Phase dependence of Thomson scattering in an ultraintense laser field

The Thomson scattering spectra of an electron by an ultraintense laser field are computed. It is found that the electron orbit, and therefore its nonlinear Thomson scattering spectra, depend critically on the amplitude of the ultraintense laser field and on the phase at which the electron sees the laser electric field. Contrary to some customary notions, the Thomson scattering spectra, in general, do not occur at integer multiples of the laser frequency and the maximum frequency is proportional to the first instead of the third power of the electric field strength for the case of an ultraintense laser. The implications of these findings are discussed. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69907/2/PHPAEN-9-10-4325-1.pd

Azimuthal clumping instabilities in a ZZ-pinch wire array

A simple model is constructed to evaluate the temporal evolution of azimuthal clumping instabilities in a cylindrical array of current-carrying wires. An analytic scaling law is derived, which shows that randomly seeded perturbations evolve at the rate of the fastest unstable mode, almost from the start. This instability is entirely analogous to the Jeans instability in a self-gravitating disk, where the mutual attraction of gravity is replaced by the mutual attraction among the current-carrying wires.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87765/2/052701_1.pd

Caterpillar structures in single-wire Z-pinch experiments

A series of experiments have been performed on single-wire Z pinches (1–2 kA, 20 kV, pulse length 500 ns; Al, Ag, W, or Cu wire of diameter 7.5–50 μm, length 2.5 cm). Excimer laser absorption photographs show expansion of metallic plasmas on a time scale of order 100 ns. The edge of this plasma plume begins to develop structures resembling a caterpillar only after the current pulse reaches its peak value. The growth of these caterpillar structures is shown to be consistent with the Rayleigh–Taylor instability of the decelerating plasma plume front at the later stage of the current pulse. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71205/2/APPLAB-83-24-4915-1.pd

Mode Competition in Relativistic Magnetrons and Injection Locking in KW Magnetrons

Both relativistic and nonrelativistic magnetrons are under experimental and theoretical investigation at U of M. Relativistic (Titan‐6‐vane) magnetron experiments (300–400 kV, 1–10 kA, 0.5 microsecond) investigate mode control with various output coupling geometries. Mode competition between the pi mode and the 2/3 pi mode has been characterized for two‐versus‐three output extractors for comparison with particle in cell simulations. Phase measurements and time‐frequency‐analysis are performed for mode identification. Peak microwave output power on the order 0.5 GW has been measured, assuming equal output from 3 waveguides. Nonrelativistic (4 kV, <1A, kW microwave power) magnetron experiments are performed on commercial oven magnetrons for an in‐depth investigation of crossed‐field injection‐locking and noise. Injection‐locking is demonstrated by utilizing an oven magnetron as a reflection amplifier. Noise generation is explored as a function of injected signal and cathode conditions. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87505/2/301_1.pd

Azimuthal wire motion and ablation dynamics in z -pinches.

This dissertation describes theoretical, simulation, and experimental work to study ablation dynamics and azimuthal 'clumping' of wires in multi-wire z-pinch arrays. First, a resistive-inductive model of discrete filamentary conductors is developed to model wires in multi-wire z-pinches. These equations have been shown to decrease computation times over similar models in literature by as much as 5 orders of magnitude. From these new equations, a simulation code called REIN (REsistive-INductive) was developed to simulate discrete wire arrays. REIN simulations revealed that wires 'clumped' azimuthally when an initial random azimuthal perturbation was placed on the wire positions. Theory was developed to describe the observed clumping. The fastest growing clumping mode was found to be the pairing of neighboring wires (pi-mode). Experiments were conducted by the author on the COBRA accelerator (1 MA, 100 ns risetime) at Cornell University to look for azimuthal wire clumping in closely spaced wires. Radiography of wire cores did not demonstrate azimuthal wire clumping. However, based on the lack of motion, the major conclusion of the experiment is that less than 7% of the total current was flowing in the wire cores. Another calculation indicated that for wire cores to remain unvaporized by resistive heating, less than 1% of the current could have flowed in the cores. The other 93--99% of the current must therefore have flowed in the coronal plasma. Results also indicated presence of axial non-uniform ablation. Experiments performed at the University of Michigan on the MIZ-3 (9-10 kA, 400 ns risetime) and MIZ-4 (18 kA, 150 ns risetime) facilities are also described. On MIZ-4, plasma electron temperatures of ablated 30 mum Al wires were measured by emission spectroscopy to be 1.5-2 eV. Comparing results to previous experiments, it is observed that electron temperature scales very weakly with current (more than 10 times increase in current amplitude resulted in less than 2 times increase in electron temperature). On MIZ-3, more intense optical emission from wires preheated with DC currents indicated greater energy deposition than in slightly preheated wires.Ph.D.Applied SciencesNuclear engineeringPlasma physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126306/2/3238098.pd