Milena A. Angelova

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.

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.

Sensitivity of

Instabilities can affect the quality of flux compression and other high-current experiments. The modeling of such experiments involves a number of numerical difficulties. One such difficulty is the selection of proper material property models. This paper investigates the effect of different electrical resistivity models on the development of the m = 0 instability in a single cylindrical conductor driven by a current typical of the Atlas generator. The nonlinear development of this magnetohydrodynamic (MHD) instability can lead to current disruption. An important challenge for modeling is to predict the maximum magnetic field on a rod surface that can be obtained prior to disruption. This problem can also be generalized to the more complicated moving liner problem. A series of 2-D MHD simulations was performed with the state-of-the-art MHRDR code to conduct a sensitivity study. Results indicate that the development and growth of m = 0 instability is sensitive to the resistivity models used. Furthermore, it was observed that models with lower values of resistivity near the metal-insulator transition produce higher growth-rate sausage instability.

m = 0

Plasma formation on the surface of thick metal in response to a pulsed multi-megagauss magnetic field is being investigated at the University of Nevada, Reno, using aluminum rods that have radii larger than the magnetic skin depth. US and Russian radiation-magnetohydrodynamic codes are being used to help interpret the experimental results such as time of plasma formation and rate of current channel expansion. The best results obtained to date with the UNR code MHRDR use a standard SESAME Maxwell-construct EOS and a Russian resistivity model, and the computed times of formation agree well with the observations across the full range of wire diameters. This leads to the conclusion that plasma formation is an MHD effect and does not involve the non-MHD processes often evoked in other contexts. The computations show that plasma is formed in low-density material that is resistive enough to expand across the magnetic field and yet conductive enough that Ohmic heating exceeds expansion cooling as the expanding material undergoes the liquid-vapor transition.

Instability to Different Resistivity Models

Summary form only given. Applying a magnetic field of several megagauss to a surface drives an interesting interplay of magnetic diffusion, hydrodynamics, and radiative energy transfer. This physics is important in wire-array Z-pinches, high current fuses, magnetically insulated transmission lines, ultrahigh magnetic field generators, magnetized target fusion, and astrophysics. To investigate such plasmas experimentally, 1 MA was driven through a 1 -mm-diameter cylindrical aluminum rod, using the UNR Zebra generator. The 70-ns current rise was sufficiently short that the current skin depth was a small fraction of the conductor radius. Diagnostics included optical imaging to a time-gated intensified CCD camera and a streak camera, magnetic field probes, photodiodes, photomultipliers, and laser shadowgraphy, schlieren, interferometry, and Faraday rotation. These yielded information on the threshold for plasma formation, the expansion of the aluminum, the temperature at the transition between optically thick and optically thin matter, and the growth of the unstable m=0 mode driven by the curvature of the magnetic field. Plasma formation due to ohmic heating was distinguished from plasma formation due to high electric fields or electrical contacts by comparing shots with wire loads vs. loads machined from a solid aluminum cylinder to have a 1-mm-diameter central length but large-diameter contacts. Time-gated images show markedly more uniform light from the machined load than from the wire load. The relatively simple experimental setup was chosen in the hope of providing a benchmark with which to test and improve radiation-magnetohydrodynamics modeling. Measurements have been compared with the results of RAVEN and MHRDR computer simulations, using various assumptions for equation of state, electrical conductivity, and radiation. The simulations yield observed quantities such as luminosity, laser shadowgraphs, and m=0 mode growth. They also yield many additional interesting details, such as the propagation of a compression wave from the surface to the axis and back, with a resultant rapid radial expansion of the surface after peak current.

Numerical simulation of metallic surface plasma formation by megagauss magnetic fields

When, where, and how plasma forms on metal surfaces driven by intense current are important questions for both basic science and applications. The thermal ionization of the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, is being investigated with detailed experiments1-3 and numerical modeling4-8. Aluminum 6061 rods with initial radii (R0 from 0.25-1.00 mm) larger than the magnetic skin depth are pulsed with the 1.0-MA, 100-ns Zebra generator. The surface is examined with time-resolved imaging, radiometry, spectroscopy, and laser shadowgraphy. The surface magnetic field (Bs) rises at 30-80 MG/us, with corresponding peak Bs of 1.5-4 MG For these rise rates, thermal plasma is observed to form when Bs reaches 2.2 MG. Optical emission from the plasma surface is initially non-uniform, but becomes quite highly uniform as TBB increases. While the current is rising linearly, the Al surfaces expand at 3-4 km/s, with no evidence, after surface plasma forms, of either re-pinching or outward acceleration. At peak current, TBB is 20 eV for R0 =3D 0.50 mm rods, but only 0.7 eV for R0 =3D 1.00 mm rods. Strong plasma fluting develops in the first case, while extremely smooth expansion occurs in the second (indicating resistive vapor). Moreover, after peak current, plasma (if formed) accelerates (to 10 km/s), while resistive vapor continues expanding at constant speed. The well-characterized experiment is providing a benchmark for radiation-MHD modeling. VNIIEF-UP and UNR-MHRDR modeling have achieved results that agree well with observations. Plasma is formed in low density material resistive enough to expand across the magnetic field, yet conductive enough that ohmic heating exceeds expansion cooling as the expanding material undergoes the liquid-vapor transition. An analytic calculation indicates ohmic heating should produce plasma, consistent with numerical and experimental observations.

Plasma Formation and Evolution from an Aluminum Surface Driven by a MG Field

When, where, and how plasma forms on metal surfaces driven by intense current are important questions for both basic science and applications. The question of the conductivity of a metal surface under pulsed megagauss magnetic field has been posed since at least 1959, when Fowler et al.1 produced fields above 10 MG. The thermal ionization of the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, is being investigated with well-characterized experiments2,3 and detailed 1-D and 2-D numerical modeling4–6. Aluminum rods with radii larger than the magnetic skin depth are pulsed with the 1.0-MA, 100-ns Zebra generator. A novel mechanical connection eliminates nonthermal precursor plasma, which in earlier experiments was produced by electric-field-driven electron avalanche and arcing electrical contacts. The surface was examined with time-resolved imaging, pyrometry, spectroscopy, and laser shadowgraphy. Thermal plasma forms when the surface magnetic field reaches 2.0 MG, in agreement with recent theoretical results4. Measurement of the surface temperature, expansion velocity, and ionization state, as a function of applied field, constrains the choice of models used in the radiation-magnetohydrodynamic simulations, which include the Eulerian MHRDR code and the Lagrangian RAVEN and UP codes. Numerical predictions can vary by orders of magnitude, but, for MHRDR modeling, the computed times of plasma formation agree well with observations if a standard SESAME Maxwell-construct EOS is used in conjunction with a VNIIEF resistivity model. An analytic calculation indicates ohmic heating should produce plasma, consistent with numerical and experimental observations.

Thermal aluminum plasma formation and evolution by pulsed megagauss field

Recent aluminum rod experiments1,2 driven by 1-MA Zebra generator at University of Nevada, Reno (UNR) have provided a benchmark for magnetohydrodynamic (MHD) modeling. The innovative ‘hourglass’ and ‘barbell’ load geometries used in the experiments made it possible to distinguish between plasma formation due to Ohmic heating, which can be studied numerically utilizing MHD codes, and plasma formation due to high electric fields, by introducing a large-diameter contact with the electrodes. This prevents the explosive electron emission (EEE) at the contacts which triggers initial plasma formation in the conventional rod explosion experiments.

Thermal ionization of an aluminum surface by pulsed megagauss field

Instabilities can affect the quality of flux compression and other high current experiments. The modeling of such experiments involves a number of numerical difficulties. One such difficulty is the selection of proper material property models. This work investigates the effect of different EOS and other material property models on the development of the m=0 instability on a single cylindrical conductor driven by a current typical of the Atlas generator. The nonlinear development of this magnetohydrodynamic instability can lead to current disruption. An important challenge for modeling is to predict the maximum magnetic field on the rod surface that can be obtained prior to disruption. This problem can also be generalized to the more complicated moving liner problem. A series of 2-D magnetohydrodynamic simulations are performed with the state-of-the-art MHRDR code to conduct a sensitivity study.

Numerical study of plasma formation from aluminum rods driven by megaampere currents

Summary form only given. Magnetically insulated transmission lines (MITLs) employ the principle of magnetic insulation to efficiently transmit energy from a source to a load. They are a part of pulsed power devices such as fast Z-pinches and particle accelerators, which operate in the regime of extremely high voltages. Cathode plasma produced as a result of strong electric fields in MITLs may significantly decrease the amount of transmitted energy or may close the MITL gap causing an electrical termination. An understanding of plasma formation and evolution in MITL gaps can help improve the efficiency of energy transmission, thereby improving the design and characteristics of some pulsed power devices. Plasma evolution in MITL gaps is complex, involving several competing processes, including magnetic field-plasma interactions and plasma radiation. This study concentrates on the understanding of plasma radiation and the ways this radiation affects the plasma evolution in the MITL. This paper will present the results of one and two-dimensional magnetohydrodynamic plasma simulations as well as the development of collisional radiative equilibrium (CRE) tables for plasma radiation, EOS, and transport coefficients