· 198 · IMP & HIRFL Annual Report 2015

4 - 4 Ultracold Plasma: An Experimental Realization∗

Zaheer U. Syed1,2, Li Yufan1,2, Zhao Dongmei1, Ma Xinwen1 and Yang Jie1

(1Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730070, China;2University Chinese Academy of Sciences, Beijing 100049, China)

Ultracold plasma has well defined initial conditions, including the particle densities of more than 105 cm−3 as

well as the electron temperature in the range of 1∼1 000 K and ion temperature from tens of millikelvin to a few

Kelvin. So the ultracold plasma provides an ideal environment for the study of strongly coupled systems, where

the Coulomb interaction is comparable with the thermal kinetic energy. Ultracold plasma can be obtained by two

techniques, photoionization of laser cooled atoms[1], and the evolution of Rydberg atoms into plasma[2]. We obtained

ultracold plasma by both the techniques as shown in Fig. 1.

In the photoionization method, as shown in Fig. 1(a), a pulse laser ionizes the cold atom directly, and a pulse

of electrons arrives at the detector by applying the 30 mV/cm DC electric fried (see the first peak in Fig. 1 (a)).

When a fraction of electrons leaves the cloud, an attractive potential well forms by the ions left in the area. This

potential well continuously deepen when more electrons leave. When the depth of the potential well is equal to the

kinetic energy of the electrons, no more electrons can escape, and the plasma is formed[1]. A few microseconds later,

the electric field is increased linearly, and a second peak is detected. The magnitude of the peak decreases with the

decrease of laser energy.

In the second method, the cold atoms are firstly excited into a Rydberg states by a pulse laser, then the Rydberg

atoms will spontaneously evolve into ultracold plasma. Fig. 1(b) shows the electron signal (up) from the Rydberg

atoms and ultracold plasma. The first small peak is from slow ionization of Rydberg atoms, which is due to the

collision between the hot atoms and Rydberg atoms. The second sharp peak is due to the plasma electrons with a

tail representing the Rydberg states which evolves into plasma at later delay time. The Fig. 1(c) shows the plasma

signals with different delay time of the applied external pulsed electric field, the inset shows the integration of the

second peaks, which represents the life time of the ultracold plasma.

Fig. 1 (color online) (a) Electron signals recorded for different laser energies, where the atoms are directly

ionized by pulse laser. (b) Electron signal and external electric field for the evolution of Rydberg atoms

into plasma. (c) Plasma signals for different delay time.

References

[1] T. C. Killian, S. Kulin, S. D Bergeson, et al., Phys. Rev. Lett., 83(1999)4776.

[2] M. P. Robinson, B. L Tolra, M. W Noel, et al., Phys. Rev. Lett., 85(2000)4466.

∗ Foundation item: National Natural Science Foundation of China(11274316, 21203216, 11404346), “One Hundred Talents Program”of Chinese Academy of Sciences