Hollow Beam Optical Ponderomotive Trap for Ultracold Neutral Plasma

Abstract

Rapidly oscillating, inhomogeneous electromagnetic field from laser exert a force that repels charged particles from regions of high light intensity. We propose and analyze a flat-bottomed hollow-beam ponderomotive optical trap for an ultracold neutral plasma (UNP), driven by a high-power CO$_2$ laser. Molecular dynamics simulations show that the plasma and Rydberg atoms are effectively trapped within a nearly uniform dark region bounded by repulsive light walls. In contrast to RF traps, flat-bottomed traps yield a small density-weighted mean ponderomotive energy per electron, while the UNP collision frequency is far below the laser frequency, thereby making collisional absorption negligible and does not limit the lifetime of the trap. This approach could enhance antimatter production and storage.

Figures (4)

• Figure 1: Optical ponderomotive potential shape and properties. (a) Cross-sections of the ponderomotive potential for electrons normalized to its peak value $U_{\text{p}0}$ for various azimuthal indices $\ell$. The dashed curve shows the initial UNP density distribution with $\sigma_0=30~\mu$m along one axis. (b) Intensity profiles of the LG$_{0\ell}$ trap for selected $\ell$ values. (c) Ponderomotive force on electrons and the force from electron thermal pressure. Solid curves represent $F_{\text{p}x}$ for $\ell=\{16,4,2,1\}$ with fixed potential depth $U_{\text{p}0}/k_\text{B}=20$ K; dashed curves show $F_{\text{exp}x}$ for $T_{\text{e}}=\{1,5,10\}$ K.
• Figure 2: Time evolution of the normalized number of ions (a), the fraction of bound states (b) and the charge imbalance (c) inside the PT ($r<r_0$) for different trap depths $U_{\text{p}0}/k_{\text{B}}$ (indicated in the legend in K). The initial number of particles $N_{\text{i,e}}(0)=500$, corresponds to an initial peak density $n_0=1.18 \times 10^9$ cm$^{-3}$, $\ell=16$ and $T_e(0)=1$ K. Curves for $U_{\text{p}0}/k_{\text{B}}=0$ and 1.6 K in panel (c) are excluded for clarity due to high uncertainties.
• Figure 3: The normalized numbers of ions (a), electrons (b), and bound states (c) inside the trap volume, as well as the electron temperature (d), vs the trap depth $U_{\text{p}0}/k_{\text{B}}$ after $t=11\tau_{\mathrm{exp}}$, for different initial UNP densities $n_0$ and various trap shapes $\ell$.
• Figure 4: (a) Peak density $n_{\text{e}}(t)$ for several trap depths $U_{\text{p}0}/k_{\text{B}}$ (in K, see legend). (b) Radial density profiles at $t=0$ and $t\!\approx\!11\tau_{\mathrm{exp}}$ for $U_{\text{p}0}/k_{\text{B}}=56$ K. During equilibration the initially Gaussian profile ($k=1$) evolves toward a flat-top shape ($k=6$) with damped oscillations; the dotted vertical line corresponds to $r_0$ value. (c) The lifetime $\tau$ of electrons (dashed line) and ions (solid line) inside the PT versus $U_{\text{p}0}/k_\text{B}$ for two different initial densities. For panels (a)--(c) $\ell=16$ azimuthal index is used. (d) Density-weighted mean ponderomotive energy per electron $\langle U_{\text{p}0}\rangle/k_{\text{B}}$ vs $\ell$, points are simulation results and solid lines are least-squares fits to $A/(\ell+1)+c$, consistent with the thin-shell model expectation $\langle U_{\mathrm{p}}\rangle \propto U_{\mathrm{p}0}/(\ell+1)$.