H. U. Rahman

Control of the Rayleigh–Taylor instability in a staged Z pinch

Z-pinch experiments and computer simulations provide evidence for enhanced stability and current transfer in a staged Z pinch, consisting of an annular krypton shell imploding onto a deuterium gas fill. Visible-streak and Schlieren imaging provide evidence for a multilayer implosion where the outer plasma shell is Rayleigh–Taylor unstable and the inner plasma column is stable. Computer simulations indicate that the discharge current diffuses through the unstable, outer Kr shell. As the discharge current layer implodes onto the deuterium, current is transferred and a stable implosion results, producing a deuterium-compression ratio of 200.

Staged Z pinch for controlled fusion

A staged Z pinch is considered in which an annular plasma shell made of a high Z material like Kr implodes onto a coaxial plasma target made of a low Z material like deuterium or a deuterium–tritium mixture. The target plasma could be made either by exploding a cryogenically extruded fiber or by filling the annular shell with a gas puff or a plasma puff. Modeling is performed with a two-dimensional (2D) radiation-MHD (magnetohydrodynamic) code. A parameter study is made to determine the sensitivity of this configuration to initial conditions of the shell and the target plasmas. An axial magnetic field is essential for a stable implosion and efficient energy coupling to the final load. Using a 50–50 mixture of deuterium–tritium as a target plasma, the fusion energy gain is optimized by adjusting the initial parameters. The calculations are based on the parameters of the University of California Irvine Z-pinch facility which has a maximum energy storage of 50 kJ.

Measurement of grain charge in dusty plasma Coulomb crystals

We report the measurement of the grain charge on individual dust grains of a dusty plasma Coulomb crystal. These dust grains, grown in a radio frequency plasma from constituent gases, are formed by the aggregation of nanometer sized crystallites. The Coulomb crystal formed from the dust grains appears to be made of mutually repulsive columns of grains confined by the walls of the electrode. A direct measurement of the grain charge was performed using a Faraday cup. The average measured value of the grain charge using this technique was found to be 2/spl times/10/sup 5/ e. This value of the grain charge is approximately consistent with estimates based on the plasma parameters. The corresponding electrostatic interaction energy between the dust grains is found to be 200 times greater than the dust thermal energy.

CLOSED CYCLE CRYOGENIC FIBER EXTRUSION SYSTEM

A fiber extrusion system is described that produces frozen fibers of almost any condensible gas. This extruder has the advantage of employing a closed‐refrigeration system. To date, this system has produced fibers of H2, D2, and Ne of a diameter ranging from 100 to 130 μm. The extrusion occurs at a specific temperature which is several degrees below the triple point of these gases. Once the fiber is extruded it can survive in vacuum for 20 min if the nozzle (extrusion) temperature is lowered to 8 K. The length of these fibers can be of the order of 1 m. D2 fibers will be used in a staged Z‐pinch experiment as a fuel for thermonuclear fusion. For this application a guiding structure is needed to position the fiber between the electrodes with millimeter precision, without significantly affecting its quality.

Shock formation in Ne, Ar, Kr, and Xe on deuterium gas puff implosions

1- and 2-D simulations of 1-cm radius, gas-puff liners of Ne, Ar, Kr, and Xe imploding onto a deuterium target are conducted using the discharge parameters for the Zebra (1 MA, 130 ns) driver using the resistive MHD code MACH2. This is an implementation of the Staged Z-pinch concept, in which the target is driven to high-energy-density first by shock compression launched by a diffused azimuthal magnetic field ( J×B force), and then by the adiabatic compression as the liner converges on axis. During the run-in phase, the initial shock heating preheats the deuterium plasma, with a subsequent stable, adiabatic compression heating the target to high energy density. Shock compression of the target coincides with the development of a J×B force at the target/liner interface. Stronger B-field transport and earlier shock compression increases with higher-Z liners, which results in an earlier shock arrival on axis. Delayed shock formation in lower-Z liners yields a relative increase in shock heating, however, the 2...

Fusion in a staged Z-pinch

This paper is dedicated to Norman Rostoker, our (FJW and HUR) mentor and long-term collaborator, who will always be remembered for the incredible inspiration that he has provided us. Norman’s illustrious career dealt with a broad range of fundamental-physics problems and we were fortunate to have worked with him on many important topics: intense-charged-particle beams, field-reversed configurations, and Z-pinches. Rostoker ’s group at the University of CA, Irvine was well known for having implemented many refinements to the Z-pinch, that make it more stable, scalable, and efficient, including the development of: the gas-puff Z-pinch [1], which provides for the use of an expanded range of pinch-load materials; the gas-mixture Z-pinch [2], which enhances the pinch stability and increases its radiation efficiency; e-beam pre-ionization [3], which enhances the uniformity of the initial-breakdown process in a gas pinch; magnetic-flux-compression [4, 5], which allows for the amplification of an axial-magnetic f...

UCI Staged Z Pinch

The Staged Z-Pinch couples energy to a target, plasma-dynamically in stages. Our analysis considers this multi-shell z-pinch configuration driven by the ZOT Facility at the University of California, Irvine with predictions for a 2 MA, 1.8 μs krypton-liner shell and DT target of: Y≈4×1015 neutrons/pulse, G∼1 gain, nτ≈1015 cm−3-s, τ=2 ns, n=7×1023 cm−3, and Tion=10 keV.

UCI staged Z pinch facility

The Staged Z Pinch couples energy to a target plasma, dynamically in stages. The present UCI experiment provides stable, multishell z pinches at 1.2 MA and 1 μs implosion time. Test-stand studies of an exploded-fiber target indicate that the fiber core is not ionized, due to current channeling in the high conductivity ablated plasma.

Injector design for liner-on-target gas-puff experiments

We present the design of a gas-puff injector for liner-on-target experiments. The injector is composed of an annular high atomic number (e.g., Ar and Kr) gas and an on-axis plasma gun that delivers an ionized deuterium target. The annular supersonic nozzle injector has been studied using Computational Fluid Dynamics (CFD) simulations to produce a highly collimated (M > 5), ∼1 cm radius gas profile that satisfies the theoretical requirement for best performance on ∼1-MA current generators. The CFD simulations allowed us to study output density profiles as a function of the nozzle shape, gas pressure, and gas composition. We have performed line-integrated density measurements using a continuous wave (CW) He-Ne laser to characterize the liner gas density. The measurements agree well with the CFD values. We have used a simple snowplow model to study the plasma sheath acceleration in a coaxial plasma gun to help us properly design the target injector.

Investigation of magnetic flux transport and shock formation in a staged Z-pinch

Target preheating is an integral component of magnetized inertial fusion in reducing convergence ratio. In the staged Z-pinch concept, it is achieved via one or more shocks. Previous work [Narkis et al., Phys. Plasmas 23, 122706 (2016)] found that shock formation in the target occurred earlier in higher-Z liners due to faster flux transport to the target/liner interface. However, a corresponding increase in magnitude of magnetic pressure was not observed, and target implosion velocity (and therefore shock strength) remained unchanged. To investigate other means of increasing the magnitude of transported flux, a Korteweg-de Vries-Burgers equation from the 1-D single-fluid, resistive magnetohydrodynamic equations is obtained. Solutions to the nondispersive (i.e., Burgers) equation depend on nondimensional coefficients, whose dependence on liner density, temperature, etc., suggests an increase in target implosion velocity, and therefore shock strength, can be obtained by tailoring the mass of a single-liner ...