Using higher ionization states to increase Coulomb coupling in an ultracold neutral plasma

Area of Science:

• Plasma Physics
• Atomic Physics
• Quantum Fluids

Background:

• Ultracold neutral plasmas offer a unique environment to study fundamental plasma physics.
• Strongly coupled plasmas exhibit complex behaviors not seen in weakly coupled systems.

Purpose of the Study:

• To investigate the time-evolving root-mean-square (rms) velocity distribution in ultracold neutral plasmas.
• To explore the effects of mixed ion species (Ca+ and Ca2+) on plasma properties.
• To determine conditions that minimize ion heating in these plasmas.

Main Methods:

• Generation of ultracold neutral Ca+ plasma via photoionization of laser-cooled atoms.
• Formation of a mixed Ca+-Ca2+ plasma by promoting a fraction of ions to the second ionization state.
• Varying time delays between ionization events to control plasma composition and heating.
• Measurements and simulations of the rms velocity distribution and Coulomb strong-coupling parameter (Γ).

Main Results:

• Achieved a minimum in ion heating by optimizing the time delay between ionization steps.
• Observed an increase in the Coulomb strong-coupling parameter (Γ) by a factor of 1.4, reaching a maximum of 3.6.
• Demonstrated that the mixed Ca+-Ca2+ plasma approaches the liquid-like correlation regime (Γ=6.8 for pure Ca2+).

Conclusions:

• Minimizing ion heating in mixed ultracold plasmas is achievable.
• Strong coupling in these plasmas can be tuned to approach liquid-like correlations.
• The study provides insights into the dynamics of strongly coupled, multi-species ultracold plasmas.