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Re: Skin Effect & Primary Current?


>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>To: tesla-at-pupman-dot-com
>Subject: Re: Skin Effect & Primary Current?
>
>Hi Jim,
>         Nice thinkin', and thanks for the information. The gap 
>electrodes were specifically chose to be tungsten carbide with this
>thought in mind (v. high melting/sublimation point). Pre-apology for 
>the various snips....
>
>>     At this current density your gaps are in the glow-discharge
>> region (don't believe me? look at the light emitted. Is it blue:
>> indicating a Nitrogen glow discharge? or Green: copper? ?/iron. =>
>> arc)
>
>Bright blue.
Glow discharge regime, not arc! On my rotary, with iron/steel bolts,
I'm eroding to gap => arc; maybe this is why I'm having so much
problem with "power arcs"?
> 

>> WHAT? Mercury at 10nS. !!!! ?????  Why are Mercury Thyratrons So Slow?
>> If low pressure mercury arcs quench in 10nS. why use blown Cu/N2
>> gaps;) I think von Engel is off here (pun intended)
>
>I have it on good authority that specially built thyratrons can indeed
>operate at moderately high break rates and but require special 
>internal construction in the form of baffles and the anode must be 
>made of a particular type of material. I'm sorry, I don't have the 
>particulars handy. 
>
A Hydrogen thyratron?

------- snippet from EG&G app note ------

2. Comparison of Characteristics {hydrogen thyratron vs spark gap jf}

The hydrogen thyratron is a unidirectional, hot cathode device that
requires some minutes of warm-up time from an external heater supply
prior to operation. {metal-hydride gas reservoir jf}  Timing precision
is relatively good.  Triggering requirements are slightly more complex
than for the spark gap.  Radiated EMI is lower, due to lower di/dt
capability {vs spark gap jf} RMS and average current ratings are
vastly superior.  Total space requirements are greater due to heater
requirements and thermal considerations.

The triggered spark gap is a bidirectional, cold cathode device that
has no warm-up time and can carry large peak currents.  Timing
precision is relatively poor.  Triggering requirements are simple.
Average power ratings are low. Mechanically, the device is robust.
Radiated EMI is high.
------- end snippet -----------------------------
Later in the application not EG&G talks about the life of hydrogen
thyratrons vs spark gaps  ---- 

I just did an ABOUT FACE!

EG&G is building spark gaps that operate in the arc regime: a glow
discharge starts, builds up, vaporizes electrode material and
transitions to an arc! 

Our's are not in the arc regime! (expect maybe the 'power arc'
phenomenon) We are building Nitrogen thyratrons that trigger by
exceeding their hold off voltage rather than by pulsing a trigger
grid. Richard's cylindrical gap works well because it has a large
enough surface are to prevent it from transitioning into the arc
regime. vis-a-vi:

>>But if (with electrode distances of <= 1cm) the
>>potential difference is of order 30V, the current density at the
>>cathode spot 10^4 to 10^6 A/cm^2, 
...implies an arc. Notice 10,000 to 1,000,000 Amp per square cm.

>> Although, if the 10^-3 sec number is correct for Cu, maybe we should
>> consider a different electrode material? (or at least determine if we
>> are close to the arc region of operation instead of just the glow
>> discharge region).
nope, glow discharge. I have less than 100 amps in my TC primary (~60)
and a lot of area.
>
>I think high m.p. is certainly the way to go. I have had no trouble 
>with power arcs or quenching with the coil in question. To some extent
>this is alleviated because in throwing an attached arc, the system 
>loses energy very quickly indeed. I have scoped the loss as occurring 
>in less than 10 cycles (very conservative).
>
Do I note that you have your rotary gap running?

	Regards,

	jim