Pulsed Energy Source Research Articles

Evaluation of ablation plasma characteristics of intense, pulsed, ion-beam evaporation

Basic characteristics of an ablation plasma produced by an intense, pulsed, ion beam have been evaluated from the measurement of ion-flux density by a biased ion collector. The target mass loss is detected by the measurement of the weight of the target in the comparison before and after the shot. A one-dimensional hydrodynamic model is introduced with the assumption of a high-power, light-ion beam-driven expansion and the following adiabatic expansion into vacuum. With this model, it is possible for us to deduce the temperature and the pressure from the data of the ion flux and the mass loss of the target, respectively. This method is also applicable to other intense pulsed energy sources such as lasers or electron beams.

Characteristics of Ablation Plasma Produced by Intense, Pulsed, Ion Beam

Characteristics of ablation plasma produced by the irradiation of an intense, pulsed, ion beam on targets, which has been known to be very effective in preparing thin films and is known as ion-beam evaporation (IBE), have been investigated experimentally by time-of-flight of ion-flux density measured by biased ion collector (BIC). With ion-beam power density of ∼109 W/cm2, the velocity and the kinetic energy of the ablation plasma were measured at various positions from the substrate using a titanium or aluminum target. An analytic solution derived from one-dimensional hydrodynamic expansion into a vacuum has been developed for the IBE process. Using such a model, it is possible to deduce the temperature in IBE from the signals of BICs. This method is also applicable to other intense pulsed energy sources such as lasers or electron beams.

Pulsed high-voltage and high-current outputs from homopolar energy storage system

Pulsed energy source with a versatile output, using self-excited homopolar generator for the initial storage of energy, has been developed. Large energy storage of this inertial–inductive system provides an attractive option for satisfying pulse power requirements associated with such applications as plasma confinement and heating, electromagnetic acceleration of projectiles, and with production of intense radiation. These applications require high rate of energy delivery to the load at specific current and voltage levels. Megajoule output pulses, alternatively, at hundreds of kilovolts or at megampere levels, have been obtained by incorporating unique current interrupting system. The overall pulser efficiency, which depends sensitively on the load characteristics was measured over a range from 10% to more than 90% for different pulser-load circuit arrangements.

Kinetic temperature measurement of the front surface of a target exposed to an intense pulsed electron beam

A measurement of surface temperature has been developed for pulsed radiation dose levels such that vaporization of the material under investigation is obtained. This method, which has been successfully used, is based on the kinetic definition of temperature, and can be used for temperatures above the vaporization threshold and beyond without changing the physical phenomena. A detector located at a distance l from the target establishes the speed distribution of atoms ejected, from which we deduce the temperature. The rise time of the leading portion of the signal obtained depends on the temperature. Compared to optical methods, this method has the advantage of being unaffected by the electromagnetic and bremsstrahlung noises inherent to electron generators. Moreover, the hydrodynamic expansion velocity can be calculated and the blowoff density estimated. This method can also be used in the case of other pulsed energy sources such as photons or ions.

Current continuity in dense plasma

The equations of current continuity have been solved for high-density (nion>1012 cm−3) gaseous plasma as a problem associated with electron-beam-controlled electrically excited pulsed laser energy sources. The procedure used first-order numerical approximations to the space and time development of current and potential gradients. The results of a laser plasma study are (a) the total electron current is not very sensitive to electron emission at the cathode because (b) the sheath multiplies both secondary electrons and cathode current by a large factor and adjusts to current requirement set by electron mobility and density in the plasma. Small changes in the sheath thickness and potential drop satisfy the variable-current condition set by gas ionization and ion recombination: I=new=ew (S/α)1/2. The potential drop in the sheath of an STP plasma is 400–2000 V for plasma currents of 1–10 A/cm2, and the sheath thickness is d≃0.003 cm.

ACOUSTICAL HOLOGRAPHY USING DIGITAL PROCESSING

A method for the reconstruction of hologram images by digital computer has been developed and its technical feasibility, demonstrated. The method for the detection of geological formations is applied to acoustic data from a series of two‐dimensional models of the earth. The models are constructed from 4 ft × 8 ft acrylic sheets and a radially pulsed (wide‐band) energy source is used. Values of elastic displacement scattered to the surface by the buried formations constitute the basic data from which the images are reconstructed. The images are displayed as computer plots of amplitude versus position. Narrow frequency bands ranging from 5 khz to 70 khz are selected from the wide‐band signals to form images at several different frequencies. Range‐gating in the time domain is demonstrated to be a useful technique for the removal of source‐generated noise. At all frequencies, the images demonstrate lateral and depth resolution near the diffraction‐limited values predicted by equations from classical optics. The method permits detection of objects with discontinuities that are sufficiently sharp to produce significant diffraction of the incident energy, such as a geological fault. Images of weakly diffracting objects, such as a triangular anticline, are more difficult to interpret because of strong ghost images of the source. The results show that the interference of diffracted and reflected energy can lead to erroneous interpretations of acoustic hologram images.