Sarah J. Messer

Spherically Imploding Plasma Liners as a Standoff Driver for Magnetoinertial Fusion

Spherically imploding plasma liners formed by merging an array of high Mach number plasma jets are a proposed standoff driver for magnetoinertial fusion (MIF). This paper gives an updated concept-level overview of plasma liner MIF, including advanced notions such as standoff methods for forming and magnetizing the fuel target and liner shaping to optimize dwell time. Results from related 1-D radiation-hydrodynamic simulations of targetless plasma liner implosions are summarized along with new analysis on the efficiency of conversion of the initial liner kinetic energy to stagnation thermal energy. The plasma liner experiment (PLX), a multi-institutional collaboration led by the Los Alamos National Laboratory, plans to explore the feasibility of forming spherically imploding plasma liners via 30 merging plasma jets. In the near term, with modest pulsed power stored energy of ≲1.5 MJ, PLX is focusing on the generation of centimeter-, microsecond-, and megabar-scale plasmas for the fundamental study of high energy density laboratory plasmas. In the longer term, PLX can enable a research and development path to plasma liner MIF ultimately requiring compressing magnetized fusion fuel to ≳100 Mbar.

A contoured gap coaxial plasma gun with injected plasma armature

A new coaxial plasma gun is described. The long term objective is to accelerate 100-200 microg of plasma with density above 10(17) cm(-3) to greater than 200 km/s with a Mach number above 10. Such high velocity dense plasma jets have a number of potential fusion applications, including plasma refueling, magnetized target fusion, injection of angular momentum into centrifugally confined mirrors, high energy density plasmas, and others. The approach uses symmetric injection of high density plasma into a coaxial electromagnetic accelerator having an annular gap geometry tailored to prevent formation of the blow-by instability. The injected plasma is generated by numerous (currently 32) radially oriented capillary discharges arranged uniformly around the circumference of the angled annular injection region of the accelerator. Magnetohydrodynamic modeling identified electrode profiles that can achieve the desired plasma jet parameters. The experimental hardware is described along with initial experimental results in which approximately 200 microg has been accelerated to 100 km/s in a half-scale prototype gun. Initial observations of 64 merging injector jets in a planar cylindrical testing array are presented. Density and velocity are presently limited by available peak current and injection sources. Steps to increase both the drive current and the injected plasma mass are described for next generation experiments.

Characteristics of the plasma impedance probe with constant bias

The impedance of a small spherical probe immersed in a uniform plasma is measured by recording the reflection coefficient of an applied signal using a network analyzer. This impedance has a resonance at the plasma frequency where the imaginary part goes to zero, a feature that has made this measurement a good way of determining electron density. When the plasma potential is positive with respect to the sphere—for example, if the sphere is electrically floating or grounded, a second resonance occurs at ω<ωpe due to the capacitance created by the depleted electron density in the sheath. A greatly increased power deposition occurs at this lower resonance, whose frequency can be controlled by applying a dc bias which changes the sheath width. As the bias is increased the value of this frequency becomes smaller until the resonance disappears completely at Vprobe=Vplasma. As the bias is further increased past the plasma potential, an electron sheath forms with its own resonance, which is at a lower frequency th...

Antenna impedance measurements in a magnetized plasma. II. Dipole antenna

This paper presents experimental impedance measurements of a dipole antenna immersed in a magnetized plasma. The impedance was derived from the magnitude and phase of the reflected power using a network analyzer over a frequency range of 1MHz–1GHz. The plasma density was varied between 107and1010cm−3 in weakly (ωce ωpe) magnetized plasmas in the Space Physics Simulation Chamber at the Naval Research Laboratory. Over this range of plasma conditions the wavelength in the plasma varies from the short dipole limit (λ≫L) to the long dipole limit (λ∼L). As with previous impedance measurements, there are two resonant frequencies observed as frequencies where the impedance of the antenna is real. Measurements have indicated that in the short dipole limit the majority of the power deposition takes place at the lower resonance frequency which lies between the cyclotron frequency and the upper hybrid frequency. These measured curves agree very well with the analytic theory for a short dipole i...

On collisionless energy absorption in plasmas: Theory and experiment in spherical geometry

An investigation of the rf impedance characteristics of a small spherical probe immersed in a laboratory plasma is ongoing in the large Space Physics Simulation Chamber [D. N. Walker et al., Rev. Sci. Instrum. 65, 661 (1994)] at the Naval Research Laboratory. The data taken are from network analyzer measurements of the reflection coefficient obtained when applying a low level rf signal to the probe which is either near floating potential or negatively dc biased in a low pressure plasma. As is well known, sheaths form around objects placed inside plasmas. The electron density is smaller inside the sheath, and the reduction in density alters the plasma impedance. Surprisingly, the impedance becomes “resistive,” even though the plasma is effectively collisionless, at frequencies below the bulk plasma frequency, thus leading to collisionless energy absorption. This behavior comes directly from Maxwells equations along with the cold fluid equations. The solutions obtained indicate that this form of plasma res...

Antenna impedance measurements in a magnetized plasma. I. Spherical antenna

The input impedance of a metal sphere immersed in a magnetized plasma is measured with a network analyzer at frequencies up to 1GHz. The experiments were done in the Space Physics Simulation Chamber at the Naval Research Laboratory. The hot-filament argon plasma was varied between weakly (ωce ωpe) magnetized plasma with electron densities in the range 107–1010cm−3. It is observed that the lower-frequency resonance of the impedance characteristic previously associated with series sheath resonance ωsh in the unmagnetized plasma occurs at a hybrid sheath frequency of ωr2=ωsh2+κωce2, where κ is a constant 0.5<κ<1. As seen in previous experiments, the higher frequency resonance associated with the electron plasma frequency ωpe in the unmagnetized plasma is relocated to the upper hybrid frequency ωuh2=ωpe2+ωce2. As with the unmagnetized plasma, the maximum power deposition occurs at the lower frequency resonance ωr.

Broadband calibration of radio-frequency magnetic induction probes

We describe broadband calibration of a magnetic induction probe using a network analyzer. Our procedure uses a small driver coil and the scattering matrix associated with the coil-probe pair. This allows rapid, high-resolution measurements of the amplitude and phase of the probe’s response to oscillating magnetic fields. Our setup gives high accuracy between 100kHz and 100MHz and less accuracy outside this range. We present over 1800 sensitivity measurements between 30kHz and 1GHz. The ideal probe response is compared to calibrations done with two different driver coils. We discuss several hurdles in probe design and calibration, as well as several high-frequency effects.

Merging of high speed argon plasma jets

Formation of an imploding plasma liner for the plasma liner experiment (PLX) requires individual plasma jets to merge into a quasi-spherical shell of plasma converging on the origin. Understanding dynamics of the merging process requires knowledge of the plasma phenomena involved. We present results from the study of the merging of three plasma jets in three dimensional geometry. The experiments were performed using HyperV Technologies Corp. 1 cm Minirailguns with a preionized argon plasma armature. The vacuum chamber partially reproduces the port geometry of the PLX chamber. Diagnostics include fast imaging, spectroscopy, interferometry, fast pressure probes, B-dot probes, and high speed spatially resolved photodiodes, permitting measurements of plasma density, temperature, velocity, stagnation pressure, magnetic field, and density gradients. These experimental results are compared with simulation results from the LSP 3D hybrid PIC code.

Interferometer density measurements of a high-velocity plasmoid

The plasmoid produced by a half-scale contoured gap coaxial plasma accelerator using ablative polyethylene capillary plasma injectors is measured using a quadrature heterodyne HeNe interferometer. The plasmoid is found to have a sharp rise in density at the leading edge, with a gradual falloff after the peak density. For this early test series, an average bulk density of 5×1014 cm−3 is observed, with densities up to 8×1014 cm−3 seen on some shots. Although plasmoid mass is only about 58 μg due to the low current and injected mass used in these tests, good shot-to-shot repeatability is attained making analysis relatively straightforward, thus providing a solid foundation for interpreting future experimental results.

Nonlinear compressions in merging plasma jets

We investigate the dynamics of merging supersonic plasma jets using an analytic model. The merging structures exhibit supersonic, nonlinear compressions which may steepen into full shocks. We estimate the distance necessary to form such shocks and the resulting jump conditions. These theoretical models are compared to experimental observations and simulated dynamics. We also use those models to extrapolate behavior of the jet-merging compressions in a Plasma Jet Magneto-Inertial Fusion reactor.