Devices called “solid-state ferroelectric-based sources” (SSFBSs) are under development as sources of electrons, ions, ultraviolet light, and x-rays for diverse applications in characterization and processing of materials. Whereas heretofore it has been necessary to use a different device to generate each of the aforementioned species of charged particles or radiation, a single SSFBS can be configured and operated to selectively generate any of the species as needed using a single source. Relative to comparable prior sources based, variously, on field emission, thermionic emission, and gaseous discharge plasmas, SSFBSs demand less power, and are compact and lightweight.
An SSFBS exploits the unique physical characteristics of a ferroelectric material in the presence of a high-frequency pulsed electric field. The basic building block of an SSFBS is a ferroelectric cathode — a ferroelectric wafer with a solid electrode covering its rear face and a grid electrode on its front face (see figure). The application of a voltage pulse — typically having amplitude of several kilovolts and duration of several nanoseconds — causes dense surface plasma to form near the grid wires on the front surface.
The shape of the applied voltage waveform determines the characteristics of the emitted charged particles and radiation and can be tailored to maximize the yield of electrons or ions. For example, one could utilize bipolar pulsing and/or a succession of pulses with different time intervals between them. Although the parameters of the surface plasma do not depend strongly on the polarity of the voltage, it is preferable to apply negative pulses to the rear (solid) electrode while keeping the front (grid) electrode at ground potential.
The plasma generates intense visible and ultraviolet light. If a grid is placed at a suitable short distance near the front face and a synchronized positive voltage pulse is applied to this grid, then electrons are extracted from the plasma and accelerated toward and through the grid. The resulting electron beam can be aimed at a target. The impingement of the energetic electrons on a suitable target can be utilized to generate x-rays. Another option is to pulse an accelerating grid negative with respect to the front face so as to extract and accelerate positive ions.
Typical parameters of an optimized design for a basic SSFBS include a ferroelectric-plasma area of 10 cm2, operating pressure of about 10 -2 torr (about 1.3 Pa), pulse-repetition frequency of 20 Hz, electron or ion current up to 10 A under an accelerating pulse of 30-kV amplitude and 50-ns duration.
This work was done by Yoseph Bar-Cohen, Stewart Sherrit, Xiaoqi Bao, Joshua Felsteiner, and Yakov Karsik of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.