The spectacular progress made during the last few years in reaching high energy densities in fast implosions of annular current sheaths (fast Z pinches) opens new possibilities for a broad spectrum of experiments, from x-ray generation to controlled thermonuclear fusion and astrophysics. Presently Z pinches are the most intense laboratory X ray sources (1.8 MJ in 5 ns from a volume 2 mm in diameter and 2 cm tall). Powers in excess of 200 TW have been obtained. This warrants summarizing the present knowledge of physics that governs the behavior of radiating current-carrying plasma in fast Z pinches. This survey covers essentially all aspects of the physics of fast Z pinches: initiation, instabilities of the early stage, magnetic Rayleigh-Taylor instability in the implosion phase, formation of a transient quasi-equilibrium near the stagnation point, and rebound. Considerable attention is paid to the analysis of hydrodynamic instabilities governing the implosion symmetry. Possible ways of mitigating these instabilities are discussed. Non-magnetohydrodynamic effects (anomalous resistivity, generation of particle beams, etc.) are summarized. Various applications of fast Z pinches are briefly described. Scaling laws governing development of more powerful Z pinches are presented. The survey contains 36 figures and more than 300 references.

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The physics of fast Z pinches

High-thermal-conductivity, mesophase-pitch-derived carbon foams: effect of precursor on structure and properties

The boundary radiation heat loss method is investigated for the flash method in measuring thermal diffusivity. Present correction procedures for heat loss are tested experimentally using AXM−5Q (POCO) graphite. In addition, a new method for radiation loss correction is presented based solely on the heating portion of the temperature rise curve. Using the new method, corrections of 35% can be made within a ±3% error. The thermal diffusivity results calculated using the adiabatic expression for various portions of the fractional rise curve and for various heat losses are also presented.

Radiation loss in the flash method for thermal diffusivity

Analytical modeling of thermal and mechanical response is a fundamental step in the design process for ultra-high-temperature ceramic components, such as nose tips and wing leading edges for hypersonic applications. The purpose of the analyses is to understand the response of test articles to high-enthalpy flows in ground tests and to predict component performance in particular flight environments. Performing these analyses and evaluating the results require comprehensive and accurate physical, thermal, and mechanical properties. In this paper, we explain the nature of the analyses, highlight the essential material properties that are required and why they are important, and describe the impact of property accuracy and uncertainty on the design process.

Material property requirements for analysis and design of UHTC components in hypersonic applications

In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, zT. Advice for improving the data quality in Seebeck coefficient, electrical resistivity, and thermal conductivity (from flash diffusivity and heat capacity) measurements are given together with methods for identifying possible erroneous data. Measurement of the Hall coefficient and calculation of the charge carrier concentration and mobility is also included due to its importance for understanding materials. It is not intended to be a complete record or comparison of all the different techniques employed in thermoelectrics. Rather, by providing an overview of common techniques and their inherent difficulties it is an aid to new researchers or students in the field. The focus is mainly on high temperature measurements but low temperature techniques are also briefly discussed.

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Measuring thermoelectric transport properties of materials

Equations are derived giving the temperature distribution within a thin slab of material which has received a short pulse of energy on one surface, for the case in which energy loss at the surfaces (by radiation or convection) is not negligible. An analysis of these equations indicates that measurement of thermal diffusivity by the pulse method should be feasible even when losses are so large that the maximum temperature of the far face is only 10 or 20% of the no‐loss value; this includes nearly all materials, and temperatures to 2500°K or higher.

Pulse Method of Measuring Thermal Diffusivity at High Temperatures

X‐ray emission was used to diagnose the plasma formation in a gas‐puff pulsed‐discharge device. X‐ray pinhole images were collected simultaneously with x‐ray spectra from Ar and Kr gases. X rays were emitted from z‐pinched regions (50–100 μm in diameter) along a collinear track formed by the plasma implosion of a hollow gas jet. Spectral analysis indicated Ar plasma formation with electron densities of ∼2×1021 cm−3. The Ar plasma temperature was ∼1 keV, based on a collisional‐radiative transport model. Li‐like satellite lines were classified in Ar XVI with atomic structure calculations. The Kr x‐ray spectrum (6–8‐A region) was dominated by Ne‐like Kr XXVII lines. New spectral transitions were classified in Na‐like Kr XXVI and F‐like Kr XXVIII. The wavelength calibrations in the Kr spectrum were obtained from exploded‐Al‐wire plasma generated in coincidence with the plasma implosion formed in the puff gas.

X‐ray spectra from a gas‐puff z‐pinch device

Single‐wire Fe spectra collected from two different exploded‐wire generators (Gamble II and Owl II) were analyzed to determined the ionization stages produced in the plasmas. The temperature for the hot‐plasma pinches for both generators was 1.4±0.2 keV at which an abundance of Fe XXIV transitions is produced. The Fe K spectra from exploded wires are basically similar to those produced in the pinched plasma generated randomly in the vacuum spark; however, the exploded wires have lower plasma temperatures than the hottest pinches produced in the vacuum spark. A detailed interpretation of the Fe L spectra formed in the exploded wires permitted line and ionization stage identifications in the 7–12‐A region. Such spectroscopic data is useful for analysis of complex Fe spectra generated in multitemperature plasma devices like Tokamaks.

Robert D. Cowan

Papers

Pulse Method of Measuring Thermal Diffusivity at High Temperatures

X‐ray spectra from a gas‐puff z‐pinch device

X-ray line emission and plasma conditions in exploded Fe wires