Micropinch Formation Research Articles

Micropinch formation dynamics in X pinches.

High temporal resolution x-ray streak camera studies of micropinch formation in Cu hybrid xpinches reveal key plasma conditions. Analysis of Ne-like Cu lines indicate an average electron temperature of about 200eV and 4.5×10^{28}m^{-3} electron density. The spectra suggest that the electron temperature jumps to about 1 keV, inferred from the continuum and the postcontinuum line emission that includes Li-like Cu lines. There is no sign of a rapid temperature change or a substantial surge in radiation emission during the 200 ps precontinuum x-ray burst, suggesting that the radiative collapse process does not play a major role in micropinch formation. Two-dimensional extended Magnetohydrodynamic (MHD) simulations, coupled to a collisional-radiative spectral analysis code, suggest the significance of the rapid radial implosion of high-temperature, low-density plasma, the axial outflow, and the dynamic plasma pressure in micropinch formation.

Micro-pinch formation and extreme ultraviolet emission of laser-induced discharge plasma* *Project supported by the Basic and Applied Basic Research Major Program of Guangdong Province, China (Grant No. 2019B030302003).

Extreme ultraviolet (EUV) source produced by laser-induced discharge plasma (LDP) is a potential technical means in inspection and metrology. A pulsed Nd:YAG laser is focused on a tin plate to produce an initial plasma thereby triggering a discharge between high-voltage electrodes in a vacuum system. The process of micro-pinch formation during the current rising is recorded by a time-resolved intensified charge couple device camera. The evolution of electron temperature and density of LDP are obtained by optical emission spectrometry. An extreme ultraviolet spectrometer is built up to investigate the EUV spectrum of Sn LDP at 13.5 nm. The laser and discharge parameters such as laser energy, voltage, gap distance, and anode shape can influence the EUV emission.

Investigation of wire-array Z-pinches by laser probing diagnostics

Laser diagnostics provides powerful tools for the investigation of dense Z-pinches. In this paper, wire-array Z-pinches are investigated at the 1 MA Zebra generator using laser diagnostics at different wavelengths coupled with x-ray diagnostics. Plasma dynamics during the ablation, implosion, and stagnation stages are observed by multiframe diagnostics. Cascading and nonprecursor implosions are studied in wire arrays. Ultraviolet diagnostics allows deep penetration into the Z-pinch plasma at stagnation. End-on probing reveals the complicated structure of the precursor. Strong magnetohydrodynamic instabilities are found in a dense pinch hidden in the trailing plasma. Small-scale instabilities are seen in the Z-pinch plasma with micrometer resolution. Probing of the pinch from four directions shows asymmetrical trailing plasma in some configurations of wire arrays. Faraday rotation diagnostics reveals the magnetic fields and the current distribution in the plasma of the precursor and Z-pinch. Redistribution of current in the trailing plasma is seen during kink and sausage instabilities in the stagnation stage. The formation of micropinches and hot spots in the Z-pinch is analyzed with coupled laser and x-ray diagnostics. Different laser diagnostics allow the study of Z-pinch plasmas in all stages, including fast dynamics and instabilities.

On the Features of X-Ray Emission of a Laser-Plasma Diode

The formation of micropinches and X-ray sources in low-voltage, laser-induced discharges with high rates of current rise is investigated.

Observation of micropinch formation in cathode jet of a low-power laser-induced vacuum discharge

The report presents the results from experimental investigation of micropinch formation in the plasma of a vacuum discharge induced by a 6 ns laser pulse of energy J = 0.5–200 mJ (at a storage voltage from 4 to 15 kV and the discharge current range of 6–26 kA, respectively). The discharge gap images were obtained using a pinhole camera in the EUV and soft X-ray ranges of 15–73 eV and 80–284 eV energy. It is shown that micropinch formation in the plasma cathode jet occurs, mainly, in the matter evaporated by the laser pulse at the discharge ignition near the moment when the current derivative reaches the maximum. It is found that the cathode jet may consist of several pinched areas, and each of them has its own structure, and the improvement of the discharge and laser radiation parameters allows us to reach a stable single pinching of plasma. The parameters of the micropinch (the plasma compression ratio, size, and position of the emitting area in the interelectrode gap) as well as the current flow through the interelectrode gap, at the given storage voltage, are completely governed by the laser radiation characteristics.

Formation of a cathode plasma jet in a laser-induced vacuum discharge

The process of formation of micropinch structures in the plasma of a vacuum discharge initiated by a laser pulse on the cathode is studied. It is demonstrated that the micropinch formation takes place mainly in the substance evaporated by the laser pulse upon the discharge initiation. The size of the hot region of the plasma jet and the distance from the cathode to the micropinch increase with increasing laser pulse energy.

Implosion dynamics and radiation characteristics of wire-array Z pinches on the Cornell Beam Research Accelerator

Experimental results are presented that characterize the implosion dynamics and radiation output of wire-array Z pinches on the 1-MA, 100-ns rise-time Cornell Beam Research Accelerator (COBRA) [J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]. The load geometries investigated include 20-mm-tall cylindrical arrays ranging from 4to16mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten, aluminum, or Invar (64% iron, 36% nickel). Diagnostics fielded include an optical streak camera, a time-gated extreme-ultraviolet framing camera, a laser shadowgraph system, time-integrated pinhole cameras, an x-ray wide-band focusing spectrograph with spatial resolution, an x-ray streak camera, a load voltage monitor, a Faraday cup, a bolometer, silicon diodes, and diamond photoconducting detectors. The data produced by the entire suite of diagnostics are analyzed and presented to provide a detailed picture of the overall implosion process and resulting radiation output on COBRA. The highest x-ray peak powers (300–500GW) and total energy yields (6–10kJ) were obtained using 4-mm-diameter arrays that stagnated before peak current. Additional findings include a decrease in soft x-ray radiation prior to stagnation as the initial wire spacing was changed from 1.6mmto785μm, and a timing correlation between the onset of energetic electrons, hard x-ray generation, and the arrival of trailing current on axis—a correlation that is likely due to the formation of micropinches. The details of these and other findings are presented and discussed.

Flow of multiple charged accelerated metal ions from low-inductance vacuum spark

Results of studies of the short-run beams of multiple charged fast ions that have been found earlier by the authors in a low voltage vacuum spark are presented. The ion emission was due to the formation of micropinches in the cathode plasma jet by the action of the self-magnetic field. A relation between the average velocity of the fast ions and that of the bulk of the ions of the cathode jet was obtained over a wide range of the discharge current amplitudes. The total yield of the multiple charged fast ions per pulse Nf was evaluated from the direct collector measurements with regard to a decrease in ion flow due to several reasons. This value was in satisfactory agreement with evaluation that was obtained from the ballistic pendulum measurements and gave Nf ≈ 5 × 1013–1014 ions per pulse at the average ion charge state of +9 at the maximum of the discharge current Id = 12 kA. Evaluation of current density for these ions gave jf ≈ 3 mA cm−2 at a distance of about 1 m from the anode.

Micropinches in plasma flame expanding into vacuum ambient

Results of experimental and theoretical studies of plasma parameters in a current-carrying plasma flame expanding into the vacuum ambient are presented. It is shown that at the initial stage of the discharge burning the short-run beams of multiply charged ions of the cathode material (with mean ion charge state up +9 for Cu) were emitted from the cathode plasma flame. The X-ray pinhole measurements showed that a small-size area of hot electron with a temperature of 100 ÷ 200 eV was formed at a range of about 2÷3 mm from the cathode. A theoretical model is developed of the process of the current-carrying cathode plasma flame expansion. A set of hydrodynamic equations was solved numerically by the particle-in-cell method. It was shown that due to the Ampere forces a neck was formed at a distance from the cathode that was developed to a micropinch, where electron temperature attained of about 100 eV and the beams of multiply charged ions of the cathode material were produced. Both the temporal and space parameters of the micropinch formation are consistent with results of the measurements. The mean ion charge state of the ions also is in agreement with the experimental data.

Micropinch actuation in the SPEED 2 plasma focus

The occurrence of hot (kilo-electron-volts), dense () and short-lived (nanoseconds) substructures in pinch plasmas (micropinches) is usually very erratic in space and time. On doping the deuterium discharge of the fast plasma focus SPEED 2 (180 kV, 1.5 MA, 400 ns) with argon, typically ten micropinches appear in an orderly manner along the axis of symmetry forming a straight chain of successive flashes within 30 ns that starts near the anode and ends after 2 cm. Four frame pictures taken in the VUV ( nm) at different times during pinch formation and careful comparison with stronger filtered x-ray pinhole pictures ( nm) show that micropinches are actuated in necked pinch regions that preferentially undergo radiative collapse via bremsstrahlung and line radiation from argon. However, not every necking leads to micropinch formation so that necking is a necessary but not sufficient condition for micropinch actuation.

Observations of plasma dynamics in the vacuum spark

Experimental observations are presented of a vacuum spark driven by a low impedance pulse forming line delivering 100 kA to the load. A pulsed laser is used to form a preionizing plasma on the cathode. The combination of axial and radial optical streak camera observations, together with the time and space resolved soft x-ray emission, permit the evolution of the plasma dynamics, density and temperature to be measured. Three kinds of behavior are observed according to axial position. A close correlation is found between the x-ray and the optical emission, with the observation of micropinch formation. A discussion is presented in which the behavior of the vacuum spark under differing operating conditions is compared.

On stream inhomogeneities in a pulsed electromagnetic accelerator

In a number of papers [1–7] carried out on various pulsed electromagnetic accelerators the appearance of a lamellar structure in the stream of accelerated ionized gas occurs having an oscillation frequency of several megahertz. The majority of authors relate the development of irregularities in the stream with local peculiarities in the flow of electric currents (secondary breakdowns [6], the formation of micropinches [3–5], the motion of a current spiral [2], etc.). Notwithstanding the generality of many properties of the irregularities recorded under various conditions, no unified point of view exists as yet concerning the nature of their development.