Sean Keely

Operation regimes of magnetically insulated transmission lines

Magnetically insulated transmission lines (MITLs) are commonly used for efficient power transport in the vacuum section of pulsed power devices. Plasma forming from metal surfaces limits the power transmitted to a load through MITLs. It eventually shunts the load, producing so-called MITL closure. Fundamental experiments are being performed on high intensity power transmission through coaxial cylindrical vacuum transmission lines. A current that rises to 1 MA in 100 ns is driven through the MITLs by a 2-MV, 2-/spl Omega/ pulse generator (Zebra). The condition of the MITL surfaces is carefully controlled and characterized before each shot. Differential B-dot probes measure the current before and after the MITL, to determine the time of gap closure. Optical imaging and laser diagnostics observe the plasma evolution in the gap with time and space resolution. The radial gap of the cylindrical vacuum transmission line has been systematically varied, and the time of MITL closure measured. They increase with the radial gap size in a discontinuous manner. Critical transitions (discontinuous jumps in closure time) appear to separate distinct MITL operation regimes. This is the first experiment and data set of this kind known to the authors. Electromagnetic-particle-in-cell and radiation-magnetohydrodynamic computer modeling assist the experiment, being used to refine the experimental design and to interpret the results.

Powerful microfocus x-ray and hard x-ray 1 MA x-pinch plasma source for imaging, spectroscopy, and polarimetry

The x-ray emission of Ti, Fe, Mo, W and Pt x-pinches are currently bieng studied at the Nevada Terawatt Facility z- pinch machine (0.9-1.0 MA, 100 ns). New x-ray diagnostics for time-resolved spectroscopy and imaging has been developed and used in x-pinch experiments. The total x- ray/EUV yield was more than 10 kJ. The minimum x-ray pulse duration was 1.1 ns (Mo, W, Pt). For Ti, Mo and W pinches x-ray pulses occurred in two or three groups in the narrow time intervals after the start of the current. The most compact emitting region has been observed for a planar-loop Mo x-pinch (the number of hot spots ranging from 1-5 with a minimum size smaller than 30 micrometers at (lambda) <1.5-2 Angstoms). Strong jets were observed (Ti, Fe, Mo) directed toward the discharge axis, perpendicular to the wires. A structure of an x-pinch includes energetic electron beams directed toward the anode and along wires. The total beam energy increases from Ti to W. A pulse of hard x-ray radiation was observed moving upwards along the axial axis with an energy of several hundred keV(Mo). The size of this source was smaller than 1 mm. The measurements of temperature and density of x-pinch plasmas were based on theoretical modeling of K-shell Ti and L-shell Mo spectra (Te=1.5 keV for Ti, 0.8 keV for Mo, Ne up to 2- 3x1022 cm-3 with 1-10% of hot electrons).

Application of new five-channel x-ray/EUV spectrometer with an imaging transmission diffraction grating for study of z-pinch plasma sources

A new five-channel spectrometer is designed for registration of x-ray spectral line emissions from plasmas with temporal resolution. All channels are independent from each other and include wide variety of dispersing elements (crystals and/or multiplayer mirrors) and detectors (Si-diodes or PCD). Sixth channel is used for device alignments with minimum adjustments can be used as channel for transmission diffraction grating spectrometer or channel for another time resolved detector. The device was used in experiments with different plasma sources in different configurations and showed its reliability and flexibility.

Multichannel fast hard x-ray spectrometer with spatial resolution capability for extended z-pinch plasma sources

A multi-channel, fast, hard x-ray diode spectrometer is being developed at the Nevada Terawatt facility. This spectrometer helps facilitate the study of the time evolution of hard x-ray emissions from hot, dense plasma. Each channel in the array can be adjusted individually with shielded view areas, allowing small areas of an x-ray source to be isolated and studied by region. This spatial resolution capability will permit a better understanding of the mechanisms present in hot, dense plasma. Results will be presented of experimental tests, and their interpretation, pertaining to the hard x-ray emissions generated from an x-pinch source in a pulse-power type device.