Ronald W. Moses

Transport implications of current drive by magnetic helicity injection

It is shown that in fusion plasma configurations sustained by electrode helicity injection, the core electron temperature (in electron volts) can, at most, be 25% to 40% of the electrode voltage (in volts). This result is obtained by assessing magnetic helicity injection as a driver of macroscopic steady-state plasma currents in magnetic confinement devices. Coaxial helicity injection using electrodes (CHI) and oscillating-field current drive (OFCD) are compared to inductive current drive. Magnetic helicity, K, is uniquely defined as the time-dependent volume integral of A⋅B when the surface components of A are purely solenoidal. Using an Ohm’s law including Hall terms, magnetic helicity transport modeling shows that no closed magnetic surfaces with time and volume averaged parallel currents can exist continuously within a plasma sustained only by CHI or OFCD. The 25% to 40% limitations are obtained by considering long and short electron mean-free-path models of parallel energy transport.

Coaxial plasma thrusters for high specific impulse propulsion

A fundamental basis for coaxial plasma thruster performance is presented and the steady-state, ideal MHD properties of a coaxial thruster using an annular magnetic nozzle are discussed. Formulas for power usage, thrust, mass flow rate, and specific impulse are acquired and employed to assess thruster performance. The performance estimates are compared with the observed properties of an unoptimized coaxial plasma gun. These comparisons support the hypothesis that ideal MHD has an important role in coaxial plasma thruster dynamics.

Magnetic nozzle design for coaxial plasma accelerators

Magnetic nozzles have great potential for improving the efficiency and performance of coaxial plasma accelerators in applications such as space propulsion and advanced manufacturing. Proper design of magnetic field geometry can improve coaxial accelerator performance in three ways. First, the applied field which intercepts the anode surface without directly connecting the two electrodes can minimize anode fall inefficiencies by improving electron conduction across the anode sheath and by opposing the Hall-induced starvation effect. Second, a properly designed magnetic geometry can provide a nozzling mechanism to permit the plasma to accelerate smoothly and efficiently from sub to super-magnetosonic flow. Third, a magnetic nozzle provides control over the flow of plasma from the accelerator. For applications such as surface modification and etching, magnetic nozzles can maximize the treatable surface area and tailor the downstream plasma energy distribution. For thrust generation, proper design of a magnetic nozzle can enable efficient detachment of the plasma from the magnetic field. >

The Genesis Solar Wind Concentrator

The primary goal of the Genesis Mission is to collect solar wind ions and, from their analysis, establish key isotopic ratios that will help constrain models of solar nebula formation and evolution. The ratios of primary interest include 17O/16O and 18O/16O to ±0.1%, 15N/14N to ±1%, and the Li, Be, and B elemental and isotopic abundances. The required accuracies in N and O ratios cannot be achieved without concentrating the solar wind and implanting it into low-background target materials that are returned to Earth for analysis. The Genesis Concentrator is designed to concentrate the heavy ion flux from the solar wind by an average factor of at least 20 and implant it into a target of ultra-pure, well-characterized materials. High-transparency grids held at high voltages are used near the aperture to reject >90% of the protons, avoiding damage to the target. Another set of grids and applied voltages are used to accelerate and focus the remaining ions to implant into the target. The design uses an energy-independent parabolic ion mirror to focus ions onto a 6.2 cm diameter target of materials selected to contain levels of O and other elements of interest established and documented to be below 10% of the levels expected from the concentrated solar wind. To optimize the concentration of the ions, voltages are constantly adjusted based on real-time solar wind speed and temperature measurements from the Genesis ion monitor. Construction of the Concentrator required new developments in ion optics; materials; and instrument testing and handling.

Efficient Numerical Modal Solutions for RF Propagation in Lossy Circular Waveguides

The propagation of electromagnetic waves is reconsidered in the context of cylindrical tunnels through material of uniform electrical properties. In the absence of internal conductors, the wave solutions reduce to well-known analytic expressions, requiring the solution of a single transcendental equation in complex phase space. Any given tunnel configuration has a wide range of solutions in phase and frequency space. Identifying these solutions can be a significant numerical challenge. Both the Newton-Raphson method and the winding number technique have been described in previous publications, and each can be prone to missing solutions. This paper reviews these two solution techniques and develops a combination of the two to provide a fast and efficient procedure for locating the roots in well-defined regions of phase space.

Computational modeling of wall-supported dense Z-pinches

In our previous computational modeling of deuterium-fiber-initiated Z-pinches intended for ohmic self-heating to fusion conditions, instability-driven expansion caused densities to drop far below those desired for fusion applications; such behavior has been observed on experiments such as Los Alamos` HDZP-II. A new application for deuterium-fiber-initiated Z-pinches is Magnetized Target Fusion (MTF), in which a preheated and magnetized target plasma is hydrodynamically compressed, by a separately driven liner, to fusion conditions. Although the conditions necessary for suitable target plasma--density O(10{sup 18} cm{sup -3}), temperature O(100 eV), magnetic field O(100 kG)--are less extreme than those required for the previous ohmically heated fusion scheme, the plasma must remain magnetically insulated and clean long enough to be compressed by the imploding liner to fusion conditions, e.g., several microseconds. A fiber-initiated Z-pinch in a 2-cm-radius, 2-cm long conducting liner has been built at Los Alamos to investigate its suitability as an MTF target plasma. Two-dimensional magnetohydrodynamic modeling of this experiment shows early instability similar to that seen on HDZP-II; however, when plasma finds support and stabilization at the outer radial wall, a relatively stable profile forms and persists. Comparison of experimental results and computations, and computational inclusion of additional experimental details is being done. Analytic and computational investigation is also being done on possible instability-driven cooling of the plasma by Benard-like convective cells adjacent to the cold wall.

Reduction of the Anode Fall in a Coaxial Plasma Thruster with an Applied Magnetic Nozzle

The Coaxial Thruster Experiment at the Los Alamos National Laboratory has demonstrated 10 ms quasi‐steady operation and ideal MHD‐like performance. Previous experiments utilized an unoptimized applied magnetic configuration in which field lines intercepted both thruster electrodes. These experiments demonstrated the ability of magnetic connection between the plasma and the electrodes to control the anode fall. In this work we report on the effect of an evolved applied magnetic nozzle which provides magnetic connection of the electrodes to the bulk of the plasma without direct magnetic connection of the electrodes. This magnetic nozzle configuration significantly reduces the anode fall fraction and thus may provide a promising means of improving MPD thruster efficiency.

The High‐Latitude Ionosphere and Its Effects on Radio Propagation

The ionosphere is indeed the place where Earth and space come together. Correspondingly, the ionosphere is subject to the details and complexities of both Earth and space. If one is to develop a logical understanding of even a limited portion of the ionosphere, that knowledge will be constructed on a foundation of many facts of nature. Awareness of those facts will in turn be supported by a vast historical array of scientific effort to ascertain the fundamentals of Earth and space that combine to form the ionosphere as we know it. Fortunately for us, R.D. Hunsucker and J.K. Hargreaves have written a book that goes from the Earth up and comes from the Sun down to arrive at a remarkably detailed physical description of the ionosphere and its impact on human activities, especially radio-frequency (RF) communications.

Comparison of the aperture and chromatic aberrations of magnetic round lenses with those of quadrupole doublets

The paraxial equation for rotationally symmetric magnetic lenses is integrated numerically by using the Numerov method. The normalized third‐order aperture and second‐order chromatic aberration coefficients (C3N and CcN) are calculated. A Fourier representation of the lens field is used, and a search for minimum C3N is carried out by varying the Fourier coefficients. Comparisons with comparable quadrupole doublets, and calculations of C3N and CcN for actual coil systems are presented. The main conclusion is that magnetic round lenses are about a factor of 2 better than comparable magnetic quadrupole doublets as far as the third‐order aperture aberration is concerned, but the chromatic aberrations in both systems are comparable.

Enhanced penetration of field‐aligned currents into a Lorentz plasma in a stochastic magnetic field

We show that anomalously fast current penetration into a plasma can result from the nonlocal electrical conductivity caused by a stochastic magnetic field. We calculate the admittance of a plasma slab subjected to an oscillating surface electric field along B. The effect of increasing stochasticity (or of increasing electron mean free path) is to make the admittance less dissipative and more inductive.