William E. Amatucci

Velocity‐shear‐driven ion‐cyclotron waves and associated transverse ion heating

Recent sounding rocket experiments, such as SCIFER, AMICIST, and ARCS-4, and satellite data from FAST, Freja, DE-2, and HILAT, provide compelling evidence of a correlation between small-scale spatial plasma inhomogeneities, broadband low-frequency waves, and transversely heated ions. These naturally arising, localized inhomogeneities can lead to sheared cross-magnetic-field plasma flows, a situation that has been shown to have potential for instability growth. Experiments performed in the Naval Research Laboratorys Space Physics Simulation Chamber demonstrate that broadband waves in the ion-cyclotron frequency range can be driven solely by a transverse, localized electric field, without the dissipation of a field-aligned current. Significant perpendicular ion energization resulting from these waves has been measured. Detailed comparisons with both theoretical predictions and space observations of electrostatic waves found in the presence of sheared cross-magnetic-field plasma flow are made.

Measurement of absolute electron density with a plasma impedance probe

A small spherical probe is used in conjunction with a network analyzer to determine the impedance of the probe-plasma system over a wide frequency range. Impedance curves are in good agreement with accepted circuit models with plasma-sheath and electron plasma frequency resonances easily identifiable. Clear transitions between capacitive and inductive modes as predicted by the model are identified. Sheath thickness and absolute electron density are determined from the location of these transitions. The absolute electron density indicated by the location of the impedance resonance is compared to measurements using the plasma oscillation method.

Contamination-free sounding rocket Langmuir probe

A technique for removing surface contaminants from a sounding rocket spherical Langmuir probe is presented. Contamination layers present on probe surfaces can skew the collected data, resulting in the incorrect determination of plasma parameters. Despite following the usual probe cleaning techniques that are used prior to a launch, the probe surface can become coated with layers of adsorbed neutral gas in less than a second when exposed to atmosphere. The laboratory tests reported here show that by heating the probe from the interior using a small halogen lamp, adsorbed neutral particles can be removed from the probe surface, allowing accurate plasma parameter measurements to be made.

Perpendicular ion heating by velocity‐shear‐driven waves

Perpendicular ion heating resulting from velocity-shear-driven ion-cyclotron waves has been measured for the first time. The experiment was performed in the Naval Research Laboratorys Space Physics Simulation Chamber (SPSC) under plasma conditions approaching those in the natural space environment. Sheared cross-field flow is induced by a controllable, inhomogeneous, transverse, DC electric field (LE ∼ (1–2)ρi) created without drawing significant levels of magnetic-field aligned current. Mode frequency data suggest that the most efficient heating occurs when the Doppler shifted frequency in the ion frame is located near a harmonic of the ion-cyclotron frequency.

Laboratory investigation of boundary layer processes due to strong spatial inhomogeneity

Laboratory experiments have been conducted to simulate the dynamics of highly localized magnetospheric boundary layers. These regions, such as the plasma sheet boundary layer and the magnetopause, are primary regions of solar wind mass, energy, and momentum transport into the near-Earth space environment. During periods of solar activity, the boundary layers can become compressed to scale lengths less than an ion gyroradius. Theoretical predictions indicate that the plasma can respond to relax these highly stressed conditions through the generation of instabilities in the lower hybrid frequency range. The experiments reported here document the characteristics of waves associated with these instabilities.

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...

A large volume microwave plasma source

We report on the design, construction, and use of a large cavity microwave plasma source. The source is designed to provide a range of space‐plasma‐like conditions in the Naval Research Laboratory Space Physics Simulation Chamber. A new feature of the source design incorporates hard anodized aluminum as the internal cavity area surface and does not use the conventional quartz cavity liner which is prone to overheating and cracking with extended use. By placing a number of small plasma outlet holes around the surface of a 36.8 cm output plate, we are able to provide a fairly radially uniform plasma; by further surrounding the production region and exit with an axial pinch magnetic field we are able to extend this region of plasma uniformity further toward the chamber walls and cover a significant portion of the experimental area. The source provides plasmas with selectable densities between 104 and 108 cm−3 and electron temperatures vary from about 0.5 to 2.0 eV.

Velocity shear-driven instabilities in a rotating plasma layer

The linear stability of a radially localized layer rotating about the cylindrical axis in a magnetized plasma is investigated using an eigenvalue analysis. The eigenvalue equation is solved numerically in a parameter regime characteristic of the Space Physics Simulation Chamber (SPSC) experiments [Amatucci et al., Phys. Rev. Lett. 77, 1978 (1996)] at the Naval Research Laboratory (NRL). Four types of instabilities are predicted. They are type-A and type-B Kelvin-Helmholtz instabilities, a transverse current-driven instability, and the inhomogeneous energy density driven instability (IEDDI). A quantitative comparison between theory and experiment indicates that an experimentally observed fluctuation in a rotating plasma layer is an IEDDI.

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...

Whistler wave propagation and whistler wave antenna radiation resistance measurements

Whistler waves are a common feature of ionospheric and magnetospheric plasmas. While the linear behavior of these waves is generally well understood, a number of interesting observations indicate that much remains to be learned about the nonlinear characteristics of the mode. For example, in space, very low frequency (VLF) emissions triggered by whistler modes launched from ground-based transmitters have been observed. Emission is assumed to come from transverse currents formed by counterstreaming electrons that are phase bunched by the triggering signal. In the laboratory, it has been shown that with increasing amplitude of the driving signal applied to an antenna, the whistler mode radiation pattern forms a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. These observations have prompted an Naval Research Laboratorys (NRL) Space Physics Simulation Chamber study of whistler wave dynamics. The goals are to investigate whistler wave ducting, self-focusing, and amplification, and to study nonlinear whistler-plasma interactions.