Observation of Fast Current Redistribution in an Imploding Plasma Column

Spectroscopic measurements of the magnetic field evolution in a Z-pinch throughout stagnation and with particularly high spatial resolution reveal a sudden current redistribution from the stagnating plasma (SP) to a low-density plasma (LDP) at larger radii, while the SP continues to implode. Based on the plasma parameters it is shown that the current is transferred to an increasing-conductance LDP outside the stagnation, a process likely to be induced by the large impedance of the SP. Since an LDP often exists around imploding plasmas and in various pulsed-power systems, such a fast current redistribution may dramatically affect the behavior and achievable parameters in these systems.

Figure 1

Schematics for B-field determination. (a) Diagnostic setup: with the side-on imaging, at the outermost chord, the LOS is parallel to $\overrightarrow{B}$ so that only the circularly polarized ${\sigma }^{+}$ and ${\sigma }^{-}$ Zeeman components are observed. The light is transformed into orthogonal, linearly polarized light by a $\lambda /4$ plate and spatially separated by a polarizing beam splitter. The two plasma images, with the different polarization properties, are defracted by a 1-m imaging spectrometer and recorded on an intensified charge-coupled device (not shown on the image). (b) Typical spectral recording of the O VI 3s–3p transition obtained at $t=100\mathrm{ns}$. The green line represents the unshifted spectral line, illustrating the deviation of the measured spectral lines from the unperturbed position, where ${\sigma }^{+}$ and ${\sigma }^{-}$ exhibit blue and red (B-dependent) shifts, respectively. (c) Spectral lineouts (markers) taken from (b) at $y=1.6\pm 0.1\mathrm{mm}$ from the upper and the lower image (bright rectangles) and their fit (dashed/dotted). The solid lines represent the calculated Zeeman pattern without line broadening. The separation between the profiles of $0.37\AA$ yields a B-field of 2.3 T, and the Stark broadening yields $n_e\approx 9\times {10}^{17}{\mathrm{cm}}^{-3}$ [40, 41].

Figure 2

Evolution of the O IV ($t=50\mathrm{ns}$) and O VI ($t\ge 100\mathrm{ns}$) emission intensity radial distribution along with the magnetic field measured at the outer chord: at $t=50–140\mathrm{ns}$, the agreement between expected (B-dot) and measured magnetic field (Zeeman) indicates the confinement of the entire discharge current in the imploding plasma. At $t=170\mathrm{ns}$ the fraction of the current that can be located in the stagnating plasma is $\lesssim 30\%$.

Figure 3

Gray: location of the plasma with $n_e>0.1n_{e,\max }$ (from continuum radiation intensity) with uncertainty in light gray. Red: locations at which a certain percentage of the discharge current was measured. At $t\ge 170\mathrm{ns}$ the measurements were obtained in an LDP with a density lower than 10% of $n_{e,\max }$. Initially, the current implodes together with the main plasma, while at stagnation (140–170 ns), a sudden current redistribution to much larger radii takes place.

Figure 4

(a) Schematic of the Z-pinch experiment with initial gas distribution, imploding and stagnating plasma (SP) and LDP featuring “zippering” effect. (b) Simplification of the 2-mm plasma element into a lumped-circuit model.

Figure 5

Simulation of the lumped-circuit equation (LCE). (a) The abrupt decrease of the LDP resistance $R_2$ (green) enables the current transfer. (b) The evolution of the current partition $I_2/I$ (blue) follows the impedance ratio (dashed black) with a delay of the current transfer time (dotted green).