Formation of a millimeter-sized spark discharge in ambient air is traced on a nanosecond time scale using multi-frame laser probing with an exposure time of 70 ps and spatial resolution of 3-4 μm. The discharge is initiated by a 25 kV voltage pulse with a rise time of 4 ns, with the pulse applied to the gap formed by a point cathode and flat anode. It is demonstrated that the gap breakdown is accompanied by the fast (∼1 ns) formation of a highly ionized homogeneous spark channel originating from the point cathode. We discover that the fast fine-scale filamentation of the homogeneous spark channel arises several nanoseconds after the breakdown and at some distance from the cathode, which results in a complex filamentary structure of the channel. We find that the growing spark channel, in fact, develops in the form of multiple (N  10) rapidlyevolving filaments that constitute micron-sized (∼10-50 μm) plasma channels with an electron density of -ñ 10 10 e 19 20 cm −3 and subnanosecond characteristic evolution time. First filaments appear at the top of the developing homogenous spark channel. Further, the growing filaments are split themselves, and their number is increased over time up to several tens. Our findings indicate that the fast fine-scale filamentation is one of the important mechanisms governing the spark channel resistance after the breakdown.

X-ray spectroscopy with high spectral (up to AA/A = lo-*) and spatial resolution (up to 1 pm) is discussed. Devices based on crystals, diffraction and Bragg-Fresnel elements and their applications in Zand X-pinches and laser plasma experiments are described.

Calibrated density measurements have been obtained of the coronal plasmas around exploding 7.5-40 m W wires carrying 15-120 kA per wire for 30-70 ns. X-ray radiographs of the exploding wire plasmas using 2.5-10 keV photons from a Mo wire X-pinch backlighter enabled measurements of areal densities of W ranging from 2ϫ10 17 /cm 2 , equivalent to 0.03 m of solid density W, to about 10 19 /cm 2 . The rapidly expanding ͑few mm/s͒ coronal plasmas surrounding the slowly expanding ͑Ͻ1 mm/s͒ residual wire cores have areal densities up to about 2ϫ10 18 /cm 2 . Single 7.5 m wires tested with 100 kA had as much as 90% of the initial wire material in the coronal plasma. Coronal plasma W number densities were estimated to be up to a few times 10 18 /cm 3 , while core W densities as low as a few times 10 20 /cm 3 were observed. With linear arrays of four ͑eight͒ 7.5 m wires carrying 30 kA ͑15 kA͒/wire, up to 35% ͑25%͒ of the initial W wire material was in the plasma around and between the wires at 46-48 ns after the current started. Preheating the wires to drive off adsorbed gases and hydrocarbons increased the W mass in the coronal plasma and made it more uniform then when wires were not preheated.

The dynamics of the dense plasma near the cross point of an X pinch has been investigated using 1 ns x-ray backlighting images at different moments relative to the start of 100 ns [full width at half maximum (FWHM)] 200 kA current pulses. If the two metal wires are fine enough (e.g., 10 μm W or 17.5 μm Mo) to form a pinch at the cross point, accompanied by an x-ray burst, with the available current pulse, then the images show three stages of development: a radial explosion/expansion phase; an implosion during which a dense Z pinch of 200–300 μm length forms at the cross point together with plasma jets which move axially away from that point; and a breaking up of the Z pinch, coincident in time with one or two x-ray bursts, after which a 300 μm gap opens up. For W, the backlighter minimum sensitivity is 1017/cm2 areal density, and the dense Z pinch is estimated to have a volume density close to 1021/cm3. Shock waves appear to be expanding at about 50 μm/ns from the end points of the collapsing Z pinch, where the plasma was the most dense.

The initial stage of the fast electrical breakdown of an air gap with a pin-to-plane electrode geometry is studied on a nanosecond time scale using multi-frame laser probing with an exposure time of 70ps and spatial resolution as high as 3-4μm. We find that the gap breakdown is associated with the fast (1 ns) formation of a micron-sized (∼10 μm) cathode spot that appears as a plasma with an electron density of » n 10 cm e 19