Volodymyr Makhin

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.

A Zebra Experiment to Study Plasma Formation by Megagauss Fields

An alternative concept for fusion energy production is magnetized target fusion using metal liners to compress a mixture of magnetic flux and plasma fuel. In liner flux compression experiments, megagauss fields are produced at peak compression that heats the surfaces of aluminum walls of the liner cavity. Some radiation magnetohydrodynamic (MHD) modeling indicates that plasma formation should occur between 3 and 5 MG; however, such modeling depends on assumed material properties, which are a topic of ongoing research. Load hardware and diagnostics have been developed to study metal vapor and plasma formation on aluminum surfaces subjected to pulsed megagauss fields on the University of Nevada Zebra facility. The experiment is designed to study this interesting threshold for plasma formation. A current of 1 MA is pulsed along a stationary central rod to generate magnetic fields of 2-4 MG. The goal is to observe and diagnose the formation of metal vapor and plasma in the vicinity of the rod. The simple geometry enables easy access by diagnostics, which include magnetic sensors, filtered photodiode measurements, optical imaging, and laser schlieren, shadowgraphy, and interferometry. From these measurements, the magnetic field, the temperature of the surface metal plasma, the radiation field, and the growth of instabilities can be inferred. The diagnostics are time resolved to individually examine the distinct phases of heating, surface plasma formation predicted by radiation MHD modeling, and instability.

Particle and heat transport in a dense wall-confined MTF plasma (theory and simulations)

Plasma beta in magnetized target fusion systems is sometimes much greater than 1, and the plasma may be in direct contact with the imploding liner. Plasma processes are strongly dominated by inter-particle collisions. Under such conditions, the plasma microturbulence, behaviour of α-particles, and plasma equilibria are very different from conventional fusion systems. This paper contains the most comprehensive analysis of the corresponding phenomena to date. Two-dimensional numerical simulations of plasma convection in the targets of a diffuse pinch type demonstrate an onset of convection in this configuration.

FRC lifetime studies for the Field Reversed Configuration Heating Experiment (FRCHX)

The goal of the Field-Reversed Configuration Heating Experiment (FRCHX) is to demonstrate magnetized plasma compression and thereby provide a low cost approach to high energy density laboratory plasma (HEDLP) studies, which include such topics as magneto-inertial fusion (MIF). A requirement for the field-reversed configuration (FRC) plasma is that the trapped flux in the FRC must maintain confinement of the plasma within the capture region long enough for the compression process to be completed, which is approximately 20 microseconds for FRCHX. Current lifetime measurements of the FRCs formed with FRCHX show lifetimes of only 7 ∼ 9 microseconds once the FRC has entered the capture region.

Magnetohydrodynamic simulation of the inverse-pinch plasma discharge

A wall confined plasma in an inverse-pinch configuration holds potential as a plasma target for Magnetized Target Fusion (MTF) as well as a simple geometry to study wall-confined plasma. An experiment is planned to study the inverse-pinch configuration using the Zebra Z pinch [B. S. Bauer et al., AIP Conference Proceedings Vol. 409 (American Institute of Physics, Melville, 1997), p. 153] of the Nevada Terawatt Facility at the University of Nevada, Reno (UNR). The dynamics of the discharge formation have been analyzed using analytic models and numerical methods. Strong heating occurs by thermalization of directed energy when an outward moving current sheet (the inverse pinch effect) collides with the outer wall of the experimental chamber. Two-dimensional magnetohydrodynamic simulations show Rayleigh–Taylor and Richtmyer–Meshkov like modes of instability, as expected because of the shock acceleration during plasma formation phase. The instabilities are not disruptive, but give rise to a mild level of turbu...

Self-organization observed in numerical simulations of a hard-core diffuse Z pinch

A hard-core Z-pinch plasma (metal conductor on axis) with an unstable pressure profile can rearrange itself through m=0 interchange motions to produce a stable pressure profile. In this paper the self-organization process is demonstrated in numerical simulations of an experimental plasma formation process, using a two-dimensional compressible two-fluid magnetohydrodynamic code. The stabilization process results in m=0 turbulence, which has a level of kinetic energy that is saturated typically at a few percent of the plasma thermal energy. Using idealized initial conditions for simulations with an axial sinusoidal density perturbation, it is possible to observe in detail the development of instability and then turbulence. At first a coherent Rayleigh–Taylor type motion grows exponentially, with localized isentropic heating and cooling associated with the motion. Then the bubble and spike structure breaks up and incoherent m=0 turbulence develops.

Operation regimes of magnetically insulated transmission lines

Magnetically insulated transmission lines (MITLs) are commonly used for efficient power transport in the vacuum section of pulsed power devices. Plasma forming from metal surfaces limits the power transmitted to a load through MITLs. It eventually shunts the load, producing so-called MITL closure. Fundamental experiments are being performed on high intensity power transmission through coaxial cylindrical vacuum transmission lines. A current that rises to 1 MA in 100 ns is driven through the MITLs by a 2-MV, 2-/spl Omega/ pulse generator (Zebra). The condition of the MITL surfaces is carefully controlled and characterized before each shot. Differential B-dot probes measure the current before and after the MITL, to determine the time of gap closure. Optical imaging and laser diagnostics observe the plasma evolution in the gap with time and space resolution. The radial gap of the cylindrical vacuum transmission line has been systematically varied, and the time of MITL closure measured. They increase with the radial gap size in a discontinuous manner. Critical transitions (discontinuous jumps in closure time) appear to separate distinct MITL operation regimes. This is the first experiment and data set of this kind known to the authors. Electromagnetic-particle-in-cell and radiation-magnetohydrodynamic computer modeling assist the experiment, being used to refine the experimental design and to interpret the results.

Investigation of a novel discharge EUV source for microlithography

A plasma discharge could be an inexpensive and efficient EUV source for microlithography, if issues of brightness, lifetime, debris, repetition rate, and stability can be resolved. A novel discharge EUV source (international patent pending) is being investigated that may offer an economical solution to these issues. The novel EUV discharge seeks to efficiently assemble a hot, dense, uniform, axially stable plasma with magnetic pressure and inductive current drive, employing resonant theta-pinch-type compression of plasma confined in a magnetic mirror. This resonantly compressed mirror plasma (RCMP) source would be continuously driven by a radio frequency oscillator, to obtain an EUV conversion efficiency greater than that of sources in which the plasma is discarded after each radiation burst. An analytic calculation indicates the novel RCMP source could provide 115 W of 13.45 nm radiation in 3.3 mm2sr etendue to an intermediate focus. Numerical modeling of RCMP dynamics has been performed with MHRDR-EUVL, a magnetohydrodynamic (MHD) numerical simulation with atomic and radiation physics. The numerical simulation demonstrates the efficacy of resonant magneto-acoustic heating. An experiment is being developed to test the new concept.

Development of an Experiment to Study Plasma Formation by Megagauss Fields

Load hardware and diagnostics have been developed to study metal vapor and plasma formed from aluminum surfaces by pulsed MG fields on Zebra. Radiation MHD modeling indicates plasma formation should occur between 3-5 MG, but such modeling depends on assumed material properties, which are a topic of ongoing research. The experiment is designed to learn about this interesting threshold for plasma formation. A current of 1 MA is pulsed along a stationary, central wire, to generate magnetic fields of 3-5 MG. The goal is to observe and diagnose the formation of metal vapor and plasma in the vicinity of the wire. The simple geometry enables easy access by diagnostics, which include magnetic sensors, filtered photodiode measurements, optical imaging, and laser schlieren, shadowgraphy, interferomerry and Faraday rotation. From these measurements the magnetic field, the density and temperature of the surface metal plasma, the radiation field, and the growth of instabilities will be inferred. Predictions of experimental data will be calculated from numerical simulations and compared with experimental results. The diagnostics are time resolved, so as to examine individually the distinct phases of compression, plasma formation, radiation-magnetohydrodynamic evolution, and instability. Diagnostics have being developed using a small HV pulser.

The Challenge of Wall-Plasma Interaction with Pulsed MG Fields Parallel to the Wall

Experiments suitable for a variety of pulsed power facilities are being developed to study plasma formation and stability on the surface of typical liner materials in the megagauss (MG) regime. Understanding the plasma properties near the surface is likely to be critical for the design of Magnetized Target Fusion experiments, where the plasma density in the region near the wall can play an important role in setting the transport from hot fuel to the cold boundary. From the perspective of diagnostic access and simplicity, the surface of a stationary conductor with large enough current to generate MG surface field offers advantages compared with studying the surface of a moving liner. This paper reports on recent experiments at UNR that have generated magnetic fields in the range of about 0.2 to 3 MG, which confirm the viability of future experiments planned at Atlas and/or Shiva Star. Diagnostics reported here involve electrical measurements, streak camera photography, and surface luminosity. Additional diagnostic measurements and numerical modeling will be reported in the future.