The radio-frequency absorption in ultracold neutral plasma

Ultracold neutral plasma (UNP) represents a new frontier in the study of neutral plasmas. We investigate the energy deposited in UNP when a radio frequency (RF) is applied. For the small size of UNP, the collisional absorption in UNP can be neglected. And during the expansion of UNP, the peak resonance absorption is observed when the center density of UNP is e1/2e1/2 times of the critical density determined by the incident RF: ne0=e1/2nc=e1/2ω02m/4πe2. From the calculations, we can quantitatively measure the evolution of UNP density by radio frequency resonance diagnostic.