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Electrical & Computer Engineering Faculty Publications

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Articles 181 - 199 of 199

Full-Text Articles in Physics

Effect Of Fluctuations On Lower Hybrid Power Deposition And Hard X-Ray Detection, George Vahala, Linda L. Vahala, Paul T. Bonoli Jan 1992

Effect Of Fluctuations On Lower Hybrid Power Deposition And Hard X-Ray Detection, George Vahala, Linda L. Vahala, Paul T. Bonoli

Electrical & Computer Engineering Faculty Publications

The hard X-ray intensity radial profiles from lower hybrid current drive experiments are interpreted as being correlated with fluctuations in the bulk plasma. This view seems to be dictated by comparing the hard X-ray data for various n║ with the Monte Carlo solutions of the lower hybrid wave energy deposition on plasma electrons. Information on internal magnetic fluctuations may, under certain conditions, be unfolded from a n║ scan of the hard X-ray profiles.

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Self-Consistent Determination Of Low-Za Pellet Ablation And Pellet Penetration, Linda L. Vahala, Glenn Gerdin, A. G. El Cashlan, Paul Parks Jan 1992

Self-Consistent Determination Of Low-Za Pellet Ablation And Pellet Penetration, Linda L. Vahala, Glenn Gerdin, A. G. El Cashlan, Paul Parks

Electrical & Computer Engineering Faculty Publications

The ablation dynamics of LiT pellets are solved self-consistently over a modest range of parameters using a surface dissociation model. The self-consistently determined parameters are then used to modify the standard low-Z pellet penetration codes. Since LiT pellets have certain advantages over carbon [in particular, Li conditioning of the walls and T for refueling a D-T reaction], the penetration of LiT into fusion plasmas is considered.

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Effect Of Magnetic And Density Fluctuations On The Propagation Of Lower Hybrid Waves In Tokamaks, George Vahala, Linda L. Vahala, Paul T. Bonoli Jan 1992

Effect Of Magnetic And Density Fluctuations On The Propagation Of Lower Hybrid Waves In Tokamaks, George Vahala, Linda L. Vahala, Paul T. Bonoli

Electrical & Computer Engineering Faculty Publications

Lower hybrid waves have been used extensively for plasma heating, current drive, and ramp-up as well as sawteeth stabilization, The wave kinetic equation for lower hybrid wave propagation is extended to include the effects of both magnetic and density fluctuations. This integral equation is then solved by Monte Carlo procedures for a toroidal plasma. It is shown that even for magnetic/density fluctuation levels on the order of 10-4, there are significant magnetic fluctuation effects on the wave power deposition into the plasma. This effect is quite pronounced if the magnetic fluctuation spectrum is peaked within the plasma. For …

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Charge-State Equilibrium And Nonequilibrium Modeling Of The Carbon-Pellet Plasma Interaction, A. G. Elcashlan, G. A. Gerdin, L. L. Vahala, P. B. Parks Jan 1992

Charge-State Equilibrium And Nonequilibrium Modeling Of The Carbon-Pellet Plasma Interaction, A. G. Elcashlan, G. A. Gerdin, L. L. Vahala, P. B. Parks

Electrical & Computer Engineering Faculty Publications

Self-consistent equilibrium and nonequilibrium charge-state models are formulated for the spherical expansion of low-Z pellet vapor as an inviscid perfect gas of constant ratio of specific heats being heated volumetrically by the incident electrons of a thermonuclear plasma. The two models are found to be in agreement in the region where the ratio of the ionization length ζj to pellet radius rp is less than unity, but a single parameter, such as the magnitude of this ratio on the sonic surface, is insufficient to determine whether an equilibrium model will be valid for all regions of the ablatant …

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Electromagnetic Wave Scattering From Magnetic Fluctuations In Tokamaks, L. L. Vahala, G. Vahala, N. Bretz Jan 1992

Electromagnetic Wave Scattering From Magnetic Fluctuations In Tokamaks, L. L. Vahala, G. Vahala, N. Bretz

Electrical & Computer Engineering Faculty Publications

Cross sections are calculated for electromagnetic wave scattering and mode transformation from magnetic and density fluctuations in a homogeneous plasma. For the special case of scattering perpendicular to the magnetic field, density fluctuations scatter ordinary to ordinary and extraordinary to extraordinary modes-but cannot transform these modes. On the other hand, magnetic fluctuations perpendicular to the field can transform modes but cannot scatter on a single branch. For incident frequencies on the order of the electron plasma frequency or gyrofrequency, the cross sections for scattering and transformation due to field and density fluctuations have a similar value. Estimates are given for …

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Effects Of Quantum Noise On A Two-Level System In A Single-Mode Cavity, Linda L. Vahala Aug 1990

Effects Of Quantum Noise On A Two-Level System In A Single-Mode Cavity, Linda L. Vahala

Electrical & Computer Engineering Faculty Publications

The effects of quantum noise on a two-level system in the bad-cavity regime are considered perturbatively in the form of closure at the pair-correlation level. It is found that pair-correlation effects can reduce the level of semiclassical chaos. However, under the rotating-wave approximation (RWA), quantum noise can lead to chaos if there is an initial population inversion, while the full RWA Hamiltonian system remains integrable.

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Application Of The Results Of Carbon Pellet Modeling To The Problem Of Plasma Penetration, L. L. Vahala, A. G. El Cashlan, G. A. Gerdin, P. B. Parks Jan 1990

Application Of The Results Of Carbon Pellet Modeling To The Problem Of Plasma Penetration, L. L. Vahala, A. G. El Cashlan, G. A. Gerdin, P. B. Parks

Electrical & Computer Engineering Faculty Publications

The assumptions of the evaporation model for low-Z pellets interacting with magnetic fusion plasmas developed by Parks are tested. These assumptions are that the vapor density profile in the region adjacent to the pellet surface falls off with radial distance as r-α , where 5 <α<6, and that the ionization time for the transition between charge states זf (i.e., for r < ~3 sonic radii). The first assumption is tested by solving a two-parameter eigenvalue problem for the evaporation cloud in the region interior to the sonic radius; the results are found to be consistent with the low-Z evaporation model. The second assumption, that זzi «זf, is tested at the sonic radius using the results from atomic physics and the low-Z evaporation model. It is found that indeed זzi «זf for plasmas with parameters close to thermonuclear conditions (e.g. CIT), but not for those of smaller Tokamaks such as …

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Influence Of Copper Doping On The Performance Of Optically Controlled Gaas Switches, St. T. Ko, V. K. Lakdawala, K. H. Schoenbach, M. S. Mazzola Jan 1990

Influence Of Copper Doping On The Performance Of Optically Controlled Gaas Switches, St. T. Ko, V. K. Lakdawala, K. H. Schoenbach, M. S. Mazzola

Electrical & Computer Engineering Faculty Publications

The influence of the copper concentration in silicon-doped gallium arsenide on the photoionization and photoquenching of charge carriers was studied both experimentally and theoretically. The studies indicate that the compensation ratio (NCu/NSi) is an important parameter for the GaAs:Si:Cu switch systems with regard to the turn-on and turn-off performance. The optimum copper concentration for the use of GaAs:Si:Cu as an optically controlled closing and opening switch is determined.

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Measurement Of Magnetic Fluctuations By O-X Mode Conversion, L. L. Vahala, G. Vahala, N. Bretz Jan 1990

Measurement Of Magnetic Fluctuations By O-X Mode Conversion, L. L. Vahala, G. Vahala, N. Bretz

Electrical & Computer Engineering Faculty Publications

The possibility of measuring magnetic fluctuations in a fusion plasma is considered by examining the O→X mode conversion. Under certain conditions and with good angular resolution, this mode conversion can be attributed to the presence of magnetic fluctuations even though the level of these fluctuations is much lower than that of density fluctuations. Some nonideal effects such as mode polarization mismatch at the plasma edge are also discussed.

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Nanosecond Optical Quenching Of Photoconductivity In A Bulk Gaas Switch, M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, S. T. Ko Jan 1989

Nanosecond Optical Quenching Of Photoconductivity In A Bulk Gaas Switch, M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, S. T. Ko

Electrical & Computer Engineering Faculty Publications

Persistent photoconductivity in copper-compensated, silicon-doped semi-insulating gallium arsenide with a time constant as large as 30 µs has been excited by sub-band-gap laser radiation of photon energy greater than 1 eV. This photoconductivity has been quenched on a nanosecond time scale by laser radiation of photon energy less than 1 eV. The proven ability to turn the switch conductance on and off on command, and to scale the switch to high power could make this semiconductor material the basis of an optically controlled pulsed-power closing and opening switch.

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Gaas Photoconductive Closing Switches With High Dark Resistance And Microsecond Conductivity Decay, M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, R. Germer, G. M. Loubriel, F. J. Zutavern Jan 1989

Gaas Photoconductive Closing Switches With High Dark Resistance And Microsecond Conductivity Decay, M. S. Mazzola, K. H. Schoenbach, V. K. Lakdawala, R. Germer, G. M. Loubriel, F. J. Zutavern

Electrical & Computer Engineering Faculty Publications

Silicon-doped n-type gallium arsenide crystals, compensated with diffused copper, were studied with respect to their application as photoconductive, high-power closing switches. The attractive features of GaAs:Cu switches are their high dark resistivity, their efficient activation with Nd:YAG laser radiation, and their microsecond conductivity decay time constant. In the authors' experiment, electric fields are high as 19 kV/cm were switched, and current densities of up to 10 kA/cm2 were conducted through a closely compensated crystal. At field strengths greater than approximately 10 kV/cm, a voltage `lock-on' effect was observed.

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An Optically Controlled Closing And Opening Semiconductor Switch, K. H. Schoenbach, V. K. Lakdawala, R. Germer, S. T. Ko Jan 1988

An Optically Controlled Closing And Opening Semiconductor Switch, K. H. Schoenbach, V. K. Lakdawala, R. Germer, S. T. Ko

Electrical & Computer Engineering Faculty Publications

A concept for a bulk semiconductor switch is presented, where the conductivity is increased and reduced, respectively, through illumination with light of different wavelengths. The increase in conductivity is accomplished by electron ionization from deep centers and generation of bound holes. The reduction of conductivity is obtained by hole ionization from the excited centers and subsequent recombination of free electrons and holes. The transient behavior of electron and hole density in a high power semiconductor (GaAs:Cu) switch is computed by means of a rate equation model. Changes in conductivity by five orders of magnitude can be obtained.

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Comment On "Orientation, Alignment, And Hyperfine Effects On Dissociation Of Diatomic Molecules To Open Shell Atoms", Mark D. Havey, Linda L. Vahala Jan 1987

Comment On "Orientation, Alignment, And Hyperfine Effects On Dissociation Of Diatomic Molecules To Open Shell Atoms", Mark D. Havey, Linda L. Vahala

Electrical & Computer Engineering Faculty Publications

A recent paper in this journal [Y. B. Band e t a l., J. Chem. Phys. 8 4, 3762 (1986)] reported parameters describing orientation and alignment produced, in an axial recoil limit, by one photondissociation of diatomic molecules. Reported also were values, applicable to the resonance transitions of the alkali atoms, for orientation and alignment depolarization coefficients. Most of the numerical values reported for the coefficients were incorrect, in some cases by as much as a factor of 2. We report a tabulation of correct depolarization coefficients applicable to the resonance transitions of common alkali isotopes. Further, the coefficients …

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Experimental Fine-Structure Branching Ratios For Na-Rare-Gas Optical Collisions, Mark D. Havey, F. T. Delahanty, Linda L. Vahala, Gary E. Copeland Oct 1986

Experimental Fine-Structure Branching Ratios For Na-Rare-Gas Optical Collisions, Mark D. Havey, F. T. Delahanty, Linda L. Vahala, Gary E. Copeland

Electrical & Computer Engineering Faculty Publications

Experimental ratios for branching into the fine-structure levels of the Na 3p multiplet, as a consequence of an optical collision with He, Ne, Ar, Kr, or Xe, are reported. The process studied is Na(3s2S1/2)+R+nhNNa(3p2Pj)+R+(n-1)hN, where R represents a rare-gas atom and where the laser frequency N is tuned in the wings of the Na resonance transitions. The branching ratios are defined as I(D1)/I(D2) where I(D1) and I(D2) are measured intensities of the atomic Na D1 and D2 lines. The ratios are determined for detunings ranging from about 650 …

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Nonadiabatic Theory Of Fine-Structure Branching Cross Sections For Na-He, Na-Ne, And Na-Ar Optical Collisions, Linda L. Vahala, P. S. Julienne, Mark D. Havey Jan 1986

Nonadiabatic Theory Of Fine-Structure Branching Cross Sections For Na-He, Na-Ne, And Na-Ar Optical Collisions, Linda L. Vahala, P. S. Julienne, Mark D. Havey

Electrical & Computer Engineering Faculty Publications

The nonadiabatic close-coupled theory of atomic collisions in a radiation field is generalized to include electron spin and is used to consider the weak-field Na–rare-gas (RG) optical collision Na(2S1/2)+RG+nhν μNa(2Pj)+RG+(n-1)hν. The effects of detuning and incident energy on the branching into the atomic Na 3p2P3/2 and 3p2P1/2 states are examined. The cross sections σ(j) are found to have a strong asymmetry between red and blue detuning as well as a complex threshold and resonance structure dependence on energy. A partial cross-section analysis …

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Systematic Estimate Of Binding Energies Of Weakly Bound Diatomic Molecules, Linda L. Vahala, Mark D. Havey Jan 1984

Systematic Estimate Of Binding Energies Of Weakly Bound Diatomic Molecules, Linda L. Vahala, Mark D. Havey

Electrical & Computer Engineering Faculty Publications

There is often insufficient spectroscopic data for a full RKR inversion to yield a potential for weakly bound diatomic molecules. In these cases, parametrized functions such as the Morse or Thakkar potentials may be used to obtain estimates of the binding energy. The Thakkar potential is more flexible, and has been used successfully on some weakly bound systems. In the more usual case, the Thakkar parameter p, which determines long range behavior R-p, is chosen by p=-a1-1, where a1 is the first Dunham coefficient; p is usually noninteger. The authors present …

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Picosecond Laser Pulse Irradiation Of Crystalline Silicon, K. L. Merkle, H. Baumgart, R.H. Uebbing, F. Phillipp Jan 1982

Picosecond Laser Pulse Irradiation Of Crystalline Silicon, K. L. Merkle, H. Baumgart, R.H. Uebbing, F. Phillipp

Electrical & Computer Engineering Faculty Publications

Morphology changes introduced by picosecond laser pulses at λ = 532 nm and 355 nm in (111) and (100) silicon samples are studied by means of optical and high-voltage electron microscopy. Depending on energy fluence, orientation and wavelength, amorphous or highly defective regions may be created. From an analysis of damage thresholds and damage depth distributions it is concluded that melting and energy confinement precedes the formation of the structural changes.

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Stable Equilibrium Statistical States For Spheromaks, George Vahala, Linda L. Vahala Jan 1979

Stable Equilibrium Statistical States For Spheromaks, George Vahala, Linda L. Vahala

Electrical & Computer Engineering Faculty Publications

Incompressible nondissipative magnetohydrodynamic turbulence is treated for spherical systems. From the absolute equilibrium expectation values of the fields one can investigate those initially quiescent states for which no large mean square velocity will develop. This stable state is force-free and gives rise to the Hill vortex structure for the magnetic flux surfaces.

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Comments On 'Fluctuations In Guiding Center Plasma In Two Dimensions', George Vahala, Linda L. Vahala, David Montgomery, Glenn Joyce Jan 1974

Comments On 'Fluctuations In Guiding Center Plasma In Two Dimensions', George Vahala, Linda L. Vahala, David Montgomery, Glenn Joyce

Electrical & Computer Engineering Faculty Publications

It is stated that the principal result of the paper by Taylor and Thompson (see abstr. A14511 of 1973) on autocorrelations in density for the electrostatic guiding center plasma in two dimensions is wrong owing to an incorrect integration. It is further stated that there is no meaningful distinction between an `interaction cutoff' and a `fluctuation cutoff'.

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