Studies Of The Creation And Verification Of Ultra-cold Plasma Based On The Magneto-optical Trap

Ultra-cold plasma is a sort of unique plasma with very low electron temperature and ion temperature, in which the Coulomb interactions among the charged particles are comparable with their thermal kinetic energies, so it provides an ideal environment for the study of strong coupling effect and few body correlated effect, which play important roles in the inertial confinement fusion, astrophysics and other physical science fields. Since the first ultra-cold plasma experiment was performed by Killian’s group in the NIST in 1999, many laboratories in the world have carried out many studies on cold Rydberg atoms and ultra-cold plasmas. So far, all the investigations on the ultra-cold plasma are focused mainly in two purposes, how to create an ultra-cold plasma having a controllable initial state, and how the ultra-cold plasma evolves. We have built a rubidium magneto-optical trap(MOT) system and used it to cool and trap cold 87 Rb atoms cloud. Then the ultra-cold plasma was created by introducing another pulsed dye laser to directly ionize the cold atoms in MOT. The effect of the initial electron temperature and density to the evaluation of ultra-cold plasma were investigated by adjusting the wavelength and intensity of the ionization laser.The MOT consists of three parts: the vacuum, the laser and the control systems. The vacuum system is carefully designed to obtain a vacuum as high as 5×10-9 mbar. The laser system is stabilized by the saturated absorption spectroscopy with bandwidth less than 1MHz. the control system synchronizes the time sequence of the laser, coils and detectors within a precision of 1μs. Using this MOT, we got as many as 1.4×106 87 Rb atoms cooled and trapped with a density of 109 cm-3 and a temperature of 533 μK.The ultra-cold plasma was created by photo ionization of cold 87 Rb atoms in 5P3/2 state with another dye laser shorter than 479 nm. The electrons from direct ionization and from the plasma were extracted by a pulsed electric field and detected. The ionization cross section and initial electron temperature are determined by the difference between the ionization laser wavelength and the ionization potential of 87Rb atom. The initial electron/ion density depends on the intensity of the ionization laser. The lower exceeding energy and the higher laser intensity achieve the lower initial electron temperature and the higher charged particle density, respectively, which are helpful for the creation of ultra-cold plasma.