125 Years of Vacuum Capacitor Technology

September 01, 2021

When Nikola Tesla filed the first patent for a vacuum capacitor on September 15, 1896 - 125 years ago - little did he know the impact this invention would have on today's modern world.

Becoming the father of electricity thanks to his cat.

Born in 1856 in a part of Austrian Empire that is now Croatia, Nikola Tesla transformed daily life at the turn of the 20th century with his inventions and contributions toward the electric age. He invented the modern AC electricity system while working for the famous inventor Thomas Edison in the US — all that without even receiving a degree.

The story tells that one day, as young Niko was petting his cat (and best childhood companion, called Macak), a spark of static electricity jumped from his cat’s fur to his hand. At that moment he decided to make studying electricity, particularly magnetism, his life’s work. Attempting to develop inventions he could patent and market, Tesla conducted a range of experiments with mechanical oscillators and generators, electrical discharge tubes and early X-ray imaging (like his fellow inventor Wilhelm Röntgen).

When Tesla filed the patent, the variable vacuum capacitor was meant to enhance the quality of electrical components for handling “high-frequency currents”, however they have only been available commercially since 1942. One of the first commercial producers of the technology was the US-based company Machlett Laboratories.

Nikola Tesla  - Image source:  Pixabay

Machlett and Comet setting the pace

Machlett had been a very important partner for Comet: acting first as licensor for the production of X-ray tubes since the early 1950s, and then – from 1965 to 1989 - for the manufacture of Comet's vacuum capacitors.

Before being sold to Eimac in 1989 Machlett had, however, already passed on the many years of experience and knowledge to Comet.

The Comet team absorbed the knowhow and started to further develop the technologies. This way vacuum capacitors have become an indispensable component in subsystems used to better control RF voltages and electrical currents in modern manufacturing processes.

Components of Machlett Vacuum Variable Capacitor - Image source: Archive, Machlett (Lars Giers)

“Although we have made major advances in terms of product lifetime and power density, the basic design for Vacuum Capacitors has remained very stable over time, not because engineers aren’t creative, but because this stability is important to customers, especially in applications where precision is crucial. Quality, repeatability, stability and reliability is what we’ve been best at all these years.”

Walter Bigler, Longstanding Vacuum Technology expert at Comet PCT

But what exactly is a vacuum capacitor?

A capacitor is a passive electrical component that is capable of storing electrical charges. A capacitor consists of two conductive surfaces called electrodes, which are usually placed very close to each other. There is an electrical insulating medium between the electrodes—in the simplest case air. The capacitor is characterised by its capacitance, which indicates how much electrical charge can be stored at a certain applied voltage. The maximum voltage that can be applied to the electrodes and the maximum alternative current the capacitor can handle are also of interest.

Some applications, such as the generation of radio waves in shortwave transmitters, require capacitors with very high dielectric strength and the ability to withstand very high currents. In addition, it is necessary for the capacitance of the capacitors used in these transmitters to be adjustable. In principle, only vacuum capacitors can fulfill these properties. Because of the much better insulating properties of vacuum compared to air, vacuum capacitors are very often used. The electrodes of vacuum capacitors are typically concentric rings or spirals that are immersed in each other. The capacitance can be varied by changing the depth of immersion.

Both the areas of application and the corresponding designs of capacitors are very diverse. They are used in high power broadcast stations to tune resonant circuits and antennas, or in medical and scientific RF applications, such as MRI or accelerators. Comet mainly uses them in RF Matching Networks for plasma applications in the semiconductor industry.

Vacuum Variable Capacitor Shown in Cut-Away View

In the semiconductor industry

In many coating and etching processes within the semiconductor industry, a plasma is used which is ignited and maintained by high-frequency energy. Because the system impedance of the RF generator and the plasma are not equal, it is necessary to match the different impedances in order to transfer as much of the generator's energy to the plasma as possible. To achieve this, RF Matching networks (also known as “Match boxes”) adjust the dynamic RF impedance of a plasma chamber to match the impedance of the RF system. This way we can ensure that the process power, yielded by the RF generator, can be supplied to the plasma process. (example in the video hereafter)

Over the past decades, Comet has established a very strong market position in the field of vacuum capacitors through superior quality, exceptional customer orientation and the most reliable delivery service. The most cutting-edge equipment in the semiconductor industry utilises Comet capacitors. This is thanks to Comet’s proven expertise in high-frequency engineering and manufacturing technology.

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Vacuum Variable Capacitors are widely used to build microchips, crucial for the digitalization of the modern world. More details in the video (minute 1:55).

Discover the full range of Comet PCT world's most precise vacuum capacitors.


Comet Group and Comet PCT, internal sources and archives   |   Service Bulletin SB-12, Vacuum variable Capacitors" (PDF)  |  Vacuum Variable Capacitors – An introduction to their Design, Rating and Installation by Lars Giers (Machlett)   |  Wikipedia: Vacuum Capacitor and Nikola Tesla  |   Google Patents patents.google.com (U.S. No. 567,818)

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