Plasmas, often called the fourth state of matter and the most common one in the universe, have parameters varying by many orders of magnitude, from temperature of a few hundred kelvin in the Earth's ionosphere to 10 16 K in the magnetosphere of a pulsar. Ultracold plasmas, produced by photoionizing a sample of laser-cooled and trapped atoms near the ionization limit, have extended traditional neutral plasma parameters by many orders of magnitude, to electron temperatures below 1 K and ion temperatures in the tens of &mu K to a few Kelvin, and densities of 10 5 cm -3 to 10 10 cm -3 . These plasmas thus provide a testing ground to study basic plasma theory in a clean and simple system with or without a magnetic field. Previous studies of ultracold plasmas have primarily concentrated on temperature measurements, collective modes and expansion dynamics in the absence of magnetic fields. This thesis presents the first study of ultracold plasma dynamics in a magnetic field. The presence of a magnetic field during the expansion can initiate various phenomena, such as plasma confinement and plasma instabilities. While the electron temperatures are very low in ultracold plasmas, we need only tens of Gauss of magnetic field to observe significant effects on the expansion dynamics. To probe the ultraocold plasma dynamics in a magnetic field, we developed a new diagnostic - projection imaging, which images the ion distribution by extracting the ions with a high voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (< 20 &mu s), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2-D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of magnetic field (up to 70 G),and observe that the transverse expansion velocity scales as B &minus1/2 , explained by a nonlinear ambipolar diffusion model that involes anisotropic diffusion in two different directions. We also present the first observation of a plasma instability in an expanding ultracold plasma. We observe periodic emission of electrons from an ultracold plasma in weak, crossed magnetic and electric fields, and a strong perturbed electron density distribution in electron time-of-flight projection images. We identify this instability as a high-frequency electron drift instability due to the coupling between the electron drift wave and electron cyclotron harmonic, which has large wavenumbers corresponding to wavelengths close to the electron gyroradius.
In strongly magnetized neutral plasmas, electron motion is reduced perpendicular to the magnetic field direction. This changes dynamical plasma properties such as temperature equilibration, spatial density evolution, electron pressure, and thermal and electrical conductivity. In this paper we report measurements of free plasma expansion in the presence of a strong magnetic field. We image laser-induced fluorescence from an ultracold neutral Ca^{+} plasma to map the plasma size as a function of time for a range of magnetic field strengths. The asymptotic expansion velocity perpendicular to the magnetic field direction falls rapidly with increasing magnetic field strength. We observe that the initially Gaussian spatial distribution remains Gaussian throughout the expansion in both the parallel and perpendicular directions. We compare these observations with a diffusion model and with a self-similar expansion model and show that neither of these models reproduces the observed behavior over the entire range of magnetic fields used in this study. Modeling the expansion of a magnetized ultracold plasma poses a nontrivial theoretical challenge.
We develop a projection imaging technique to study ultracold plasma dynamics. We image the charged particle spatial distributions by extraction with a high‐voltage pulse onto a position‐sensitive detector. Measuring the 2D width of the ion image at later times (the ion image size in the first 20 μs is dominated by the Coulomb explosion of the dense ion cloud), we extract the plasma expansion velocity. These velocities at different initial electron temperatures match earlier results obtained by measuring the plasma oscillation frequency. The electron image size slowly decreases during the plasma lifetime because of the strong Coulomb force of the ion cloud on the electrons, electron loss and Coulomb explosion effects.
We measured the velocity distribution of ions in the ultracold neutral plasma (UNP) during its early evolution by the technique of velocity map imaging. The ion temperature during the phase of ion equilibrium was obtained experimentally through this method. The Coulomb coupling parameter of ions in the UNP after disorder-induced heating was also determined to be 2.1, which agreed well with the prediction from a charged particle tracing simulation. In addition, the ion expansion during the ion equilibration phase was observed. Notably, the experimentally observed expansion speed is larger than the value obtained from the self-similar expansion model, indicating the involvement of additional mechanisms, besides the electron thermal pressure, in driving the ion expansion. We have also discussed the contributions of ion–ion correlations and charge imbalance to the plasma expansion.
We present a study of the expansion of an ultracold neutral plasma (UCNP) with an initial density distribution that decays exponentially in space, created by photoionizing atoms shortly after their release from a quadrupole (or biconic cusp) magnetic trap. A characteristic ion acoustic timescale is evident in the evolution of the plasma size and velocity, indicating that the dynamics are reasonably well described by a model of hydrodynamic expansion of a quasi-neutral plasma. However, for low plasma density and high initial electron temperature, excess ion kinetic energy in the vicinity of the central density peak suggests significant local non-neutrality at early times. Observations are compared to the well-understood self-similar expansion of a UCNP with an initial Gaussian density distribution, and a similar scaling law describes the evolution of plasma size for both cases.
We report a systematic experimental and numerical study of the expansion of ultracold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities,
v0, of ultracold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature,
Te,0=mionv02/kB (where
mion is the
Rb+ ion mass), as a function of the original Rydberg atom density and binding energy,
Eb,i. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range ${10}^{7}--{10}^{9} {\mathrm{cm}}^{\ensuremath{-}3}$, for states with principal quantum number
n>40,Te,0 is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity
kBTe,0 is strongly correlated with the fraction of atoms which ionize, and is in the range $0.6\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|\ensuremath{\lesssim}{k}_{B}{T}_{e,0}\ensuremath{\lesssim}2.5\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|$. On the other hand, plasmas from Rydberg samples with $n\ensuremath{\lesssim}40$ evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where
Te,0 is determined by their Coulomb coupling parameter, ${\mathrm{\ensuremath{\Gamma}}}_{e}\ensuremath{\sim}0.01$.