Measurement of the threshold ion number in the formation of an ultracold plasma

We present threshold ion number measurement in the formation of an ultracold plasma in a rubidium magneto-optical trap using the photoionization of laser-cooled atoms technique. The density and temperature of the trapped atoms in the magneto-optical trap were about 1010 cm-3 and 500 μK, respectively. The measured value of the lifetime of the ultracold plasma was exactly matched with the calculated value. The threshold ion number in the formation of ultracold plasma was obtained from the linear relationship between ion and electron numbers for different laser energies. The dependence of the threshold ion number on the depth of the potential well was investigated experimentally. The experimental results were explained by the Coulomb potential well model.