Gas-puff Implosions Research Articles

Neutron production and implosion characteristics of a deuterium gas-puff Z pinch

Experiments on the Z accelerator with deuterium gas puff implosions have produced up to 3.9×1013(±20%) neutrons at 2.34 MeV (±0.10MeV). Experimentally, the mechanism for generating these neutrons has not been definitively identified through isotropy measurements, but activation diagnostics suggest multiple mechanisms may be responsible. One-, two-, and three-dimensional magnetohydrodynamic (MHD) calculations have indicated that thermonuclear outputs from Z could be expected to be in the (0.3–1.0)×1014 range. X-ray diagnostics of plasma conditions, fielded to look at dopant materials in the deuterium, have shown that the stagnated deuterium plasma achieved electron temperatures of 2.2keV and ion densities of 2×1020cm−3, in agreement with the MHD calculations.

Z-pinch plasma neutron sources

A deuterium gas-puff load imploded by a multi-MA current driver from a large initial diameter could be a powerful source of fusion neutrons, a plasma neutron source (PNS). Unlike the beam-target neutrons produced in Z-pinch plasmas in the 1950s and deuterium-fiber experiments in the 1980s, the neutrons generated in deuterium gas-puffs with current levels achieved in recent experiments on the Z facility at Sandia National Laboratories could contain a substantial fraction of thermonuclear origin. For recent deuterium gas-puff shots on Z, our analytic estimates and one- and two-dimensional simulations predict thermal neutron yields ∼3×1013, in fair agreement with the yields recently measured on Z [C. A. Coverdale et al., Phys. Plasmas (to be published)]. It is demonstrated that the hypothesis of a beam-target origin of the observed fusion neutrons implies a very high Z-pinch-driver-to-fast-ions energy transfer efficiency, 5 to 10%, which would make a multi-MA deuterium Z-pinch the most efficient light-ion accelerator. No matter what mechanism is eventually determined to be responsible for generating fusion neutrons in deuterium gas-puff shots on Z, the deuterium neutron yield is shown to scale as Yn∼Im4, where Im is the peak current of the pinch. Theoretical estimates and numerical modeling of deuterium gas-puff implosions demonstrate that the yields of thermonuclear fusion neutrons that can be produced on ZR and the next-generation machines are sufficiently high to make PNS the most powerful, cost- and energy-efficient laboratory sources of 2.5−14MeV fusion neutrons, just like plasma radiation sources are the most powerful sources of soft and keV x rays. In particular, the predicted deuterium-tritium thermal neutron-producing capability of PNS driven by the next-generation ZR and ZX accelerators is ∼5×1016 and ∼1018, respectively.

Experimental study of an argon-hydrogen Z pinch plasma radiation source

Experiments with Ar-H/sub 2/ double gas puffs have been conducted on the GIT-12 generator at the current level of 2.1-2.4 MA and 250-350 ns implosion times. The argon-hydrogen mixture was used as a working medium in the outer shell of a double gas puff. The goal of the experiments was to verify whether the use of the argon-hydrogen mixture in the outer shell can improve the stability of a double gas puff implosion and provide increased argon K-shell radiation yields. The experiments showed that hydrogen admixture results in the change of implosion dynamics: decreased implosion velocities and compression ratios. The experimental data does not allow a conclusion that the use of an Ar-H/sub 2/ mixture in the outer shell of a double gas puff significantly improves the implosion stability. An increase in the hydrogen percentage in the outer shell leads to a decrease in plasma density and temperature, and as a result, reduced K-shell radiation yields and powers.

Long time implosion experiments with double gas puffs

Long time implosion experiments with argon double gas puffs have been conducted on the GIT-12 [S. P. Bugaev et al., Izv. Vyssh. Uchebn. Zaved., Fiz. 40, 38 (1997)] generator at the current level of 2.2–2.4 MA. A double gas puff was used as one of the alternative ways to improve implosion stability at implosion times from 230 to 340 ns. The results of these experiments were compared with two-dimensional snowplow simulations. The experiments and the simulations show that the final pinch is sufficiently stable when the inner-to-outer shell mass ratio is greater than 1. The maximum argon K-shell yield obtained in the experiments is 740 J/cm with 220 GW/cm radiation power. At the long implosion times, the K-shell yield obtained in the double gas puff implosion is twice the K-shell yield of a 4-cm-radius single gas puff, with more than an order of magnitude increase in radiation power.

Hydromagnetic Rayleigh–Taylor instability in high-velocity gas-puff implosions

Experiments using the Saturn pulsed power generator have produced high-velocity z-pinch plasma implosions with velocities over 100 cm/μs using both annular and uniform-fill gas injection initial conditions. Both types of implosion show evidence of the hydromagnetic Rayleigh–Taylor instability with the uniform-fill plasmas producing a more spatially uniform pinch. Two-dimensional magnetohydrodynamic simulations including unsteady flow of gas from a nozzle into the diode region have been used to investigate these implosions. The instability develops from the nonuniform gas flow field that forms as the gas expands from the injection nozzle. Instability growth is limited to the narrow unstable region of the current sheath. For the annular puff the unstable region breaks through the inner edge of the annulus increasing nonlinear growth as mass ejected from the bubble regions is not replenished by accretion. This higher growth leads to bubble thinning and disruption producing greater nonuniformity at pinch for the annular puff. The uniform puff provides gas to replenish bubble mass loss until just before pinch resulting in less bubble thinning and a more uniform pinch.

On stabilization of gas puff implosion: experiment and simulation

A double gas puff was used to study the mitigation of the magneto-Rayleigh-Taylor (RT) instabilities for long implosion times (up to 250 ns). The experiments have been performed on the inductive storage GIT-4 (1.7 MA, 120 ns) generator. Current division between the outer and inner shells was controlled using magnetron-discharge preionization. The implosion of the a double gas puff, with the improved preionization, results in the formation of a uniform plasma column. The results of two-dimensional (2-D) radiation-magnetohydrodynamic simulations support the experimental results: a double gas puff implosion mitigates the RT instabilities, leading to the development of only small-amplitude waves. The 2-D simulation allowed us to explain the halo effect seen in the experiments: the use of the low hybrid conductivity in the calculation demonstrated the existence of the high density plasma core surrounded by a low density plasma halo.

Observation of the bremsstrahlung generation in the process of the Rayleigh–Taylor instability development at gas puff implosion

The electron magnetohydrodynamic model predicts the appearance of anode–cathode voltage in the process of Rayleigh–Taylor instability development during gas puff implosions. The appearance of the anode–cathode voltage should be accompanied by the accelerated electron flow and the generation of the bremsstrahlung radiation. Experiments with neon and krypton gas puffs were performed on the GIT-4 [S. P. Bugaev, et al., Plasma Sci. 18, 115 (1990)] generator (1.6 MA, 120 ns) to observe the bremsstrahlung radiation during the gas puff implosion. Two spikes of the bremsstrahlung radiation were observed in the experiments. The first spike is connected with the gas breakdown; the second one is connected with the final stage of the implosion. The development of the RT instabilities does not initiate the bremsstrahlung radiation, therefore, the absence of anode–cathode voltage is demonstrated.

Suppression of Rayleigh-Taylor Instability in Z-Pinch Loads with Tailored Density Profiles.

A load structure with tailored density profile which delays the onset of the Rayleigh-Taylor instability development in imploding {ital Z} pinches by inverting acceleration of the magnetic field/plasma interface is proposed and studied numerically. This approach makes it possible to start gas-puff implosions from large radii (say, 8cm) and produce significant {ital K}-shell yield with current pulse duration of 250ns and longer. It could also be used to mitigate imprinting of initial perturbations into laser fusion targets. {copyright} {ital 1996 The American Physical Society.}

A heuristic model for the nonlinear Rayleigh–Taylor instability in fast Z pinches

A simple, heuristic model for the early nonlinear phase of the Rayleigh–Taylor instability (RTI) in thin-cylindrical-shell Z-pinch implosions has been developed. This model is based on the fact that, as the field–plasma interface is deformed, there is a component of the applied force that acts to move mass from the low mass per unit area bubble region into the higher mass per unit area spike region. The resulting reduced mass per unit area of the bubble causes it to be preferentially accelerated ahead of the spike. The pinch begins to radiate as the bubble mass first reaches the axis, and it continues to radiate while the mass that is entrained within the spikes and within unperturbed parts of the shell also arrives on axis. This model relates the time at which the bubble arrives on axis to an initial wavelength and amplitude of a single mode of the RTI. Then, by comparing this to the time at which the unperturbed mass reaches the axis, one estimates pinch thermalization time, a quantity that is determined experimentally. Experimental data, together with analytic models, have been used to choose appropriate initial wavelength and amplitude both for foils and for certain gas puff implosions. By noting that thermalization time is a weak function of these parameters, it is argued that one may use the same values for an extrapolative study of qualitatively similar implosions.

Argon gas puff implosion experiments and two-dimensional modeling

The K-shell x ray yields from argon gas puff Z pinches are observed to increase from 3.5±1.0 kJ to 13±1.0 kJ when inwardly tilted nozzles are used on a 6 TW, 4 MA generator. This increase is associated with the elimination of the zipper effect and the achievement of higher density plasmas, as confirmed by x-ray diagnostics. Two-dimensional (2-D) magnetohydrodynamic modeling of the gas puff implosions indicates that the collapsing shell reaches a higher density and smaller diameter when the axial zipper is eliminated. The calculations show that axial mass flow in the zippering cases result in nonhollow collapses which limit the final pinch radii and densities. The calculations also indicate that narrower nozzle exits contribute to increasing the assembled plasma density.

Neon gas puff implosions on a high-current microsecond generator with and without a plasma opening switch

Implosions of 2.5-cm-diam neon gas shells on a 1.2 μs quarter-period, 3 MA driver, FALCON, have produced no more than 7.6 kJ of kilovolt neon K-shell radiation. The incorporation of a plasma opening switch produces faster current pulses: up to 1.8 MA in 190 ns. With the higher rate of rise of current, neon gas puffs have produced up to 13.5 kJ of kilovolt K-shell radiation. Numerical calculations indicate that this increase in radiation is due to the achievement of a higher kinetic energy per ion at higher current levels. Spectroscopic measurements confirm that a significant fraction of the plasma is heated into the K-shell ionization states and that the faster current pulses increase this fraction up to 40%.

Measurements of axial nonuniformities in gas-puff implosions

Large-scale-length axial nonuniformities in imploding z pinches have been measured with a time-resolved framing pinhole camera and a filtered x-ray diode. The nonuniform initial conditions of the annular gas column result in a nonsimultaneous stagnation along the pinch axis. By tailoring the initial gas-puff density distribution we have reduced the time interval of stagnation from 10 ns to below 4 ns for a 2-cm-long pinch.

THE SANDIA X-RAY LASER PROGRAM

The Sandia x‐ray laser program is based on the intense keV radiation produced by stagnating gas puff implosions. The laser scheme uses this radiation to photoionize Ne‐like ions to F‐like ions, and produce a 3p‐3s population inversion via recombination. An annular stagnation shell is used to separate the pump from the lasant. Converter technology is being developed to try the Na‐Ne line matching scheme. Experimental and computational results will be presented.

Large-scale-length nonuniformities in gas puff implosions

Because they are less susceptible to the hydromagnetic Rayleigh–Taylor instability than other fast Z-pinch imploding liner systems, gas puffs offer the possibility of higher implosion velocity. This higher specific energy appears necessary for optimizing high-energy x rays required in a photoionization-pumped soft x-ray laser. Nevertheless, large-scale-length nonuniformities created as the gas flows from the nozzle across the electrode gap are a potential problem. One- and two-dimensional calculations suggest that gas near the nozzle will implode before that which is further from the nozzle, leading to an effect described as ‘‘zippering.’’ Because the number of such two-dimensional calculations that can be done is limited and because the density distribution of nozzles is uncertain, we have developed a simple quasi-two-dimensional interface code that is able to quickly survey the effect of arbitrary initial gas distributions on the implosion dynamics. Results of this survey suggest that zippering contributes significantly to thermalization time, and we propose two methods to counteract this problem. These techniques, each of which involves tailoring the initial density distribution to offset effects of nonuniformities, appear promising. Nevertheless, we will never completely eliminate these nonuniformities, therefore, they must be accounted for in x-ray laser target design.

A gas puff soft x-ray laser target design

An x-ray laser target to be driven by a Xe gas puff implosion powered by the Proto II machine at Sandia is proposed. The laser material is Sn pumped to the Ni-like ionization state. The main laser transition (4d-4p) is calculated to be near 110.5 eV.