WHAT ARE VACUUM CAPACITORS?

Vacuum capacitors use a high vacuum as dielectric. Vacuum is the strongest insulator, with no problems such as contamination, heating, moisture absorption, and dielectric aging. These capacitors have exceptional performance, especially where voltage, current, and RF loss all matter at once. Absence of any physical material between electrodes allows extremely high voltages, and very low dielectric losses, as also excellent high frequency performance.

Construction

Vacuum capacitor has two parallel metallic plates, usually stainless steel, mounted on insulating material- either glass or ceramic material The assembly is placed in a glass ot ceramic envelope. Most vacuum capacitors use concentric metal cylinders or interleaved cylindrical plates enclosed in a sealed ceramic or glass envelope. The figure below shows construction of vacuum fixed capacitor with concentric cylinder electrodes.

Vacuum capacitors have a substantially smaller ESR and inductance, which helps them handle RF current efficiently and stay cooler under load. Capacitance is determined by area of electrodes, space between them, and permittivity of vacuum.  Rated voltages can vary from 5 kV to 100 kV, and operating frequencies in hundreds of megahertz.

Greatest advantages of vacuum capacitors are low losses and high breakdown resistance. They have very low equivalent series resistance and can handle large RF currents with very little heating. Hence these are widely used in transmitters, tuning networks, filtering stages, coupling circuits, etc.

Peak voltage of a vacuum capacitor is limited by mechanical design, and does not appreciably vary with frequency. Breakdown voltage depends on physical separation distance between plates, and level of vacuum. Capacitance may vary from as low as 5 pF to 5000 pF and beyond. Vacuum capacitors can withstand high temperature of up to 125°C. These capacitors are usually made with a tolerance of ±10%, while those below 100 pF may be made with closer tolerance of ±5%.

Vacuum Variable capacitors

Vacuum variable capacitors form a greater portion of vacuum capacitor usage. These capacitors may last a lifetime of over 2 million cycles of extreme value adjustments.

Vacuum variable capacitors use concentric cylindrical or ring-shaped electrodes inside a sealed glass or ceramic envelope with bellows or other vacuum-sealed adjustment mechanism. This changes the overlap of electrodes, thereby changing capacitance. The construction makes vacuum capacitors electrically more robust, but more complex and expensive. Ratio of maximum capacitance to minimum value can be as high as 150:1.

First vacuum variable capacitor was developed by Jo Emmett Jennings (U.S.A.) in 1940, and were commercialized since 1942. A major application is in plasma generators, where frequencies from 2 MHz to 160MHz are common.

Vacuum capacitors are available from values from 20 pf to over 5000 pf, with voltages ranging from 5 kV to 100 kV. Current capabilities may go to few hundred amperes at frequencies even up to hundreds of megahertz range. Vacuum variable capacitors are common in high voltage applications of 5000 Volts and above. These capacitors are also manufactured with motorized control of multi-turn screw for moving electrode.

There is a special class of these capacitors (vacuum trimmers) which can be varied within a small range of overall nominal value. For example, a 100pF capacitor may be variable from a minimum of 95 pF to maximum 100 pF.

Advantages of Vacuum Capacitors

• High working voltage up to tens of kV
• Low power losses, ideal for high frequency
• Durable & maintenance-free, with hardly any aging effect
• High capacitance stability even under large temperature or load changes

Limitations of vacuum capacitors

• Larger & more expensive
• Mechanically sensitive due to glass/ceramic envelope
• Economically viable only for specialized applications

Their main downsides are cost, size, and mechanical complexity. Vacuum capacitors are more expensive to manufacture because they need a sealed vacuum enclosure and precision moving parts.

Applications of vacuum capacitors

Vacuum capacitors are preferred in radio-frequency and high-voltage equipment due to low loss, high current handling, and strong resistance to arcing. They are especially useful wherever the circuit must tune precisely under high voltage and high current without failing from heat or discharge.

Range of applications for vacuum capacitors include:        

• Broadcasting communication equipment: Vacuum capacitors are used as tuning, coupling, filtering, neutralization and DC isolation components in medium-wave, short-wave and ultra-short-wave transmitters.
• Semiconductor manufacturing equipment: deposition and etching equipment for plasma
• High Frequency Industrial Equipment: Used for High Frequency Medium Heating and Welding, etc.
• Medical devices: medical analyzers and therapeutic instruments, etc.
• High Energy Physics: High Energy Particle Accelerator, etc.
• Power equipment: testing equipment for dielectric loss
• Research facilities and particle accelerators, where precision tuning and high voltage are required.