Pinch Implosion Research Articles

Scenario of a magnetic dynamo and magnetic reconnection in a plasma focus discharge

The paper discusses a possible energy transformation that leads to the acceleration of fast ions and electrons. In plasma-focus discharges that occur during deuterium filling, which have a maximum current of about 1 MA, the accelerated deuterons produce fast fusion neutrons and fast electrons hard X-ray emissions. Their total energy, which is of the order of several kilojoules, can be delivered by the discharge through a magnetic dynamo and self-organization to the ordered plasma structures that are formed in a pinch during the several hundreds of nanoseconds of the pinch implosion, stagnation, and evolution of instabilities. This energy is finally released during the decay of the ordered plasma structures in the volume between the anode face and the umbrella front of the plasma and current sheath in the form of induced electric fields that accelerate fast electrons and ions.

Hybrid MHD/PIC simulation of a metallic gas-puff z pinch implosion

We present the hybrid MHD/PIC simulations of the time evolution of a bismuth metallic gas-puff z pinch in external axial magnetic field (AMF). Recent experiments with IMRI-5 generator (450 kA, 450 ns) [1] show the certain effect of an axial magnetic field on radiation energy produced during z pinch implosion. In order to perform the numerical simulation of gas puff z pinch a hybrid model was developed. The hybrid model treats the electrons as a massless fluid and ions as macroparticles. The macroparticle dynamic is calculated with the use of PIC method. Ion-ion Coulomb collision is considered with the use of MC method. The radiation transfer of bismuth plasma was accounted in the framework of P1 method. The interelectrode gap pumping by plasma of 8 μs 80 kA bismuth arc with the following plasma implosion by 450 kA / 450 ns current pulse in different external AMF was modelled. The obtained results are in a reasonable agreement with the experimental results.

High-Quality Implosion of Overmassed <inline-formula> <tex-math notation="LaTeX">$Z$ </tex-math> </inline-formula>-Pinch in the Experiment With Magnetocumulative Generator

This paper presents the results of the experiment with an overmassed wire-array $Z$ -pinch load that is powered by a current pulse (amplitude ~4.3 MA and rise time $\sim 0.9~\mu \text{s}$ ) from a multidisk magnetocumulative generator. Interesting feature of the experimentally recorded soft X-ray (SXR) pulse is short (~10 ns) duration at a microsecond current rise time with total energy of several hundreds of kilojoules. Obtained results are the evidence of high-quality implosion of the current-carrying plasma. The 2-D magnetohydrodynamic code FLUX-rz was used for the numerical simulation and analysis of the obtained results. The calculations indicated that only ~30% of the initial load mass took part in the SXR generation. The main peculiarities of the overmassed $Z$ -pinch implosion are discussed in this paper.

Energy balance in a Z pinch with suppressed Rayleigh–Taylor instability

At present Z-pinch has evolved into a powerful plasma source of soft x-ray. This paper considers the energy balance in a radiating metallic gas-puff Z pinch. In this type of Z pinch, a power-law density distribution is realized, promoting suppression of Rayleigh–Taylor (RT) instabilities that occur in the pinch plasma during compression. The energy coupled into the pinch plasma, is determined as the difference between the total energy delivered to the load from the generator and the magnetic energy of the load inductance. A calibrated voltage divider and a Rogowski coil were used to determine the coupled energy and the load inductance. Time-gated optical imaging of the pinch plasma showed its stable compression up to the stagnation phase. The pinch implosion was simulated using a 1D two-temperature radiative magnetohydrodynamic code. Comparison of the experimental and simulation results has shown that the simulation adequately describes the pinch dynamics for conditions in which RT instability is suppressed. It has been found that the proportion of the Ohmic heating in the energy balance of a Z pinch with suppressed RT instability is determined by Spitzer resistance and makes no more than ten percent.

Coulomb explosion of “hot spot”

The study presented in this paper has shown that the generation of hard x rays and high-energy ions, which are detected in pinch implosion experiments, may be associated with the Coulomb explosion of the hot spot that is formed due to the outflow of the material from the pinch cross point. During the process of material outflow, the temperature of the hot spot plasma increases, and conditions arise for the plasma electrons to become continuously accelerated. The runaway of electrons from the hot spot region results in the buildup of positive space charge in this region followed by a Coulomb explosion. The conditions for the hot spot plasma electrons to become continuously accelerated have been revealed, and the estimates have been obtained for the kinetic energy of the ions generated by the Coulomb explosion.

Effect of the axial magnetic field on a metallic gas-puff pinch implosion

The effect of an axial magnetic field Bz on an imploding metallic gas-puff Z-pinch was studied using 2D time-gated visible self-emission imaging. Experiments were performed on the IMRI-5 generator (450 kA, 450 ns). The ambient field Bz was varied from 0.15 to 1.35 T. It was found that the initial density profile of a metallic gas-puff Z-pinch can be approximated by a power law. Time-gated images showed that the magneto-Rayleigh–Taylor instabilities were suppressed during the run-in phase both without axial magnetic field and with axial magnetic field. Helical instability structures were detected during the stagnation phase for Bz < 1.1 T. For Bz = 1.35 T, the pinch plasma boundary was observed to be stable in both run-in and stagnation phases. When a magnetic field of 0.3 T was applied to the pinch, the soft x-ray energy was about twice that generated without axial magnetic field, mostly due to longer dwell time at stagnation.

Influence of asynchronous actuation of the Angara-5-1 facility modules on the penetration of the magnetic field inside a wire array

The influence of asynchronous actuation of the Angara-5-1 facility modules on the implosion symmetry of the wire array plasma in the Z-pinch mode is studied. It is shown that an increase in the r.m.s. jitter in the start times of the modules leads to an appreciable azimuthal asymmetry of magnetic field penetration inside the wire array and, as a consequence, to an increase in the duration of the soft X-ray pulse, a decrease in its power, and a shift of the Z-pinch with respect to the array axis. Necessary conditions for axisymmetric pinch implosion are determined. Experimental data on the magnitude and azimuthal distribution of the current (magnetic field) inside the wire array were obtained from magnetic probe measurements. The position of the Z-pinch with respect to the wire array axis was determined from two-dimensional X-ray images and radial optical streak images.

Electromagnetic pulse emission produced by Z pinch implosions

In this paper, we represent the radiation characteristics of electromagnetic pulse generated by Z pinch implosion. Magnetic energy which couples with motions of metallic wire arrays or solid liners driven by Z pinch can radiate away. Theoretical results indicate that the radiation power of electromagnetic pulse is determined by both load current and implosion trace. Experiments are carried on primary test stand facility at Institute of Fluid Physics where a current rising to 7 MA in (10%–90%) 65 ns is used to drive a wire array Z pinch. The measured load current and implosion trace show that the Z pinch can deliver about 1 GW, 10 ns full width, 20–70 MHz central frequency, broadband electromagnetic pulse with an energy conversion efficiency of 10-7. Parameters of electromagnetic pulse are much smaller than those of X-ray with a power of 50 TW and an energy of 0.5 MJ. In the approximation of weak relativistic case, the power of electromagnetic pulse which is proportional to sixth power of load current, dramatically increases with current increasing. Soft X-ray radiation is an important channel for dissipating a considerable fraction of energy provided by facility. The results presented here demonstrate that electromagnetic pulse emission in the case of higher load current can cause significant damage to diagnostic devices. Moreover, intense electromagnetic pulse produced by this method may have many potential applications.

Matter–Antimatter Relativistic Ambiplasma Pinch

An ambiplasma is a plasma made from matter and antimatter. We study here the property of such plasma at relativistic energy, which is made of counterstreaming electron and positron beams, and its extension to a hydrogen-antihydrogen ambiplasma pinch configuration, where the pinch current is made up from counterstreaming electrons and positrons at relativistic energy. Being very much as intense as relativistic electron beams, which are largely immune against the classical pinch instabilities, the same can hopefully be expected for the counterstreaming relativistic electron and positron beams. By radiation cooling, such electron-positron ambiplasma can collapse to a very small diameter, which, in the limit, is determined by Heisenberg's uncertainty principle, reaching enormous densities. Because the counterstreaming electrons and positrons move at relativistic energy, their cross section for mutual annihilation is substantially reduced, making the lifetime of this ambiplasma large in comparison with the pinch implosion time. In a similar way, one can obtain a proton-antiproton ambiplasma pinch with two counterstreaming intense multigigaelectronvolt proton and antiproton beams. To overcome the technical problem to create such beams, as an easier way, one may consider a hydrogen-antihydrogen ambiplasma pinch where only the electrons and positrons carry the current. The current is produced by electron-positron runaway through the application of a high voltage pulse from a super Marx generator.

Architecture of petawatt-classz-pinch accelerators

We have developed an accelerator architecture that can serve as the basis of the design of petawatt-class $z$-pinch drivers. The architecture has been applied to the design of two $z$-pinch accelerators, each of which can be contained within a 104-m-diameter cylindrical tank. One accelerator is driven by slow ($\ensuremath{\sim}1\text{ }\ensuremath{\mu}\mathrm{s}$) Marx generators, which are a mature technology but which necessitate significant pulse compression to achieve the short pulses ($\ensuremath{\ll}1\text{ }\ensuremath{\mu}\mathrm{s}$) required to drive $z$ pinches. The other is powered by linear transformer drivers (LTDs), which are less mature but produce much shorter pulses than conventional Marxes. Consequently, an LTD-driven accelerator promises to be (at a given pinch current and implosion time) more efficient and reliable. The Marx-driven accelerator produces a peak electrical power of 500 TW and includes the following components: (i) 300 Marx generators that comprise a total of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{4}$ capacitors, store 98 MJ, and erect to 5 MV; (ii) 600 water-dielectric triplate intermediate-store transmission lines, which also serve as pulse-forming lines; (iii) 600 5-MV laser-triggered gas switches; (iv) three monolithic radial-transmission-line impedance transformers, with triplate geometries and exponential impedance profiles; (v) a 6-level 5.5-m-diameter 15-MV vacuum insulator stack; (vi) six magnetically insulated vacuum transmission lines (MITLs); and (vii) a triple-post-hole vacuum convolute that adds the output currents of the six MITLs, and delivers the combined current to a $z$-pinch load. The accelerator delivers an effective peak current of 52 MA to a 10-mm-length $z$ pinch that implodes in 95 ns, and 57 MA to a pinch that implodes in 120 ns. The LTD-driven accelerator includes monolithic radial transformers and a MITL system similar to those described above, but does not include intermediate-store transmission lines, multimegavolt gas switches, or a laser trigger system. Instead, this accelerator is driven by 210 LTD modules that include a total of $1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ capacitors and $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 200-kV electrically triggered gas switches. The LTD accelerator stores 182 MJ and produces a peak electrical power of 1000 TW. The accelerator delivers an effective peak current of 68 MA to a pinch that implodes in 95 ns, and 75 MA to a pinch that implodes in 120 ns. Conceptually straightforward upgrades to these designs would deliver even higher pinch currents and faster implosions.

Diagnostics on the COBRA pulsed power generator

The COBRA pulsed power generator has a variable current pulse wave form and amplitude (95–180ns rise time, up to 1MA peak current). It was designed to study wire array Z pinches and X pinches, including plasma formation, pinch implosion dynamics, and pinch plasma parameters as a function of current rise time. These loads have been studied using an extensive set of diagnostics with spatial and/or temporal resolution. The set of electrical diagnostics on the COBRA generator includes Rogowski coils to monitor the total load current and the current through individual return current posts, and there is also an inductive voltage monitor. A set of extreme ultraviolet and x-ray detectors is used to study the load radiation. Wire array and X pinch plasma formation and dynamics are studied using two-frame, point projection X-pinch x-ray imaging as well as with multiframe laser probing. Flat potassium acid phtalate crystal (KAP), convex, extreme luminosity imaging conical spectrograph, and focusing spectrograph with spatial resolution with mica crystal, pinhole cameras, and a camera with a slit and a step filter set (slip step-wedge camera) can be used in each pulse to monitor the x-ray emission from the X pinch(es) and arrays in several spectral bands.

Multiwire$X$-Pinches at 1-MA Current on the COBRA Pulsed-Power Generator

The rebuilt COBRA pulsed-power generator, which has a variable current-pulse waveform and amplitude (95-150-ns rise time and ~1-MA peak current), has extended the range of the current-pulse parameters that can be used to study the X-pinches. X-pinches with 4-12 wires of several different wire materials (from Al to W) with diameters from 25 to 75 mum have been studied. The influence of the rate of the rise of current and the X-pinch wire mass on the X-pinch plasma formation and pinch implosion dynamics has been studied using a set of diagnostics with spatial and/or temporal resolution. Multiwire X-pinches were placed in the diode center, and/or two four-wire X-pinches were placed in one of the four parallel return-current circuits of the diode. The experiments showed that the most important factor determining the first X-ray burst timing is the linear mass density of the X-pinch wires, and an optimal value for obtaining a single high-energy X-ray burst was found. Radiographic images of the different test objects, wires in wire-array Z-pinches, and the X-pinches, themselves, were obtained using a micrometer-scale spatial resolution

Theoreticalz-pinch scaling relations for thermonuclear-fusion experiments

We have developed wire-array z -pinch scaling relations for plasma-physics and inertial-confinement-fusion (ICF) experiments. The relations can be applied to the design of z -pinch accelerators for high-fusion-yield (approximately 0.4 GJ/shot) and inertial-fusion-energy (approximately 3 GJ/shot) research. We find that (delta(a)/delta(RT)) proportional (m/l)1/4 (Rgamma)(-1/2), where delta(a) is the imploding-sheath thickness of a wire-ablation-dominated pinch, delta(RT) is the sheath thickness of a Rayleigh-Taylor-dominated pinch, m is the total wire-array mass, l is the axial length of the array, R is the initial array radius, and gamma is a dimensionless functional of the shape of the current pulse that drives the pinch implosion. When the product Rgamma is held constant the sheath thickness is, at sufficiently large values of m/l, determined primarily by wire ablation. For an ablation-dominated pinch, we estimate that the peak radiated x-ray power P(r) proportional (I/tau(i))(3/2)Rlphigamma, where I is the peak pinch current, tau(i) is the pinch implosion time, and phi is a dimensionless functional of the current-pulse shape. This scaling relation is consistent with experiment when 13 MA < or = I < or = 20 MA, 93 ns < or = tau(i) < or = 169 ns, 10 mm < or = R < or = 20 mm, 10 mm < or = l < or = 20 mm, and 2.0 mg/cm < or = m/l < or = 7.3 mg/cm. Assuming an ablation-dominated pinch and that Rlphigamma is held constant, we find that the x-ray-power efficiency eta(x) congruent to P(r)/P(a) of a coupled pinch-accelerator system is proportional to (tau(i)P(r)(7/9 ))(-1), where P(a) is the peak accelerator power. The pinch current and accelerator power required to achieve a given value of P(r) are proportional to tau(i), and the requisite accelerator energy E(a) is proportional to tau2(i). These results suggest that the performance of an ablation-dominated pinch, and the efficiency of a coupled pinch-accelerator system, can be improved substantially by decreasing the implosion time tau(i). For an accelerator coupled to a double-pinch-driven hohlraum that drives the implosion of an ICF fuel capsule, we find that the accelerator power and energy required to achieve high-yield fusion scale as tau(i)0.36 and tau(i)1.36, respectively. Thus the accelerator requirements decrease as the implosion time is decreased. However, the x-ray-power and thermonuclear-yield efficiencies of such a coupled system increase with tau(i). We also find that increasing the anode-cathode gap of the pinch from 2 to 4 mm increases the requisite values of P(a) and E(a) by as much as a factor of 2.

Grazing-incidence mirror streak camera diagnostic for emission measurements of imploding z pinches on the Sandia Z-machine

A soft x-ray (0.1–1 keV) streak camera using a grazing-incidence mirror has been developed for the Sandia Z facility, a 20 MA, 100 ns rise-time accelerator that can generate &amp;gt;200 TW,2 MJ, x-ray pulses. The streak camera is used to measure with one dimension of spatial resolution the continuous time history of sub-kilo-electron-volts emission from z-pinch and radiation flow experiments. Radiation &amp;gt;1 keV is eliminated by the use of a grazing-incidence mirror and transmission filters. The diagnostic has a magnification of 1.22, a 20 mm field-of-view and a spatial resolution of ∼350 μm. The recording length of the instrument is variable in duration up to a maximum of 33 ns, making it well suited for the typical 100 ns Z pinch implosion times. The spatial resolution can readily be changed to be along either the axial or the radial direction.

Energy spectra measurements of X-ray emission from electron interaction in a dense plasma focus device

Electrons generated during a pinch implosion in a hollow anode Mather-like plasma focus device (PF) are considered as a possible X-ray source via the impinging of those particles on medium and high- Z targets. A usual PF device has been slightly modified to optimise the X-ray production and their measurements by means of a suitable and non-invasive spectrometer. This ensemble allows measurements of X-rays generated booth by electrons turning back to the anode and by target collision of the so-called relativistic electron beam. The spectrum of the emitted photons is evaluated by using a differential absorption based technique. The X-ray spectrometer consists of a stack of LiF dosimeters which act both as detectors and filters to give curves of attenuated intensities. Finally, the energy distribution is calculated from such attenuation curves using an iterative procedure based on spectral algebra formalism.

Feasibility of X-ray interstitial radiosurgery based on plasma focus device

A narrow beam of relativistic electrons (REB) generated during a pinch implosion (shot) in a Mather-like plasma focus (PF) device and impinging on a high Z target is considered as X-ray brachytherapy source. The main feature of the REB consists of a narrow back warded small divergence beam with an initial cross section of few mm. The apparatus includes a PF device of about 20 cm diameter and 30 cm length. Attached to the housing that contains the inner anode in the PF, an electron pipe is at first realised by several collimations which define a target spot of about 5 mm when the target is posed at 20 cm far from the pinch region. With the actual configuration, this means a practical length of the X-ray probe of about 10 cm. First experimental results with a tungsten target show that the energy spectra of the photons consist mainly of characteristic L-X rays of mean energy around 10 keV superimposed to a continuous low intensity Bremsstrahlung which photon energy extend up to several tens of keV. A selective K shell ionisation assessment performed by using target from Fe to Cd indicates a mean energy of the REB in the range 35–40 keV. The dose rate delivered in a LiF dosimeter at 10 mm far from the X-ray source has been estimated around 4.5 Gy/shot. The obtained dose rate greater than 10 8 Gy/s and the relatively high repetition rate of the PF devices makes this proposal suitable for interstitial radiosurgery.

Quantum-mechanical calculation of Stark widths of Ne VIIn=3, Δn=0transitions

The Stark widths of the Ne VII $2s3s\ensuremath{-}2s3p$ singlet and triplet lines are calculated in the impact approximation using quantum-mechanical convergent close-coupling and Coulomb-Born-exchange approximations. It is shown that the contribution from inelastic collisions to the linewidths exceeds the elastic width contribution by about an order of magnitude. Comparison with the linewidths measured in a hot dense plasma of a gas-liner pinch indicates a significant difference which may be naturally explained by nonthermal Doppler effects from persistent implosion velocities or turbulence developed during the pinch implosion. Contributions to the linewidth from different partial waves and types of interactions are discussed as well.

Analyzing time-resolved spectroscopic data from an azimuthally symmetric, aluminum-wire array,z-pinch implosion

A 90-wire, aluminum, $z$-pinch experiment was conducted on the Saturn accelerator at the Sandia National Laboratories that exhibited azimuthally symmetric implosions and two x-ray bursts, a main burst and a subsidiary one. These bursts correlated with two consecutive radial implosions and are consistent with predicted magnetohydrodynamics behavior. A variety of time-resolved, accurately timed, spectroscopic measurements were made in this experiment and are described in this paper. These measurements include (1) the pinch implosion time, (2) time-resolved pinhole pictures that give sizes of the $K$-shell emission region, (3) time-resolved $K$-series spectra that give the relative amounts of hydrogenlike to heliumlike to continuum emission, (4) the total and the $K$-shell x-ray power outputs, and (5) time-resolved photoconducting diode measurements from which continuum slopes and time-resolved electron temperatures can be inferred. Time-resolved Ly-\ensuremath{\alpha} and Ly-\ensuremath{\beta} linewidths are obtained from the spectra and inferences about time-resolved ion temperatures are also made. All of these data correlate well with one another. A method is then presented of analyzing this data that relies on the complete set of time-resolved measurements. This analysis utilizes one-dimensional radiative magnetohydrodynamic simulations of the experiments, which drive $z$-pinch implosions using the measured Saturn circuit parameters. These simulations are used to calculate the same x-ray quantities as were measured. Then, comparisons of the measured and calculated data are shown to define a process by which different dynamical assumptions can be invoked or rejected in an attempt to reproduce the ensemble of data. This process depends on the full data set and provides insight into the structure of the radial temperature and density gradients of the on-axis pinch. It implies that the first implosion is composed of a hot plasma core, from which the kilovolt emissions emanate, surrounded by a cooler, denser shell, and it provides details about the structure of the temperature and density gradients between the core and shell regions. These results are found to be broadly consistent with an earlier, less detailed, data analysis in which plasma gradients are ignored. However, the ability to reproduce the full spectroscopic data in the present analysis is found to be sensitively dependent on the radial gradients that are calculated.

MHD Instabilities in the dynamic model of plasma open switches

The dynamic model of plasma open switches is proposed in this article. It is based on the initiation and development of force instabilities (pinches and necks) in spatially inhomogeneous plasma accelerated by the magnetic field pressure. The model proposed does not require the consideration of subtle effects at the pinch implosion stage. It allows one to account for main features of the operation of plasma current open switches within the framework of magnetic hydrodynamics. It also provides quantitative estimates in good agreement with experimental results.

Methods for producing ultrahigh magnetic fields

A general class of methods is suggested for producing controlled magnetic fields up to the order of 100 MG. The methods involve radially imploding plasmas in which longitudinal magnetic fields have been entrained. Specific methods of implosion suggested are (a) gas-puff Z pinch and (b) ablative implosion of a liner by a laser. Applications include reducing fusion ignition thresholds, producing beams of gamma radiation, accelerating particles, and producing high-energy densities under controlled conditions.