Roger D. Jones

Fast ions and hot electrons in the laser−plasma interaction

Data on the emission of energetic ions produced in laser–matter interactions have been analyzed for a wide variety of laser wavelengths, energies, and pulse lengths. Strong correlation has been found between the bulk energy per AMU for fast ions measured by charge cups and the x‐ray‐determined hot electron temperature. Five theoretical models have been used to explain this correlation. The models include (1) a steady‐state spherically symmetric fluid model with classical electron heat conduction, (2) a steady‐state spherically symmetric fluid model with flux limited electron heat conduction, (3) a simple analytic model of an isothermal rarefaction followed by a free expansion, (4) the lasnex hydrodynamics code [Comments Plasma Phys. Controlled Fusion 2, 85 (1975)], calculations employing a spherical expansion and simple initial conditions, and (5) the lasnex code with its full array of absorption, transport, and emission physics. The results obtained with these models are in good agreement with the experi...

Self-focusing and filamentation of laser light in high Z plasmas

Self‐focusing and filamentation of short wavelength laser light in high Z plasmas of interest to laser fusion are discussed. It is found that self‐focusing behavior is very dependent on the details of the characteristics of the laser beam, the plasma conditions, and the energy transport processes. Laser light absorption and self‐focusing are strongly competitive processes. At. 0.26 μm wavelength the collisional absorption is often so great that there is no intensity amplification of the beam despite the fact that strong self‐focusing is present. Wide variations are found in laser light penetration, affected by several factors. Diverging optics reduce the likelihood of self‐focusing. Large scale length density gradients have little effect on focusing behavior. The self‐focusing behavior is very dependent on beam shape. Large scale hot spots can have a significant effect on whole beam self‐focusing early in the pulse. The behavior of small scale hot spots can be qualitatively different than the standard pic...

Kinetic theory, transport, and hydrodynamics of a high‐Z plasma in the presence of an intense laser field

A systematic treatment of collisional kinetic theory and transport for a high‐Z plasma in the presence of an intense laser field is presented. The time‐averaged electron kinetic equation is obtained in three ways. Collisionally induced harmonics of the laser frequency in the electron distribution function examined. In the case of high and low laser intensities, the kinetic equation is shown to possess self‐similar solutions. In the low field limit, the solution found earlier by Langdon is recovered. In the high field limit, under special conditions, the solution is shown to be a Maxwellian. Monte Carlo calculations are found to corroborate these findings. In the low field limit, it is shown that the anisotropic part of the distribution follows the isotropic part adiabatically. An expression for the anisotropic part is obtained. The inverse bremsstrahlung heating rate is derived and shown to agree with Monte Carlo results. Memory effects in transport are discussed. The bremsstrahlung emission rate and the ...

Laser-produced plasmas in medicine

The authors examine those areas of laser medicine in which plasmas (ionized gases) are produced. In fact, the presence of a plasma is essential for the various applications to succeed. The authors consider examples of the plasmas produced in ophthalmology (e.g., lens membrane destruction following cataract surgery), in urology and gastroenterology (e.g., kidney and gall stone ablation and fragmentation), and in cardiology and vascular surgery (e.g., laser ablation and removal of fibro-fatty and calcified arterial plaque). Experimental data are presented, along with some results from computer simulations of the phenomena. Comments on future directions in these areas are included. >

Laser induced density profiles in an isothermal plasma

The absorption and scattering of laser light in laser fusion targets is very dependent on the exact form of the laser induced density profiles. The possible profiles that can exist are examined using a simple isothermal model. It is shown that four different structures are possible. The flow speed profiles and the speed of the critical surface are determined for each of these structures. It is shown that two of these structures are locally overdense and can exist in an underdense plasma, and thus may have a large effect on the amount of light reaching the critical surface.

‘‘Flicker’’ in small scale laser–plasma self‐focusing

Small amplitude, short wavelength ion acoustic waves in laser‐produced plasmas cause fluctuations in the trajectories of light rays that can lead to time‐dependent, self‐sustaining shifting of focal spots and a somewhat random redistribution of the light near the critical surface. This flickering is seen in simulations involving small scale beam inhomogeneities over a uniform background laser profile, which model the center of a realistic laser beam. The effect can cause significant intensity multiplication in long scale length high‐Z plasmas with only modest beam imperfections.

Laser-induced profile modification: Effect of electron response

Physical processes which occur at the critical surface such as resonance absorption and scattering of the incident laser light are very sensitive to the exact form of the laser induced density profile. The presence of ‘‘overdense bumps’’ in density profiles has been seen experimentally and calculated theoretically using a simple isothermal model. These structures are important, in that they could, in an experimental situation, reflect a significant amount of light before it reaches the critical surface and considerably affect the physics occuring within a laser pellet target. A generalization of a previous calculation by using an arbitrary equation of state (density response of the electrons to the electrostatic potential φ) of the form ρ=ρ(φ) is obtained. In particular, two models for the electronic response (one including the effects of the presence of trapped electrons within the overdense shock structures) calculating the resulting effect on the produced density profiles are investigated. Qualitativel...

Kinetic theory of electron drift vortex modes

Magnetic electron drift vortex modes are examined using a kinetic theory. The dispersion is found to be much different than that obtained from the fluid theory. For frequencies much smaller than the electron plasma frequency, the waves are heavily Landau damped. The modes are weakly damped at frequencies just below the plasma frequency and hence have an electromagnetic rather than magnetic character.

Constants of motion in a helical magnetic field

The relationship between the ’’screw’’ symmetry of a helical magnetic field and the constants of motion of the system is obtained. In particular, the integral of motion for the symmetry is derived and the relationship between this integral and one obtained by Laird and Knox is demonstrated.

Electrostatic electron surface modes on a plasma–vacuum interface of finite width

The electron surface mode dispersion relation, including Landau damping, is obtained for a vacuum–plasma interface. Unlike previous work, the interface is permitted to have a finite width and no wall boundary conditions are assumed. When the density gradient scale length Ln is large compared with a Debye length k−1D and small compared with a surface mode wavelength 2πk−1, then the mode frequency is ω=(ωp/21/2) (1+kLn/6), and the Landau damping rate is γ =−[6/(2π)1/2]ωp/(kDLn). These expressions are much different than the comparable expressions for a wall‐confined plasma.