Steady-State Ultracold Plasma Created by Continuous Photoionization of Laser Cooled Atoms

In this Letter we discuss our approach that makes possible creation of the steady-state ultracold plasma having various densities and temperatures by means of continuous two-step optical excitation of calcium atoms in the magneto-optical trap. A strongly coupled ultracold plasma can be used as an excellent test platform for studying many-body interactions associated with various plasma phenomena. The parameters of the plasma are studied using laser-induced fluorescence of calcium ions. The experimental results are well described by a simple theoretical model involving equilibration of the continuous source of charged particles by the hydrodynamical ion outflux and three-body recombination. The ultracold plasma with the peak ion density of $2.7\times {10}^6{\mathrm{cm}}^{-3}$ and the minimum electron temperature near 2 K has been prepared. Our steady-state approach in combination with the strong magnetic confinement of the plasma will make it possible to reach extremely strong coupling in such system.

Figure 1

(a) Energy levels of the ${}^{40}\mathrm{Ca}$ atom; $E_I$ is ionization threshold, $E_e$ is initial electron energy. (b) Energy level of the ${}^{40}{\mathrm{Ca}}^{+}$ ion. (c) Scheme of the laser beams, used to create and illuminate the plasma. Two of the MOT beams are directed along the $z$ axis and are not shown. (d) LIF spectrum of the plasma ions obtained by scanning the 397 nm laser frequency. The peak signal corresponds to a zero detuning ${\mathrm{\Delta }}_{397}=0$ of the 397 nm from the ${{}^{2}S}_{1/2}-{{}^{2}P}_{1/2}$ transition frequency. The blue dashed curve corresponds to the best Lorentz fit. (e) Typical fluorescence image of a stationary ion cloud using an exposure time of 1 s and fluorescence spatial distribution profile of ions along the horizontal axis.

Figure 2

(a) The dependence of LIF of MOT signal on time. $u$ is the relative reduction of the MOT atomic fluorescence signal after the ionizing laser is switched on. (b) The dependence of the peak LIF of ions signal on time. An established MOT and ions fluorescence signal level after the ionizing laser beam is turned on is shown with dashed lines.

Figure 3

(a) The dependence of the total ions number on the initial electron energy $E_e$. Points are the experimental data, solid curves are the theoretical calculations taking into account three-body recombination (TBR) and dashed curves are the calculations without the TBR. $u$ is the relative reduction of the MOT fluorescence after the ionizing laser is switched on. $u=0.36$, 0.24, 0.16 correspond to the 390 nm laser intensity 153, 109, $66\mathrm{mW}/{\mathrm{cm}}^2$ accordingly. “A” is the experimental point that corresponds to maximum ions peak density of about $2.7\times {10}^6{\mathrm{cm}}^{-3}$. (b) Decay time of the plasma cloud versus $E_e$. In the inset the dependence of the established electron temperature on $E_e$ is shown.