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OBSERVATION OF ULTRACOLD HEAVY ION BEAMS WITH MICROMETER SIZE BY SCRAPING

M. Steck, K. Beckert, P. Béller, B. Franzke, F. Nolden

Published Jan 1, 2004• 0 citations
OpenAlex

Abstract

A method to measure the horizontal beam size of elec-tron cooled ion beams with micrometer resolution has been developed. It exploits small variations of the beam momen-tum for micrometer displacement in a dispersive section of the storage ring. The method has been applied to low in-tensity beams which are supposed to undergo a phase tran-sition to an ordered state. The measurements evidenced that the low intensity beam does not only collapse to small momentum spread, but also shrinks abruptly in the trans-verse degree of freedom. This confirms the hypothesis that a one-dimensional ordered structure is formed.

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A method to measure the horizontal beam size of elec-tron cooled ion beams with micrometer resolution has been developed. It exploits small variations of the beam momen-tum for micrometer displacement in a dispersive section of the storage ring. The method has been applied to low in-tensity beams which are supposed to undergo a phase tran-sition to an ordered state. The measurements evidenced that the low intensity beam does not only collapse to small momentum spread, but also shrinks abruptly in the trans-verse degree of freedom. This confirms the hypothesis that a one-dimensional ordered structure is formed.
At S-LSR, abrupt reduction of momentum spread of 7 MeV proton beam to ~2 times 10-6at proton number of -2000 has been observed, which indicates phase transition to 1 dimensional ordered state. Attained proton temperatures after transition are 26 mueV and 1 meV for longitudinal and transverse directions, respectively, which compared with the corresponding values of 20 mueV and 34 meV for electron beam, indicates the magnetization of electron. Laser cooling of24Mg+has also been started and momentum spread of ~108ions is reduced to 2 times 10-4, saturated with the momentum transfer from transverse degree of freedom by intra-beam scattering.
Nanobeam is an advanced technology for preparing charged ion beams with spot diameters on a nanometer scale, and mainly used for high-resolution and high-precision ion beam analysis, ion beam fabrication and ion beam material modification research. The nanobeam devices play an important role in realizing material analysis, micro/nano fabrication, microelectronic device manufacturing and quantum computing. The high-quality ion source is one of the key components of nanobeam device, the performance of which directly affects the resolution and precision of the nanobeam system. However, the traditional ion source used in this system is limited to available ionic species, large energy spread and complex structure. These issues hinder their ability to meet emerging application scenarios that require multi-ion types and high resolution. This emphasizes the importance of creating newion sources as soon as possible. With the development of laser cooling technology, ultracold ions with temperatures in the range of mK or even μK can be obtained based on photoionization of cold atoms and laser cooling of ions. The typical characteristics of low temperature and easy operation greatly promote the emergence of ultracold ion sources. The ultracold ions exhibit extremely small transverse velocity divergence, which can significantly enhance the brightness and emittance quality parameters of the ion source, bringing great opportunities for innovating nano-ion beam technology. Therefore, the research on ultracold ion sources is of great significance for achieving high-quality ion sources with higher brightness, smaller size, lower energy dispersion, more diverse ion species, and simplified structure. Here, we introduce the important achievements in basic research and application technology development of magneto-optical trap ion sources, cold atomic beam ion sources, and ultracold single ion sources from the aspects of preparation principles, generation methods, and typical applications, and review the recent research progress of ultracold ion sources. Finally, we provide an outlook on the future development and application prospects of ultracold ion sources.
We observed phase transitions and ordered structures of laser-cooled Mg+{\mathrm{Mg}}^{+}Mg+ ions stored in a radio-frequency quadrupole storage ring with a diameter of 115 mm. The ions are confined two dimensionally by a quadrupole field and can move freely along the ring circumference. By cooling the ions with tangentially intersecting laser beams, we observed a linear string of ions aligned along the center line of the quadrupole field. For higher ion densities the linear structure changes into a helical structure. The dynamics of the stored ions was studied.
At present, the smallest spot size which can be achieved with state-of-the-art focused ion beam (FIB) technology is mainly limited by the chromatic aberrations associated with the 4.5 eV energy spread of the liquid-metal ion source. Here we numerically investigate the performance of an ultracold ion source which has the potential for generating ion beams which combine high brightness with small energy spread. The source is based on creating very cold ion beams by near-threshold photoionization of a laser-cooled and trapped atomic gas. We present ab initio numerical calculations of the generation of ultracold beams in a realistic acceleration field and including all Coulomb interactions, i.e., both space charge effects and statistical Coulomb effects. These simulations demonstrate that with existing technology reduced brightness values exceeding 105 A m−2 sr−1 V−1 are feasible at an energy spread as low as 0.1 eV. The estimated spot size of the ultracold ion source in a FIB instrument ranges from 10 nm at a current of 100 pA to 0.8 nm at 1 pA.
S-LSR is a compact ion storage ring constructed at Kyoto University several years ago. The ring is equipped with a Doppler laser cooling system aimed at beam crystallization. Bearing in mind hardware limitations in S-LSR, we try to find an optimum set of primary experimental parameters for the production of an ultracold heavy ion beam. Systematic molecular dynamics simulations are carried out for this purpose. It is concluded that the detuning and spot size of the cooling laser should be chosen around |−-−|42 MHz and 1.5 mm, respectively, for the most efficient cooling of 40 keV |24^{24}24|Mg|+^++| beams in S-LSR. Under the optimum conditions, the use of the resonant coupling method followed by radio-frequency field ramping enables us to reach an extremely low beam temperature on the order of 0.1 K in the transverse degrees of freedom. The longitudinal degree of freedom can be cooled to close to the Doppler limit; i.e., to the mK range. We also numerically demonstrate that it is possible to establish a stable, long one-dimensionally ordered state of ions.
The energy distribution of an ultracold rubidium ion beam, which is intended to be used as the source for a focused ion beam instrument, is measured with a retarding field analyzer. The ions are created from a laser-cooled and compressed atomic beam by two-step photoionization in which the ionization laser power is enhanced in a build-up cavity. Particle tracing simulations are performed to ensure the analyzer is able to resolve the distribution. The lowest achieved full width 50% energy spread is (0.205±0.006)\left(0.205\pm0.006\right)(0.205±0.006) eV. The energy spread originates from the variation in the ionization position of the ions which are created inside an extraction electric field. This extraction field is essential to limit disorder-induced heating which can decrease the ion beam brightness. The ionization position distribution is limited by a tightly focused excitation laser beam. Energy distributions are measured for various ionization and excitation laser intensities and compared with calculations based on numerical solutions of the optical Bloch equations including ionization. A good agreement is found between measurements and calculations.