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Induced m=0 instability in fast ablative capillary discharges and possible utilisation for X-ray lasers

Ellwi S., Andreic Z., Ferri S., Juschkin L., Koshelev K., Kunze H.

Nukleonika

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2001

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Vol. 46, nr 1

1-3

EN

High current linear discharges confined by their own magnetic field are subject to magneto-hydrodynamic instabilities which perturb a straight plasma column. An example is the m=0 mode, which is characterised by the development of necks contracting rapidly towards the axis with the ion sound speed. Discharges through capillaries were assumed to be stable hitherto, but by cutting capillaries lengthwise after a few shots hot spot traces clearly imprinted on the inner wall of the capillary are observed. They are interpreted as marks of an m=0 instability, and this interpretation is substantiated by a series of time-gated pinhole images, which show that the hot plasma region is clearly detached from the wall at the second current maximum and concentrated on the axis thus making the development of the instability possible. The instability occurs only with a specific sample of polyacetal as wall material, and its axial wavelength increases with the length of the capillary. By modulating respectively the inner wall of the capillary the wavelength can be imposed within limits. This is exploited for a soft X-ray laser scheme based on charge exchange pumping of bare carbon ions of hot plasma streaming from the necks and colliding with cold plasma outside the neck regions. Exponential growth of the Balmer-alfa line of CVI at 18.22 nm is realised.

2

Technical setup and physical properties Technical setup and physical properties of a gas-liner pinch

Büscher S., Wrubel T., Kunze H.

Nukleonika

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2000

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Vol. 45, nr 3

167-172

EN

he present technical status and physical properties of the gas-liner pinch device in Bochum are presented and discussed. This comprises spectroscopic as well as diagnostic achievements. A short review on the measurements performed at this light source will also be given.

3

On the influence of a carbon solid on the properties of a helium plasma in a theta pinch discharge

Freese W., Kunze H.

Nukleonika

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2000

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Vol. 45, nr 3

173-177

EN

A helium plasma produced in a 82 kJ theta pinch discharge has been seeded with carbon ions by a carbon solid rod positioned on the axis. The first shock front of the discharge ablates carbon atoms from the solid surface which are ionized and interact with the plasma. Some shock front properties (namely thickness, compression velocity and its shape) were analyzed with an ICCD-camera system taking 2-dimensional photos of the plasma continuum emitted along the axis with a time duration of 100 ns. While the influence on the shape was not found, the temporal behaviour was drastically changed when the rod was added. The shock front occurs earlier and compresses faster. An explanation remains to be given. Furthermore, the influence of the carbon impurities on the plasma parameters has been investigated by measuring the electron temperature and density by means of plasma spectroscopy comparing these results with those obtained from discharges in helium without rod. We found a decrease in the electron density about 500 ns after the shocked plasma has started to interact with the rod surface. Simultaneously increasing continuum emission and decreasing intensity of the HeII Pa line indicates a regime of recombination. No influence was found on the electron temperature.