A sharpened rod on top of the topload, pointing at nothing in particular. It looks like an afterthought and it is one of the few parts you can actually control the discharge with.
What it is and why
Electric field concentrates at a point. A smooth toroid holds its charge with the field spread 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, so the air there breaks down first, and at a lower voltage than the toroid alone would have needed.
Two things follow. The discharge starts sooner, which is often what you want. And it starts where you put the spike, instead of wherever the toroid happens to be dirtiest or has the sharpest fingerprint.
It is a consumable. The tip carries every discharge and changes shape as it does, so its behaviour drifts over an evening. Builders who chase length change them the way you change a drill bit. RAIKIRI's is tungsten, because the copper ones kept melting.
What you decide
How far past the toroid it stands
Criterion: whether the tip is still inside the toroid's own field.
A short spike, barely past the outline of the toroid, works better than a long one. A tip just outside the outline sits in that field, and the field pushes the discharge up and out. Extend the spike far enough and the tip leaves the toroid's field altogether, at which point the nearest earthed object decides where the arc goes instead of you.
How the tip is finished
Criterion: how many channels you want, and you want one.
A smooth tip gives one large channel. A rough one gives several short ones. All the energy in one channel gives the maximum length, while the same energy split between three gives three short arcs, so a single channel is a design goal rather than an accident.
What will get you
The numbers
- Length: barely past the toroid's outline.
- Capacitance to the topload: far larger than the topload's own capacitance to the world. The failed case measured 12.6 to 20 pF, which was the same order and therefore a divider.
- Material, at the top end: tungsten. RAIKIRI's, after copper melted.
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.
- The arc goes to the nearest earthed object rather than up. The spike is too long and its tip has left the toroid's field.
- Performance drifts over an evening with nothing changed. The tip eroded. It is a consumable.
- Nothing comes off it at all. That is its own checklist, and it 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 a needle rather than a ball, and it is short, for the reason in the middle of this page.