A breakout point is a sharpened rod on top of the topload. It looks like an afterthought, but it is one of the few parts you can use to control where and when the discharge starts.
What it is and why
Electric field concentrates at a point. A smooth toroid spreads its field evenly over its whole surface, and the voltage has to get very high before anything breaks down. Put a spike on it and the field at the tip is several times higher than anywhere else. The air there breaks down first, at a lower voltage than the toroid alone would have needed.
The discharge therefore starts sooner, which is often what you want. It also starts where you put the spike, instead of wherever the toroid happens to be dirtiest or has the sharpest fingerprint.
The tip is a consumable. It carries every discharge and changes shape as it does, so its behaviour drifts over an evening. Builders who chase length change tips the way you change a drill bit, and where heat is damaging the tip they go to tungsten.
What you decide
How the tip is finished
Criterion: how many channels you want. You want one.
A smooth tip gives one large channel and a rough one gives several short ones. All the energy in one channel gives the most length. Split the same energy between three and you get three short arcs. So aim for a single channel on purpose.
Smooth and blunt are separate things. The tip should be sharp, because concentrating the field is what the part is for. The surface should be even, because roughness anywhere along the rod sprays corona instead of feeding one channel. A sharp tip on a rough shaft is the common mistake, and an easy one to make with a file.
Pick the material rather than using whatever was in the drawer. Where heat is the trouble, use tungsten, as on the RAIKIRI below, whose copper points kept melting.
For the same reason we would not use a soft joint at the tip. This is our own reasoning, not something a builder wrote down. Every discharge leaves through that one point, the tip wears like a consumable, and the fix when it melts is a metal picked for not melting. Solder has the lowest melting point anywhere in the assembly, and it would sit right where the energy leaves. Fix the point mechanically and keep soft alloys out of the last centimetre.
What will get you
The numbers
- Capacitance to the topload: far larger than the topload's own capacitance to the world. In the taped case above it was not, and that is what made the divider.
- Material, at the top end: tungsten, where heat is damaging the tip.
The taped case used to come with a measured capacitance in picofarads. We could not trace that figure to any source we can fetch, so it is gone. The case still shows the order of magnitude: comparable, not far larger. What a topload costs still prints the number, and it is the same untraced measurement there. The tungsten tip belongs to Daniel Eindhoven's RAIKIRI, remade in tungsten after the copper ones kept melting. That row is on the records wall flagged stale: his page is still up, and no longer carries any of it.
What goes wrong
- Arcing inside the structure and an unstable coil. The capacitive divider above.
- Three short arcs instead of one long one. A rough tip, or several competing points.
- Corona spray along the rod instead of one arc off the tip. The surface rather than the geometry: the shaft has to be even too, not only the tip.
- Performance drifts over an evening with nothing changed. The tip eroded. It is a consumable.
- Nothing comes off it at all. See its own checklist, which starts a long way from the tuning.
Where next
- What a topload costs, whether you want a toroid under it at all.
- An arc grows as fast as charge arrives, what happens once the discharge leaves.
- No sparks on breakout.
The breakout on the department diagrams is drawn as a needle, not a ball, because a point concentrates the field.