Re: Skin Effect & Primary Current?

various people wrote:

>>     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"?


While it is true that glow discharges emit the characteristic 
spectral lines of the gas that constitutes them, this isn't a
good way to determine if the process is really an arc or a glow.
I use magnetron sputter guns to deposit metal films here at OSU.
These guns operate in the glow discharge regime with current
densities of about 0.1 Amp/cm^2. [dischare current density is the
total current/discharge (not electrode) area]. Still, these glow
discharges emit very strongly in BOTH the gas and electrode
spectral lines. 

Most of the spark gap dischages I've seen for tesla coils and the
like are not dominated by the spectral lines of the gas or electrode.
Instead they are rather white/blue - indicating extreme temperatures
associated with arc phenomena.

Glow discharges also are distinguished from arcs (roughly) in another
way. Glow discharges tend to envelop the entire electrode surface so
that the current density is relatively uniform just near the electrode
surface. They also tend to emit light throughout a large volume 
between electrodes rather than in a quasi 1-d path like lightning.
Since the current is spead uniformly in a glow, the current densities
are also low and electrode errosion occurs mainly by 'sputtering'.
[sputtering happens when the gaseous ions lose their  kinetic
energy though collisions with the electrode and subsequently
eject electrode atoms]

Arcs, being more 1-d on the other hand, tend to form hot spots on the 
electrode surfaces where the current density can be very high. [Arcs
typically contact the electrode in fractions of square mm in area] These
hot spots are actually points where the electrode has reached it's
boiling point [9000 deg F in the case of tungsten, 4500 F for copper oxide]
and is literally
boiled off the electrode surface. This can cause the formation of pits 
in the electrode material. This pitting does not happen in the case of
glow discharges since the electrodes are 'sputtered' much more
uniformly. Since the temperature of an arc is so high, unlike the glow,
one should also see a black-body radiation curve superimpodsed on the
spectral lines of the gas and electrode materials. This black-body
radiation is responsible for much of the 'whiteness' of arcs.

Finally, arcs usually have lowwer voltage drops than do
glows. Typical arcs might have 50-200volts drop depending heavily
on the current, whereas glow dischage voltage drops are 
*charateristically* independent of current and are typically 200-
1000 volts. ( neon gas will 'glow' down to ~70v, however).

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 the discharge is very concentrated- only a fraction of the
electrode is active at any one time - note the pitting! Therefore
the current density is high.


>>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.


If you have 100 amps and the discharge area is typically (~0.5mm)^2 then
you have a current density of 40,000 amps/cm^2. That's hot! I'm 
pretty sure you're really in the arc regime. Remember, it's not the
total electrode area, only the area of the electrode which is actually
surporting the discharge at any one time ( the bright spots).

You might want to look at Gaseous Conductors by J. D. Cobine.

-Ed Harris