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
The Ultra-HGI was developed in response to researchers' needs for smaller insulators; however, smaller size used to mean breakdown at a lower voltage. During the high-voltage conditions that cause a conventional insulator to break down, electrons that are emitted from the insulator surface drift in the electrical field but can impact the surface. This collision liberates a greater number of electrons in a secondary emission, which leaves a net positive charge on the insulator surface that attracts yet more electrons. The result is an escalating cycle of electron collisions and liberations, called a secondary emission avalanche. The avalanche creates a dense plasma that prevents the insulator from sustaining an electric field; the insulator short-circuits and breaks down.
The Livermore insulator designers postulated that an interruption of the electron collisions would avert the avalanche, and breakdowns would not occur. They configured such an interruption by making an insulator with conductive layers that alternate with insulating layers. The conductive layers are spaced at distances equivalent to electron travel distances, which isolates the insulating layers from electron charges.
The Ultra-HGI is wholly different from today's conventional insulators, which are fabricated as single pieces or as multisection designs. In a single-piece insulator, any flaws make the insulator susceptible to premature failure because the flaw becomes the focus of extremely high-voltage gradients, which initiate and then propagate failure mechanisms. In the conventional multisection insulator, failure in one section causes all the voltage to be redistributed across the few remaining good sections, thereby making them more susceptible to failure. In comparison, the Ultra-HGI's alternating layers divide voltages so finely that the chances of failure are small, and when failures do occur, they are confined to a very small portion of the insulator. An Ultra-HGI insulator comprising a 2.5-millimeter-thick layered stack could have more than 100 conductive "capacitor" layers, each of which receives only a small portion of the high-voltage gradient. In the event of a failure or breakdown of one of the capacitors, the entire voltage is reapportioned across enough remaining capacitors that each receives only a small increase in voltage, and the insulator remains viable. Thus, any breakdown is temporary, no permanent damage is caused, and the insulator can be reconditioned for further use.

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.

--Gloria Wilt

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 (stoddard1@llnl.gov).

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