Gas-puff Z-pinch Research Articles

Soft X-Ray Images of Krypton Gas-Puff Z-Pinches

A series of experiments has been carried out on Qiang-guang I generator to study the dynamics of krypton gas-puff Z-pinches. The generator was operated at a peak current of 1.5 MA with a rise-time of 80 ns. The specific linear mass of gas liner was about 20 μg/cm in these experiments. In the diagnostic system, a four-frame x-ray framing camera and a pinhole camera were employed. A novel feature of this camera is that it can give time-resolved x-ray images with four frames and energy-resolved x-ray images with two different filters and an array of 8 pinholes integrated into one compact assemble. As a typical experimental result, an averaged radial imploding velocity of 157 km/s over 14 ns near the late phase of implosion was measured from the time-resolved x-ray images. From the time-integrated x-ray image an averaged radial convergence of 0.072 times of the original size was measured. An averaged radial expansion velocity was 130 km/s and the maximum radial convergence of 0.04 times of the original size were measured from the time-resolved x-ray images. The dominant axial wavelengths of instabilities in the plasma were between 1 and 2 mm. The change in average photons energy was observed from energy spectrum- and time-resolved x-ray images.

Optical Observation of the Instability in Microsecond Gas-Puff Z-Pinch Plasmas

The occurrence of fluid instabilities in gas-puff z-pinch plasmas have been investigated using laser diagnostic systems. An interferometer system was employed to investigate the temporal evolution of the structure in the plasma column during the pinch. The z-pinch plasma was driven by a fast bank, which provided a current of 160 kA for 1.5 µs quarter period of oscillation. Initially the plasma was formed in a conical shape due to the initial gas distribution. The plasma layer moved toward the z-axis and its shape gradually changed into columnar. The plasma layer splits into two layers on the way to implosion. The inner layer, which was indicated to be shock front, propagated toward the z-axis faster than the outer layer. The outer layer seemed quite smooth while the implosion by the inner layer took place at the z-axis. After the blast from the implosion expanded and came across the outer layer, the disturbance occurred at the outer layer. It is concluded that the plasma instability is significantly influenced by the shockwave rather than the initial gas formation in the case of a microsecond implosion.

Three-dimensional hydromagnetic simulation of a high-velocity gas-puff Z-pinch

Experiments on the Sandia National Laboratories Saturn pulsed power machine have produced high-velocity Z-pinch plasma implosions of krypton gas puffs from a circular nozzle. We use the three-dimensional (3-D), time-dependent MACH3 magnetohydrodynamics code to simulate these experiments. Generally, the quality of Z-pinch implosions is degraded by hydromagnetic Rayleigh-Taylor instabilities. The images displayed in this paper illustrate the growth in time of this instability in three spatial dimensions from a modest (and realistic) initial 3-D perturbation.

Soft X-ray spectroscopic study of a gas-puff Z-pinch argon plasma

X-ray radiation characteristics of argon plasma produced by a gas-puff Z-pinch device were investigated using an X-ray crystal spectrometer, an X-ray diode, and an extreme ultraviolet (XUV) spectrometer. Using a germanium crystal we have observed spectral emission from Ar XVII produced by hot spots at the pinched stage. With the help of a 2-m grazing incidence XUV spectrometer, the spectrum of 30 to 250 /spl Aring/ were obtained. Strong lines from Ar VIII to Ar XIII were observed with a continuum whose peak is around 120 /spl Aring/. The radiation energy in the spectral range is estimated to be about 23 joule which is about 0.6% of the electrical energy stored in capacitors.

Evolution of the gas-puff Z-pinch column

The evolution of a neon implosion of low energy discharge (4 kJ, 40 kV, 150 kA, 1.1 /spl mu/s) was studied using X-ray, schlieren, and high speed electrooptical visible gated quadro camera diagnostics. The geometry, an helical structure of pinched column, two steps of pinching, and X-ray emission have been studied. The hypothesis of an axial component of magnetic field generation, the helical shape of the magnetic and electric field lines, and the release of magnetic energy during the second pinching of the column are discussed.

2-D Code for Simulating a Hollow Gas Puff Z-Pinch

We compare the dynamics of plasma in a hollow gas puff Z-pinch device, obtained with Schlieren pictures, with the predictions of a 2-D MHD Lagrangian code. We show that agreement between the model and experiment require an axially variable line density and width of the injected gas.

Energy Transfer to Gas-Puff Z-Pinch and X-Ray Radiation

Gas-puff z-pinch plasmas were generated using rare gases (Ne, Ar and Kr). The intensities of XUV and X-ray radiations increased with atomic number Z of the gas. The net energy transferred to the plasma also increased with Z of the gas, which is a direct result of strong radial compression of the plasma column. Multiple ionization plays an important role in the strong compression of the high Z z-pinch plasma.

VUV ablation of polymers by emission from gas-puff Z-pinch plasmas

We have made experiments of the polymer ablation utilizing the gas-puff Z-pinch plasmas which provide a highly bright emission extending over a broad wavelength range from visible lights to X-rays. When the emission was irradiated through a mask onto a polytetrafluoroethylene (PTFE) film, the irradiated area was ablated and a pattern of the mask was copied on the surface of the film. A detail observation with an atomic force microscope showed that the irradiated area was precisely processed with no thermal damage. A chlorinated polyvinylchloride (Cl-PVC) pellet was also irradiated. The analysis of unirradiated and irradiated surface was performed by FTIR-ATR. The FTIR-ATR spectrum after irradiation shows an appearance of a new peak attributed to C5C bonds.

Evolution of gas-puff Z-pinch during implosion and pinch phase

The four-frame high-speed electrooptical camera backed by an automatic image capturing and processing system has been used to investigate the evolution of the gas-puff Z-pinch. The sequence of four images of the pinch continuum radiation (the exposure of each frame was 1 ns, the time separation of frames was 10 or 20 ns) enabled the study of the pinch-forming phase (zipper effect, compression), the stagnation of plasma on the axis, as well as the pinch decay (disintegration, collapsing of the plasma column) and the determination of the velocities of individual processes.

Fusion conditions in a finite-thickness gas-puff staged Z-pinch

We investigate the implosion of a dense τ-pinch plasma driven by an annular finite-thickness gas-puff Z-pinch. The imploding Z-pinch traps an axial magnetic field Bz, compressing it to large values in an extremely short time. The temporal variation of Bz then induces an azimuthal τ current on the surface of a fibre placed on the axis, with a rise time an order of magnitude shorter than the rise time of the Z-pinch current. Our numerical results demonstrate that, for a thick gas-puff layer, maximum compression occurs before the current peaks.We also find that at peak compression, fuel densities of the order of 1025 cm-3 and temperatures above 10 keV can be achieved on a time scale of the order of 0.1 ns. Thus a Lawson parameter nτ ≈ 1014 s cm-3 for a DT fibre becomes achievable. The snowplough effect in the Z-pinch exercises a stabilization effect on the growth of sausage and Rayleigh—Taylor instabilities. In the limit of a very thin gas-puff layer, previous results are fully recovered.

Phenomenological modeling of turbulence in Z-pinch implosions

A phenomenological investigation into the effects of magnetohydrodynamic (MHD) turbulence on the initial stagnation dynamics of aluminum wire array and argon gas puff Z-pinch implosions is performed. The increases that turbulence produces in the plasma viscosity, heat conductivity, and electrical resistivity are modeled by using multipliers for these quantities in one-dimensional (1-D) MHD calculations. The major effect of these increases is to soften the 1-D implosions by decreasing the densities that are achieved on axis at stagnation. As a consequence, a set of multipliers can be found that reasonably duplicates the average electron temperatures, ion densities, and mass of the K-shell emission region that were measured at stagnation for a variety of Physics International aluminum wire array and argon gas puff experiments. It is determined that the dependence of these measured quantities on the multipliers is weak once a level of enhancement is reached, where agreement between calculations and experiments is attained. The scaling of K-shell yield with load mass for a fixed implosion velocity is then reexamined, and the minimum load mass needed to efficiently produce K-shell emission by thermalization of kinetic energy is calculated for aluminum and argon using this phenomenological soft implosion modeling. The results show an upward shift in the minimum mass by a factor of 6 when compared to the original nonturbulent hard implosion calculations.

Multichannel soft X-ray diagnostics of hot plasma evolution in nitrogen-puff Z-pinch

Soft X-ray emission from nitrogen plasma of a small gas-puff Z-pinch is studied. A multilayer-mirror polychromator with time-resolved registration by positive-intrinsic-negative (PIN) diodes is installed, and its relatively narrow spectral windows are aligned to the important parts of the nitrogen spectrum between 390 and 710 eV. The electron temperature evolution (T/sub e/ approximately=400 eV in hot spots) is determined from the slope of the recombination continuum of He-like nitrogen. The T/sub e/ of bulk plasma ( approximately=100 eV) is compared with T/sub e/ calculated from yields of K-shell lines. Pinhole cameras are used for better understanding the radiation phase of the pinch. Time sequences of peaks in X-ray signals corresponding to a number of bright spots on photographs are presented. >

X-ray emission scaling and energy balance of small gas-puff Z-pinch

Gas-puff Z-pinch is discussed as an intensive pulsed source of soft X-rays. Measurements of total X-ray yield YT and the yield YK from K shells of highly ionized atoms af argon, neon, and nitrogen are presented. The radiation produced by a small-scale (4.3 kJ) gas-puff Z-pinch confirmed the YK approximately=I4 scaling (well known from large devices) for pinch currents I=120-200 kA. Some basic considerations about the energy balance of the Z-pinch as well as similarities between terawatt and gigawatt devices are expressed.

XUV emission of nitrogen-puff Z-pinch

In order to increase X-ray output from the K shells of a highly ionized plasma in a small gas-puff Z-pinch, work with nitrogen or carbon instead of neon or argon is suggested. Basic measurements of the radiation of such a plasma, made by vacuum X-ray diodes and thin filters, are presented. Two working regimes with nitrogen at pinch currents 160 and 200 kA, with different contributions of line and continuous spectra in the K shell output, are discussed. It is shown that at a typical energy input of small Z-pinches (several kilo Joules) the suggested lighter elements can be ionized to He-like and H-like states not only in the hot spots, as has been observed with neon and argon, but also in the bulk. The yield of nitrogen K-shell radiation (about 10 J per shot) is therefore substantially higher than that of neon. Besides the pinch physics and radiation dynamics, the K-shell emission of the lighter elements may also be interesting in X-ray sources in the spectral region of a 'water window'.

Operating Regions of a Gas-Puff Z-Pinch and Its X-Ray Radiation

A z-pinch experiment is carried out with a hollow-shaped Ar gas puff. Three types of discharge are found depending on the delay time of discharge from gas-puff detection, and the pinch time is compared with the measured gas pressure profile. Strong X-ray radiation is emitted from spots, a cloud and the anode surface. The number of spikes in the X-ray signal is correlated with spot images of the X-ray photograph. An X-ray burst is sometimes observed in the short-delay-time operation.

The Effect of the Puffed Gas Shell Profile on Plasma Dynamics in a Small Gas-Puff Z-Pinch

An argon annular gas-puff Z-pinch has been experimentally investigated with three different nozzles. The puffed gas shell profiles and the plasma implosion have been measured using a fast ionization gauge and a differential interferometer. These experiments confirm that both the puffed gas density and its distribution may effect the compression process of Z-pinch plasma.

Experimental study on the dynamics of a cylindrical plasma sheath in a gas-puff z-pinch device

The dynamics of a hollow cylindrical plasma sheath produced in a gas-puff Z-pinch device were studied with a differential interferometer. The velocity of the imploding plasma during three stages of compression the minimum radius of the pinched plasma column, the hot spots and the X-ray emissions were measured. The effect of the parameters of the gas cylinder on the plasma properties and X-ray emission was investigated with three different nozzles. Not only the gas density but also the distribution may influence the process of implosion. The experiments show that a thinner gas sheath produces a higher velocity of compression and higher X-ray yield. An interesting phenomenon of multiple pinch was observed. X-rays may be emitted at different locations and different instants.

Dynamics of sausage instabilities of a gas-puff Z-pinch

The early stage of the sausage instability in a gas-puff Z-pinch has been registered in VUV and soft X-rays with a 10 ns framing camera. We hypothesize that the rings of plasma expanding from the sausage instability enable an alternative current path to dominate the formation of “micropinches”.

Lithium niobate stress gauge current diagnostic for noninductive measurement of fast-rise-time multimegampere currents

Accurate modeling of load behavior in Z-pinch plasma radiation sources driven by high-current generators requires the measurement of fast-rise-time multimegampere currents close to the load. Conventional current diagnostics mounted in inductive cavities (such as B-dot loops and Rogowski coils) fail at small radius because of electrical breakdown produced by high dI/dt. In this paper, we describe the use of large-signal, nanosecond-time-resolution lithium niobate piezoelectric stress gauges to directly measure the magnetic pressure B2/2μ0=μ0I2/8π2r2 generated at radius r by a current I flowing in a radial transmission line. Current measurements have been performed at radius r=2.54×10−2 m on Sandia National Laboratories’ Proto-II (10 TW) and SATURN (30 TW) gas puff Z-pinch experiments with maximum currents of 10.1 MA and dI/dt to 2.1×1014 A/s. Comparisons with Faraday rotation and B-dot current diagnostic measurements at large radius are presented. Bremsstrahlung noise problems unique to the SATURN gas puff source are discussed. For a Y-cut lithium niobate stress gauge on a pure tungsten electrode, current densities up to I/2πr=78 MA/m can be measured before the electrode yield strength and the piezoelectric operating stress limit are exceeded. Above the Hugoniot elastic limit of the electrode material, the dynamic range and accuracy of the diagnostic are greatly reduced, but it appears that the technique can be extended to higher current densities using an X-cut quartz piezoelectric element and a tungsten-sapphire electrode impedance stack.

Selected properties of terawatt Z-pinches from temporally and spatially resolved spectroscopy

X-ray diagnostics such as time-resolved pinhole cameras and time-resolved crystal and grazing incidence spectrometers are used to examine the dynamics of the implosion and radiation phases of gas puff, wire array and single wire Z-pinches. Unique measurements include: the measurement of a radial collapse velocity of 40 cm/μs from Doppler splitting of argon XUV lines; the axial variation in the ratio of the 2P 1 2 and 2P 3 2 fine structure components of the aluminium Ly-α line due to localized dense spots; and the time evolution of the plasma electron density from the temporal behavior of the ratio of the aluminium He-like resonance to intercombination lines. Moreover, the electron densities on hot spots in aluminium and titanium Z-pinches are found to be between 5×10 21 to 10 22cm -3.