P. Colestock

Low frequency electrostatic instability in a helicon plasma

Recent discoveries in a helicon plasma show a decrease in equilibrium plasma density as magnetic field strength is increased. This can be explained in the framework of a low frequency electrostatic instability. However, quiescent plasma behavior in helicon sources has been hitherto accepted. To verify the existence of an instability, extensive measurements of fluctuating quantities and losses as a function of magnetic field were implemented. Furthermore, a theoretical model was developed to compare to the measurements. Theory and measurement show very good agreement; both verifying the existence of a low frequency instability and showing that it is indeed responsible for the observed density characteristic.

Quiescent and unstable regimes of a helicon plasma

Known for their ability to produce high densities at low power, helicon discharges have found many uses. However, it has been discovered that the plasma density saturates, and even falls for light ion gases, as the magnetic field is increased. Detailed measurements of fluctuations in plasma density reveal the onset of a strong, low-frequency electrostatic instability. This onset correlates well with density saturation and is predicted from a linear theory.

On the dynamics of hot air plasmas related to lightning discharges: 1. Gas dynamics

In this paper, we first study the dynamics of hot shocks in air in cylindrical geometry coupled to multiband radiation transport and detailed air chemistry. The wide energy and length scale ranges which are covered herein includes and exceeds the ones of first and subsequent return strokes happening during lightning discharges. An emphasis is put on the NOx production and the optical power emitted by strong shocks as the ones generated by Joule heating of the air from intense current flows. The production rate of NOx, which is useful for atmospheric global modeling, is found to be between 4.5 × 1016 and 8.6 × 1016 molecules/J for all computed cases, which is in agreement with the literature. Two different radiation transport methods are used to characterize the variability of the results according to the radiation transport method. With the exact radiation solver, we show that between 15 and 40% of the energy is lost by radiation, with a percentage between 20 and 25% for averaged lightning energies. The maximal visible peak is between 7 × 108 W/m and 3 × 107 W/m obtained for, respectively, a 19 kJ/cm and a 28 J/cm energy input. The mean radiated powers in the visible range are found between 9 × 106 W/m and 2 × 105 W/m for the energies just mentioned. We discuss the agreement of these values with previous studies.

Review of ICRF Experiments

The current status of ICRF experiments is reviewed with emphasis on high-power heating results in tokamaks. The rationale for the use of the fast magnetosonic wave is described along with simplified models for the wave coupling and power absorption. Experimental results are presented which confirm the essential features of the theoretical model and suggest a highly favorable prognosis for scaling the heating process to a reactor.

A Method to Estimate Whistler Wave Vector from Polarization Using Three-Component Electric Field Data

Satellites in the Earths magnetosphere can be used to record the rich electromagnetic wave activity due to terrestrial lightning, typically up to several tens of kilohertz. With simultaneous recordings of the three components of wave electric field E and of the three components of wave magnetic field B, the entire wavefield, polarization, and wave vector can be specified without any appeal to a priori assumptions about the wave mode and without any reliance on the validity of a dispersion relation. However, some satellites lack such a complete suite of measurements. We develop a method which assumes the theoretical dispersion relation for whistler waves then uses recordings of the three components of wave electric field E but no magnetic components to derive the wave polarization and the wave vector (up to a sign ambiguity on the latter). The method can work only because the dispersion relation, which is assumed, already contains information from the full Maxwells equations. We illustrate the method with 12 s duration simultaneous recordings, at 32 kilosample/s, of three orthogonal components of wave electric field E from the C/NOFS satellite in low-Earth orbit. Our particular example in this article is shown to contain two broadband whistler features in the range of 4–15 kHz, whose wave vectors differ both according to their polar angles from the geomagnetic field B0 and according to their azimuth around the geomagnetic field B0.

Lower hybrid to whistler mode conversion on a density striation

When a wave packet composed of short wavelength lower hybrid modes traveling in an homogeneous plasma region encounters an inhomogeneity, it can resonantly excite long wavelength whistler waves via a linear mechanism known as mode conversion. An enhancement of lower hybrid/whistler activity has been often observed by sounding rockets and satellites in the presence of density depletions (striations) in the upper ionosphere. We address here the process of linear mode conversion of lower hybrid to whistler waves, mediated by a density striation, using a scalar-field formalism (in the limit of cold plasma linear theory) which we solve numerically. We show that the mode conversion can effectively transfer a large amount of energy from the short to the long wavelength modes. We also study how the efficiency scales by changing the properties (width and amplitude) of the density striation. We present a general criterion for the width of the striation that, if fulfilled, maximizes the conversion efficiency. Such a criterion could provide an interpretation of recent laboratory experiments carried out on the Large Plasma Device at UCLA.

Comparison of methods of determining meteoroid range rates from linear frequency modulated chirped pulses

[1] In this paper we present three methods for evaluating range rates of meteoroids passing through the ionosphere, using linear frequency modulated (LFM) chirped pulse data from the ALTAIR radar. The first method is based on the simple calculation of range differences divided by interpulse intervals. The second method utilizes the dual‐frequency capability of ALTAIR to solve for range rates based on the difference in the measured ranges due to range‐Doppler coupling. The third method utilizes a simplified form of integer programming in order to unwrap the phase differences of the matched filter time response, with reliance on the rough approximation available from the first method to disambiguate the solution set. The results of the three methods, with error bounds, are given for a large set of meteoroid head echoes taken from a data collection conducted with ALTAIR in 2007. Citation: Loveland, R., A. Macdonell, S. Close, M. Oppenheim, and P. Colestock (2011), Comparison of methods of determining meteoroid range rates from linear frequency modulated chirped pulses, Radio Sci., 46, RS2007,

Tunnel-diode loaded split-ring resonators as a foundation for nonlinear metamaterials

The nonlinear electromagnetic response is one of the foundations of modern technology and it arises in natural materials at the atomic scale. We briefly present some of the fundamentals of nonlinearity in natural materials and then we present experimental studies of analogous behavior in meta-atoms, the fundamental building block of metamaterials. Specifically tunnel-diode loaded, microwave split-ring resonators are shown to enable various nonlinear phenomena including self-sustained oscillation, harmonic/comb generation, frequency locking/pulling, and quasi-chaos generation. We discuss the possible adaptation of these unit cells to create bulk nonlinear metamaterials.

RF Signatures of Hypervelocity Impacts on Spacecraft

We investigate the radio frequency (RF) signature resulting from a hypervelocity impact on a spacecraft. While hypervelocity impactors, consisting of meteoroids, dust and orbital debris, are known to cause mechanical damage to satellites, very little is understood about the electrical effects associated with an impact. We present a theory to describe the behavior of the plasma produced from a hypervelocity impact, which results from ionization of both the particle as well as a small part of the spacecraft. The plasma, which does not penetrate the spacecraft chassis, produces a strong electromagnetic pulse (EMP) at a broad frequency spectrum, which can produce electrical anomalies or even catastrophic damage depending on the amount of electrical shielding. We compare our theory to data taken by the Cassini spacecraft in order to understand the resulting RF signature. Our analyses show that RF emission should be a routine outcome from impacts by particles but may only cause anomalies or failures when the impactor exceeds a velocity of 12 km/s.

Electromagnetic Response of Finite Terahertz Metafilm Arrays Excited on Total Internal Reflection Boundaries

Resonant excitation of planar terahertz metamaterials using attenuated total reflection is demonstrated. Experimental results reveal an anomalous increase in the resonance strength while the sample is illuminated near the edge of the metamaterial array with a finite-size terahertz beam. A re-radiation signal at the fundamental metamaterial resonance is observed on the transmission side of the total internal reflection interface where no signal was expected. Multiple theoretical approaches address the physical origins of this re-radiation signal and rich behavior has been simulated with numeric simulations. Although models indicate that surface waves could exist, radiation coupled across the total internal reflection surface appears predominately mediated by finite currents oscillating in resonators at the edge of the metafilm array. The observations could lead to a better understanding of boundary effects in finite, planar metamaterials and more accurate modeling of MM-mediated total reflection spectroscopy.