Research On Dynamics Of Ion Crystal In A Linear Radio-frequency Trap

The fine structure constant is a physical quantity used to characterize the strength of electromagnetic interaction. The theory of quantum electrodynamics (QED) is an important part of the Standard model of particle physics. The determination of fine structure constant and the test of quantum electrodynamics are the essential study subject in physics at present. The precise measurement of fine structure in simple atomic systems is an important method for determination of the fine structure constant and testing the quantum electrodynamics. With the Doppler cooling, the trapped ions in the radio frequency (RF) field can be cooled to few mK and form Coulomb crystal. The realization of sympathetic cooling makes the cooling independent of the energy level structure of ion itself. Combining with the laser cooling, sympathetic cooling has become a universal method to obtain a low temperature ion system. Based on the corrections of QED, the spectrum of Li+ can be calculated accurately. Therefore, a precise measurement of fine structure of cooled Li+ becomes one of important and feasible schemes to determine the fine structure constant and test the QED theory, as well as one of the significant ways to study the atomic structure information. The dynamics characteristics of the ions play significant role in the precise measurement of the spectrum. This thesis summarizes systematically and fundamentally the experimental and theoretical studies of dynamics characters of 40Ca+ ion crystal by laser cooling, and Li+ by sympathetic cooling, same as CaOH+. The topics covered by this thesis are categorized as follow.1. A linear Paul trap was built and optimized. We developed a three-sectioned linear Paul trap (r0=3.50 mm, R0=4.00 mm) which is installed in a vacuum system with a base pressure lower than 7.5×10-9Pa. The trapping RF field of the ions was setup with frequency Ω varying from 3.50 to 4.50 MHz, and Urf varying from 60 to 450 V, with the controlling accuracy of 0.3%. An electrostatic field control system with a precision of 1 mV was built. The photo-ionization, cooling and spectral detection laser systems of the trapped 40Ca+ and 7Li+ were constructed and optimized.2. 40Ca+ ion system with different quantity were successfully trapped and cooled in the linear Paul trap. The fluorescence spectrum and spatial configuration of ion crystal were observed by a photomultiplier tube (PMT) and an electron multiplying charge coupled devices (EMCCD), respectively. The three-dimensional translational and shape control of ion crystal were realized with a precision of 1 μm. The purification and selective-control of ion crystal were realized. A new method of compensating for the excess micromotion along two directions with one micrometer-precision in three-dimensional Coulomb crystals was proposed; this method is based on shape control and optical imaging of a Coulomb crystal in the sectioned linear ion trap. The temperature of the ion crystal was about 5 mK. The three-dimensional ion crystal can stay in the RF trap for more than 15 days. The typical characteristics of pure ion crystal such as space configuration, temperature distribution, oscillation frequency, and structural phase transition were studied systematically. A method of determination of the ion quantity is estimated rapidly and with accuracy of 4% was proposed. This method is based on the low-temperature ion density theory and combined with the measurement of the ion crystal size obtained from experiment with the precision of a micrometer.3. The dynamics process of laser cooling and sympathetic cooling were analyzed in detail. The typical characteristics of mixed ion crystal such as space configuration, efficiency and temperature-temporal evolution of sympathetic cooling were studied systematically. The relation between cooling efficiency, equilibrium temperature, trapping potential, cooling and heating force, and mass to charge ratio of ions were studied quantitatively. A method for improving the efficiency of sympathetic cooling of mixed ion system was proposed, which is adding the third ions, whose mass to charge ratio is between the cooled ions and sympathetic cooled ions, and this method is very useful in the condition of big difference of mass to charge ratio of cooled ions and sympathetic cooled ions, and lower equilibrium temperature of mixed ion system can be achieved.