Vacuum capacitor in pulse application?

[Al (astral highway)]

Hi folks,

I have experience of various capacitors in high-voltage, high current applications, and I know that Strontium titanate doorknobs capacitors are pretty hard to beat in RF pulse applications.

I do have a lovely ex-Soviet vaccuum capacitor (fixed) of the correct value for a tank circuit I need to build, but it is physically huge, so not mechanically trivial to mount it. With my capacities still restricted to some extent, I don't want to invest time concocting an elaborate, suitably strong assembly for lacklustre results.

So before trying it, I wondered if anyone can say what sort of performance I could expect in an HF tank circuit? It's 150pF, the glass envelope is 7cm in diameter and around 10cm long. It is terminated axially with big aluminium connectors with screw threads.

It can obviously deal with peak currents in the 100s of A, but is a fixed vacuum capacitor actually designed for pulse use, or more typically as an RF bypass capacitor or similar?

The alternative is a 15kV rated doorknob capacitor, again ex-Soviet NOS, containing silver. It is huge and virtually indestructable, but I don't yet own it. That of course is very easy to manipulate by comparison.

Thank you folks!

[russell_w_b]

Vacuum caps are used almost universally in high-power broadcast HF tank-circuits so should be OK for your application. We used Jennings, Comet, EEV devices fixed and variable, water-cooled and simple glass or ceramic envelope devices.

The small 50pF - 150pF vac caps (about 3" to 6" long and 2" diameter) are rated at 35kV - 50kV. They can occasionally go 'soft' when they'll glow purple.

[Al (astral highway)]

Hi Russell,

Thank you , that's reassuring! Is there any X-ray risk with perhaps 15kV pulses in the tank circuit?

[Malcolm G6ANZ]

probably not. You only get x rays when free electrons strike a solid target, usually tungsten. In a tank circuit the electrons are all contained in the wiring or capacitors so no free electrons so no x rays. There might be a small chance of some low level x rays if it arced over in some fashion but even then the energy would be so low as to be insignificant and the UV from the arc more noticeable.
Malcolm

[Herald1360]

Originally Posted by Malcolm G6ANZ

You only get x rays when free electrons strike a solid target, usually tungsten.

You can also get them from a beam of electrons accelerated up to about 2000 x rest mass when they go round the bends in a synchrotron.

When that beam was "lost" and the electrons hit the structure you got a gamma ray "flash"

[Malcolm G6ANZ]

True, your'e talking about Bremmenstralung (sp) radiation. This radiation is emitted whenever a stream of ,fast moving, electrons are bent in a magnetic field. There is usually a lot of lead shielding to absorb the radiation around the bend magnets.
CRT users need not worry
Malcolm