R. C. Hazelton

Characterization of magnetohydrodynamic effects in a plasma opening switch

Plasma dynamics are characterized in a long‐conduction time plasma opening switch [Phys. Fluids B 4, 2368 (1992)] operated with densities and currents near the theoretical intersection of the magnetohydrodynamic (MHD) [IEEE Trans. Plasma Sci. PS‐19, 400 (1991)] and electronmagnetohydrodynamic (EMH) [Phys. Fluids B 3, 1908 (1991)] regimes. In agreement with MHD theory, a hydrodynamic snowplow is observed to translate axially and reach the load end of the switch at opening. The axial motion of the front agrees with one‐dimensional analytic predictions if the carbon plasma possesses an average ionization of 1, but is significantly less than theory if higher ionization levels are present. The axial motion of the plasma center‐of‐mass is significantly less than that expected from theory for any ionization level. The reduced center‐of‐mass motion is attributed to an ion loss mechanism in the unconfined switch plasma, an effect also observed in the total particle inventory, which saturates while the source plasm...

Effect of Material Parameters on the Charging Characteristics of Irradiated Dielectrics

In order to accurately predict the charging characteristics of dielectric materials used on spacecraft surfaces, a knowledge of material parameters determined under irradiation conditions is required. The relative importance of these parameters can then be determined by comparison with numerical models. This paper reports the measurements of the charging characteristics and material parameters of Teflon, Kapton and fused silica samples irradiated by a monoenergetic electron beam and the comparison of these measurements with the predictions of a numerical model. The experimental system is configured to closely approximate the one dimensional geometry of the numerical model. Time dependent surface potentials are inferred from the measurement of the energy spectra of secondary electrons emitted during irradiation. Bulk conductivities are inferred from equilibrium surface potentials and equilibrium substrate currents. The measured secondary electron emission coefficients for Teflon and Kapton are in agreement with values in the literature as is the bulk conductivity of Teflon. The conductivity of Kapton is observed to be larger than values reported in the literature and has an energy flux dependence. Comparisons with the numerical model predictions indicate that secondary electron emission is the controlling material characteristic for Teflon and fused silica whereas the secondary emission and bulk conductivity are the controlling factors for Kapton.

Resonant, heterodyne laser interferometer for state density measurements in atoms and ions

A resonant, two-wavelength heterodyne interferometer has been developed to measure state densities of specific atomic populations. Using a tunable diode laser, one of the wavelengths is set near an absorption line and is sensitive to resonant enhancement of the refractive index. The other wavelength is at least several linewidths away and is used to measure nonresonant effects. The subtraction of the phase shifts of the two interferometers allows one to measure the population density in the lower state of the line being investigated. The interferometer system has been tested using a plasma, created by an inverse pinch plasma source, to measure the state density of Hn=2 by making resonant observations near the Hα (6563A) transition. Average line-of-sight densities of 5.4×1011cm−3 were measured with a minimum resolvable density of 7.3×109cm−3. The transverse spatial resolution was 1–2mm. The technique described is applicable for measuring lower state densities in any atom or ion where the oscillator strengt...

Streaked laser absorption measurements of density and temperature profiles in a high density Z pinch

A nonresonant laser absorption technique is described for studying the density and temperature profiles in a high density z‐pinch discharge. A long pulse (700‐ns) dye laser backlights the pinch and a streak camera records the transmitted laser intensity. Spatially and temporally resolved absorption coefficients are inferred from the fractional transmission through the plasma. The measured values are related to plasma density and temperature profiles using theoretical calculated absorption coefficients and equations of state. The results demonstrate the fine spatial resolution that can be obtained with this technique, reveal a complex radial structure in z‐pinch discharges not observable in emission light streak photographs, and establish the practicality of using the technique to evaluate and benchmark theoretical model predictions.

Conduction Processes in Kapton H Irradiated by an Electron Beam

Current conduction in Kapton H films irradiated by a non-penetrating electron beam was studied by comparing the observed current-voltage characteristics with theoretical models. The functional dependence of the current on the electric field was found to be in relatively good agreement with the dependence predicted by the Poole-Frenkel conduction and Schottkey emission model. A beam energy dependence was observed that is not accounted for by these models. No correlation was obtained with space-charge-limited conduction model that assumes a zero electric field at the injecting electrode. This result was observed both for a field independent conductivity as well as for a Poole-Frenkel conductivity. A modified space-charge-limited conduction model that includes a finite electric field at the injection electrode appears to describe the observed dependence. This model incorporates any beam energy dependence in a natural way through a beam dependent variation in electric field that is predicted to occur at the boundary of the unirradiated region.

A Fast Exhaust-Gas Analyzer for the ITER Fusion Experiment Divertor

This paper presents a first demonstration of a radio-frequency (RF)-excited optical gas analyzer (RF-OGA) designed to quantitatively measure minority species inside the neutralization region of the ITER fusion experiment divertor. The sensor head, which creates its own plasma excitation and plasma light emission, is designed to operate in a strong magnetic field, and the RF coupling leads to bright light emission. It also allows for operation at low voltages, avoiding the radiation-enhanced breakdowns expected when high voltages are present in the ITER environment. Furthermore, the preferred sensor head features full isolation of the metal RF electrodes from the induced plasma. This ¿electrodeless¿ operation will permit long operation without frequent maintenance. The testing of a first experimental RF-OGA with an electrodeless design in a strong (~2-T) magnetic field showed a mostly linear response of the He I-6678 A¿ line emission to the He concentration in a hydrogen background, which would produce a He concentration measurement accurate to within 2% of the helium-to-hydrogen ratio.

Resonant laser diagnostics of a planar plasma opening switch

The plasma opening switch (POS) is an integral part of inductive store pulsed power systems. Using flashboards coated with BaO and a dye laser tuned to the 493.4 nm ground state transition of the singly-ionized barium ion, resonant laser diagnostics have been employed to image the switch plasma and provide a measurement of the plasma density during conduction and opening. Gaps open during the conduction phase with their position in the inter-electrode region depending upon the initial fill plasma. There is little axial motion of the plasma, contrary to the predictions of analytical hydro model calculations performed using the measured switch parameters. This discrepancy may be due to a finite lifetime for ions in the switch that is less than the conduction time resulting in a larger effective mass. From a functional point of view modified bipolar model calculations best fit the data. >

High resolution measurements of ion temperatures in z-pinch plasmas

The Doppler broadening of ion line profiles emitted by z-pinch plasma provides information about the thermalization of the implosion kinetic energy and the radiation efficiency of the pinch. Measurements of these line profiles are often complicated by source broadening in the instrument and opacity broadening of the emitted radiation. A high resolution concave crystal spectrometer in the Johann geometry was used to record the time averaged spectra of optically thin trace elements in the load. An imaging slit provided radially resolved but axially averaged spectra. The measurements indicate that lower ion temperatures (3–5 keV) are observed for Al wire loads on both the Saturn and Double EAGLE accelerators in the short current pulse mode (60–100 ns) than in the long pulse mode (125–225 ns) where values of 6.3–9.5 keV are observed. These values are smaller than those observed on Saturn by others. Furthermore, the wavelength at the line center of axially resolved ion line profiles on the DM-2 accelerator at ...

INVESTIGATION OF CURRENT CHANNEL MIGRATION IN A CONDUCTING PLASMA BETWEEN PLANAR ELECTRODES

Magnetic‐field penetration is characterized in a current conducting plasma between planar electrodes used as a plasma opening switch [G. G. Spanjers, E. J. Yadlowsky, R. C. Hazelton, and J. J. Moschella, J. Appl. Phys. 77, 3657 (1995)]. The experiment is performed in a regime where Hall effects [Amnon Fruchtman, Phys. Fluids B 3, 1908 (1991)] are predicted to describe the current channel migration. Measurements of the magnetic‐field penetration in two cases with opposite electrical polarity indicate that the Hall effects are not the dominate process. A one‐dimensional resistive magnetohydrodynamic model is used to show that current channels in agreement with those measured can be predicted through a coupling of a plasma snowplow motion with resistive diffusion.

Interferometric measurement of physical phenomena during the implosion phase of a puff-on-puff Z-pinch load on Double-EAGLE

Theoretical studies have predicted that the disruptive role of the Rayleigh-Taylor (R-T) instability on the current conduction and implosion characteristics of annular Z-pinch loads will be mitigated by mass accretion if uniform fill or multiple annular shell loads are used. Holographic interferometry was used to study these physical processes during the implosion phase of puff-on-puff loads on a terawatt accelerator. Both axial (r-z) density perturbation and azimuthal (r-/spl theta/) filamentation modes of the R-T instability were observed. Significant ionization (Z/spl ap/3-10) of the inner gas puff atoms was observed below the anode grid before the outer puff had imploded to this radial position. Radiation hydrodynamic calculations indicate that photoionization by radiation from the outer current carrying shell could not account for this ionization. Current flowing on the inner gas puff could be the source of this ionization. The effect of these physical processes on the radiation yield from z-pinches warrants further investigation.