Dense Z-pinch Research Articles

K-shell radiation physics in low- to moderate-atomic-number z-pinch plasmas on the Z accelerator

Dense z-pinches produced by 100 ns implosions of wire arrays or gas puffs produce substantial soft X-ray power. One class of z-pinch radiation sources includes low- to moderate-atomic-number K-shell radiators, such as aluminum and iron. These loads are designed for 1–10 keV K-shell X-ray generation, and offer opportunities for crystal spectroscopy that can reveal fundamental properties of the plasma when studied using plasma spectroscopic modeling. Typically these plasmas are characterized by ion densities of ∼ 10 20 cm - 3 , diameters of 1–5 mm, electron temperatures up to several keV, and a range of opacities of the K-shell lines. Measurements from wire arrays on Sandia's 20 MA Z accelerator are presented along with collisional radiative and hydrodynamic simulations. The impact of opacity and 3D structure on non-LTE, non-diffusive radiation transport and X-ray production is discussed.

Comparative analysis of time-resolved and time-integrated x-ray data from long pulse Z-pinch implosions on Saturn

Two series of Al:Mg wire array Z-pinch implosions were performed on the Saturn pulsed-power generator [R. B. Spielman et al., in Proceedings of the 2nd International Conference on Dense Z-Pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3]. In one series, the wire number was varied, in the other, the load mass was varied. In all cases an initial array diameter of 40 mm was maintained. Sufficient x-ray data were obtained in these experiments to allow the plasma conditions to be inferred by matching x-ray data to that calculated by a collisional-radiative equilibrium (CRE) model. In fitting the data, the plasma has been assumed to be either uniform [zero dimension (0D)] or to vary one-dimensionally (1D) with radius. On one shot, a comparison of these two methods of data analysis performed with both time-resolved and time-integrated data has been carried out. While the same trends in plasma conditions as a function of load mass and wire number are seen, the uniform plasma assumption yields higher electron temperatures, lower core densities, but higher K-shell radiating mass fractions than those obtained from the 1D assumption. When comparing time-resolved and time-integrated spectroscopic data, a pronounced difference is seen in a density-sensitive line ratio whose time-resolved values corroborate experimental evidence that much of the K-shell radiation is emitted from a high density core plasma of spatial extent less than about 300 μm.

High-resolution X-ray imaging spectroscopy diagnostic of hollow ions in dense plasmas

The formation of hollow ions in dense Z-pinch and laser produced plasmas is investigated by means of high-resolution X-ray spectroscopy and atomic data calculations. Dense plasma effects are found to result in highly populated excited states which open up excitation channels not accessible in low-density plasma sources. This makes fs-laser radiation interacting with matter an important tool for modern atomic physics investigations.

Steady State of Dense Sheet Z-Pinch with Thermal Conduction

The problem of steady state of the sheet Z-pinch was solved, incorporating the thermal conductivity effects. In particular, solutions which incorporate low plasma pressure and low temperature on the sheet boundary are investigated. Three types of temperature profiles were identified. Dependence of the solution type on the pinch plasma parameters was determined.

Study of hydrodynamic instabilities of a Z-pinch during a high-current explosion of a thin wire

Results are presented from laboratory and numerical experiments on the influence of the core and associated hydrodynamic instabilities on the high-current implosion of a plasma of exploding metal wires. The experimental investigation of the discharge structure was carried out using the multiframe X-ray backlighting technique with high temporal and spatial resolution (<1 ns and 1 µm, respectively); X-pinches were used as small-sized radiation sources. The implosion of a dense Z-pinch was modeled by the free-point method with the use of a two-dimensional radiative MHD code. The onset of instabilities at the corona-core boundary was modeled by the NUTCY Eulerian code. The results show that hydrodynamic processes in the core are primarily responsible for the formation of small bright regions observed in X-rays. After the reflection of a shock wave from the axis, the rapid onset of hydrodynamic instabilities can occur at the corona-core boundary.

Propulsion application of non-neutronic fusion chain reactions in Petawatt laser ignited shear flow stabilized dense plasma z-pinches

Shear flow stabilized z-pinches promise the attainment of very high densities. At these high densities non-neutronic fusion chain reactions, ideally suited for propulsion, can be ignited with petawatt lasers. To drive the dense pinch discharges large capacitors with short discharge times are required, favoring high voltages. To reduce the weight of the capacitors, and at the same time to have high voltages, use can be made of the high vacuum of space which in combination with magnetic insulation has the potential for gigajoule, gigavolt capacitors.

KeV Ion Beam Generation from Z-Pinches Produced in Plasma Focus like Geometry

Dense Z-pinches produce strong soft X-ray and VUV emission with ns exposure when working with moderate current at the moment of maximum compression. Within these Z-pinches the Plasma focus geometry offers an efficient production of hot and dense plasmas (kT e more than 100 eV respectively N ion greater than 10 18 cm -3 ) in which also fast thermal ions are created. In order to design such a plasma focus of 1 m 3 laboratory size, which is equivalent to deliver a pinch current of around 200 kA, and a micro plasma focus, which gives 20 kA using a cm 3 scaled device, are investigated. Different direct and indirect diagnostic methods are presented in order to work out the required conditions for efficient ion beam production.

Laser Ignition of an Isentropically Compressed Dense Z-Pinch

A dense z-pinch generated by a high voltage discharge over a corrugated helical sawtooth-shaped capillary tube with a solid DT core, is by shear flow stabilized against the m = 0 and m= 1 magnetohydrodynamic instabilities, and by rotational flow against the Rayleigh-Taylor instability. The shear-and rotational flow result from jet formation by the corrugated surface. A programmed voltage pulse can then isentropically compress the DT core to high densities, and if ignited at one end by a petawatt laser pulse, a thermonuclear detonation wave can be launched propagating along the z-pinch channel. The proposed z-pinch burn should also work without tritium as a thermonuclear detonation wave in deuterium.

Pulse Power Compression by Cutting a Dense Z-Pinch with a Laser Beam

Abstract A thin cut made through a z-pinch by an intense laser beam can become a magnetically insulated diode crossed by an intense ion beam. For larger cuts, the gap is crossed by an intense relativistic electron beam, stopped by magnetic bremsstrahlung resulting in a pointlike intense x-ray source. In either case, the impedance of the pinch discharge is increased, with the power delivered rising in the same pro-portion. A magnetically insulated cut is advantageous for three reasons: First, with the ion current com-parable to the Alfvèn ion current, the pinch instabilities are reduced. Second, with the energy deposit-ed into fast ions, a non-Maxwellian velocity distribution is established increasing〈σ ν〉 value for nuclear fusion reactions taking place in the pinch discharge. Third, in a high density z-pinch plasma, the intense ion beam can launch a thermonuclear detonation wave propagating along the pinch discharge channel. For larger cuts the soft x-rays produced by magnetic bremsstrahlung can be used to drive a thermonuclear hohlraum target. Finally, the proposed pulse power compression scheme permits to use a cheap low power d.c. source charging a magnetic storage coil delivering the magnetically stored energy to the pinch discharge load by an exploding wire opening switch.

Numerical Investigation of the Stationary State of Dense Sheet Z-Pinches

Numerical Investigation of the Stationary State of Dense Sheet Z-Pinches

Propagating Thermonuclear Burn by Laser Ignition of a Dense Z-Pinch

Abstract A linear pinch discharge above the Pease-Braginskii current and stabilized by axial shear flow can radiatively collapse to high densities. A thermonuclear detonation wave can then be launched from one end of the discharge channel by ignition with a powerful laser pulse. Axial shear flow stabilization may be realized by injecting a fast moving jet along the pinch discharge channel, possibly in combination with a frozen DT fiber positioned on the pinch axis.

Characterization of energy flow and instability development in two-dimensional simulations of hollow z pinches

A two-dimensional (2-D) Eulerian Radiation-Magnetohydrodynamic (RMHD) code has been used to simulate imploding z pinches for three experiments fielded on the Los Alamos Pegasus II capacitor bank [J. C. Cochrane et al., Dense Z-Pinches, Third International Conference, London, United Kingdom 1993 (American Institute of Physics, New York, 1994), p. 381] and the Sandia Saturn accelerator [R. B. Spielman et al., Dense Z-Pinches, Second International Conference, Laguna Beach, 1989 (American Institute of Physics, New York, 1989), p. 3] and Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)]. These simulations match the experimental results closely and illustrate how the code results may be used to track the flow of energy in the simulation and account for the amount of total radiated energy. The differences between the calculated radiated energy and power in 2-D simulations and those from zero-dimensional (0-D) and one-dimensional (1-D) Lagrangian simulations (which typically underpredict the total radiated energy and overpredict power) are due to the radially extended nature of the plasma shell, an effect which arises from the presence of magnetically driven Rayleigh–Taylor instabilities. The magnetic Rayleigh–Taylor instabilities differ substantially from hydrodynamically driven instabilities and typical measures of instability development such as e-folding times and mixing layer thickness are inapplicable or of limited value. A new measure of global instability development is introduced, tied to the imploding plasma mass, termed “fractional involved mass.” Examples of this quantity are shown for the three experiments along with a discussion of the applicability of this measure.

Accelerator plasma-target-based fusion neutron source

The University of Illinois inertial electrostatic confinement (IEC) device provides 10 7 2.5-MeV D–D neutrons per second (n s −1), when operated with a steady-state deuterium discharge at 70 kV (G.H. Miley et al., Inertial electrostatic confinement neutron/proton source, in: M. Haines, A. Knight (Eds.), 3rd Int. Conf. Dense Z-pinches, AIP Conf. Proc. 299, AIP Press, New York, 1994, pp. 675–689). Being compact and lightweight, the IEC potentially represents an attractive portable neutron source for activation analysis applications (R.A. Anderl et al., Development of an IEC neutron source for NDE, 16th IEEE/NPSS Symp. Fusion Engineering, IEEE, Piscataway, NJ, 1996, pp. 1482–1485). The plasma discharge in the IEC is unique, using a spherical grid in a spherical vacuum vessel with the discharge formed between the grid and the vessel wall, while the cathode grid also serves to extract high-energy ions. Two key features of the IEC discharge physics are discussed: (1) the formation of ion `microchannels' that carry the main ion flow through grid openings; and (2) the potential well structure formed in the dense central core.

Observations of vacuum spark dynamics from its X-ray emission

The behavior of a medium energy (/spl sim/1 kJ) pulse-power-driven vacuum spark is shown to depend on the electrode material and form of the anode in otherwise similar conditions of operation. The dynamical evolution of the discharge is followed from its soft X-ray emission. The electrode materials compared are titanium and aluminum with a form of anode that is tubular or conical. The use of a tubular anode favors a more uniform sheath and a better formation of a dense Z-pinch and the ensuing hot spots are found to be at least twice as bright. The position of the brightest spots differs according to the material and the electrode shape, and is found to coincide with the shape of the sheath. The energy output is measured and the density of the plasma in the hot spots may be calculated.

Space and time resolved electron density and current measurements in a dense plasma focus Z-pinch

Plasma density and current profiles in a Z-pinch are important parameters to understand the implosion and radiation physics. This paper describes measurements of electron density and current at radii of /spl ges/200 /spl mu/m from the axis of a dense plasma focus (DPF) pinch plasma that is imploded by a /spl ap/0.3 MA current pulse. These measurements use laser interferometry and polarimetry. The electromagnetic wave propagating through a current carrying plasma will change its phase, polarization state, and propagation direction. Refraction by electrons bends the wave fronts and changes the propagation direction; Faraday rotation due to the magnetic field and electron density rotates the laser polarization vector. By measuring these quantities simultaneously, the magnetic field and electron density can be separately determined. Although the DPF used here is a low current device, the measured densities (/spl les/10/sup 20/ cm/sup -3/) and magnetic fields (/spl sim/100 T) are similar to values expected just outside higher current but larger radius Z-pinches, so this technique should be applicable there as well. The techniques described here do not require access to the core of the pinch to work; just outside these pinches the coronal density and self-magnetic field are high enough to give reliable data but not so high as to make the measurements difficult.

Nitrogen laser system for diagnosing z-pinch and x-pinch plasmas

A nitrogen laser system built to probe dense z-pinch and x-pinch plasmas is described. This low-cost system consists of an atmospheric pressure oscillator and a low pressure amplifier. It generates a 1-cm-diam 3371 Å laser beam with approximately 0.33 mJ in a ≲1 ns pulse, and with a coherence length of at least 5 cm. This laser has been used in several modes of operation, such as for simultaneous schlieren and interferometry imaging of dense z-pinch and x-pinch plasmas. Example results are presented.

Inertial confinement fusion ablator physics experiments on Saturn and Nova

The Saturn pulsed power accelerator [R. B. Spielman et al., in Proceedings of the 2nd International Conference on Dense Z-pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3] at Sandia National Laboratories (SNL) and the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] at Lawrence Livermore National Laboratory (LLNL) have been used to explore techniques for studying the behavior of ablator material in x-ray radiation environments comparable in magnitude, spectrum, and duration to those that would be experienced in National Ignition Facility (NIF) hohlraums [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)]. The large x-ray outputs available from the Saturn pulsed-power-driven z pinch have enabled us to drive hohlraums of full NIF ignition scale size at radiation temperatures and time scales comparable to those required for the low-power foot pulse of an ignition capsule. The high-intensity drives available in the Nova laser have allowed us to study capsule ablator physics in smaller-scale hohlraums at radiation temperatures and time scales relevant to the peak power pulse for an ignition capsule. Taken together, these experiments have pointed the way to possible techniques for testing radiation-hydrodynamics code predictions of radiation flow, opacity, equation of state, and ablator shock velocity over the range of radiation environments that will be encountered in a NIF hohlraum.

A Monte-Carlo technique for the reconstruction of time dependent spectra of short-pulse neutron sources

Abstract A method is presented which yields time-resolved energy spectra of neutrons emitted in intense bursts by an extension of time of flight spectroscopy. The experiment consists of recording the signals of a number of detectors placed at various optimized distances from the source. The evaluation is based on a new Monte-Carlo reconstruction method which is also a powerful tool in designing and optimizing the experiment. Extensive numerical tests which simulate the application of the method for determining the time-resolved energy spectrum of fusion neutrons emitted by a plasma focus generator (pulse duration 20–500 ns, energy domain centered at 2.5 MeV and energy widths of 100–1000 keV) are presented. Time resolution better than 10 ns and energy resolution of about 50 keV is possible. It is also shown that in the case of dense Z-pinch experiments characterized by shorter emission times (20–40 ns) the method still works for flight bases not longer than a few meters. The application of the Monte-Carlo reconstruction method to an experimental data set collected at the high voltage plasma focus SPEED 2, operated at 180 kV bank voltage, 1.5 MA pinch current and 65 kJ capacitor energy, is illustrated.

Instability heating of a solid fiber Z-pinch

A dense Z-pinch formed by the electrical breakdown of solid CD2 fibers in an 800 kA, 100 ns risetime pulse generator has been studied with optical and radiation diagnostics. It has been found that, contrary to calculations based on classical joule heating of the plasma that predict approximate dynamic equilibrium, the pinch always expands explosively while displaying intense m=0 hydromagnetic instability activity. Excellent agreement with the observed expansion rate as well as with measured electron temperatures and neutron yield has been obtained by including in a simulation code the direct heating of ions by turbulence arising from instability growth.

Nernst and Ettinghausen effects in the dense Z-pinch: their impact upon equilibria and runaway electrons

The effects of the Nerst term in Ohm's law and Ettinghausen heat flow are examined for the dense Z-pinch. The Ettinghausen term causes radial electron heat flow in the direction of j V-product B whereas the Nernst term redistributes the axial current density according to the radial electron temperature gradient. The effects of these two terms on Z-pinch equilibria are the eradication of both the surface peak in current and the associated surface maximum in electron temperature. In dense Z-pinches formed from solid fibres in vacuum, the plasma density must fall to zero at the edge. This means that near the edge the equilibration timescale exceeds the ohmic heating timescale and the electron and ion temperatures become decoupled. Two temperature simulations of such pinches show the electron temperature apparently becoming extremely large near the pinch surface. However, inclusion of the Ettinghausen and Nernst terms eradicates this problem. Therefore these terms are important in determining whether the threshold conditions for the onset of micro-instabilities are exceeded in the surface plasma of fibre Z-pinches. The Nernst and Ettinghausen effects arise from the velocity dependence of the collision frequency. The electrons in the energetic tail of the distribution function flow preferentially radially inwards due to an Ez/Btheta drift and, upon reaching the axis, become demagnetized and are free to run away with singular orbits about the local field null. The generation of electron beams by this mechanism will be particularly prevalent if runaway is assisted by a very low density on axis such as an in gas-puff pinches.