Ioana Paraschiv

Linear analysis of sheared flow stabilization of global magnetohydrodynamic instabilities based on the Hall fluid model

A systematic study of the linear stage of sheared flow stabilization of Z-pinch plasmas based on the Hall fluid model with equilibrium that contains sheared flow and an axial magnetic field is presented. In the study we begin with the derivation of a general set of equations that permits the evaluation of the combined effect of sheared flow and axial magnetic field on the development of the azimuthal mode number m=0 sausage and m=1 kink magnetohydrodynamic (MHD) instabilities, with the Hall term included in the model. The incorporation of sheared flow, axial magnetic field, and the Hall term allows the Z-pinch system to be taken away from the region in parameter space where ideal MHD is applicable to a regime where nonideal effects tend to govern stability. The problem is then treated numerically by following the linear development in time of an initial perturbation. The numerical results for linear growth rates as a function of axial sheared flow, an axial magnetic field, and the Hall term are reported.

Scattering of electromagnetic waves by vortex density structures associated with interchange instability: Analytical and large scale plasma simulation results

The presence of plasma turbulence can strongly influence propagation properties of electromagnetic signals used for surveillance and communication. In particular, we are interested in the generation of low frequency plasma density irregularities in the form of coherent vortex structures. Interchange or flute type density irregularities in magnetized plasma are associated with Rayleigh-Taylor type instability. These types of density irregularities play an important role in refraction and scattering of high frequency electromagnetic signals propagating in the earth ionosphere, in high energy density physics, and in many other applications. We will discuss scattering of high frequency electromagnetic waves on low frequency density irregularities due to the presence of vortex density structures associated with interchange instability. We will also present particle-in-cell simulation results of electromagnetic scattering on vortex type density structures using the large scale plasma code LSP and compare them with analytical results.

Linear and nonlinear development of m=0 instability in a diffuse Bennett Z-pinch equilibrium with sheared axial flow

The effect of sheared axial flow on the Z-pinch sausage instability has been examined with two-dimensional magnetohydrodynamic simulations. Diffuse Bennett equilibria in the presence of axial flows with parabolic and linear radial profiles have been considered, and a detailed study of the linear and nonlinear development of small perturbations from these equilibria has been performed. The consequences of both single-wavelength and random-seed perturbations were calculated. It was found that sheared flows changed the internal m=0 mode development by reducing the linear growth rates, decreasing the saturation amplitude, and modifying the instability spectrum. High spatial frequency modes were stabilized to small amplitudes and only long wavelengths continued to grow. Full stability was obtained for supersonic plasma flows.

2D MHD computer modeling of dense plasma focus accelerators

Dense Plasma Focus (DPF) discharge is a very promising mechanism to achieve high neutron production yield (about 10/sup 11/-10/sup 12/ per pulse) from the D-T reactions. Focus is conjectured to be a finite 2D Z-pinch formation near the end of the coaxial plasma accelerator, typically having the density above the level of 10/sup 19//cm/sup 3/ and temperature of a few keV during 100 to 150 ns. Interest to DPF appeared since the pioneer experiments of Filippov and the first theoretical reviews of Dyachenko and Imshennik and Mather. During the last decades theoretical investigation of DPF lagged behind the experiments, giving scant explanation of the experimental results. At the same time modeling and diagnostic capabilities have been dramatically improved. This presentation shows that the computer simulation by the code MHRDR (Magneto Hydro Radiative Dynamic Research) can help meet the goals and challenges of the LANL-Bechtel Nevada Dense Plasma Focus Accelerator project. Theoretical estimations, made in this report, represent the bounds between the parameters of source generator, geometry of the electrodes and feeding circuit and the initial density of the background gas in the form of the simple scaling laws.

Modeling of the inverse z-pinch dynamics

The two-dimensional MHD numerical simulation MHRDR has been applied to develop and investigate a new possible fusion scheme, and design experiments to test it. The confinement of magnetized high-beta plasma directly by material walls holds considerable promise for fusion. An interesting prospective Magnetized Target Fusion (MTF) target plasma is the cylindrical inverse pinch, which is, in theory, an MHD-stable, self-organized plasma. An inverse pinch consists of coaxial, metal, current-carrying cylinders with plasma between them. Important insight into this plasma has been obtained using the MHRDR simulation. First, simulations observe that interchange m=0 modes rearrange the plasma into a pressure profile that is stable to m=0 (the Kadomtsev stable profile). Such plasma self-organization is very encouraging for the development of a robust practical device, since the pressure profile does not have to be created in a very particular manner to satisfy the Kadomtsev criterion. Second, the plasma beta can be adjusted by using an initial bias current on the central conductor to magnetize the gas before it is ionized. In this way, the plasma beta can be kept below 40%, so that, according to theory, the troublesome m=1 mode is also stabilized. (The r-z MHRDR code does not analyze the three-dimensional kink motion.) Although the convection associated with self-organization enhances thermal transport, the kinetic energy of turbulent motion is small compared to the thermal energy, and the energy transport is globally Bohm-like, which is acceptable for MTF. The MHRDR modeling is guiding the design of an experiment on the 2-TW Zebra z-pinch at UNR to test the inverse-pinch concept. For the parameters of the designed experiment, MHRDR simulations predict the 2-MV, 1-MA Marx generator will produce a deuterium plasma with B/spl sim/4T, n/spl sim/10/sup 22/m/sup -3/, T/spl sim/300 eV, and a lifetime of 10-50 microseconds. Understanding of the energy transport in this simple wall-confined plasma will increase confidence in the design of eventual integrated liner-on-plasma experiments.

Two-terawatt Zebra Z-pinch at the Nevada terawatt facility

A high-repetition-rate, 2-TW Z-pinch (Zebra or HDZP-II from LANL: 2 MV, 1.2 MA, 100 ns, 200 kJ, 1.9 ohm) has been assembled to investigate the early-time evolution of a current-driven wire, the plasma turbulence around and between wires, the acceleration of a plasma current sheet by a magnetic field, and the suppression or reduction of plasma instabilities, and to generate radiation for applications. The heating, expansion, and dynamics of wires driven by current prepulses similar to those at SNL-Z is being examined in isolated wires and soon in SNL-Z wire arrays. 290 trillion watts of X-rays can now be generated by a few cubic millimeters of plasma. The source of this plasma is the Z-pinch. This plasma confinement device drives a giant current through a tiny load, compressing and heating it with extreme current-produced magnetic fields. The Z-pinch suffers from plasma instabilities that limit its performance. The ultimate performance limit of the Z-pinch is unknown: another order of magnitude increase in X-ray power levels may be possible. Such an improvement would open up new applications. Understanding the dense Z-pinch is vital to the search to ameliorate it. This article describes the activation of the 2-TW Zebra Z-pinch, the development of diagnostics, and an initial single-wire experiment.

Kinetic modeling of x-ray laser-driven solid Al plasmas via particle-in-cell simulation

Solid-density plasmas driven by intense x-ray free-electron laser (XFEL) radiation are seeded by sources of nonthermal photoelectrons and Auger electrons that ionize and heat the target via collisions. Simulation codes that are commonly used to model such plasmas, such as collisional-radiative (CR) codes, typically assume a Maxwellian distribution and thus instantaneous thermalization of the source electrons. In this study, we present a detailed description and initial applications of a collisional particle-in-cell code, picls, that has been extended with a self-consistent radiation transport model and Monte Carlo models for photoionization and KLL Auger ionization, enabling the fully kinetic simulation of XFEL-driven plasmas. The code is used to simulate two experiments previously performed at the Linac Coherent Light Source investigating XFEL-driven solid-density Al plasmas. It is shown that picls-simulated pulse transmissions using the Ecker-Kröll continuum-lowering model agree much better with measurements than do simulations using the Stewart-Pyatt model. Good quantitative agreement is also found between the time-dependent picls results and those of analogous simulations by the CR code scfly, which was used in the analysis of the experiments to accurately reproduce the observed Kα emissions and pulse transmissions. Finally, it is shown that the effects of the nonthermal electrons are negligible for the conditions of the particular experiments under investigation.

Dynamics of electromagnetic drift-flute waves in high-beta plasmas in the presence of sheared plasma flows

Investigation of the mechanisms of penetration of high-beta plasma flows across the magnetic field is important for numerous applications. These studies are relevant to the physics of z-pinches, space physics (e.g. solar wind), and astrophysics (e.g. supernovae).

Linear and Nonlinear Development of the M=0 Instability in z-pinch Equilibria with Axial Sheared Flows

Summary form only given. Sheared flows in plasmas can play an important role in reducing the growth of instabilities. Axial sheared flows in cylindrical configurations are especially of interest in studying Z-pinch plasmas with applications to fusion energy and fusion propulsion, as well as in understanding the behavior of astrophysical jets. We use a 2D MHD simulation code (MHRDR) to study the linear and nonlinear development of m=0 modes. In order to allow the modeling of high Mach number plasma flows, the code was modified to include the use of periodic boundary conditions and a Van Leer monotonic advection treatment. The initial equilibrium was initialized either with pure instability modes or with random perturbation modes. The growth, saturation, and mode interaction was studied using FFT analysis. This study extends the linear theory results previously obtained by solving the ideal MHD linearized equations.

MHD Simulation Studies of Z-Pinch Perturbed Equilibria in the Presence of Sheared Axial Flows

Summary form only given. The growth and saturation of the m=0 magnetohydrodynamic (MHD) instability is numerically studied in a cylindrical Bennett equilibrium in the presence of sheared plasma flows with the aid of a 2D MHD code (MHRDR). Using Fourier analysis the amplitude of different axial modes is followed from the linear to the nonlinear regime. Their linear growth is measured and compared to the results obtained by solving the ideal MHD linearized equations and to the results obtained using a 3D hybrid simulation code