Joseph A. Johnson

Neutral particle analyzer measurements on the SSPX spheromak

A neutral particle analyzer is used to measure the time-resolved energy spectrum of neutral hydrogen leaving a spheromak plasma. A gas cell filled with 10-50 mTorr of helium is used to strip electrons from incoming neutral hydrogen, lowering the minimum detectable energy well below that obtained with thin foils. Effective neutral particle temperature is calculated by fitting a Maxwellian energy distribution to the measured energy spectrum above and below approximately 300 eV. A computational model with approximated profiles of plasma density and neutral density is used with the measured neutral hydrogen flux to estimate the ion temperature. Measurement of the power flux due to neutral hydrogen emitted at the measurement location is extended to the whole plasma surface to estimate the total charge exchange power loss from the plasma. The initial results indicate that the charge exchange power loss represents only 2% of the total input gun power during the sustainment phase of the discharge.

Turbulent microwave plasma thermodynamics for fundamental fluctuation modes

Microwave plasmas are generated in helium, neon, argon, krypton, and xenon at a range of microwave powers from 300 to 1800 W. A floating Langmuir double probe is employed to determine plasma electron density and temperature for all five species. The standard turbulence analysis is carried out by using time resolved neutral line emission data form these gases at a sampling rate of 100 MHz. From the Fourier power spectrum of the data, the strongest fluctuation frequency is found to be consistently the fundamental or a second harmonic of a turbulence characteristic frequency in the spectra. In all five species the strongest frequency is not influenced by increased microwave power even though other thermodynamic parameters are changed. The low chaotic dimension for all species seems independent of microwave power and of turbulent fluctuation energy. The phase space trajectories show simplicity and periodicities are consistent with the low chaotic dimension and with the peak frequencies obtained from the fluct...

Evidence of inverse bremsstrahlung in laser enhanced laser-induced plasma

Plasmas created by a Nd:yttrium aluminum garnet laser show systematic changes in local electron temperature when bathed by a continuous wave laser of increasing irradiance. By monitoring the local electron density, the laser light absorption coefficient, and the signal to noise ratio in neutral emissions, we explain the changes in electron temperature and signal to noise to be a consequence of inverse bremsstrahlung in this new system of laser enhanced laser-induced plasmas.

Critical turbulent energy reductions in plasmas using weak magnetic fields

With an arc-driven shock tube, laser induced fluorescence, and a multipoint density diagnostic technique, we study the turbulence behind an ionizing shock wave in the presence of a magnetic field. The magnetic field is directed either parallel to or antiparallel to the direction of the shock wave’s propagation, and is configured in such a way as to couple with turbulent velocity fluctuations in the plane perpendicular to the direction of flow. We find that the magnetic field can be used to reduce the turbulent energy in a plasma system. Further, when the evolution to turbulence is treated as a second-order phase transformation, the critical turbulent energy decreases with increasing magnetic field.

Interaction of turbulent plasma flow with a hypersonic shock wave

A transient increase is observed in both the spectral energy decay rate and the degree of chaotic complexity at the interface of a shock wave and a turbulent ionized gas. Even though the gas is apparently brought to rest by the shock wave, no evidence is found either of prompt relaminarization or of any systematic influence of end-wall material thermal conductivities on the turbulence parameters.

SECOND VISCOSITY ENHANCEMENT IN TURBULENT NONEQUILIBRIUM FLOW

Using a zeroth‐order entropy production model, a relationship between the second viscosity, the relaxation time for a nonequilibrium process, and turbulence is determined from the macroscopic entropy rate equation. For turbulence generated reaction rate distortion, enhanced second viscosities can be predicted for turbulent fluid systems with long relaxation times and nonzero velocity divergence.

Evidence of natural closure in compressible turbulent flow

Using a Ludwieg tube wind tunnel, first evidence is obtained of a vanishing triple correlation in supersonic compressible turbulent flow. The results take advantage of a new technique for very high frequency direct estimation velocimetry from density measurements through laser induced fluorescence and seem to confirm early theoretical predictions of Tsuge and Sagara.

Low energy ion distribution measurements in Madison Symmetric Torus plasmas

Charge-exchange neutrals contain information about the contents of a plasma and can be detected as they escape confinement. The Florida A&M University compact neutral particle analyzer (CNPA), used to measure the contents of neutral particle flux, has been reconfigured, calibrated, and installed on the Madison Symmetric Torus (MST) for high temperature deuterium plasmas. The energy range of the CNPA has been extended to cover 0.34–5.2 keV through an upgrade of the 25 detection channels. The CNPA has been used on all types of MST plasmas at a rate of 20 kHz throughout the entire discharge (∼70 ms). Plasma parameter scans show that the ion distribution is most dependent on the plasma current. Magnetic reconnection events throughout these scans produce stronger poloidal electric fields, stronger global magnetic modes, and larger changes in magnetic energy all of which heavily influence the non-Maxwellian part of the ion distribution (the fast ion tail).

Note: A high Mach number arc-driven shock tube for turbulence studies

A high Mach arc-driven shock tube has been built at the Center for Plasma Science and Technology of Florida A&M University to study shock waves. A larger apparatus with higher voltage was built to study more stable shock waves and subsequent plasmas. Initial measurements of the apparatus conclude that the desired Mach numbers can be reached using only two-thirds the maximum possible energy that the circuit can provide.

Mass dependency of turbulent parameters in stationary glow discharge plasmas

A direct current glow discharge tube is used to determine how mass changes the effects of certain turbulence characteristics in a weakly ionized gas. Helium, neon, argon, and krypton plasmas were created, and an axial magnetic field, varied from 0.0 to 550.0 Gauss, was used to enhance mass dependent properties of turbulence. From the power spectra of light emission variations associated with velocity fluctuations, determination of mass dependency on turbulent characteristic unstable modes, energy associated with turbulence, and the rate at which energy is transferred from scale to scale are measured. The magnetic field strength is found to be too weak to overcome particle diffusion to the walls to affect the turbulence in all four types of plasmas, though mass dependency is still detected. Though the total energy and the rate at which the energy moves between scales are mass invariant, the amplitude of the instability modes that characterize each plasma are dependent on mass.