Paul E. Adamson

Modeling positrons in molecular electronic structure calculations with the nuclear-electronic orbital method.

The nuclear-electronic orbital (NEO) method was modified and extended to positron systems for studying mixed positronic-electronic wavefunctions, replacing the mass of the proton with the mass of the positron. Within the modified NEO framework, the NEO-HF (Hartree-Fock) method provides the energy corresponding to the single-configuration mixed positronic-electronic wavefunction, minimized with respect to the molecular orbitals expressed as linear combinations of Gaussian basis functions. The electron-electron and electron-positron correlation can be treated in the NEO framework with second-order perturbation theory (NEO-MP2) or multiconfigurational methods such as the full configuration interaction (NEO-FCI) and complete active space self-consistent-field (NEO-CASSCF) methods. In addition to implementing these methods for positronic systems, strategies for calculating electron-positron annihilation rates using NEO-HF, NEO-MP2, and NEO-FCI wavefunctions were also developed. To apply the NEO method to the positronium hydride (PsH) system, positronic and electronic basis sets were optimized at the NEO-FCI level and used to compute NEO-MP2 and NEO-FCI energies and annihilation rates. The effects of basis set size on NEO-MP2 and NEO-FCI correlation energies and annihilation rates were compared. Even-tempered electronic and positronic basis sets were also optimized for the e+LiH molecule at the NEO-MP2 level and used to compute the equilibrium bond length and vibrational energy.

Toward Simultaneous 2D ACAR and 2D DBAR: Sub-Pixel Spatial Characterization of a Segmented HPGe Detector Using Transient Charges

Sub-pixel spatial characterization of an Ortec HPGe double-sided strip detector was measured in preparation for its planned use in a positron annihilation spectrometer (PAS) to simultaneously measure the two-dimensional Angular Correlation (2D ACAR) and Doppler Broadening (2D DBAR) of Annihilation Radiation. Sr-85 514-keV photons were finely collimated onto the center of 25, equally spaced sub-pixels within a single 5-mm by 5-mm intrinsic pixel, using a collimator with an aperture diameter of 0.15 ± 0.05 mm. The location of each full-energy recorded event was determined by analyzing the relative sizes of coincident transient charges on charge collection strips adjacent to the strip recording full-energy events. Interpolation, using ratios of the coincident transient charge sizes, was used to determine two-dimensional locations of full-energy events. Radial location data was fit to a function which describes a Gaussian point spread function uniformly distributed over a circular aperture. The standard deviation of the point spread function, 0.22 mm, is a measure of the spatial resolution of this detector system. The relative efficiency for detection of 514-keV photons across the intrinsic pixel was also measured.

Field Reversed Configuration (FRC) formation, translation and compression

Experiments on FRC formation and translation into the interior of a metal shell or liner have been conducted at AFRL. Flux exclusion, collimated light, and interferometer data on magnetized plasma injection will be presented. These are a pre-requisite for FRC compression by liner implosion, experiment progress on which will also be presented. FRC translation, capture, and compression experiments all use primarily axial ∼ 2 Tesla guide and mirror fields established inside the liner, using ∼ 5 millisecond rise time discharges into an array of pulsed magnet coils surrounding the liner implosion portion of the device. A 12 MA, 4.5 MJ axial discharge drives the liner implosion for compression experiments. The FRC capture experiments use 3 capacitor discharges into a segmented theta coil surrounding the FRC formation region to establish a bias field, accomplish pre-ionization of deuterium gas, and provide the reverse field main theta discharge (∼ 1 Megamp) which forms the FRC. This is aided by two cusp field discharges. The guide and mirror fields enable translation of the FRC and its capture in the liner interior region. Diagnostics include pulsed power (current and voltage), magnetic field, field exclusion, He Ne laser interferometry, imaging and spectroscopy, radiography, and both activation and time-of-flight neutron detection. Design features and operating parameters are guided by 2D-MHD simulations.

Three-dimensional electron-positron momentum distribution of O3+-irradiated 6H SiC using two positron spectroscopy techniques simultaneously

A three-dimensional (3D) positron annihilation spectroscopy system (3DPASS) capable of determining 3D electron-positron (e−-e+) momentum densities from measurements of deviations from co-linearity and energies of photons from e−-e+ annihilation events was employed to examine the effects of O-atom defects in 6H SiC. Three-dimensional momentum datasets were determined for 6H SiC irradiated with 24 MeV O3+ ions. Angular correlation of annihilation radiation (ACAR) and coincidence Doppler-broadening of annihilation radiation (CDBAR) analyses are presented. In addition, a novel technique is illustrated for analyzing 3D momentum datasets in which the parallel momentum component, p|| (obtained from the CDBAR measurement) is selected for annihilation events that possess a particular perpendicular momentum component, p- observed in the 2D ACAR spectrum.

Application of GAMESS/NEO to quantum calculations of muonic molecules

The General Atomic and Molecular Electronic Structure System (GAMESS) has been modified to perform studies involving negative muons. This system, coupled with the Nuclear-Electronic Orbital (NEO) method enables the ab-initio study of muonic atoms where both the negative muon and the positive nuclei are modeled as quantum particles. This is of particular usefulness in the study light nuclei, muonic atoms, such as is encountered in muon-catalyzed fusion. NEO was also modified to allow the inclusion of positive exotic-particles to be studied using open and closed shell Hartree-Fock and Configuration Interaction. Capitalizing on these modified methods, the muon density and vibrational dynamics of some light muonic molecules have been analyzed.

Experimental Studies of an Ultrahigh-Speed Plasma Flow

In 1991, Turchi et al. reported evidence for a 2000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS). The 2008 article by Turchi et al. posits that if such high Z plasma could instead be composed of deuterium or a deuterium-tritium mixture, then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several experiments executed in an effort to achieve an ultrahigh-speed flow in a deuterium plasma. The first phase of this research concentrated on extension of the earlier work to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. The experiments met with limited success, exhibiting evidence of a 550 km/s plasma flow which convected a small fraction of the total magnetic field. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low-energy shots were conducted to examine the impact of several proposed design modifications on current delivery to the armature. These experiments demonstrated that the hardware assembled for this investigation was unlikely to forestall breakdown in the injected gas as required by Turchi et al. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low-energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultrahigh-speed plasma flow concept.

Adventures in the experimental development of an ultrahigh speed plasma flow

In 1991, Turchi et al. [1] reported evidence for a 2,000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS) [2]. The 2008 article by Turchi et al. [3] posits that if such high Z plasma could instead be composed of deuterium or a deuterium-tritium mixture then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several exploratory tests executed in an effort to achieve significant energy transfer from a plasma flow switch to a deuterium plasma. The first phase of this research concentrated on extension of the earlier work [1, 2] to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. In analyzing the armature behavior, the initial conditions used in 2-D axisymmetric MHD simulations to approximate the wire-array/polymer film composite armature resulted in significant uncertainty in the validity of the calculations. This uncertainty confounded efforts to improve the opening switch behavior of the armature. Low density foams, commonly used in other high energy density plasma experiments, were seen as a candidate material for the armature that would facilitate greater fidelity between simulations and the experiment. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low energy shots were conducted to examine the impact of several proposed design modifications on current delivery to the armature. One conclusion of these experiments was that it has been very difficult to forestall breakdown in the injected gas as required by Turchi et al. [3]. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultra-high-speed plasma flow concept.

Overview of FRC translatin experimental collaboration for magnetized target fusion

We present and overview the experimental high density Field Reversed Configurationi (FRC) approach for application to a physics demonstration of magnetized target fusion (MTF). This MT target plasma continues to be developed at the Los Alamos FRC experiment FRXL. The first translated FRXL FRC data will be shown, where the translation speeds exceed 15cm/usec, which yields a translation time substantially shorter than the FRC lifetimes. The conical theta coil is expected to generate toroidal magnetic field and helicity and increase stability and lifetime. The implications of the present data for MTF experiments will be discussed, along with the hardware, diagnostics, and pre-compression plasma formation and trapping experiments.

Experiments on field reversed configuration (FRC) formation and their compression using liners

Three types of experiments developing FRC formation, injection, and compression are described: field-compression, FRC formation-translation-capture, and FRC formation—translation—capture—compression. All involve the generation of primarily axial guide and mirror magnetic fields with ∼ 2 Tesla peak fields, using ∼5 ms rise time discharges into 9 pulsed magnet coils surrounding the liner implosion portion of the device. The field compression and FRC compression experiments use 12 MA, 4.5 MJ Shiva Star capacitor bank axial discharges to drive implosions of 30 cm tall, 10 cm diameter, 1 mm thick Al shells or liners. The FRC capture experiments are a pre-requisite for the destructive FRC compression experiments. All FRC experiments use 3 capacitor discharges into a segmented theta coil surrounding the FRC formation region to establish a bias field, accomplish pre-ionization of deuterium gas, and provide the reverse field main theta discharge (∼ 1 MA) which forms the FRC. This is aided by two cusp field discharges. The guide and mirror fields enable translation of the FRC and its capture in the liner interior region. Diagnostics include pulsed power (current and voltage), magnetic field, field exclusion, laser interferometry, imaging and spectroscopy, radiography, and both activation and time-of-flight neutron detection. Design features and operating parameters are guided by 2D-MHD simulations. Supported by DOE-OFES.

Positron annihilation lifetime spectroscopy of dodecaborate cage molecules in aqueous nitrate solutions

A fast-fast PALS spectrometer incorporating two BaF<inf>2</inf> scintillation detectors was characterized. The PALS spectrometer was employed to measure a model explosives system having nitrate oxidizing sites and B<inf>12</inf>H<inf>12</inf>⁻² reducing sites. Positron annihilation lifetimes and intensities were measured for nitrogen-purged aqueous solutions containing various concentrations of potassium dodecahydrododecaborate K<inf>2</inf>B<inf>12</inf>H<inf>12</inf> and potassium nitrate with added carrier-free ²²NaCl. Data analysis was accomplished using the PALSfit software program. The influence of electron scavenging and competition by nitrate ions was measured for various concentrations of potassium nitrate. Suppression of positronium formation by nitrate ions at 5 M concentration was greater for o-Ps than for p-Ps in 0.42 M dodecahydrododecaborate solution. At concentrations above 10⁻² M dodecahydrododecaborate only solutions the B<inf>12</inf>H<inf>12</inf>⁻² anion suppressed p-Ps formation.