arXivHTMLPDF

Abstract

We perform a detailed experimental study of resistive cooling of large ensembles of highly charged ions such as ${\mathrm{Ar}}^{13+}$ in a cryogenic Penning trap. Different from the measurements reported by Vogel et al. [M. Vogel et al., Phys. Rev. A 90, 043412 (2014)], we observe purely exponential cooling behavior when conditions are chosen to allow collisional thermalization of the ions. We provide evidence that in this situation resistive cooling time constants and final temperatures are independent of the initial ion energy and that the cooling time constant of a thermalized ion ensemble is identical to the single-ion cooling time constant. For sufficiently high ion number densities, our measurements show discontinuities in the spectra of motional resonances which indicate a transition of the ion ensemble to a fluidlike state when cooled to temperatures below approximately 14 K. With the final ion temperature presently being 7.5 K, ions of the highest charge states are expected to form ion crystals by mere resistive cooling, in particular not requiring the use of laser cooling.

DOI
10.1103/physreva.98.023423

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We perform a detailed experimental study of resistive cooling of large ensembles of highly charged ions such as Ar13+{\mathrm{Ar}}^{13+}Ar13+ in a cryogenic Penning trap. Different from the measurements reported by Vogel et al. [M. Vogel et al., Phys. Rev. A 90, 043412 (2014)], we observe purely exponential cooling behavior when conditions are chosen to allow collisional thermalization of the ions. We provide evidence that in this situation resistive cooling time constants and final temperatures are independent of the initial ion energy and that the cooling time constant of a thermalized ion ensemble is identical to the single-ion cooling time constant. For sufficiently high ion number densities, our measurements show discontinuities in the spectra of motional resonances which indicate a transition of the ion ensemble to a fluidlike state when cooled to temperatures below approximately 14 K. With the final ion temperature presently being 7.5 K, ions of the highest charge states are expected to form ion crystals by mere resistive cooling, in particular not requiring the use of laser cooling.
We have performed systematic measurements of the resistive cooling behavior of bunches of highly charged ions in a Penning trap after injection from an external source. In particular, we have been able to measure the exponential cooling rate of the axial center-of-mass motion and experimentally show its linear dependence on the ion number within the bunch, as expected from theory. The common center-of-mass energy of the ions is reduced by more than four orders of magnitude on the time scale of milliseconds while the single-ion cooling occurs on the scale of seconds, representing a highly effective way to remove the dominating part of the ion kinetic energy. Published by the American Physical Society 2024
A Penning trap which is optimized for transverse resistive cooling yields a cooling time constant for trapped He{sup +} ions of as low as 3 seconds, depending on cooling duty cycle. Opposite quadrants of the ring electrode resonate (Q=1800 at 7.4 MHz) with Nb/Ti inductors at 4.2 K. The f{prime} motion is resonantly cooled by operation at 2 Tesla axial field and trap voltages from 12 to 38 Volts. Ion signals are magnetically coupled to a 4 K GaAs fet preamplifier, wide-band 300 K amplifier and linear diode detection system. The rate of cooling is set by fractional time on resonance during sweep of the trap voltage. Shifting and broadening due to space charge are demonstrated. Other interesting features of the ion spectra, such as peak structure, response to heating by f{sup +} - f{sup {minus}}, and growth/decay of the peak components, will be presented.
In this work, laser-cooled ion crystals containing 1000 to 100000 singly charged magnesium ions (Mg+) were prepared in a Penning trap. The properties of the ion crystals and their structure displaying long-range ordering were analyzed by various non-destructive techniques. After creation of the Mg + ions in the form of ion bunches in an external source, the ions were injected into the Penning trap where their temperature was reduced by eight orders of magnitude within seconds using a combination of buffer gas cooling and Doppler laser cooling. The achieved temperatures in the millikelvin-regime were close to the theoretical Doppler-cooling limit and sufficiently low to induce the transition to a crystal phase exhibiting long-range ordering. The structure of these mesoscopic ion crystals is in agreement with a model describing the crystal as a set of planar shells. This allows for a derivation of properties such as the charge density or the temperature of the observed crystals. For the process of combined buffer-gas and Doppler laser cooling an analytical model has been developed, which explains the time development of the temperature and the fluorescence signal in agreement with the experimental results.
We have performed laser cooling of Mg+{}^{+}+ ions confined in a Penning trap. The externally produced ions were captured in flight, stored, and laser cooled. Laser-induced fluorescence was observed perpendicular to the cooling laser axis. Optical detection down to the single-ion level together with the electronic detection of the ion oscillations inside the Penning trap have been used to acquire information on the ion storage time, ion number, and ion temperature. Evidence for the formation of ion crystals has been observed. These investigations are an important prerequisite for the sympathetic cooling of simultaneously stored highly charged ions and the precision laser spectroscopy of forbidden transitions in these.
Penning traps, with their ability to control planar crystals of tens to hundreds of ions, are versatile quantum simulators. Thermal occupations of the motional drumhead modes, transverse to the plane of the ion crystal, degrade the quality of quantum simulations. Laser cooling using electromagnetically induced transparency (EIT cooling) is attractive as an efficient way to quickly initialize the drumhead modes to near ground-state occupations. We numerically investigate the efficiency of EIT cooling of planar ion crystals in a Penning trap, accounting for complications arising from the nature of the trap and from the simultaneous cooling of multiple ions. We show that, in spite of challenges, the large bandwidth of drumhead modes (hundreds of kilohertz) can be rapidly cooled to near ground-state occupations within a few hundred microseconds. Our predictions for the center-of-mass mode include a cooling time constant of tens of microseconds and an enhancement of the cooling rate with increasing number of ions. Successful experimental demonstrations of EIT cooling in the NIST Penning trap [Jordan et al., [Phys. Rev. Lett. 122, 053603 (2019)] validate our predictions.
We have determined the temperature and heating rate of laser-cooled ions in a Penning trap using Doppler laser spectroscopy. Between and ions are trapped in a Penning trap and Doppler laser cooled to temperatures of a few millikelvin, where they form ion crystals. This system is an example of a strongly coupled one-component plasma. The ion temperature was measured as a function of time after turning off the laser-cooling. In the solid phase, we measured a heating rate of . Information about possible heating mechanisms was obtained directly from temperature measurements, and also from measurements of the rate of radial expansion of the ion plasma. We determined that the observed heating is due to collisions with the residual gas of our vacuum system.