Scott D. Kovaleski

Surface instability of multipulse laser ablation on a metallic target

Large scale wavelike patterns are observed on an aluminum surface after it is ablated by a series of KrF laser pulses (248 nm, 40 ns, 5 J/cm2). These surface structures have a wavelength on the order of 30 μm, much longer than the laser wavelength. We postulate that these wave patterns are caused by the Kelvin–Helmholtz instability at the interface between the molten aluminum and the plasma plume. A parametric study is given in terms of the molten layer’s thickness and of the spatial extent and kinetic energy density in the laser-produced plasma plume. Also included is an estimate of the cumulative growth in a multipulse laser ablation experiment. These estimates indicate that the Kelvin–Helmholtz instability is a viable mechanism for the formation of the large scale structures.

Ferroelectric plasma thruster for microspacecraft propulsion

This paper presents a technology in microthruster design: the ferroelectric plasma thruster (FEPT). The FEPT utilizes an applied rf electric field to create plasma on the surface of a ferroelectric dielectric. Acceleration of ions from this plasma provides thrust. Advantages of the FEPT include emission of both electrons and ions leading to self-neutralization, creation of plasma, and acceleration of ions with a single power supply, and application of thrust in a short amount of time. We present the concept of the thruster, operational physics, as well as experimental results demonstrating plasma creation and ion acceleration. These results along with plasma spectroscopy allow us to calculate thruster parameters.

Electron beam ablation of materials

The channelspark, a low accelerating voltage, high current electron beam accelerator, has been used for ablation of materials applied to thin film deposition. The channelspark operates at accelerating voltages of 10 to 20 kV with ∼1500 A beam currents. The electron beam ionizes a low-pressure gas fill (10–20 mTorr Ar or N2) to compensate its own space charge, allowing ion focused transport. Ablation of TiN, Si, and fused silica has been studied through several plasma diagnostics. In addition, thin films of SiO2 have been deposited and analyzed. Strong optical emission from ionized species, persisting for several microseconds, was observed in the electron beam ablated plumes. Free electron temperatures were inferred from relative emission intensities to be between 1.1 and 1.2 eV. Dye-laser-resonance-absorption photography showed Si atom plume expansion velocities from 0.38 to 1.4 cm/μs for several pressures of Ar or N2 background gas. A complex, multilobed plume structure was also observed, yielding strong...

Ion acceleration in a radio frequency driven ferroelectric plasma source

Ion emission from ferroelectric plasma sources driven by radio frequency (RF) applied voltage is studied. An experimental investigation of particle emission from lead zirconate titanate ferroelectric ceramics driven by bursts of seven cycles of RF voltage at 248 kHz has revealed significant ion current emission. Measured electron to ion peak current ratios ranged from 11 to 19, which is much lower than the expected thermal current ratio. These results indicate that the ions are being preferentially accelerated. An analysis of the ponderomotive force on the ions and electrons reveals that it may be responsible for acceleration of the ions. The ferroelectric plasma is examined as a possible micropropulsion thruster. Performance estimates based on the ponderomotive acceleration calculations predict specific impulse of 2000-3000 s, thrust of 0.1-1 mN, and specific power of 15 W/mN. These estimates are compared to existing micropropulsion concepts, revealing the attractiveness of ferroelectric plasma sources.

Electron beam ablation versus laser ablation : plasma plume diagnostic studies

Experiments have been performed to compare XeCl laser ablation plume characteristics to those produced by electron . beam ablation. Potential advantages of electron beams include higher electrical efficiency ; 30% , and the ability to process materials with high optical reflectivity or transparency. The electron beam is generated by a channelspark with parameters: peak voltage of 15-20 kV, current of 1.5-1.7 kA, and pulse length of about 200 ns. The electron beam is ion focused to about 2 mm diameter by an argon background gas. Initial diagnostic experiments have utilized optical emission spectroscopy to characterize the ionization dynamics of the ablation plumes of Fe targets. Spectra taken during electron beam ablation are composed of singly ionized iron, with negligible emission from neutral iron. This is in sharp contrast with XeCl excimer laser ablation, which is composed of both neutral and ion species, the neutrals persisting strongly after the laser pulse. In addition to Fe ion emission, the channelspark emission spectrum also exhibits a high degree of excitation and ionization of the Ar background gas. Strong emission from Ar q ,A r 2q , and Ar 3q has been measured. q 1998 Elsevier

Investigation of the Piezoelectric Effect as a Means to Generate X-Rays

The piezoelectric effect is analyzed as a means to produce X-rays. A mass of crystalline piezoelectric material is used to convert a low-voltage input electrical signal into a high-voltage output signal by storing energy in a longitudinally vibrating mechanical wave. Output energy is extracted in the form of a high-voltage electron beam using a field-emission diode mounted on the surface of the crystal. The electron beam produces X-rays via bremsstrahlung interactions with a metallic surface.

Thrust Measurements of the Ferroelectric Plasma Thruster

This paper details the calculation and measurement of average thrust in the ferroelectric plasma thruster. The effect of applied voltage on thrust is evaluated. In addition, the construction and calibration of a pulsed thrust stand is presented. Characteristics of the thruster include an average power of 6 W, thrust of 68-87 muN, calculated specific impulse of 1450 s, and a thruster mass of 5.7 g. Additional benefits of the thruster include impulse bits less than 1 nN ldr s, only one power supply needed for operation, and the ability to emit both electrons and ions.

Piezoelectric-Resonance Effect in a Radio-Frequency-Driven Ferroelectric Plasma Source

The effect of driving frequency on the minimum voltage required to form plasma in a radio-frequency ferroelectric plasma source is examined. When the driving frequency of the plasma source is less than the translational effect radial resonant frequency, the applied electric field and background pressure have an increased effect on plasma formation. Applied-voltage frequencies that are close to the radial mechanical resonance of a ferroelectric/piezoelectric disk cause plasma formation to occur at reduced voltages. In one case, a 75% reduction in the breakdown voltage is realized by altering only the frequency of the applied voltage.

Study of laser target triggering for spark gap switches

The use of laser targets as a method to decrease the required laser energy to trigger a laser triggered gas switch has been investigated at the University of Missouri. Target materials were identified based on durability, melting point, reactivity and reflection coefficient. Laser targets were placed into a cathode of a laser triggered gas switch. The switch was pulse charged by the Tiger pulsed power machine to between 185 kV and 330 kV. The switch was triggered by directing a 1064 nm or 266 nm wavelength laser pulse from an Nd:YAG laser onto a laser target to ablate material and create plasma, closing the switch. The goal of the project was to trigger a high voltage gas switch with less than 1 mJ of laser energy while maintaining a switch jitter comparable to present electrically triggered switches for LTD based systems. The study successfully demonstrated that triggering the switch using a 1 mJ infrared pulse and a graphite target resulted in a jitter less than 5 ns. Findings will be used in the design of switches for LTD based systems.

Experimental study of the multichanneling self-break section of the rimfire switch

Many accelerators at Sandia National Laboratories utilize the Rimfire gas switch for high-voltage, high-power switching. In addition, there are many multichannel closing switches used in pulsed power applications. This paper presents a study of the multichanneling section of the Rimfire switch. The electrical effects of multichanneling and a method to force multichanneling are presented. In addition, an objective curve fitting method is utilized to deduce switch inductance from the measured data.