Re: Vacuum jar rotary spark gap / thyratron (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 16 Aug 2007 23:52:37 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Vacuum jar rotary spark gap / thyratron (fwd)

Hi Scott, Gary, Tim and all,

When a high vacuum spark gap breaks down, current is initially carried 
by electrons that were liberated via field emission (sometimes 
explosively at the microscopic level), and then subsequent thermionic 
emission from one (or more) incandescent cathode spots. Evaporated metal 
ions, initially from cathode spots, but under high/prolonged current 
flow, from the anode as well, also contribute to the conductivity of the 
plasma. The plasma "comes" from the electrodes, not the vacuum.

At current zeros, the conductive species in the plasma rapidly disperse 
from the reaction zone, redepositing themselves on the electrodes and 
insulating walls of the vacuum gap. A vacuum arc quenches very rapidly - 
considerably more rapidly than higher pressure gas switches, including 
atmospheric/high pressure spark gaps, glow-pseudospark gaps, Hg vapor 
ignitrons, and even hydrogen/deuterium thyratrons.

The original poster also mentioned potentially using thyratrons instead 
of spark gaps. Unfortunately, most thyratrons will conduct current only 
in one direction - any significant reverse flow rapidly destroys cathode 
material. Although there are specially constructed (hollow anode) 
thyratrons that can conduct reverse current, these only conduct about 
half the initial forward current. Thus, in order to support high 
bidirectional RF current flow in the primary (during "ring-up" of the 
secondary), you will need to either use back-to-back thyratrons with 
appropriate trigger circuitry, or use a hydrogen diode (a 
self-triggering thyratron) or use a fast, high current HV semiconductor 
rectifier chain across the thyratron. Special triggering circuitry (to 
trigger the thyratron on successive positive RF cycles during ring-up) 
may also be required.

The complexity and costs of a thyratron switched system very rapidly 
outweigh the benefits. For these reasons, several coilers on this list 
developed various more efficient solid state designs using medium 
voltage (kV level) semiconductors to serve as solid state spark gaps (as 
in OLTC or SISG designs) or to provide active RF pumping during ring-up 
(SSTC's and DRSSTC's).

Bert
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Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Thu, 16 Aug 2007 17:20:23 -0700
> From: huil888 <huil888@xxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Vacuum jar rotary spark gap / thyratron (fwd)
> 
> Gary -
> 
> I'm not sure if this is entirely true ....
> 
> I have watched a dry-pig-powered coil in operation that utilized a single 
> vacuum-contactor (mechanically operated, high-voltage, high current switch) 
> as a "sealed" static spark gap. In their intended use as a switch for very 
> large AC motors, the vacuum extinguishes the arc at the zero-crossing of the 
> AC current as the contacts are separated. Normally the contacts are pulled 
> apart rapidly to open the circuit, but in the Tesla coil application the 
> contacts were mechanically held at some small fixed distance to function as 
> single static gap. One end of the ceramic vacuum bottle is sealed with a 
> metal bellows to allow one contact to be moved. Typical ratings are 400-800A 
> at 7.5KV, with a flashover voltage of 40-60 KV. Currently, there are a 
> number of 3-phase vacuum-contactors listed on eBay,  and also several sets 
> of the vacuum-bottles themselves (search "vacuum contactor").
> 
> In my desktop mini-coil I use a series of Victoreen 2.5KV vacuum 
> surge-supressor spark gaps (at least I think they are evacuated) as a 
> multi-segment static gap, and the coil runs very well. The small (~1/2" dia 
> X ~2" long) ceramic-cased gaps heat up fairly quickly, but seem to quench 
> quite well.
> 
> Regards,
> Scott Hanson
>