K. Gomberoff

Observations of two-dimensional magnetic field evolution in a plasma opening switch

The time dependent magnetic field distribution was studied in a coaxial 100-ns positive-polarity Plasma Opening Switch (POS) by observing the Zeeman effect in ionic line emission. Measurements local in three dimensions are obtained by doping the plasma using laser evaporation techniques. Fast magnetic field penetration with a relatively sharp magnetic field front (⩽1 cm) is observed at the early stages of the pulse (t≲25). Later in the pulse, the magnetic field is observed at the load-side edge of the plasma, leaving “islands” of low magnetic field at the plasma center that last for about 10 ns. The two-dimensional (2-D) structure of the magnetic field in the r,z plane is compared to the results of an analytical model based on electron-magneto-hydrodynamics, that utilizes the measured 2-D plasma density distribution and assumes fast magnetic field penetration along both POS electrodes. The model results provide quantitative explanation for the magnetic field evolution observed.

Magnetic field penetration due to the Hall field in (almost) collisionless plasmas

The fast penetration of magnetic fields into plasmas due to the Hall field is described. The penetration occurs in nonuniform plasmas of a characteristic length smaller than the ion skin depth, it is much faster than the ion motion, and its rate is independent of the resistivity. Some previous results are described: a shock penetration of the magnetic field accompanied by a large energy dissipation, and field expulsion from an initially magnetized plasma. It is then shown how the Hall field can enhance the penetration into a plasma surrounded by vacuum. Finally, it is demonstrated how the evolution of the magnetic field in a plasma that conducts current between electrodes depends crucially on its evolution near the electrodes, when a realistic density profile is taken into account.

Fast magnetic field penetration into a cylindrical plasma of a nonuniform density

The penetration of a magnetic field into a cylindrical plasma of a density that varies both radially and axially is studied. The magnetic field penetrates rapidly due to the Hall field, along constant nr2 lines (n is the dimensionless plasma density and r is the dimensionless radial coordinate). For a plasma that conducts between two cylindrical electrodes, it is shown that there is magnetic field penetration for both positive and negative polarity cases as long as there is penetration along the electrodes. The magnetic field evolution is found, analytically and numerically, for different time behaviors of the magnetic field at the boundaries. Ion velocities are also calculated.

Magnetic field penetration and electron heating in weakly nonuniform plasmas

The simultaneous magnetic field penetration and electron heating in plasmas of nonuniform density and of azimuthal cylindrical symmetry are studied. The penetration is caused by both the convective skin effect and the pressure gradient. The shock penetration of the current layer into a cold unmagnetized plasma is shown to result in equal magnetic field energy and electron thermal energy. A direct relation is shown between the heating of an electron along its orbit and the deviation from the frozen‐in law. General Hugoniot relations are presented for the shock penetration into warm magnetized plasmas. The profiles of the magnetic field and of the electron thermal energy are found in the steady skin layer that is formed in the case of no penetration.

Fast decay of plasma return currents due to whistler waves

The evolution of the return current induced by a charged particle beam in a magnetized plasma is studied. The beam current is perpendicular to the background magnetic field. The return current is shown to depart from the beam along the background magnetic field with a whistler rather than a diffusion or an Alfven velocity. In a plasma bounded by two conductors the return current oscillates with the whistler period. Analytical expressions for the evolution of the magnetic field and of the plasma return current are derived for a beam with a finite width and with various rise time dependences. When the whistler time is shorter than the rise time of the beam current, the plasma return current does not grow beyond the whistler time.

Effect of Hall currents on the steady convection of a current‐carrying cylindrical plasma

The effect of Hall currents on the steady convection of a fully ionized current‐carrying cylindrical plasma column surrounded by perfectly conducting walls is studied. In general, when the Hall term is large, it can affect the magnetohydrodynamic (MHD) stability problem significantly, but as long as it is small the instability persists and the marginal modes remain the same as without Hall currents. In some cases, though, even if the Hall term is large, the convective character of some modes is not changed. Thus, it is shown that convection due to the combined effect of resistivity and heat conductivity is not affected by Hall currents. However, convection due to heat conductivity and viscosity is no longer possible in the presence of Hall currents. When the combined effect of heat conductivity, resistivity, and viscosity is considered, the Hall term changes the stability properties of the system in such a way that there are no modes that are both marginal and stationary. Consequently, there are no convec...

Evolution of a magnetic field and plasma pushing in the presence of a parallel magnetic field

The evolution in the plasma of a magnetic field that is fast‐rising at the plasma boundary, and the simultaneous pushing of the plasma by that magnetic field, are studied for the case that a parallel magnetic field is present in the plasma. It is shown that initially the magnetic field propagates into the plasma in the form of a whistler wave. The magnetic field evolution is then governed by the Schrodinger equation for a free particle, as described in the previous simplified analysis [Phys. Fluids B 3, 1546 (1991)]. Later, the gas‐dynamics shock propagation exceeds the magnetic field propagation. If the magnetic field in the plasma is initially oblique and not parallel, a quasiperpendicular fast (super‐Alfvenic) shock propagates in the plasma, following a whistler precursor. The width of the current channel is on the scale of the ion skin depth.

The effect of displacement current on whistler propagation of a fast-rising magnetic field

The effect of the displacement current on the magnetic field propagation into plasmas in the form of a whistler wave is studied. For times between the electron and the ion cyclotron periods and if the plasma is tenuous so that the electron plasma frequency is smaller than the cyclotron frequency, the magnetic field is shown to be governed by the Telegraph equation with complex coefficients. The propagation of a fast‐rising magnetic field is examined and at early times the magnetic field is shown to propagate as a left‐polarized wave with the light velocity. At a later time the magnetic field propagates as the dispersive whistler wave and is governed by the diffusion equation with a complex coefficient [Fruchtman and Maron, Phys. Fluids B 3, 1546 (1991)]. As expected, the front of the wave keeps propagating with the finite velocity of light. The effect of collisional resistivity is also considered.

2D magnetic field evolution and electron energies in plasma opening switches

Summary form only given, as follows. We present measured time dependent 2-D structures of the magnetic field in two plasma opening switches (on 100 ns and 0.5 /spl mu/s time scales), together with an analytical model based on EMHD used to explain the data for the short time switch. The magnetic field, local in 3-D, is obtained by doping the plasma with various species and observing the Zeeman splitting of their spectral lines, using polarization spectroscopy. In the coaxial 100-ns switch, fast (/spl sim/10/sup 8/ cm/s) field penetration with a relatively sharp field front is observed, together with a non-monotonic axial distribution later in the pulse. The model developed, that utilizes the measured 2D plasma density distribution and assumes fast field penetration along both POS electrodes, provides quantitative explanation for the observed field evolution. In the planar 0.5 /spl mu/s switch, a 3-D mapping of the field is obtained. The current channel, 2.5 cm wide, is found to propagate downstream at a velocity /spl sim/3/spl times/10/sup 7/ cm/s. The electron energy in the current channel is studied spectroscopically. The replacement of electrons and the energy dissipation due to the field penetration will be discussed. A comparison of the result for the two time-scale switches and for opposite charging polarities in the coaxial switch will be presented.

Convection in a cylindrical plasma column with a free boundary

It is shown that in a current‐carrying cylindrical plasma with a free boundary, resistivity and thermal conductivity can lead to large‐scale steady convection. Convection can take place for any azimuthal wave number m, provided that ‖k‖a≫1 or ‖k‖a≪1, where k is the longitudinal wave number and a is the radius of the plasma column. Otherwise, convection occurs only for large m values. The states that can trigger large‐scale stationary convection are incompressible and satisfy the heat conduction equation for arbitrary finite adiabaticity coefficient, γ, provided that η/κ=8π/3, where η is the resistivity and κ is the thermal conductivity. The convection cells are helically twisted tubes, and the number of convection cells is equal to 2m. The situation is very similar to the case when the plasma column is limited by perfectly conducting walls.