Publications

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Flute mode turbulence plays an important role in numerous applications, such as tokamak, Z-pinch, space and astrophysical plasmas. In a low beta plasma flute oscillations are electrostatic and in the nonlinear stage they produce large scale density structures co-mingling with short scale oscillations. Large scale structures are responsible for the enhanced transport across the magnetic field and appearance of short scales leads to ion heating, associated with the ion viscosity. In the present paper nonlinear equations which describe the nonlinear evolution of the flute modes treated as compressible electromagnetic oscillations in a finite beta inhomogeneous plasma with nonuniform magnetic field are derived and solved numerically. For this purpose the 2D numerical code FLUTE was developed. Numerical results show that even in a finite beta plasma flute mode instability can develop along with formation of large scale structures co-existing with short scale perturbations in the nonlinear stage. © 2008 Global-Science Press.

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The immersed- Bz diode is being developed as a high-brightness, flash x-ray radiography source at Sandia National Laboratories. This diode is a foil-less electron-beam diode with a long, thin, needlelike cathode that is inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces forward-directed bremsstrahlung. In addition, electron beam heating of the anode produces surface plasmas allowing ion emission. Two different operating regimes for this diode have been identified: a nominal operating regime where the total diode current is characterized as classically bipolar and an anomalous operating regime characterized by a dramatic impedance collapse where the total diode current greatly exceeds the bipolar limit. Data from a comprehensive series of experiments fielded at 4 and 5 MV, where the diode operates in the nominal or stable impedance regime, with beam currents ranging from 20-40 kA on target are presented. In this mode, both the measured diode current and experimental radiation production are consistent with physics based models including two-dimensional particle-in-cell simulations. The analysis indicates that intermediate mass ions (e.g., 12-18 amu) control the nominal impedance evolution rather than expected lighter mass ions such as hydrogen. © 2007 American Institute of Physics.

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Minimization of the radiographic spot size and maximization of the radiation dose is a continuing long-range goal for development of electron beam driven X-ray radiography sources. In collaboration with members of the Atomic Weapons Establishment(AWE), Aldermaston UK, the Advanced Radiographic Technologies Dept. 1645 is conducting research on the development of X-ray sources for flash core-punch radiography. The Hydrodynamics Dept. at AWE has defined a near term radiographic source requirement for scaled core-punch experiments to be 250 rads{at}m with a 2.75 mm source spot-size. As part of this collaborative effort, Dept. 1645 is investigating the potential of the Self-Magnetic-Pinched (SMP) diode as a source for core-punch radiography. Recent experiments conducted on the RITS-6 accelerator [1,2] demonstrated the potential of the SMP diode by meeting and exceeding the near term radiographic requirements established by AWE. During the demonstration experiments, RITS-6 was configured with a low-impedance (40 {Omega}) Magnetically Insulated Transmission Line (MITL), which provided a 75-ns, 180-kA, 7.5-MeV forward going electrical pulse to the diode. The use of a low-impedance MITL enabled greater power coupling to the SMP diode and thus allowed for increased radiation output. In addition to reconfiguring the driver (accelerator), geometric changes to the diode were also performed which allowed for an increase in dose production without sacrificing the time integrated spot characteristics. The combination of changes to both the pulsed power driver and the diode significantly increased the source x-ray intensity.

In particle-based plasma simulation, when dealing with source terms such as ionization, emission from boundaries, etc., the total number of particles can grow, at times, exponentially. Additionally, problems involving the spatial expansion of dynamic plasmas can result in statistical under representation of particle distributions in critical regions. Furthermore, when considering code optimization for massively parallel operation, it is useful to maintain a uniform number of particles per cell. Accordingly, we have developed an algorithm for coalescing or fissioning particles on 2D and 3D orthogonal grids that is based on a method of Assous et al. [F. Assous, T. Pougeard Dulimbert, J. Segre, J. Comput. Phys. 187 (2003) 550]. Here, we present the algorithm and describe in detail its application to particle-in-cell simulations of gas ionization/streamer formation and dynamic, expanding plasmas.

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The main physical processes responsible for plasma ablation in multiwire Z pinches are considered via eigensolutions to one-dimensional steady state magnetohydrodynamics. A double scale-length structure of the plasma accelerating layer is demonstrated. The width of the resistive scale-length that defines the current layer structure is significantly larger than the thermal scale-length, where transport of energy toward the cores and plasma pressure play important roles. The transport of energy is provided mainly by radiation, though electron thermal conduction is also important very close to the plasma-core interface. Another type of solution of the steady state problem is revealed, when local Ohmic heating is important down to the interface. Selection between these two types of solutions is considered from multiple points of view. Although the one-dimensional problem is mainly considered in this paper, it is shown how the one-dimensional results may help to understand results of two-dimensional models.

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