A. R. Mingaleev

Density measurements in exploding wire-initiated plasmas using tungsten wires

Calibrated density measurements have been obtained of the coronal plasmas around exploding 7.5–40 μm 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 2×1017/cm2, equivalent to 0.03 μm of solid density W, to about 1019/cm2. The rapidly expanding (few mm/μs) coronal plasmas surrounding the slowly expanding (<1 mm/μs) residual wire cores have areal densities up to about 2×1018/cm2. Single 7.5 μm 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 1018/cm3, while core W densities as low as a few times 1020/cm3 were observed. With linear arrays of four (eight) 7.5 μm wires carrying 30 kA (15 kA)/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....

Studies of plasma formation from exploding wires and multiwire arrays using x-ray backlighting

Plasma formation from exploding individual wires and multiwire arrays, powered by a 450 kA, 100 ns pulsed power generator has been studied with x-ray direct backlighting using one or more X pinches as a source of radiation. Experiments have been carried out with many different wire materials ranging in Z from Al to Au. A point source of x rays is used to cast a shadow image of the object plasma directly on film (“point-projection” backlighting). Spatial resolution in the range 1–5 μm, and temporal resolution of 0.7–2 ns, has been achieved using 2–4 keV x rays from Mo wire X pinches. Different schemes for multiframe imaging are described. We also present observations of micron scale structure on residual dense wire cores 40–70 ns after the start of the current pulse.

Evolution of the structure of the dense plasma near the cross point in exploding wire X pinches

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, whe...

Bright spots in 1 MA X pinches as a function of wire number and material

Bright, intense x-ray sources with extreme plasma parameters (micropinch plasmas) have previously been characterized at 0.1–0.4MA, but the scaling of such sources at higher current is poorly understood. The x-ray source size and radiation power of 1MA X pinches were studied as a function of wire material (Al, Ti, Mo, and W) and number (1-, 2-, 8-, 32-, and 64-wire configurations). The smallest bright spots observed were from 32-wire tungsten X pinches, which produced ⩽11–16μm, ∼2J, 1–10GW sources of 3–5keV radiation.

Measurements of high-current electron beams from X pinches and wire array Z pinches

Some issues concerning high-current electron beam transport from the X pinch cross point to the diagnostic system and measurements of the beam current by Faraday cups are discussed. Results of computer simulation of electron beam propagation from the pinch to the Faraday cup give limits for the measured current for beams having different energy spreads. The beam is partially neutralized as it propagates from the X pinch to a diagnostic system, but within a Faraday cup diagnostic, space charge effects can be very important. Experimental results show evidence of such effects.

A source of hard X-ray radiation based on hybrid X pinches

X pinches are well known to produce very small, dense plasma pinches (“hot spots”) that emit sub-nanosecond bursts of 1–8u2009keV radiation. Hard X-ray radiation in the range from 8 to 300u2009keV or more is also emitted, and only a small portion of which is associated with the X-pinch hot spot. In hybrid X-pinches (HXP), the u200210u2009ns hard X-ray pulse is terminated by fast closure of the gap between the two conical electrodes of the HXP by rapidly expanding electrode plasmas. The temporal, spectral, and spatial properties of this higher energy radiation have been studied. This radiation was used for point-projection imaging with magnification between 1.5 and 6, and spatial resolution of 20–100 μm was demonstrated.

Structure on the dense residual wire core during exploding wire plasma formation

Summary form only given. High resolution (1-5 micron, 0.7-2 ns) X-ray (2-4 keV) backlighting has been used to study the formation of plasma when single wire and multiwire arrays are exploded using the /spl sim/450 kA XP pulser at Cornell University. The wire plasmas were imaged using 13-25 micron Mo wire, and 20 micron Pd wire, 150-200 kA X-pinch X-ray backlighters which were <5 microns in size and about 1 ns in duration. It is well established now that an exploding wire plasma produced by a /spl sim/100 ns pulsed power generator initially consists of a relatively low density coronal plasma surrounded by a dense residual wire core. Two images of the wire cores separated in time by 5-20 ns have been obtained 40-70 ns after the start of the current pulse with 2-4 keV X-rays using different geometries. Fine structure on the wire core surface with a spatial scale of 10-20 microns has been recorded on film at times when the wire core is about ten times that diameter. The structure, which appears to be dominantly azimuthally symmetric, is reminiscent of the Rayleigh-Taylor instability, and it is observed to increase in amplitude from the first backlighter frame to the second.

Measurements of Electron Beam Intensity and Characteristic Line Radiation from Wire Array Z-pinches on the 1 MA COBRA Pulsed-Power Generator

The majority of the X-ray emission from wire array Z-pinches is concentrated in the soft (sub-keV) part of the spectrum. In addition, the soft X-ray pulse is accompanied by significantly harder emission (multi-keV) that is generated at almost the same time. Important questions for wire array Z-pinch physics are: what fraction of the radiated energy is connected with soft versus hard X-rays? What is the nature of the hard X-rays? Previous experiments showed that hard X-rays are connected with energetic electrons generated in wire array Z-pinches as well as X-pinches. Recent experiments have investigated the relationship between electron beam current through the anode of wire array Z-pinches and the characteristic X-ray line intensity from the Z-pinches on the 1 MA COBRA pulsed-power generator. The electron beam current was measured by a low-impedance Faraday cup with a 20 mm diameter collector positioned 25 mm above the anode. The Faraday cup was filtered by thin foils of 15 mum Al, 25 mum stainless steel, or mu50 m stainless steel. X-ray radiation with E > 5 keV was monitored using a Si diode. A wide energy band spectrograph with three spherically bent mica crystals (WB-FSSR) was used to record thermal and characteristic line radiation. We measured electron beams and hard X-rays with sub-nanosecond temporal resolution and spectral lines with ~200 mum spatial resolution. We compared the intensity and timing of soft and hard x-rays, the intensity and relative positions of thermal and characteristic lines, and the intensity and energy of the electron beam in each shot. Through these comparisons, we attempt to determine factors which influence the generation of the electron beams.

Direct Measurements of Electron Beams in Symmetric and Asymmetric X pinches

Summary form only given. It is well known that X pinches produce very small, dense plasma micropinches that emit short bursts of 1.5-8 keV radiation. Immediately after the softer X-ray burst, higher energy emission in the range 8-100 keV is seen and attributed to energetic electrons accelerated in the gaps that are visible in the X pinch structure in the images recorded using X pinch point projection radiography. Electron beam parameters in the X pinches (current, average energy, and divergence) were not previously measured. Such measurements on symmetric X pinches are compromised by strong absorption and electron scattering that occurs between the X pinch crossing point and the anode. Cold, relatively high density plasma fills this space as a result of the ablation of the wire cores. An asymmetric X pinch scheme that decreases plasma density and size along the path of propagation of the electron beam has been tested. In this configuration, the anode side of the X pinch was made 4-7 times shorter than the cathode side and the angle between wires was almost 180 deg. Electron beam parameters were determined using two low impedance coaxial Faraday cups (with the filter of the inner cup working as the collector for the outer one). The filters on these cups enabled simultaneous measurements of the beam current in two energv bands. For two-wire. 17 mum, asymmetric, tungsten X pinches the electron current reached 5-10 kA with a pulse duration of 3-5 ns and a peak electron energy of more than 80 keV. These electrons were emitted in a solid angle of about 10-2 steradians. In symmetrically configured X pinches the measured electron beam current is much less and has much higher divergence. The asymmetric configuration may also be useful for ion beam studies by reversing the pulse polarity.

Optical Streak Camera-Based Studies of Wire-Array Z-Pinch Implosion Dynamics on the 1-MA COBRA Pulsed Power Generator

Summary form only given. Experimental results showing wire array z-pinch implosions on the 1-MA, 100-ns rise time COBRA pulsed power generator are presented. The principal diagnostic used for these studies was an optical streak camera system, while other supporting diagnostics include a time-gated framing camera, a laser backlighting system, time-integrated pinhole cameras with various filters, and silicon diodes and diamond photoconducting devices for monitoring X-ray production. The data produced by the entire suite of diagnostics is analyzed and presented to provide an overall picture of implosion dynamics and timing on COBRA. In particular, the implosion timing relative to the start of the current pulse on COBRA is compared to that which is predicted by the ablation/implosion model developed for wire array experiments on MAGPIE, a pulsed power generator that has a similar peak current to that of CORBRA, but with a longer, 240-ns rise time.