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Journal of the Physical Society of Japan

Pulsed Evaporative Cooling for Trapped Highly Charged Ions

Tohru Kinugawa, Frederick John Currell, Shunsuke Ohtani

Published Dec 15, 1999• 1 citation
Publisher

Abstract

We propose the application of pulsed evaporative cooling, which has been used successfully for Bose-Einstein condensation of neutral atoms, to highly charged ions. Practically, this cooling technique can be highly efficient and favorable for ions in an electron beam ion trap (EBIT). By computer simulation, we have numerically estimated the performance of this cooling scheme on the basis of the standard evaporation theory. The result is very promising; from the initial temperature of 500 eV, Kr +30 can be cooled to room temperature within a few seconds.

DOI
10.1143/jpsj.68.3763

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We propose the application of pulsed evaporative cooling, which has been used successfully for Bose-Einstein condensation of neutral atoms, to highly charged ions. Practically, this cooling technique can be highly efficient and favorable for ions in an electron beam ion trap (EBIT). By computer simulation, we have numerically estimated the performance of this cooling scheme on the basis of the standard evaporation theory. The result is very promising; from the initial temperature of 500 eV, Kr +30 can be cooled to room temperature within a few seconds.
We report on a highly efficient evaporative cooling optimized experimentally. We successfully created sodium Bose-Einstein condensates with 6.4\ifmmode\times\else\texttimes\fi{}107{}^{7}7 atoms starting from 6.6\ifmmode\times\else\texttimes\fi{}109{}^{9}9 thermal atoms trapped in a magnetic trap by employing a fast linear sweep of radio frequency at the final stage of evaporative cooling so as to overcome the serious three-body losses. The experimental results such as the cooling trajectory and the condensate growth quantitatively agree with the numerical simulations of evaporative cooling on the basis of the kinetic theory of a Bose gas carefully taking into account our specific experimental conditions. We further discuss theoretically a possibility of producing large condensates, more than 108{}^{8}8 sodium atoms, by simply increasing the number of initial thermal trapped atoms and the corresponding optimization of evaporative cooling.
Externally, in an electron beam ion trap, generated Ar16+ ions were retrapped in a Penning trap and evaporatively cooled in their axial motion. The cooling was observed by a novel extraction technique based on the excitation of a coherent axial oscillation which yields short ion bunches of well-defined energies. The initial temperature of the ion cloud was decreased by a factor of more than 140 within 1 s, while the phase-space density of the coldest extracted ion pulses was increased by a factor of up to about 9.
We study the optimization of evaporative cooling in trapped bosonic atoms on the basis of quantum kinetic theory of a Bose gas. The optimized cooling trajectory for 87Rb{}^{87}\mathrm{Rb}87Rb atoms indicates that the acceleration of evaporative cooling around the transition point of Bose-Einstein condensation is very effective against loss of trapped atoms caused by three-body recombination. The number of condensed atoms is largely enhanced by the optimization, more than two orders of magnitude in our present calculation using relevant experimental parameters, as compared with the typical value given by the conventional evaporative cooling where the frequency of radio-frequency magnetic field is swept exponentially. In addition to this optimized cooling, it is also shown that highly efficient evaporative cooling can be achieved by an initial exponential and then a rapid linear sweep of frequency.
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We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature as low as 9 K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise CPT test on trapped antihydrogen is a long-standing goal.