ELECTRONICS advancements throughout the last half century have been rapid, improving lives and commerce in amazing ways. But one technology crucial to many electronics and electrical products--the insulator--has not kept up with these dramatic improvements. Rather, insulator technology has evolved in a steadier, incremental fashion; its improvements have come from using better fabricating materials and reducing manufacturing flaws.|
The pace of insulator improvement has now taken a leap forward. Lawrence Livermore researchers led by Stephen Sampayan, together with a research team at AlliedSignal, led by Mike Krogh, have invented the Ultra High Gradient Insulator, or Ultra-HGI, a device that can reliably withstand electrical voltages four times greater than before. That means it will be a smaller, less bulky component in high-tech instrumentation such as accelerators, x-ray machines, semiconductor production tools, and large microwave tubes. Such instrumentation in turn can be designed to be smaller, thereby reducing capital and operating costs. The Ultra-HGI will allow technologies to be advanced in ways never before possible.
How It Works
Making the Insulator|
The prototypes that the research teams made to test their new insulator concept required labor-intensive fabrication. The insulator's layers--which use, for example, copper, chromium, or aluminum as conductive materials and polycarbonate, glass, or alumina as insulating materials--must be very fine. The thickness of the conducting layers is less than 1 micrometer, while that of the insulating layers is under 1 millimeter. A 1-centimeter thickness of insulator material may contain as many as 40 layers.
One prototype fabrication used perfectly flat, 0.25-millimeter-thick glass plates onto which 0.5-micrometer-thick chromium layers were deposited on both the top and bottom surfaces of the plates. A 2.5- to 3-micrometer layer of gold was added over that.
The metallized plates were aligned and stacked. In a furnace, the stack was subjected to pressure and heated long enough for the plates to form a strong bond. Then the engineers used an ultrasonic abrasive drill to shape the stack into a cylinder and cut a hole through its center. The result was a stack of rings of laminated insulating layers in planes perpendicular to the axis of the cylinder. Each ring was equivalent to a thin high-voltage capacitor; the stack of flat rings constituted a capacitive voltage divider.
The insulator development team tested their prototype by subjecting it to several low-voltage conditioning pulses in a vacuum chamber. They increased the pulses by small amounts until the insulators broke down; the tests were repeated to check the consistency of results. The table above shows the threshold at which the Ultra-HGIs and conventional insulators break down.
Such prototype Ultra-HGIs have been expensive to produce, but the team is now developing designs for mass production and has experimented with several more prototypes of the insulator. While formulating these more producible designs, they are also experimenting with other ways to prevent or control voltage breakdowns.
Many Present and Future Applications|
One reason the insulator was developed was that a new linear accelerator concept proposed by Livermore scientists required a compact insulator. The Dielectric Wall Accelerator will be an order of magnitude smaller than current linear accelerators, but it will deliver similar energy. The narrow separations between the insulator's conductive layers also have been shown to attenuate microwave power, modifying the microwave resonances that cause beam instabilities in accelerators. The Ultra-HGI reduces these resonances by a factor of 4.
While the Ultra-HGI will revolutionize linear accelerators, it will also be important for particle accelerators such as x-ray machines. It should reduce the size--and thus cost--of using such machines for lithography and medicine. It will allow improved performance of high-powered microwaves and neutron sources used for oil-well logging and for detecting explosives. The smaller size and tolerance of higher voltages provided by the Ultra-HGI should make new, smaller designs feasible and economically viable.
Key Words: capacitor, electron emission, graded insulator, insulator fabrication, R&D 100 Award, secondary emission avalanche, Ultra High Gradient Insulator (Ultra-HGI), voltage breakdown, voltage divider.
For further information contact Bob Stoddard (510) 422-4877 (firstname.lastname@example.org).