J. S. De Groot

Thomson scattering from laser plasmas

Thomson scattering has recently been introduced as a fundamental diagnostic of plasma conditions and basic physical processes in dense, inertial confinement fusion plasmas. Experiments at the Nova laser facility [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)] have demonstrated accurate temporally and spatially resolved characterization of densities, electron temperatures, and average ionization levels by simultaneously observing Thomson scattered light from ion acoustic and electron plasma (Langmuir) fluctuations. In addition, observations of fast and slow ion acous- tic waves in two-ion species plasmas have also allowed an independent measurement of the ion temperature. These results have motivated the application of Thomson scattering in closed-geometry inertial confinement fusion hohlraums to benchmark integrated radiation-hydrodynamic modeling of fusion plasmas. For this purpose a high energy 4{omega} probe laser was implemented recently allowing ultraviolet Thomson scattering at various locations in high-density gas-filled hohlraum plasmas. In partic- ular, the observation of steep electron temperature gradients indicates that electron thermal transport is inhibited in these gas-filled hohlraums. Hydrodynamic calcula- tions which include an exact treatment of large-scale magnetic fields are in agreement with these findings. Moreover, the Thomson scattering data clearly indicate axial stagnation in these hohlraums by showing a fast rise of the ion temperature. Its timing is in good agreement with calculations indicating that the stagnating plasma will not deteriorate the implosion of the fusion capsules in ignition experiments.

Dynamics of a high-power aluminum-wire array Z-pinch implosion

Annular Al-wire Z-pinch implosions on the Saturn accelerator [D. D. Bloomquist et al., Proceedings, 6th Pulsed Power Conference (Institute of Electrical and Electronics Engineers, New York, 1987), p. 310] that have high azimuthal symmetry exhibit both a strong first and weaker second x-ray burst that correlate with strong and weaker radial compressions, respectively. Measurements suggest that the observed magnetic Rayleigh–Taylor (RT) instability prior to the first compression seeds an m=0 instability observed later. Analyses of axially averaged spectral data imply that, during the first compression, the plasma is composed of a hot core surrounded by a cooler plasma halo. Two-dimensional (2-D) radiation magnetohydrodynamic computer simulations show that a RT instability grows to the classic bubble and spike structure during the course of the implosion. The main radiation pulse begins when the bubble reaches the axis and ends when the spike finishes stagnating on axis and the first compression ends. These ...

Charge order in LuFe2O4: an unlikely route to ferroelectricity.

We present the refinement of the crystal structure of charge-ordered LuFe2O4, based on single-crystal x-ray diffraction data. The arrangement of the different Fe-valence states, determined with bond-valence-sum analysis, corresponds to a stacking of charged Fe bilayers, in contrast with the polar bilayers previously suggested. This arrangement is supported by an analysis of x-ray magnetic circular dichroism spectra, which also evidences a strong charge-spin coupling. The nonpolar bilayers are inconsistent with charge order based ferroelectricity.

Dielectric Properties of Charge-Ordered LuFe2O4 Revisited: The Apparent Influence of Contacts

We show results of broadband dielectric measurements on the charge ordered, proposed to be multiferroic material LuFe(2)O(4). The temperature and frequency dependence of the complex permittivity as investigated for temperatures above and below the charge-order transition near T(CO)≈320  K and for frequencies up to 1 GHz can be well described by a standard equivalent-circuit model considering Maxwell-Wagner-type contacts and hopping induced ac conductivity. No pronounced contribution of intrinsic dipolar polarization could be found, and thus the ferroelectric character of the charge order in LuFe(2)O(4) has to be questioned.

Competing ferri- and antiferromagnetic phases in geometrically frustrated LuFe2O4.

We present a detailed study of magnetism in LuFe(2)O(4), combining magnetization measurements with neutron and soft x-ray diffraction. The magnetic phase diagram in the vicinity of T(N) involves a metamagnetic transition separating an antiferro- and a ferrimagnetic phase. For both phases the spin structure is refined by neutron diffraction. Observed diffuse magnetic scattering far above T(N) is explained in terms of near degeneracy of the magnetic phases.

Distributed absorption model for moderate to high laser powers

An analytic model is presented of a planar plasma heated by moderate to high laser powers such as are used to drive fusion pellets. Unlike previous models, this model explicitly includes the temporal evolution of the heat conduction region. It is shown that previous steady‐state models apply only to a narrow range of the parameters (laser energy flux and pulse width) used in laser pellet fusion. The new model is shown to agree with flux‐limited hydrodynamics simulations. The model and hydrodynamic simulations for classical heat transport show that the mass ablation rate and ablation pressure are essentially independent of laser wavelength for parameters relevant to laser fusion.

Electron heat transport in a steep temperature gradient

Temporal and spatial measurements of electron heat transport are made in the University of California Davis AURORA device (J. H. Rogers, Ph.D. dissertation, University of California, Davis, 1987). In AURORA, a microwave pulse heats a region of underdense, collisional, plasma (n/ncr ≲1, where ncr =1.8×1010 cm−3 is the critical density, Te0 ≊0.15 eV, and the electron scattering mean free path λ⊥≳2 cm). In this region, strong thermal heating (Tc ≲0.7 eV) as well as suprathermal heating (Th≊3 eV) is observed. The strong heating results in a steep temperature gradient that violates the approximations of classical heat diffusion theory (LT/λ⊥≳3 for thermal electrons, where LT=Tc(∂Tc/∂z)−1 is the cold electron temperature scale length. The time evolution of the electron temperature profile is measured using Langmuir probes. The measured relaxation of the temperature gradient after the microwave pulse is compared to calculations using the Fokker–Planck International code [Phys. Rev. Lett. 49, 1936 (1982)] and the...

Homogeneous electrical explosion of tungsten wire in vacuum

Experimental results on Joule energy deposition upon initiation of a fast electrical explosion of 16-μm tungsten wire in vacuum at current densities of more than 108 A/cm2 are reported. We have found that explosion with a fast current rise time (∼170 A/ns into a short) results in homogeneous and enhanced deposition of electrical energy into the tungsten before surface flashover. The maximum tungsten wire resistivity reaches a value of up to ∼185 μΩ cm before surface flashover that significantly exceeds the melting boundary and corresponds to a temperature of ∼1 eV. The highest values for light radiation and expansion velocity of wire ∼1 km/s were observed for the fast explosion. For the explosion mode with a slower current rise time (∼22 A/ns into a short), we observed the existence of an “energy deposition barrier” for tungsten wire. In the slow explosion mode, the current is reconnected to the surface shunting discharge before melting. The maximum tungsten wire resistivity in this case reaches the value of ∼120 μΩ cm, which is less than indicative of melting. Also, the energy deposition along the wire is strongly inhomogeneous, and wire is disintegrated into parts. We attribute the early reconnection of the current to the surface discharge for the slow explosion to high electron emission from the wire surface, which starts before melting.

Eigenvalue solution for the ion‐collisional effects on ion‐acoustic and entropy waves

The linearized ion Fokker–Planck equation is solved as an eigenvalue problem under the condition of collisionless electrons in the quasineutral limit (φ=0) for ionization‐temperature ratios, ZTe/Ti=2, 4, and 8 for entropy waves and ionization‐temperature ratios, ZTe/Ti=4, 8, 16, 32, 48, 64, and 80 for ion‐acoustic waves. The perturbed ion distribution function for the ion‐acoustic and entropy waves is formed from a Legendre polynomial expansion of eigenvectors and can be used to calculate collisionally dependent macroscopic quantities in the plasma such as gamma (Γ=Cp/Cv), the ratio of specific heats, and the ion thermal conductivity (κi).

Particle simulations of plasma and dielectric Čerenkov masers

A new particle simulation model has been developed to investigate Cerenkov masers. The novel aspects of the code are briefly described, and results of simulations of two types of Cerenkov masers are presented. The first device uses a conventional dielectric lining as its slow‐wave structure. The second is a new type of Cerenkov maser in which a circular waveguide is partially filled with a dense annular plasma instead of a dielectric layer. Both simulations agree well with experimental results and linear theory calculations. Saturation of the instability is shown to be due to trapping of the beam electrons. The relative merits of each system are discussed.