A few interesting publications

Here are two excellent articles related to Zach Etienne's colloquium




Selected Publications

Thumbnail of figure from publication
By S. D. Bergeson (et al.)
Abstract: A hypervelocity-microparticle-impacts (HMI) laboratory has been developed at the Ion Beam Facility (IBF) of the Los Alamos National Laboratory (LANL) using a 6-MV Van de Graaff accelerator. The purpose of the laboratory is to characterize physical phenomena associated with hypervelocity impacts. Submicrometer-sized particles with velocities ranging from less than 1 km/s to greater than 100 km/s have been produced and detected. The technology development program is emphasized, and the results of a few preliminary experiments such as impact cratering and the determination of conducting-polymer size distributions are reported.
Thumbnail of figure from publication
By Chanhyun Pak, Virginia Billings, Matthew Schlitters, and Scott D. Bergeson (et al.)
Abstract:

Magnetic fields influence ion transport in plasmas. Straightforward comparisons of experimental measurements with plasma theories are complicated when the plasma is inhomogeneous, far from equilibrium, or characterized by strong gradients. To better understand ion transport in a partially magnetized system, we study the hydrodynamic velocity and temperature evolution in an ultracold neutral plasma at intermediate values of the magnetic field. We observe a transverse, radial breathing mode that does not couple to the longitudinal velocity. The inhomogeneous density distribution gives rise to a shear velocity gradient that appears to be only weakly damped. This mode is excited by ion oscillations originating in the wings of the distribution where the plasma becomes non-neutral. The ion temperature shows evidence of an enhanced electron-ion collision rate in the presence of the magnetic field. Ultracold neutral plasmas provide a rich system for studying mode excitation and decay.

Thumbnail of figure from publication
By Chanhyun Pak, Matthew J. Schlitters, and Scott D. Bergeson
Abstract:

We report frequency-comb-based measurements of Ca Rydberg energy levels. Counterpropagating laser beams at 390 and 423 nm excite Ca atoms from the 4s2 1S0 ground state to 4sns1S0 Rydberg levels with n ranging from 40 to 110. Near-resonant two-photon two-color excitation of atoms in a thermal beam makes it possible to eliminate the first-order Doppler shift. The resulting line shapes are symmetric and Gaussian. We verify laser metrology and absolute accuracy by reproducing measurements of well-known transitions in Cs, close to the fundamental wavelengths of our frequency-doubled Ti:sapphire lasers. From the measured transition energies we derive the ionization potential of Ca, EIP=1478154283.42±0.08(statistical)±0.07(systematic) MHz, improving the previous determinations by a factor of 11.

Theses, Captstones, and Dissertations

Figure from thesis
Inertial confinement fusion (ICF) involves initiating nuclear fusion reactions by imploding a small fuel pellet, causing the fuel to reach high densities and temperatures. A novel triple-shell pellet design has been proposed that is composed of three concentric spherical shells encasing an inner volume of Deuterium-Tritium fuel. This design, an alternative to the more widely tested single shell design, addresses different physics constraints. For example, the triple shell's inner-most shell is composed of a heavy metal allowing it to trap electromagnetic radiation inside the fuel, which minimizes radiative losses. Additionally, the other shells smooth out small irregularities that form during the implosion process, such as Rayleigh-Taylor (RT) instabilities. To perform its intended purpose, the optimal physical dimensions of each shell must be determined, including shell radii and thicknesses. This research presents the results of a Bayesian optimization procedure that uses one-dimensional (1D) radiation-hydrodynamic simulations to determine the optimal triple shell design parameters. This procedure advances the research in triple shell designs by determining dimensions that best avoid hydrodynamic instability growth and optimize the total energy output of implosions. The optimization process began by generating a set of simulation data by randomly querying simulations from parameter space. This initial dataset was used as the starting point for a Bayesian optimization algorithm. The yield optimized design specifying the radius and thickness of each shell is p_in=271.47, p_th=48.438, d_in=491.48, d_th=21.193, a_in=893.01, a_th=206.99 in units of microns. When fielded in ICF experiments, a capsule with parameters outlined in this paper will produce high energy output and low RT instability presence in implosions.
Figure from thesis
We study the expansion velocity and ion temperature evolution of ultracold neutral plasmas (UNPs) of calcium atoms under the influence of a uniform magnetic field that ranges up to 200 G. In the experiments, we use a magneto-optical trap (MOT) to capture the neutral atoms and laser-induced fluorescence (LIF) to take images of the plasma. We vary the magnetic field strengths and the initial electron temperatures and observe the plasma evolution in time. We compare the ion temperature evolution to the theory introduced in the paper by Pohl et. al. [Phys. Rev. A 70, 033416 (2004)]. The evolution of the gradient of expansion velocity suggests the presence of ion acoustic waves (IAWs). We speculate that our measurements showing that the ion temperature remains relatively high throughout the evolution is a biproduct of the IAW.
Figure from thesis
In ultracold neutral plasma research, it is necessary to have an imaging system that guarantees a known magnification. Early experimentation concluded that a simple imaging system consisting of identical lenses guarantees 1:1 imaging when the lenses are well aligned and the image is resolved. The purpose of this experiment is to develop a model describing the impact object position and alignment have on the magnification and confirm this model through experimentation. To achieve this, images of the 1951 USAF resolution test chart, a \SI{50}{\micro\metre} pinhole, and a \SI{100}{\micro\metre} pinhole are taken using the imaging system for different alignments and compared to a mathematical model derived from an ABCD matrix calculation. Magnification values for the images are calculated and fitted to the mathematical model, predicting the alignment of the system. The results of this experiment confirm the model when the lens pair separation is equal to or less than $2f$. However, when the lens separation is greater than $2f$, the agreement between the experiment and model decreases for the 50 $\mu$m pinhole.