Publications

Abstract:

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.

Abstract:

New facilities such as the National Ignition Facility and the Linac Coherent Light Source have pushed the frontiers of high energy-density matter. These facilities offer unprecedented opportunities for exploring extreme states of matter, ranging from cryogenic solid-state systems to hot, dense plasmas, with applications to inertial-confinement fusion and astrophysics. However, significant gaps in our understanding of material properties in these rapidly evolving systems still persist. In particular, non-equilibrium transport properties of strongly-coupled Coulomb systems remain an open question. Here, we study ion-ion temperature relaxation in a binary mixture, exploiting a recently-developed dual-species ultracold neutral plasma. We compare measured relaxation rates with atomistic simulations and a range of popular theories. Our work validates the assumptions and capabilities of the simulations and invalidates theoretical models in this regime. This work illustrates an approach for precision determinations of detailed material properties in Coulomb mixtures across a wide range of conditions.

Abstract:

Understanding ion transport in plasma mixtures is essential for optimizing the energy balance in high-energy-density systems. In this paper, we focus on one transport property, ion–ion temperature relaxation in a strongly coupled plasma mixture. We review the physics of temperature relaxation and derive a general temperature relaxation equation that includes dynamical correlations. We demonstrate the fidelity of three popular kinetic models that include only static correlations by comparing them to data from molecular dynamics simulations. We verify the simulations by comparing with laboratory data from ultracold neutral plasmas. By comparing our simulations with high fidelity kinetic models, we reveal the importance of dynamical correlations in collisional relaxation processes. These correlations become increasingly significant as the ion mass ratio in a binary mixture approaches unity.

Abstract:

Charged particle transport plays a critical role in the evolution of high energy-density plasmas. As high-fidelity plasma models continue to incorporate new micro-physics, understanding multi-species plasma transport becomes increasingly important. We briefly outline theoretical challenges of going beyond single-component systems and binary mixtures as well as emphasize the roles experiment, simulation, theory, and modeling can play in advancing this field. The 2020 Division of Plasma Physics mini-conference on transport in Transport in Non-Ideal, Multi-Species Plasmas was organized to bring together a broad community focused on modeling plasmas with many species. This special topics issue of Physics of Plasmas touches on aspects of ion transport presented at that mini-conference. This special topics issue will provide some context for future growth in this field.

Abstract:

We report an all-solid-state gamma-ray scintillation detector comprised of a NaI(Tl) crystal and a scientific-grade CMOS camera. After calibration, this detector exhibits excellent linearity over more than three decades of activity levels ranging from 10 mCi to 400 nCi. Because the detector is not counting pulses, dead-time correction is not required. Compared to systems that use a photomultiplier tube, this detector has similar sensitivity and noise characteristics on short time scales. On longer time scales, we measure drifts of a few percent over several days, which can be accommodated through regular calibration. Using this detector, we observe that when high activity sources are brought into close proximity to the NaI crystal, several minutes are required for the measured signal to achieve a steady state.

Abstract: In this paper, we present ideas that were part of the miniconference on the crossover between High Energy Density Plasmas (HEDP) and Ultracold Neutral Plasmas (UNPs) at the 60th Annual Meeting of the American Physical Society Division of Plasma Physics, November 2018. We give an overview of UNP experiments with an emphasis on measurements of the time-evolving ion density and velocity distributions, the electron-ion thermalization rate, and plasma self-assembly—all just inside the strongly coupled plasma regime. We also present theoretical and computational models that were developed to understand a subset of HEDP experiments. However, because HEDP experiments display similar degrees of strong coupling, many aspects of these models can be vetted using precision studies of UNPs. This comparison is important because some statistical assumptions used for ideal plasmas are of questionable validity in the strongly coupled plasma regime. We summarize two theoretical approaches that extend kinetic theories into the strong-coupling regime and show good agreement for momentum transfer and self-diffusion. As capabilities improve, both computationally and experimentally, UNP measurements may help guide the ongoing development of HEDP-appropriate plasma models. Future opportunities in viscosity, energy relaxation, and magnetized plasmas are discussed.

Abstract: Transport properties of high-energy-density plasmas are influenced by the ion collision rate. Traditionally, this rate involves the Coulomb logarithm, ln Λ. Typical values of ln Λ are ≈ 10–20 in kinetic theories where transport properties are dominated by weak-scattering events caused by long-range forces. The validity of these theories breaks down for strongly coupled plasmas, when ln Λ is of order one. We present measurements and simulations of collision data in strongly coupled plasmas when ln Λ is small. Experiments are carried out in the first dual-species ultracold neutral plasma (UNP), using Ca+ and Yb+ ions. We find strong collisional coupling between the different ion species in the bulk of the plasma. We simulate the plasma using a two-species fluid code that includes Coulomb logarithms derived from either a screened Coulomb potential or a the potential of mean force. We find generally good agreement between the experimental measurements and the simulations. With some improvements, the mixed Ca+ and Yb+ dual-species UNP will be a promising platform for testing theoretical expressions for ln Λ and collision cross-sections from kinetic theories through measurements of energy relaxation, stopping power, two-stream instabilities, and the evolution of sculpted distribution functions in an idealized environment in which the initial temperatures, densities, and charge states are accurately known.

Abstract:

Plasmas are supposed to be hot. Hydrogen nuclei undergo fusion in the Sun because plasma temperatures and pressures are so high. On page 61 of this issue, Langin et al. (1) report on a completely different kind of plasma by photoionizing a laser-cooled gas of strontium atoms. The ion temperature is a chilly 0.05 K, so thermal speed of the ions is equivalent to a person taking a brisk walk. Surprisingly, the properties of this low-density, low-temperature plasma provide clues about the workings of high–energy-density physics relevant for fusion power research.

Abstract: We report nearly continuous beta-decay-rate measurements of Na-22, Cl-36, Co-60, Sr-90, and Cs-137 over a period of 2.7 years using four Geiger-Müller tubes. We carefully control the ambient pressure and temperature for the detectors, sources, and electronics in order to minimize environmentally-dependent systematic drifts in the measurement chains. We show that the amplitudes of an annual oscillation in the decay rates are consistent with zero to within 0.004%.

Abstract: We present measurements of the hyperfine splitting in the ¹⁷³Yb 6s6p1Po1(F′=3/2,7/2) states that disagree significantly with those measured previously by Das and Natarajan [Phys. Rev. A 76, 062505 (2007)]. We point out inconsistencies in their measurements and suggest that their error is due to optical pumping and improper determination of the atomic line center. Our measurements are made using an optical frequency comb. We use an optical pumping scheme to improve the signal-to-background ratio for the F′=3/2 component.

Abstract: We report on measurements of relative beta-decay rates of Na-22, Cl-36, Co-60, Sr-90, Cs-137 monitored for more than one year. The radioactive samples are mounted in an automated sample changer that sequentially positions the five samples in turn, with high spatial precision, in front of each of four Geiger–Müller tubes. The sample wheel, detectors, and associated electronics are housed inside a sealed chamber held at constant absolute pressure, humidity, and temperature to isolate the experiment from environmental variations. The statistical uncertainty in the count rate approaches a few times 0.01% with two weeks of averaging. Other sources of error are on a similar scale. The data are analyzed in variety of ways, comparing count rates of the various samples on one or more detectors, and comparing count rates of a particular sample across multiple detectors. We observe no statistically significant variations in the ratios of decay rates, either annual or at higher-frequency, at a level above 0.01%. 

Abstract: We present an analysis of ion temperatures in laser-produced plasmas formed from solids with different initial lattice structures. We show that the equilibrium ion temperature is limited by a mismatch between the initial crystallographic configuration and the close-packed configuration of a strongly-coupled plasma, similar to experiments in ultracold neutral plasmas. We propose experiments to demonstrate and exploit this crystallographic heating in order to produce a strongly coupled plasma with a coupling parameter of several hundred.

Abstract:

We describe an experimental setup for making precision measurements of relative β-decay rates of Na-22, Cl-36, Mn-54, Co-60, Sr-90, Ba-133, Cs-137, Eu-152, and Eu-154. The radioactive samples are mounted in two automated sample changers that sequentially position the samples with high spatial precision in front of sets of detectors. The set of detectors for one sample changer consists of four Geiger-Müller (GM) tubes and the other set of detectors consists of two NaI scintillators. The statistical uncertainty in the count rate is few times 0.01% per day for the GM detectors and about 0.01% per hour on the NaI detectors. The sample changers, detectors, and associated electronics are housed in a sealed chamber held at constant absolute pressure, humidity, and temperature to isolate the experiment from environmental variations. The apparatus is designed to accumulate statistics over many years in a regulated environment to test recent claims of small annual variations in the decay rates. We demonstrate that absent this environmental regulation, uncontrolled natural atmospheric pressure variations at our location would imprint an annual signal of 0.1% on the Geiger-Müller count rate. However, neither natural pressure variations nor plausible indoor room temperature variations cause a discernible influence on our NaI scintillator detector count rate.

Abstract: Ultracold neutral plasmas (UNPs) are plasmas generated through a rapid photoionization process of a laser-cooled atomic gas. Because of the very low initial ionic temperature (Ti(0) ∼mK), UNPs are extremely strongly coupled. Following the formation of correlations, UNPs settle into a coupling regime with Γ ∼ 1, where Γ is the usual Coulomb coupling parameter. The observation of a wider range of plasma phenomena requires experimental control over the details of this process. We describe the generation and diagnosis of UNPs in the strongly coupled plasma regime with Γ ≥ 1 using calcium in a magneto-optical trap. We discuss four avenues to achieve such couplings, including the use of electron screening, multiple ionization to higher ionization states, Rydberg atom dynamics, and direct laser-cooling of the ions. Electron screening mitigates the initial Coulomb repulsion, but also impacts the final effective coupling. We illustrate this by calculating the structural properties of UNPs for different strengths of electron screening for typical values of Γ. Molecular dynamics (MD) simulations are used to reveal the dynamical impacts of electron screening, and show that the final Γ is readily increased whereas the effective coupling remains of order unity. Similarly, we perform MD for a double ionization process in which the second ionization is timed carefully to correspond to a minimum in the time evolution of g(r,t). In addition to their intrinsic interest, UNPs can provide a platform for exploring basic plasma physics relevant to a wide range of seemingly disparate plasmas, including fusion-class plasmas.

Abstract: We report measurements of a neutral strongly coupled plasma generated by focusing a femtosecond-duration laser pulse into a room-temperature gas jet. The ion temperature in this plasma is determined by the plasma density through the disorder-induced heating effect. We present measurements of the mass, radius, and energy dependence of the time-varying ion density as the plasma expands. Molecular dynamics model indicate that higher values of the strong coupling parameter could be achieved if the plasma is ionized again by a second laser pulse that follows the first one. However, the final value of the coupling parameter appears to be only weakly dependent on the final ionization state.

Abstract: Ultracold neutral plasmas are strongly coupled Coulomb systems that are generated by photoionizing lasercooled atoms close to the ionization threshold. The strong coupling parameter Γ is limited at times later than ∼100 ns by disorder-induced heating. A recent simulation predicted that higher values of Γ can be realized in ultracold neutral plasmas if the plasma ions are excited to higher ionization states. In this paper we present recent results from an experiment that increases the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Using laser-induced fluorescence we map out the ion velocity distribution of the Ca+ ions in a partially doubly ionized plasma and show that the heating due to the second ionization depends on the timing of the second ionization laser pulses. We compare our results to MD simulations, which estimate that Γ increases from approximately 2.5 to 3.6.

Abstract: We report measurements and simulations of the time-evolving rms velocity distribution in an ultracold neutral plasma. A strongly coupled ultracold neutral Ca +   plasma is generated by photoionizing laser-cooled atoms close to threshold. A fraction of these ions is then promoted to the second ionization state to form a mixed Ca + −Ca 2+   plasma. By varying the time delay between the first and the second ionization events, a minimum in ion heating is achieved. We show that the Coulomb strong-coupling parameter Γ  increases by a factor of 1.4 to a maximum value of 3.6. A pure Ca 2+   plasma would have Γ=6.8 , moving these strongly coupled systems closer to the regime of liquid-like correlations.

Abstract: We present a simple method for precision spectroscopy using an optical frequency comb. One mode of a 1 GHz repetition rate mode-locked ti:sapphire laser is offset-locked to a Rb-stabilized diode laser. This partially stabilized frequency comb stays locked, unattended, for hours at a time. Using the measured offset frequency and repetition rate, we calculate the frequency of each comb mode with an absolute uncertainty of about 10 kHz in a 10 second measurement window. We demonstrate the capabilities and limitations of this approach with measurements in Rb, Cs, and Ca.

Abstract: The authors report on an approach to the construction of long-lasting rubidium atomic vapor cells. The method uses pinch-off copper cold-welds, low temperature solders, and electroplated copper to create long-lasting hermetic seals between containment chambers of dissimilar geometries and materials. High temperature epoxy, eutectic lead/tin solder, and indium solder were considered as sealing materials. These seals were analyzed using accelerated lifetime testing techniques. Vapor cells with epoxy and bare metal solder seals had a decrease in the rubidium atomic density within days after being heated to elevated temperatures. They also exhibited broadened spectra as a result of rubidium reacting with the seals. However, indium solder seals with a passivation coating of electroplated copper did not exhibit a significant decrease in linewidth or atomic density after being held at 95 degrees C for 30 days. The authors conclude that this particular seal has no rubidium chemical reaction failure mode and when used in combination with copper cold welding has the potential to create multiplatform vapor cells with extremely long lifetimes. (C) 2013 American Vacuum Society.

Abstract: We show that strong coupling between ions in an ultracold neutral plasma is limited by electron screening. While electron screening reduces the quasiequilibrium ion temperature, it also reduces the ion-ion electrical potential energy. The net result is that the ratio of nearest-neighbor potential energy to kinetic energy in quasiequilibrium is constant and limited to approximately 1 unless the electrons are heated by some external source. We support these conclusions by reporting measurements of the ion velocity distribution in an ultracold neutral calcium plasma. These results match previously reported simulations of Yukawa systems. Theoretical considerations are used to determine the screened nearest-neighbor potential energy in the plasma. DOI: 10.1103/PhysRevE.87.033101

Abstract: The effect of matrix on the formation and focusing of a Ca ion beam in the second vacuum stage of an inductively coupled plasma mass spectrometer has been evaluated with the use of planar laser induced fluorescence. A cross section of the beam was imaged near the entrance to the mass analyzer of a commercial instrument Characteristics of the beam from a solution containing only the Ca analyte closely matched those predicted by simulation software. The individual addition of three matrix species, Mg, Cs, and Pb, had minor effect on beam shape. Cs and Pb both affected the beam trajectory. The most pronounced effect was with the Pb matrix, which caused an order-of-magnitude drop in the Ca signal intensity at the electron multiplier of the mass spectrometer. The loss in signal was due primarily to a shift in the direction and location of the Ca ion beam that caused it to miss the entrance into the mass analyzer. (C) 2012 Elsevier B.V. All rights reserved.

Abstract: We report a measurement of the electron temperature in a plasma generated by a high-intensity laser focused into a jet of neon. The 15 eV electron temperature is determined using an analytic solution of the plasma equations assuming local thermodynamic equilibrium, initially developed for ultracold neutral plasmas. We show that this analysis method accurately reproduces more sophisticated plasma simulations in our temperature and density range. While our plasma temperatures are far outside the typical "ultracold" regime, the ion temperature is determined by the plasma density through disorder-induced heating just as in ultracold neutral plasma experiments. Based on our results, we outline a pathway for achieving a strongly coupled neutral laser-produced plasma that even more closely resembles ultracold neutral plasma conditions.

Abstract:

Abstract: Ultracold plasmas are formed by ionizing laser-cooled gases. The spatial disorder in the initial plasma state gives rise to rapid ion heating called disorder-induced heating. The heating rate depends on the electron temperature because lower temperature electrons more effectively shield neighbouring ion charges from one another. We illustrate the effect of electron shielding on the ion thermalization rate in ultracold plasmas.

Abstract: An analysis showing that phase noise in a master diode laser is converted to amplitude noise in an injection-locked Ti:sapphire power amplifier was recently published ["Intensity noise of an injection-locked Ti: sapphire laser: analysis of the phase-noise-to-amplitude-noise conversion," J. Opt. Soc. Am. B, 23, 1276-1286 (2006)]. As this analysis might discourage the broad implementation of injection locking, we report amplitude noise and laser linewidth measurements in such a system and note that these lasers have sufficiently low noise to be useful in a wide range of experiments in atomic, molecular, and optical physics. A low-power diode laser is amplified to 1.6W at 846nm. Amplitude noise is measured using a high-speed photodiode. Frequency noise is measured relative to a low-noise commercial Ti: sapphire laser using an offset lock and heterodyne technique. Under optimal conditions, the relative rms amplitude noise is 1%. The linewidth of the injection-locked laser is 300 kHz. As others in this field have shown, the amplitude and frequency noise characteristics depend critically on the lock circuit characteristics. (C) 2011 Optical Society of America

Abstract: We report measurements and simulations of disorder-induced heating in ultracold neutral plasmas. Fluorescence from plasma ions is excited using a detuned probe laser beam while the plasma relaxes from its initially disordered nonequilibrium state. This method probes the wings of the ion velocity distribution. The simulations yield information on time-evolving plasma parameters that are difficult to measure directly and make it possible to connect the fluorescence signal to the rms velocity distribution. The disorder-induced heating signal can be used to estimate the electron and ion temperatures similar to 100 ns after the plasma is created. This is particularly interesting for plasmas in which the electron and ion temperatures are not known.

Abstract: We report measurements and simulations of laser-induced fluorescence in ultracold neutral plasmas. We focus on the earliest times, when the plasma equilibrium is evolving and before the plasma expands. In the simulation, the ions interact via the Yukawa potential in a small cell with wrapped boundary conditions. We solve the optical Bloch equation for each ion in the simulation as a function of time. Both the simulation and experiment show the initial increase in ion fluorescence, disorder-induced heating, and coherent oscillation of the rms ion velocity. Detailed modeling of the fluorescence signal makes it possible to use fluorescence spectroscopy to probe ion dynamics in ultracold and strongly coupled plasmas.

Abstract: We present measurements of the velocity distribution of calcium atoms in an atomic beam generated using a dual-stage laser back-ablation apparatus. Distributions are measured using a velocity selective Doppler time-of-flight technique. They are Boltzmann-like with rms velocities corresponding to temperatures above the melting point for calcium. Contrary to a recent report in the literature, this method does not generate a subthermal atomic beam.

Abstract: We present the first measurements and simulations of recombination fluorescence from ultracold neutral calcium plasmas. This method probes three-body recombination at times less than 1 mu s, shorter than previously published time scales. For the lowest initial electron temperatures, the recombination rate scales with the density as n(0)(2.2), significantly slower than the predicted n(0)(3). Recombination fluorescence opens a new diagnostic window in ultracold plasmas. In most cases it probes deeply bound level populations that depend critically on electron energetics. However, a perturbation in the calcium 4snd Rydberg series allows our fluorescence measurements to probe the population in weakly bound levels that result just after recombination.

Abstract: We report on the development of a compact commercial instrument for measuring carotenoids in skin tissue. The instrument uses two light-emitting diodes (LEDs) for dual-wavelength excitation and four photomultiplier tubes for multichannel detection. Bandpass filters are used to select the excitation detection wavelengths. The f/1.3 optical system has high optical throughput and single photon sensitivity, both of which are crucial in LED-based Raman measurements. We employ a signal processing technique that compensates for detector drift and error. The sensitivity and reproducibility of the LED Raman instrument compares favorably to laser-based Raman spectrometers. This compact, portable instrument is used for noninvasive measurement of carotenoid molecules in human skin with a repeatability better than 10%.

Abstract: We have developed a compact portable instrument for resonance Raman spectroscopy of carotenoid molecules in skin tissue. Our application focuses on the 1525 cm-1 Raman line common to all carotenoids. We use a divided shifted Raman spectroscopy (DSRS) technique that reduces sensitivity to detector drift and error. Two wavelength-narrowed LEDs illuminate the sample, and scattered light in four different wavelength channels is measured. This multi-spectral approach has single-photon sensitivity and compares favorably with laser-based Raman measurements in terms of accuracy, repeatability, and measurement time.

Abstract: We report a new absolute frequency measurement of the Cs6s–8s two-photon transition measured using frequency comb spectroscopy. The fractional frequency uncertainty is 5×10−11 , a factor of 6 better than previous results. The comb is derived from a stabilized picosecond laser and referenced to an octave-spanning femtosecond frequency comb. The relative merits of picosecond-based frequency combs are discussed, and it is shown that the AC Stark shift of the transition is determined by the average rather than the much larger peak intensity.

Abstract: We describe a Fourier-transform spectrometer appropriate for use in an undergraduate optics laboratory. The modular design, which uses off-the-shelf equipment, is suitable for determining the spectra of ultrashort pulsed and continuous light sources. The spectrometer does not require equal step sizes for the motion of the mirror. An algorithm interpolates the phase of both a reference and an unknown light beam at equal intervals before performing the Fourier transform. The interferometer scan and the phase and spectral analysis are performed in a few seconds, making the apparatus a useful tool for teaching concepts,of temporal coherence and Fourier spectroscopy. (C) 2006 American Association of Physics Teachers.

Abstract: We demonstrate a compact solid-state laser source for high-resolution two-photon spectroscopy of the 1S-2S transition in atomic hydrogen. The source emits up to 20 mW at 243 nm and consists of a 972 nm diode laser, a tapered amplifier, and two doubling stages. The diode laser is actively stabilized to a high-finesse cavity. We compare the new source to the stable 486 nm dye laser used in previous experiments and record 1S-2S spectra using both systems. With the solid-state laser system, we demonstrate a resolution of the hydrogen spectrometer of 6x10(11), which is promising for a number of high-precision measurements in hydrogenlike systems.

Abstract: We report new detailed density profile measurements in expanding strongly coupled neutral calcium plasmas. Using laser-induced fluorescence techniques, we determine plasma densities in the range of 10(5) to 10(9) cm(-3) with a time resolution limit as small as 7 ns. Strong coupling in the plasma ions is inferred directly from the fluorescence signals. Evidence for strong coupling at late times is presented, confirming a recent theoretical result.

Abstract: An isothermal model of ultracold neutral plasma expansion is extended to systems without spherical symmetry. It is used to interpret new fluorescence measurements on ultracold neutral calcium plasmas. For a self-similar expansion, the fluid equations are solved both analytically and numerically. The density and velocity solutions are used to predict fluorescence signals induced by a laser beam weakly focused into the plasma. Despite the simplicity of the model, predicted fluorescence signals reproduce major features of the experimental data.

Abstract: We have demonstrated several inexpensive methods that can be used to measure the deflection angles of prisms with microradian precision. The methods are self-referenced, where various reversals are used to achieve absolute measurements without the need of a reference prism or any expensive precision components other than the prisms under test. These techniques are based on laser interferometry and have been used in our laboratory to characterize parallel-plate beam splitters, penta prisms, right-angle prisms, and corner cube reflectors using only components typically available in an optics laboratory. (c) 2005 Optical Society of America.

Abstract: Absolute x-ray calibration of laser-produced plasmas was performed using a focusing crystal von Hamos spectrometer. The plasmas were created by an Nd-YAG laser (0.53 μm/200 mJ/3 ns/10 Hz) on massive solid targets (Mg, Cu, Zn, Sn, Mo, Ta, Ti, Steel). Cylindrical mica crystal (radius of curvature R=20 mm) and a CCD linear array detector (Toshiba model TCD 1304AP) were used in the spectrometer. Both the mica crystal and CCD linear array were absolutely calibrated in the spectral range of λ=7-15 Å. The spectrometer was used for absolute spectral measurements and the determination of the plasma parameters. The unique target design allowed for multiple instruments to observe the plasma simultaneously which improved analysis. High spectrometer efficiency allows for the monitoring of absolute x-ray spectra, x-ray yield and plasma parameters in each laser shot. This spectrometer is promising for absolute spectral measurements and for monitoring laser-plasma sources intended for proximity print lithography.

Abstract: We report an optical dipole trap for calcium. The trap is created by focusing a 488-nm argon-ion laser beam into a calcium magneto-optical trap. The argon-ion laser photoionizes atoms in the trap because of a near-resonance with the 4s4f (1)F(3) level. By measuring the dipole-trap decay rate as a function of argon-ion laser intensity, we determine the (1)F(3) photoionization cross section at our wavelength to be approximately 230 Mb.

Abstract: We present a method of external-cavity diode-laser grating stabilization that combines the high output power of the Littrow design with the fixed output, pointing of the Littman-Metcalf design. Our new approach utilizes a Faraday-effect optical isolator inside the external cavity. Experimental testing and a model that describes the tuning range and optimal tuning parameters of the laser are described. Preliminary testing of this design has resulted in a short-term linewidth of 360 kHz and a side-mode suppression of 37 dB. The laser tunes mode hop free over 7 GHz, and we predict that much larger tuning ranges are possible. (C) 2004 Optical Society of America.

Abstract: Photoionizing laser-cooled atoms produces ultracold neutral plasmas with initial temperatures of 1-1000 K and densities as high as 10(10) cm(-3). Applied radio frequency fields can excite plasma oscillations that are used to monitor the expansion of the unconfined plasma. Significant three-body recombination of electrons and ions into Rydberg atoms takes place during the plasma expansion. Previous experiments have been done with xenon, but a new experiment is planned with laser-cooled strontium. The strontium ion has an optically allowed transition at a convenient blue wavelength. This will allow direct imaging of the plasma through fluorescence or absorption, and may enable laser cooling and trapping of the plasma.

Abstract: Cold plasma theory is used to calculate the response of an ultracold neutral plasma to an applied rf field. The free oscillation of the system has a continuous spectrum and an associated damped quasimode. This quasimode dominates the driven response and is resonant in the tail of the density distribution. Recent experiments used the plasma response to an applied rf field to determine the plasma density in an expanding ultracold plasma. The comparison between experiment and theory indicates that this method accurately determines the expansion velocity and underestimates the initial plasma density by a factor of 3 in weakly collisional plasmas.

Abstract: The ultracold world has fascinated and surprised scientists since 1911, when Heike Kamerlingh Onnes discovered superconductivity in mercury at 4.2 K. Now physicists routinely achieve temperatures millions of times colder. When atoms are cooled this close to absolute zero, they fall into the lowest possible quantum state – with bizarre consequences.

Abstract: We report on a 1-W injection-locked cw titanium:sapphire ring laser at 846 nm. Single-frequency operation requires only a few milliwatts of injected power. This relatively simple and inexpensive system can be used for watt-level single-frequency lasers across most of the titanium: sapphire gain region. A brief review of injection-locking theory is given, and conclusions based on this theory indicate ways to improve the performance of the system. (C) 2002 Optical Society of America.

Abstract: We report measurements of thermal self-locking of a Fabry-Perot cavity containing a potassium niobate (KNbO3) crystal. We develop a method to determine linear and nonlinear optical absorption coefficients in intracavity crystals by detailed analysis of the transmission line shapes. These line shapes are typical of optical bistability in thermally loaded cavities. For our crystal we determine the one-photon absorption coefficient at 846 nm to be alpha = (0.0034 +/- 0.0022) m(-1), the two-photon absorption coefficient at 846 nm to be beta = (3.2 +/- 0.5) X 10(-11) m/W, and the one-photon absorption coefficient at 423 nm to be (13 +/- 2) m(-1). We also address the issue of blue-light-induced infrared-absorption and determine a coefficient for this excited-state absorption process. Our method is particularly well suited to bulk absorption measurements where absorption is small compared with scattering. We also report new measurements of the temperature dependence of the index of refraction at 846 nm. and compare with values in the literature. (C) 2001 Optical Society of America.

Abstract:

We present two experiments in closed-loop feedback control. In the first experiment, students control the pointing angle of a laser to "lock" the laser onto a "target." In the second, students stabilize the pathlength difference in two arms of a Michelson interferometer. These experiments are appropriate for electronics and optics laboratory classes for junior and senior level students.

Abstract: We study the formation of Rydberg atoms in expanding plasmas at temperatures of 1-1000 K and densities from 10(5)-10(10) cm(-3). Up to 20% of the initially free charges recombine in about 100 mus, and the binding energy of the Rydberg atoms approximately equals the increase in the kinetic enemy of the remaining free electrons. Three-body recombination is expected to dominate in this regime, yet most of our results are inconsistent with this mechanism.

Abstract: We describe techniques for laser spectroscopy in the vacuum-UV (VUV) spectral region that combine high spectral resolution with high absolute accuracy. A nearly transform-limited nanosecond laser source at 120 nm is constructed with difference-frequency mixing. This source is used to perform the first, to our knowledge, Doppler-free VUV measurement. We measure the inherently narrow 1(1)S-2(1)S two-photon transition in atomic helium with a spectral resolution of 7 parts in 10(8) (180 MHz), the narrowest line width so far observed at such short wavelengths. Careful measurements of optical phase perturbations allow us to determine the absolute frequency of the line center to a fractional uncertainty of 1 part in 10(8). Improvements now in progress should reduce this uncertainty to 2 parts in 10(9). (C) 2000 Optical Society of America [S0740-3224(00)01009-2].

Abstract: We report the observation of plasma oscillations in an ultracold neutral plasma. With this collective mode we probe the electron density distribution and study the expansion of the plasma as a function of time. For classical plasma conditions, i.e.,weak Coulomb coupling, the expansion is dominated by the pressure of the electron gas and is described by a hydrodynamic model. Discrepancies between the model and observations at low temperature and high density may be due to strong coupling of the electrons.

Abstract: We report the creation of an ultracold neutral plasma by photoionization of laser-cooled xenon atoms. The charge carrier density is as high as 2 X 10(9) cm(-3), and the temperatures of electrons and ions are as low as 100 mK and 10 mu K, respectively. Plasma behavior is evident in the trapping of electrons by the positive ion cloud when the Debye screening length becomes smaller than the size of the sample. We produce plasmas with parameters such that bath elections and ions ale strongly coupled.

Abstract: A High Sensitivity Absorption experiment which is used to measure UV and VUV oscillator strengths of both atoms and ions has been developed at the University of Wisconsin–Madison. This experiment incorporates a hollow cathode discharge as an absorbing sample, the Aladdin storage ring at the Synchrotron Radiation Center as a continuum source, and a 3 m focal length vacuum echelle spectrometer equipped with a CCD detector array. The experiment achieves spectral resolving powers of 350,000 and sensitivities to fractional absorptions much smaller than 1% at deep UV and VUV wavelengths. Although absorption techniques have long been used in measuring oscillator strengths, detector arrays vastly increase the sensitivity, and synchrotron radiation extends the wavelength coverage, of such experiments. Column densities of sputtered metal atoms and ions as small as 3.0 × 108 cm-2 can be detected. Our experiment is used to measure relative oscillator strengths for lines from a common lower level, usually a VUV transition relative to a well known UV transition. An accurate absolute scale for all of these measurements is established using radiative lifetimes from laser induced fluorescence measurements on atomic/ionic beams. This experiment is applicable to essentially every element in the periodic table, both neutral atoms and atomic ions. The experiment is used to measure oscillator strengths for the most important UV and VUV resonance lines of Fe+, Co+, and Ni+. These resonance lines are prominent in absorption spectra on the Interstellar Medium (ISM) recorded using the Hubble Space Telescope.

Abstract: High-resolution laser-based measurements of energy levels have spurred the development and refinement of quantum electrodynamic calculations. We have extended very-high-resolution laser spectroscopy techniques into the vacuum ultraviolet wavelength region at λ ≈ 120 nm to determine the Lamb shift in the ground state of He I. The importance of this measurement to high accuracy quantum electrodynamics calculations is discussed. Details of the present measurement and future improvements are presented.

Abstract: Using 40 ns laser pulses, we probe the real-time dynamics of ultracold ionizing collisions in metastable xenon. We time resolve both shielding and enhancement effects, and observe the production of Xe-2(+) molecular ions through associative ionization. We estimate the rate of molecule formation in excited-state collisions, and directly measure the role of both flux enhancement and excited state survival in the collisional enhancement process. Conceptually simple theoretical models are used to predict the dynamics of the collisional shielding.

Abstract: We have extended two-photon Doppler-free spectroscopy to the vacuum ultraviolet spectral region, to accurately measure the He 1 S-1-2 S-1 transition at 120 nm. Our result is 4984872315(48) MHz. This yields a ground state Lamb shift of 41104(48) MHz, in fair agreement with theory and other experiments. This approach has the potential for significantly better accuracy once improvements in the laser and the wavelength metrology are implemented.

Abstract: New experimental branching fractions and transition probabilities are reported for 56 transitions in Fe II. The branching fractions are measured with a Fourier transform spectrometer and also with a high-resolution grating spectrometer on an optically thin hollow cathode discharge. Highly accurate experimental radiative lifetimes from the recent Literature provide the normalization required to convert our branching fractions into absolute transition probabilities. Results are compared with experimental and theoretical values in the literature. Our new transition probabilities will establish the absolute scale for relative absorption oscillator strengths of vacuum ultraviolet lines measured using a new high-sensitivity absorption experiment at the University of Wisconsin.

Abstract: We report the first measurements of UV oscillator strengths (f-values) in Fe II from a high-sensitivity absorption experiment developed at the University of Wisconsin. The accuracy of our measurements is demonstrated by our reproducing well-known f-value ratios in Fe I and Fe II. The first laboratory f-value measurement of the 160.845 nm transition in Fe II is presented and compared to values in the literature. While this paper focuses on Fe II, the high-sensitivity absorption method that we have developed is applicable to essentially every element in the periodic table, for both neutral and singly ionized species, over a broad range of wavelength and line strength.

Abstract: High sensitivity absorption spectroscopy involves the use of modern diode and CCD (charge coupled device) detector arrays to observe fractional absorptions of ultraviolet (UV) and vacuum ultraviolet (VUV) radiation as small as 0.00001. Stable arc lamps provide a continuum in some experiments, but experiments at very high spectral resolution or at VUV wavelengths require the greater spectral radiance of a synchrotron. Absolute densities of excited atoms, atomic ions, and molecular radicals are measured in both processing and lighting plasmas. Basic spectroscopic data needed for the analysis of astrophysical observations from the Hubble Space Telescope are measured using absorption of Fe+ in a hollow cathode discharge.

Abstract: New radiative-lifetime measurements based on time-resolved laser-induced fluorescence are reported for 133 odd-parity and 2 even-parity levels of Co I, ranging in energy from 28300 to 59400 cm(-1). Our lifetimes agree with earlier, but much less extensive, lifetime measurements based on laser-induced fluorescence. Satisfactory agreement is also found with the critical compilation of atomic transition probabilities from the U.S. National Bureau of Standards [J. Phys. Chem. Ref Data 17, Suppl 4 (1988)]. Our measurements provide a reliable absolute normalization for a much more comprehensive determination of Co I atomic transition probabilities.

Abstract: The absorption oscillator strength of the xenon 147-nm resonance transition is measured to be 0.264±0.016. This value is from direct absorption measurements with equivalent widths from ≊1 to ≊10 cm-1. This f-value measurement is compared to others in the literature and is used in Monte Carlo simulations of trapped decay rates. The simulations include an angle-dependent partial frequency redistribution. The simulation results are compared to trapped decay rates in the literature.

Abstract: We report accurate experimental absorption oscillator strengths (f-values) for transitions out of the ground level of Fe II to the z(4)D(7/2)(0) and z(4)F(9/2)(0) levels at 224.918 and 226.008 nm (air wavelengths) to be 0.00182(14) and 0.00244(19), respectively. The number in parenthesis is the uncertainty in the last digits. These two lines are important for studying Fe abundances and grain depletions in the interstellar medium. These f-values are determined by combining emission branching fractions with radiative lifetimes. Branching fractions are measured using classical spectroradiometry on an optically thin source. Radiative lifetimes are from the literature.

Abstract: Intensity ratios of lines of the spin-changing ''intersystem'' multiplet of Si II (4P --> 2P(o)) at 234 nm have been used to determine electron densities and temperatures in a variety of astrophysical environments. However, the accuracy of these diagnostic calculations have been limited by uncertainties associated with the available atomic data. We report the first laboratory measurement, using an ion-trapping technique, of the radiative lifetimes of the three metastable levels of the 3s3p2 4P term of Si II. Our results are 104 +/- 16, 406 +/- 33, and 811 +/- 77 mus for lifetimes of the J = 1/2, 5/2, and 3/2 levels, respectively. A-values were derived from our life-times by use of measured branching fractions. Our A-values, which differ from calculated values by 30% or more, should give better agreement between modeled and observed Si II line ratios.

Abstract: We report Si II experimental log (gf)-values of -2.38(4) for the 180.801 nm line, of -2.18(4) for the 181.693 nm line, and of - 3.29(5) for the 181.745 nm line, where the number in parenthesis is the uncertainty in the last digit. The overall uncertainties (approximately 10%) include the 1 sigma random uncertainty (approximately 6%) and an estimate of the systematic uncertainty. The oscillator strengths are determined by combining branching fractions and radiative lifetimes. The branching fractions are measured using standard spectroradiometry on an optically thin source; the radiative lifetimes are measured using time-resolved laser-induced fluorescence.

Abstract:

Transition probabilities of 100 Ti-II emission lines, originating from 7 different atomic levels, have been determined by combining branching fractions with radiative lifetimes. The branching fractions were measured using Fourier transform spectroscopy on a hollow cathode. The radiative lifetimes of these 7 - and 35 additional - levels were measured using time resolved laser-induced fluorescence on a slow Ti ion beam. The transition probabilities of 21 very weak lines have been used to derive a solar titanium abundance of a(Ti) = log(N(Ti)/N(H)) + 12 = 5.04 +/- 0.04 dex, which is insensitive to the solar model. This value is in disagreement with the meteoritic titanium abundance (4.93 +/- 0.02).

Abstract: New absolute atomic transition probability measurements are reported for 12 transitions in Cr ii and two transitions in Zn II. These transition probabilities are determined by combining branching ratios measured by classical techniques and radiative lifetimes measured by time-resolved laser-induced fluorescence. The measurements are compared with branching fractions, radiative lifetimes, and transition probabilities in the literature. The 206 nm resonance multiplets in Cr II and Zn II are included in this work. These multiplets are very useful in determining the distribution of the elements in the gas versus grain phases in the interstellar medium.

Abstract: New radiative lifetime measurements based on time-resolved laser-induced fluorescence are reported for 57 odd-parity and 9 even-parity levels of Ni I, ranging in energy from 29000 to 57000 cm-1. Our lifetimes agree with earlier measurements based on laser-induced fluorescence but are much more extensive than earlier laser measurements. These lifetimes are found to agree with the recent critical compilation of atomic transition probabilities from the U.S. National Bureau of Standards [J. Phys. Chem. Ref. Data 17, Suppl. 4 (1988)]. These lifetimes are also compared with lifetimes derived from the highly accurate (+/-0.7%) relative oscillator strengths measured by the Oxford group [Mon. Not. R. Astron. Soc. 236, 235 (1989)]. Our lifetimes provide an improved (+/-2%) absolute scale for the Oxford measurements.

Abstract: Advanced experimental techniques for measuring oscillator strengths of atomic and ionic transitions in the vacuum ultraviolet (VUV) are described. A VUV time-resolved laser-induced-fluorescence experiment for radiative lifetime measurements on atoms and ions in a beam is operational. Recent work on VUV transitions of Si I and B I is described. These lifetimes provide the essential absolute normalization for converting relative oscillator strengths to absolute transition probabilities. Emission measurements of branching fractions at VUV and longer wavelengths are proposed. A large echelle spectrograph equipped with a CCD detector array will be used. This experiment will provide the sensitivity, resolving power, and data handling capability required for extensive high quality emission branching fraction measurements. We further propose to use absorption measurements on hollow cathode discharges to determine relative absorption oscillator strengths. A demonstration of a new technique for absorption spectroscopy on glow discharges is reported. The new technique provides the sensitivity, dynamic range, and data handling capability required for extensive high quality absorption measurements. Relative absorption and emission oscillator strengths will be least-square adjusted using the bowtie method and normalized with accurate radiative lifetimes.

Abstract: In conjunction with the Los Alamos National Laboratory hypervelocity microparticle impact (HMI) team effort to produce higher impact velocities and to understand the physics of crater formation and momentum transfer, we have implemented a low noise microphone as a momentum detector on both the 6 MV Van de Graaff and 85 KV “test stand” particle accelerators. Calculations are presented showing that the impulse response of a circular membrane. When used as a momentum impulse detector, the microphone theoretically may detect impulses as small as 8.8 × 10−15 N s. Sensitivity of the microphone in this application is limited by the noise threshold of the electronic amplifiers and the ambient microphonic vibration of the system. Calculations lead us to anticipate detection of particles over the full range of the Van de Graaff acceleration capability and up to 7 km/s on the test stand. We present momentum enhancement data in the velocity range between 10 km/s and 20 km/s. Preliminary work is presented on momentum impulse calibration of the microphone using laser-pulse photon momentum as an impulse source.

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.

Abstract:

Bronin et al. [Phys. Rev. E 108, 045209 (2023)] recently reported molecular-dynamics simulations of ultracold neutral plasmas expanding in a quadrupole magnetic field. While the main results are in agreement with prior experimental measurements, we present data showing oscillations not captured in the simulations of Bronin et al. Plasmas formed using pulsed or continuous-wave ionization processes have similar confinement times.

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.

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 4s^{2 1}S₀ 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, E_IP=1478154283.42±0.08(statistical)±0.07(systematic) MHz, improving the previous determinations by a factor of 11.