Diagnostic characterization of a prepulsed carbon fiber Z pinch using spectral line intensity analyses

Abstract

Plasma formation and x-ray generation efficiency of a carbon fiber Z pinch are dramatically improved by means of an optimized current prepulse. In this article results are presented of time-integrated and also time-resolved electron temperature measurements using the optical line emission of the coronal plasma surrounding a still, cold fiber. Two phases of discharges with current prepulse are studied: the preionization phase only in which time-integrated spectra of C II–C IV emission are recorded and time resolution of the heating phase after onset of the main current pulse using C III and C IV line emission. Experimentally obtained, n=3–4, 3–3, line ratios are fitted with collisional radiative, ADAS computations of line intensity ratios as a function of electron temperature, density, ne and variable confinement, τ. All levels up to n=5 are included with the influence of higher levels also accounted for. Deviations from coronal balance give an effective value for the product 〈neτ〉. As a result, the electron temperature of the preionized coronal plasma is estimated at Te=5 eV at a density of ne≈1017 cm−3. The temperature rises to about 12 eV at the same density after the start of the main current with prepulse. Similar analytic techniques are applied to time-integrated C V and C VI XUV resonance lines from the main pinch and give plasma parameters in accord with the x-ray continuum diagnostics.