摘要:

Laser heating for an infinite solidx>0 is modeled by imposing the heat source distributionE0(1−R)&agr; exp(−&agr;x) within the material. In order to overcome the problem of superheating in the solid, when the intensity is strong enough to cause a change of phase, a ‘‘mushy zone’’ of partially molten material is postulated between the fully liquid and the solid phases. The model is applied to laser heating of a solid, initially at constant temperature, together with ablation of any fully liquefied material. The equations are transformed so as to be amenable to numerical methods and the resulting solutions analyzed. The model seems particularly appropriate to optically thin materials.

ISSN:0021-8979    

DOI:10.1063/1.335842

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The use of pulse powering for electrostatic precipitators has shown several advantages but the underlying physics has been insufficiently understood to allow any optimization. A numerical model has been assembled which allows the controlling corona process to be simulated in both time and space to permit estimation of the ion densities and field distributions to be expected for a wide range of transient electrical parameters and gas properties. Supporting laboratory measurements have been undertaken under dc and pulse conditions both to verify the simulation and to provide corona spot density data not readily obtainable by other means.

ISSN:0021-8979    

DOI:10.1063/1.335843

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Light ions deposit their energy in target materials by interaction with bound and free electrons. As the target heats toward inertial confinement fusion temperatures a progression of ionization states will be encountered. The stopping power of each ion created in this process will depend upon details of the respective bound electron states. In general, the net ion stopping power will increase compared to cold matter due to the free electron contribution. We report an experimental and theoretical study of enhanced ion stopping powers in targets heated by 0.5–1.4 TW/cm2proton beams. The experiments were performed on the Proto‐I accelerator with aluminum and nickel foil targets. The theoretical effort incorporated free and bound electron stopping terms in hydrocode simulations of the target response. At these intensities we observe and calculate stopping power enhancements of 100% for aluminum and 50% for nickel.

ISSN:0021-8979    

DOI:10.1063/1.335844

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The excitation mechanism of mercury atoms in a low‐pressure mercury‐argon discharge was investigated, employing an afterglow method. The experimental results combined with theoretical calculations have proved that the dominant process to populate the Hg 61P1level is the stepwise excitation via excited levels (63P−61P1) under conditions typical of fluorescent lamps (d=18.5–28.5 mm,I=0.3–0.5 A). When a Maxwell distribution of electron energy is assumed, the stepwise excitation is estimated to be about 73% of the total populating rate of the 61P1level in the stationary state. This value is about twice that found in literature. In the afterglow, the percentage goes beyond 97% within 10 &mgr;s after the termination of the current. By considering a non‐Maxwellian effect, the above partitions are estimated to decrease down to about 53% for the stationary state and to 93% for the afterglow. The stepwise excitation cross section was found to be in the order of 10−20–10−19m2.

ISSN:0021-8979    

DOI:10.1063/1.335845

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Simplified basic equations describing ionization waves excited in the positive columns of glow discharges are numerically solved as an initial value problem under a certain boundary condition. The nonlinear dispersion relation with the finite wave amplitude in the basic equations is also calculated. The frequency decrease with increasing wave amplitude is obtained from the relation. We can predict the evolution of envelope solitons by evaluating the coefficients of the nonlinear Schro¨dinger equation from the backward wave property and the frequency decrease. By applying an envelope solutions as an initial value, the soliton propagates by keeping its initial waveform in contrast to spreading the width of an arbitrary wave packet with a small amplitude.

ISSN:0021-8979    

DOI:10.1063/1.335846

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A study of the steady‐state discharges and the spatial distributions of plasma parameters vertical to the magnetic field was made in an rf magnetron with oval cross section. Langmuir probes were used for the major diagnostics. The device generates low‐ion energy, high‐ion flux stable plasma (Ei∼102eV, &Ggr;i∼5 mA/cm2at 1 W/cm2rf power). Ion transport toward the positive sheath is enhanced by the rf induced dc electric field in the uniform glow. The state of the glow is independent of rf power except ion densityni∝ (rf power)&agr;. The voltage drop across the sheath followsV∝(rf power) &bgr;with 0<&agr;<&bgr;<1. The discharge strongly depends on magnetic field strength and system pressure, and weakly depends on the gas chemistry. Generally, the state of the discharge is determined by charged particle generation, transport and loss processes, and in turn controlled by the system operating parameters. The experimental results obey the conservation laws and the steady‐state criterion. The correlations between plasma parameters and system parameters are investigated and discussed.

ISSN:0021-8979    

DOI:10.1063/1.335847

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Laser preionization is a method whereby the formative lag time and jitter of spark gaps can be reduced, and breakdown can be initiated at voltages significantly below the self‐breakdown value. In the spark gaps of interest (100‐ns spark duration), the plasma column expands from an initial laser preionized diameter of approximately 50 &mgr;m to an arc of diameter in excess of 1 mm, and conducts greater than 12‐kA peak current. Since the time required for the resistive collapse of the plasma is comparable to the spark duration, the spark gap represents a non‐negligible resistive loss in the circuit. The objective of this study is to understand the basic mechanisms by which laser‐triggered spark gaps develop and to provide a basis to optimize their design and minimize their resistive losses. To study the expansion and conduction phases of laser‐triggered spark gaps, a first principles model has been developed. This model includes gas dynamics, electron collision kinetics, radiation transport, and external circuitry in a self‐consistent formulation. The formulation of the model is discussed and results are compared to experimental data. We find that growth of the spark column is dominated by gas dynamic expansion of the hot ionized core, augmented by photoionization and thermal ionization at the plasma column boundary. The plasma column is confined within a high‐density cylindrical shell of neutral gas that traps ionizing radiation in a region of lowE/N(electric field divided by gas density), thereby inhibiting expansion by nonhydrodynamic means (electron avalanche).

ISSN:0021-8979    

DOI:10.1063/1.335848

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Gas mixtures for possible use in diffuse discharge switching applications require both high dielectric strength and specific electron transport properties in the conducting and the opening stages of the operation of the switch. In the conducting stage, the electron drift velocity must be large, and the electron loss processes (e.g., due to electron attachment and recombination) must be low so as to maximize the current in the discharge while maintaining low‐discharge impedance. In the opening stage, strong electron attachment along with high dielectric strength is required of the gas mixtures in order to extinguish the discharge as quickly as possible (and, thus, achieve a fast opening time) and to prevent arcing occurring between the switch electrodes due to the high voltages induced across the switch in the opening phase. Measurements of the electron drift velocity, attachment, ionization and transverse diffusion coefficients, and high‐voltage dielectric strengths of several gas mixtures proposed as candidates for use in diffuse discharge switching applications have been made and are reported and discussed.

ISSN:0021-8979    

DOI:10.1063/1.335849

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Spontaneous emission in a tapered free‐electron laser is considered for the case where the magnetic field of the undulator is described by an expression that varies discretely from one half‐period to the next. This geometry permits the utilization of the knowledge of the electron trajectories in untapered undulators. The method can be applied for any conceivable configuration of tapered undulators with electrons propagating either on or off axis and yields the spectrum of radiation at arbitrary angles including harmonics. The approach is developed for linear polarization; however, circular polarization may be treated as well.

ISSN:0021-8979    

DOI:10.1063/1.335850

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Numerical results are presented on transmission, reflection, and color of the recently described, supertwisted birefringence effect for highly multiplexed liquid‐crystal displays. The electro‐optical properties, which are investigated for normal and oblique incidence of light assuming a multiplexed duty cycle of 1/100, are compared to those of a 270° layer operating in the Mauguin mode and to 270° twisted Heilmeier and White–Taylor guest‐host schemes.

ISSN:0021-8979    

DOI:10.1063/1.335851

出版商:AIP    

年代:1985

数据来源: AIP