F. Davanloo

Production of high‐energy photons from flash x‐ray sources powered by stacked Blumlein generators

Described here is the design and construction of a pulse‐power generator capable of discharging at high repetition rates. It consists of eight triaxial Blumleins stacked in series at one end. These lines are charged in parallel and synchronously commuted with a single thyratron at the other end to produce an open circuit voltage across a stack of six times the charging voltage. An x‐ray diode has been constructed and matched to this pulse‐power source making possible the emission of an average bremsstrahlung exposure rate of 17 R/S from a sequence of 40‐ns pulses. When operated at 60‐kV charging voltage, direct spectral measurements show the output to be a true continuum, peaking at intensities in excess of 5×108 photons/keV/shot and containing useful intensities of photons having energies of 300 keV.

Flash x‐ray source excited by stacked Blumlein generators

Eight triaxial Blumleins have been stacked in series at one end while being synchronously commuted at the other end with a single hydrogen thyratron. Significant voltage gain has been maintained in the stack for periods of the order of 20 ns. Having no other switching elements, operation has been possible at repetition rates to 100 Hz. An x‐ray diode has been matched to this pulse power source, making possible the emission of an average bremsstrahlung dose of 5.8 R/s from a sequence of 20‐ns pulses. In less than 4 min of operation at 100 Hz, a dose of 1 kR could have been delivered to a target sample. When operated at 50‐kV charging voltage, spectral measurements show the output to be a true continuum, peaking at intensities in excess of 2×108 keV/keV/shot and containing useful intensities of photons having energies of 150 keV.

Protective coatings of nanophase diamond deposited directly on stainless steel substrates

Composed of sp 3 bonded nodules of carbon, nanophase diamond films are deposited in vacuum onto almost any substrate by condensing carbon ions carrying keV energies. These multiply charged ions are obtained from the laser ablation of graphite at intensities in excess of 10 11 W cm −2 . The high energy of condensation provides both the chemical bonding of such films to a wide variety of substrates and low values of residual compressive stress. Coatings of 2–5 μm thickness have extended lifetimes of materials such as Si, Ti, ZnS, ZnSe, and Ge against the erosive wear from high-speed particles by factors of tens to thousands. In this research emphasis has been placed on studies of the bonding and properties realized by the direct deposition of nanophase diamond films on stainless steel substrates. Examinations of interfacial layers showed deep penetrations of carbon atoms into steel substrates. Resistances to low and high impact wear estimated by a tumbler device and a modified sand blaster, respectively, and results indicated significant increases in the lifetime of stainless steel samples. The characterization studies in this work demonstrated nanophase diamond as an attractive material for use as a protective coating in current industrial applications.

Third continuum of argon in high pressure plasmas excited by dielectric controlled discharge

Abstract In this work we report what we believe to be the first observation of the third continuum of argon in plasmas, at pressures between 100 and 4000 Torr, created by discharge excitation. This continuum, centered at 188 nm, has been clearly seen not only in pure argon, but also in presence of various reactants and especially in argon-helium mixtures which has not been reported before.

Amorphic diamond/silicon semiconductor heterojunctions exhibiting photoconductive characteristics

Amorphic diamond deposited from an intense laser plasma of C3+ and C4+ exhibits semiconductor properties. Although this material has a measured electrical breakdown strength of 3×109 V/m, it is shown to form a heterojunction with both p- and n-type Si. Regardless of the doping type and its concentration in the Si, current is rectified in the same direction with the diamond layer acting as the cathode. A photoconductive effect is observed for these junction devices and current levels from the heterojunction vary with the amount of reverse bias and the illumination. The spectral response is estimated to mostly lie in the range of 600–900 nm.

High Power Repetitive Waveforms Generated By Compact Stacked Blumlein Pulsers

The repetitive stacked Blumlein pulse power sources developed at the University of Texas at Dallas (UTD) consist of several triaxial Blumleins stacked in series at one end. The lines are charged in parallel and synchronously commuted with a single thyratron at the other end. This allows switching t o take place at a low charging voltage relative t o the pulser output voltage. In this report, opportunities for packaging these pulsers into light and compact devices are investigated by studying Blumlein spacings on small scale prototypes. Considerable reductions in size and weight are demonstrated without significant degradations in the pulser performances. The promising capabilities of photoconductive switches fo r the use in these Blumlein pulsers are discussed.

Progress in the characterization of nanophase diamond films uniquely produced by a laser plasma discharge source

Abstract Films of nanophase diamond can be deposited at room temperature with a laser plasma discharge source of multiply charged carbon ions, without the use of any catalyst in the growth mechanism. The beam from a pulsed Nd:YAG laser is focused on graphite at intensities in excess of 1011W cm-2 and the resulting plasma ejects carbon ions carrying energies of about 1 keV through a discharge space to the substrates to be coated. The high energies of condesation produce interfacial layers between the film and substrate materials, which provide levels of adhesion which can support the protection of substrates subjected to harsh environmental conditions. In this paper, recent advances in the characterization of nanophase diamond films are given. Emphasis has been placed on studies of the bonding and properties realized in one example; i.e. the deposition of nanophase diamond on stainless steel. Measurements with Rutherford backscattering spectrometry showed that the diamond coatings deposited from laser plasmas were bonded to the stainless steel substrates through interfacial layers with significant thickness. The resistance to wear was estimated with a modified sandblaster and it was shown that a coating of only 2 μm of nanophase diamond can increase the lifetime of the sample by a factor of better than 36. The results of other mechanical measurements, such as those obtained by friction tests, are also given.

A Novel Repetitive Stacked Blumlein Pulse Power Source

In this report we review the characteristics of the Blumlein pulse power sources developed at the University of Texas at Dallas. These devices consist of several triaxial Blumleins stacked in series at one end. The lines are charged in parallel and synchronously commuted with a single thyratron at the other end. In this way relatively low charging voltages are multiplied to give the desired discharge voltage without the need for complex Marx bank circuitry. Progress in the scaling of these Blumlein pulse generators to obtain open circuit voltages in excess of 0.5 MV is described. Application of these devices to produce high power x-ray pulses is discussed.

SCRATCH ADHESION TESTING OF NANOPHASE DIAMOND COATINGS ON STEEL AND CARBIDE SUBSTRATES

Films of nanophase diamond are deposited in vacuum onto almost any substrate by condensing multiply charged carbon ions carrying keV energies. These ions are obtained from the laser ablation of graphite at intensities in excess of 10 11 W cm −2 . The high energies of condensation produce interfacial layers between the film and substrate materials, resulting in levels of adhesion that can support the protection of substrates subjected to harsh environmental conditions. In this article, we give details of the scratch adhesion testing performed on steel and carbide substrates coated with nanophase diamond. A commercially available scratch tester was used and a data analysis was presented to quantitatively assess and measure the adhesion of films on these important substrates. The characterization studies in this work demonstrated nanophase diamond as a highly adherent coating suitable for industrial applications.

Switching the stacked Blumlein pulsers status and issues

The repetitive stacked Blumlein pulse power generators developed at the University of Texas at Dallas consist of several triaxial Blumleins stacked in series at one end. The lines are charged in parallel and synchronously commuted with a single switch at the other end. In this way, relatively low charging voltages are multiplied to give a high discharge voltage across an arbitrary load. To date, the stacked Blumlein pulsers have produced high power waveforms with risetimes and repetition rates in the range of 5-50 ns and 1-200 Hz, respectively, using a conventional thyratron or spark gap. To generate waveforms with sub-nanosecond risetimes at kilo-Hertz repetition rates, fast switching devices such as photoconductive switches must be utilized. This paper describes the feasibility of an intense pulse power source based upon stacked Blumlein technology by adapting the design for use with photoconductive switches.