Trapping and Evolution Dynamics of Ultracold Two-Component Plasmas

We demonstrate the trapping of a strongly magnetized, quasineutral ultracold plasma in a nested Penning trap with a background field of 2.9 T. Electrons remain trapped in this system for several milliseconds. Early in the evolution, the dynamics are driven by a breathing-mode oscillation in the ionic charge distribution, which modulates the electron trap depth. Over longer times scales, the electronic component undergoes cooling. Trap loss resulting from $\mathbf{E}\times \mathbf{B}$ drift is characterized.

Figure 1

(a) Electrodes that generate Penning trap, dc bias, and plasma extraction electric fields. Only the bottom half is shown. (b) Axial electric-potential profile in the two-component plasma trap. To extract electrons from the trap, the voltage on the electrode $E2$ changes in time. (c) Dotted line: Electron trap depth on an axial line through the trap center as a function of the voltage difference $\Delta V$ between electrodes $E2$ and $E3$. Solid line: Average electron trap depth sampled by a distribution of electrons ${\rho }_{e^{-}}(x,y)=(\pi s^2{)}^{-1}\exp [-(x^2+y^2)/s^2]$, where $s=1.5\mathrm{mm}$.

Figure 2

(a) Periodic modulation in the electron current due to the ionic motion in the nested Penning trap ($0<t<450\mu \mathrm{s}$). At $t=450\mu \mathrm{s}$ after photoionization, an applied electric field extracts remaining plasma electrons. (b) The modulation frequency changes as curvature of the Penning-trapping electric potential for ions varies. (c) Effect of the initial ion number on the extraction signal taken under a fixed trap curvature. Initial ion numbers are shown for each signal ($N_0$ is in the ${10}^5–{10}^6$ range).

Figure 3

Electron signal (left axis) versus time (onset of extraction ramp at $t=0$) for $V_{\mathrm{in}}=-1\mathrm{V}$ and the indicated extraction delay times relative to photoexcitation. The bold curves and the dotted curves show the potential difference $\Delta V$ between $E2$ and $E3$ (right axis; $t_f$ denotes time of flight to detector). Inset: Average arrival time versus extraction delay time.

Figure 4

(a) Electron extraction signal ($V_{\mathrm{in}}=-2\mathrm{V}$) for delay times up to 1.8 ms. (b) Extraction signal between 3 and 4 ms.

Figure 5

Trapped electron number versus time for the indicated values of $V_{\mathrm{in}}$. The $1/e$ lifetimes $\tau$ are obtained from fits to the data.

Figure 6

(a) Spatial distribution of electrons at indicated extraction delay times. Initially, the plasma is prepared in either a vertical (upper row) or a horizontal (lower row) geometry. (b) Field geometry and resultant $\mathbf{E}\times \mathbf{B}$ drift-velocity field.