Dynamics of colliding ultracold plasmas

Formation of a secondary plasma of ${\mathrm{NO}}^{+}$ ions and electrons within an ultracold plasma produces an observable change in the hydrodynamics of the system. Direct photoionization adds energetic electrons, which increases the rate of expansion. The introduction of a secondary Rydberg gas has the opposite effect. In both cases, the added ions create an inertial drag that acts initially to retard the expansion of the electron gas. A cold-ion hydrodynamic shell model, which accounts well for the effect of energy added by photoionization electrons, predicts the formation of collisionless shock waves.

Figure 1

Experimental apparatus. A skimmed supersonic molecular beam enters a nominally field-free region through an aperture in grid $G_1$. There it intersects counterpropagating laser beams indicated as ${\omega }_1$ and ${\omega }_2$. Downstream, laser pulses, ${\omega }_1^{\prime }$ and ${\omega }_2^{\prime }$, timed to intercept the the initially excited volume, excite residual ground-state NO. As the excitation volume transits the plane defined by grid $G_2$, a microchannel plate detector (MCP) situated behind grid $G_3$ collects the signal of extracted plasma electrons. The carriage holding $G_2$, $G_3$, and the MCP detector translates in vacuum within a 20 cm interval under stepper motor control.

Figure 2

Black trace: Waveform observed when the illumination volume formed by laser pulses, ${\omega }_1+{\omega }_2$ at position reaches the detection grid, $G_2$. Gray trace: Typical waveform observed when laser pulses ${\omega }_1^{\prime }+{\omega }_2^{\prime }$ intercept this volume at position 2.

Figure 3

Central trace: Electron RMS half width, $\sigma \left(t\right)$, of the electron signal waveform measured for the plasma that evolves from a Rydberg gas created at position 1. Upper trace: $\sigma \left(t\right)$ of of the electron signal waveform measured for the plasma perturbed by injecting hot electrons at position 2 after an elapsed time, $t=10\mu \mathrm{s}$. Square points obtained for a shell model of the concentric colliding plasmas (see text). Lower trace: $\sigma \left(t\right)$ of of the electron signal waveform measured for the plasma perturbed by injecting a secondary Rydberg gas with $n_0=60$ at position 2 after an elapsed time, $t=10\mu \mathrm{s}.$ Solid line represents a Vlasov expansion commencing at $t=0\mu \mathrm{s}$ for an electron temperature of 5 K.

Figure 4

Schematic diagram representing a shell model for the hydrodynamics of concentric cold-ion neutral plasmas undergoing ambipolar expansion. (Lower left) Representation of shells for a primary plasma expanding with shell velocity, $v\left(r_i\right)$ determined by 10 $\mu \mathrm{s}$ of self-similar ambipolar expansion, and shells formed at $t=10\mu \mathrm{s}$ by secondary excitation. (Upper left) Initial spherical Gaussian charge distributions for the primary plasma (blue) and the secondary plasma (red). (Right) Charge distributions after 20 and 35 $\mu \mathrm{s}$ of expansion. In all cases, a black curve describes the electron charge distribution.