Re: Why do streamers happen?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:
>Original poster: G <bog-at-cinci.rr-dot-com>
>Hello List,
>I was showing a co-worker some pics of my coil, and he asked me why the 
>sparks shoot out into the air, with no apparent receiving electrode. I 
>have to admit, I'm at a loss to explain the phenomenon, except for 'the RF 
>field becomes too strong to contain the electrons.' What is the scientific 
>answer to his question?
>Thank you!
>Gregory

Hi Gregory,

Your explanation is partially correct - the electric field near the surface 
of the HV terminal must initially be sufficient to cause the air to break 
down. Once initial breakdown has been achieved, the air next to the HV 
terminal begins to ionize, forming corona at points of highest E-field 
stress. Corona initially form at a somewhat lower voltage when the HV 
terminal is negatively polarized then when it is positively polarized. If 
the HV terminal voltage continues to increase, an electrically conductive 
spear of air may suddenly poke outward from the terminal, terminating at 
the far end in a countless number of faintly luminous filaments, called 
streamers. As this occurs, a bit of excess electrical charge in the HV 
terminal is suddenly transferred into the surrounding uncharged air. While 
the actual discharge is only nanoseconds in duration, the peak current is 
typically several amperes. The sudden discharge injects a batch of charge 
into a nearby region of air, creating a "space charge" which has the same 
polarity as the HV terminal that created it. Once the space charge is 
created, the E-field stress "seen" at the terminal plummets, and the 
discharge abruptly stops.

Now suppose that the terminal voltage continues to rapidly increase or 
suddenly reverses polarity (as in a TC secondary during ring-up). In this 
case a series of additional streamer discharges can occur, each trying to 
redistribute the charge differences between the HV terminal and its 
surroundings. While the "root" of these streamers usually follow the same 
path blazed by previous streamers, the thread-like ends will seek regions 
having the lowest opposite potential. Under repeated discharges, the common 
root of the streamer discharges heats up, becoming much more conductive and 
arc-like. This hotter, brighter conductive channel is technically known as 
a "leader" (originating from the stepped-leader theory of lightning 
propagation). The brighter portions of the discharges you see nearest the 
TC HV terminal are actually leaders, while the fuzzy bluish haze that 
extends outward from the leader tips are made up of countless little 
streamers that feed current between the leader and the space charge region.

When the HV terminal quickly reverses polarity while maintaining a similar 
potential, the presence of nearby space charges tend to increase the 
effective E-field seen at the HV terminal above the field that would be 
predicted by terminal voltage alone. This means that, once initial 
breakdown has occurred, future breakdown will occur at a lower terminal 
voltage on the next RF half cycle. This is the reason why air seems to have 
a lower breakdown voltage for RF voltages versus low frequency AC or DC. 
Because they can follow the hot channels left behind by their predecessors, 
leaders may continue to grow over tens or hundreds of bangs in a disruptive 
coil.

The current that flows within the leaders and streamers are "displacement 
currents". While displacement currents are most often associated with 
charging/discharging capacitors, they flow whenever an electrical field is 
being redistributed within a dielectric. Leaders and streamers are a 
manifestation of electrical charge being rapidly redistributed between the 
surrounding space charges (or previously uncharged regions) and the HV 
terminal. A disruptive coil provides a unique combination of high voltage 
RF oscillations imbedded within a rapidly increasing voltage envelope. 
These (fortunately) conspire to create spectacular discharges that can 
become much longer that the HV terminal voltages would suggest. The 
displacement currents that flow during these discharges are not small - 
they range from amperes to tens of amperes, even though they seemingly 
terminate into thin air!

But if you think about it for a while, the situation is not terribly 
different than the grand displacement currents that also occur in thin air 
between space charge regions during cloud-to-cloud lightning.

Hope this helped a bit,

-- Bert --