R. N. Sudan

One-dimensional intense laser pulse solitons in a plasma

A general analytical framework is developed for the nonlinear dispersion relations of a class of large amplitude one-dimensional isolated envelope solitons for modulated light pulse coupled to electron plasma waves, previously investigated numerically [Kozlov et al., Zh. Eksp. Teor. Fiz. 76, 148 (1979); Kaw et al., Phys. Rev. Lett. 68, 3172 (1992)]. The analytical treatment of weakly nonlinear solitons [Kuehl and Zhang, Phys. Rev. E 48, 1316 (1993)] is extended to the strongly nonlinear limit.

Nonlaminar multicomponent models for electron flow in positive polarity multigap accelerators

Electron flow in multigap positive‐polarity inductive accelerators is studied by numerical simulation and modeling. The objective of this work is to determine the operating principles of the electron flow such that an optimally efficient design of such machines can be achieved for intense ion beam generation. Because the electrons emitted in different gaps have different energies and canonical momenta, the theory of single‐component magnetic insulation has to be extended in order to describe such multicomponent electron flows. A two‐dimensional electromagnetic particle‐in‐cell code is used to simulate multicomponent electron flow in multigap accelerators with two, three, and four gaps. Observations from these simulations lead to new one‐dimensional, time‐independent models for these flows that incorporate the time‐averaged effects of diamagnetic electron vortices. Equivalent circuits are constructed for simulated accelerators using voltage–current relations predicted by the models. These circuit models ar...

Current neutralization and focusing of intense ion beams with a plasma‐filled solenoidal lens. I

The response of the magnetized plasma in an axisymmetric, plasma‐filled, solenoidal magnetic lens, to intense light ion beam injection is studied. The lens plasma fill is modeled as an inertialess, resistive, electron magnetohydrodynamic (EMHD) fluid since characteristic beam times τ satisfy 2π/ωpe,2π/Ωe≪τ≤2π/Ωi (ωpe is the electron plasma frequency and Ωe,i are the electron, ion gyrofrequencies). When the electron collisionality satisfies νe≪Ωe, the linear plasma response is determined by whistler wave dynamics. In this case, current neutralization of the beam is reduced on the time scale for whistler wave transit across the beam. The transit time is inversely proportional to the electron density and proportional to the angle of incidence of the beam with respect to the applied solenoidal field. In the collisional regime (νe>Ωe) the plasma return currents decay on the normal diffusive time scale determined by the conductivity. The analysis is supported by two‐and‐one‐half dimensional hybrid particle‐in‐c...

A self‐consistent quasistatic equilibrium for non‐neutral diamagnetic electron vortices

A self‐consistent quasistatic equilibrium for a non‐neutral cylindrical electron vortex has been found using the two‐dimensional relativistic electron fluid equations. While other work on electron vortices considered a regime where the vortex radius is much smaller than the collisionless skin depth λ=c/ωp, this equilibrium is valid for large‐radius, diamagnetic vortices and predicts a maximum radius of 23/2λ for a highly relativistic electron vortex. The vortex model shows good agreement with observations of diamagnetic electron vortices in two‐dimensional electromagnetic particle‐in‐cell simulations of magnetically insulated transmission lines.

Formation of field‐reversed ion rings in a magnetized background plasma

In typical field‐reversed ion ring experiments, an intense annular ion beam is injected across a magnetic cusp into neutral gas immersed in a solenoidal magnetic field. In anticipation of a new experimental thrust to create strong field‐reversed ion rings the beam evolution is investigated in a preformed background plasma on a time scale greater than an ion cyclotron period, using a new two and a half‐dimensional (21/2‐D) hybrid, particle‐in‐cell (PIC) code FIRE, in which the beam and background ions are treated as macro‐particles and the electrons as a massless fluid. It is shown that under appropriate conditions axial beam bunching occurs in the downstream applied field and a compact field‐reversed ring is formed. It is observed that the ring is reflected in a ramped magnetic field. Upon reflection its axial velocity is very much less than that expected from a single particle model due to the transfer of the mean axial momentum to the background ions. This increases the time available to apply a pulsed ...

Conventional solar flare theory re-examined

Major objections are raised to the fundamental paradigm underlying conventional solar flare theory, viz., that the required free energy can be stored in situ at the requisite density in the corona in nonpotential magnetic fields by the action of photospheric convective motion, that it can be released rapidly through magnetic reconnection by a triggering event, and that a significant fraction of this energy is converted to x rays. An alternative explanation that avoids these difficulties requires that magnetic energy in the form of strongly sheared flux tubes be stored subphotospherically prior to the flare. It is the emergence of this subphotospherically stored magnetic shear energy into the photosphere and above, and its rapid conversion through internal magnetic reconnection to other forms that constitutes an actual flare.

Field reversed ion rings

In typical field-reversed ion ring experiments, an intense annular ion beam is injected across a plasma-filled magnetic cusp region into a neutral gas immersed in a ramped solenoidal magnetic field. Assuming the characteristic ionization time is much shorter than the long ({ital t}{approx_gt}2{pi}/{Omega}{sub {ital i}}) beam evolution time scale, we investigate the formation of an ion ring in the background plasma followed by field reversal, using a 21/2-D hybrid, PIC code FIRE, in which the beam and background ions are treated as particles and the electrons as a massless fluid. We show that beam bunching and trapping occurs downstream in a ramped magnetic field for an appropriate set of experimental parameters. We find that a compact ion ring is formed and a large field reversal {zeta}={delta}{ital B}/{ital B}{approx_gt}1 on axis develops. We also observe significant deceleration of the ring on reflection due to the transfer of its axial momentum to the background ions, which creates favorable trapping conditions. {copyright} {ital 1995 American Institute of Physics.}