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Re: Still more H2 Thyratron

Gary and all,

I can only describe the theory of operation at this time, since I've not
yet used these in practice. When a thyratron is fully conducting, the
space between the cathode and anode is filled with a highly conductive
plasma. However, once the anode is forced to a negative voltage with
respect to the cathode, this plasma very rapidly decays back to a
neutral gas and the tube recovers its hold-off voltage. In order to
re-assume grid control, the space between the control grid and the anode
must be thoroughly swept free of remaining electrons and ions, and the
control grid voltage's electrostatic field must extend across the gaps
in the grid aperature so as to effectively shield the cathode from the
anode. The speed at which recovery occurs is a function of the
recombination time of the particular gas used in the device, and the
spacing between the control grid and the anode. Quick recovery requires
that the anode potential be driven negative with respect to the cathode
(which is no problem for the ringing tank circuit in a TC!). 

The inherent recovery time is very rapid in the case of a hydrogen or
deuterium thyraton, typically of the order of microseconds. Only
hydrogen/deuterium-based thyratrons can recover quickly enough to
"quench" a primary circuit of a Tesla Coil. As long as the TC's
operating frequency is comparatively low, there should be ample time for
recovery - at least in theory...  :^)

Recovery in a mercury or xenon thyraton is much slower, making them
suitable for powerline frequency commutation, but not for fast pulse
switching or TC use. 

-- Bert --


Tesla List wrote:
> 
> Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>
> 
> Hi Bert:
> 
> I understand the basics of SCR operation, but it's not clear if the turn-off
> criteria for thyratrons is met with each tank circuit voltage reversal,
> which would be exactly what we want.  Is it, or is such rapid turn-off
> behavior limited to just the most exotic thyratrons?
> 
> Gary Lau
> Waltham, MA USA
> 
>                 Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
> 
>                 Hydrogen thyratrons are used for high voltage, moderate
> current
>                 switching, where the average current levels are
> comparatively low. When
>                 combined with a DC charging source and HV pulse forming
> network, they
>                 can be used to provide pulsed power for high-energy
> experiments or for
>                 pulsed RF (RADAR, partical acellerators). A thyratron is the
> vacuum tube
>                 analog to a Silicon Controlled Rectifier (SCR) - once turned
> on, current
>                 flow through the device must be stopped or the applied
> voltage reversed
>                 in order to turn off the device. A thyratron is triggered by
> a positive
>                 pulse on the control grid, which is normally biased to a
> negative DC
>                 potential versus the cathode. Once the thyratron begins to
> conduct, the
>                 grid no longer has any control over the device
> 
>                 <snip>