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Re: [TCML] TC Max Spark Length & Output Voltage

Dear Udo,

Thank-you very much for your detailed explanation of the plasma channel, how it is created & how it allows the discharge to grow.

Obviously you have much experience in this phenomenon, and have analyzed it in much detail.

Now I can appreciate how much more complicated the arc length vs. voltage is than I ever could have imagined!

So, again, I thank you very much for the time and effort you took to field my question!

Best regards,


Sent by Mačak's humble servant.

> On Jan 17, 2019, at 6:38 AM, Udo Lenz <udo_lenz@xxxxxxxxxxxxxx> wrote:
> Arc voltages come in a wide range. For a spark gap with rounded
> electrodes you need about 30kV per cm for a spark to fire.
> In an arc lamp, voltages around a 100V for a 1cm gap are sufficient.
> An arc lamp has to be ignited, though, e.g. in an antique carbon lamp,
> the electrodes first are touching and then pulled apart.
> The difference between these 2 situations results from the way,
> free electrons, which are the carriers of conductivity, are created.
> In the spark gap, free electrons have to be accelerated by the electric field
> against the resistance of air molecules to be able to hit air
> molecules fast enough to liberate new electrons from them.
> That causes an avalanche of electrons and requires high voltages.
> In an arc lamp, electrons are liberated by high temperature
> air molecules hitting each other. Thus voltage (and current)
> is required only to keep the air hot enough.
> Tesla coil arcs are special, because they can end in mid air.
> Once the arc breaks out, which requires several tens of kV
> by the means of electron acceleration, it produces a plasma
> channel. The once created plasma channel has a lot lower
> voltage drop such as in an arc lamp.
> That channel transports most of the top load voltage to the tip of
> the arc, so that it can grow from there. A TC arc is a mixture
> of both the arc lamp and spark gap situations, i.e. around 100V/cm near
> the breakout point and 30kV/cm at the arc tip.
> The voltage-length relationship in a TC arc is complicated. It also
> depends on the duration of the burst, since arc growth takes some
> time. Also involved is the frequency of the coil. A higher frequency
> will cause more charge movement along the arc, increasing power
> dissipation along the arc, which implies a better conductivity of it.
> So less voltage is then required for a given arc length.
> Udo
> ----- Original Message ----- From: "William Fox" <wm9fox@xxxxxxxxx>
> To: <tesla@xxxxxxxxxxxxxxxxxx>
> Sent: Wednesday, January 16, 2019 3:08 AM
> Subject: [TCML] TC Max Spark Length & Output Voltage
> Hello Everyone,
> There has been much discussion over the decades about estimating maximum TC spark length & corresponding voltage... To my knowledge,
> much investigation has been done by our fellow members, John Freau (Length= 1.7 * SQRT(kVA in) & Greg Leyh V = 200 kV/m)... Thanks,
> guys!  But with the advent of so many different types of TC’s beyond the SGTC (e.g. Class E, DRSSTC, QRSSTC, SSTC, VTTC, etc.) these
> issues are getting much more complicated.  It would be very interesting, if those who are much more knowledgeable in this area than
> I, take a shot at tabulating field data & see if, from that data, some equations of approximation might be derived.  Assuming
> discharge is issuing from a breakout point on topload or pointed terminal.  For example, QRSSTC’s maintain the arc channel longer,
> time-wise, allowing the discharge to grow to 5 feet or so from a very short, squatty coil, but actual voltage as low as 75 kV (this
> from offline discussion with Bert Hickman)... Thanks, Bert!  What do you members have to say about this?
> Thanks,
> Bill
> Sent by Mačak's humble servant.
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