What a topload does, and what it costs in volts

A topload is not an upgrade. It is a trade, and the thing you pay with is voltage, which is the thing that makes the arc long.

What it is and why

Z = sqrt(L / C),   and volts at a given power go as sqrt(P · Z)

More capacitance, lower impedance, fewer volts. What you buy with those volts is a resonant frequency that does not run away when the arc grows.

So the whole question is how much your particular coil detunes, and that is not a property of toploads. It is a property of the machine underneath one.

What you decide

Whether to fit one at all

Criterion: how far the arc drags the resonance.

• A slayer exciter at a few watts has nothing to detune. A point, and nothing else.
• A 4 MHz HFSSTC at 500 W is stable enough that Richie Burnett runs his resonator with no toroid at all, having not found large ones to help a CW design. His experience is at megahertz; carry it to a 300 kHz CW coil carefully.
• A DRSSTC detunes noticeably, and wants one.
• A QCW throwing a metre detunes catastrophically, and goes the other way entirely: capacitance is added, sometimes inside the secondary, so that the arc is a smaller fraction of the total.

One toroid or a stack

Small below, large above is the usual arrangement, and the two do different jobs. The small one screens the top turns of the secondary, where the potential is highest and the gradient sharpest. The large one carries the capacitance and holds the breakout, which decides where the discharge goes. The lower one takes the induction heating, being nearer the primary.

The spacing is the knob, and it does not behave the way a calculator says: see the next section.

How much clearance it gets

A gap between primary and toroid of about the secondary's radius, and metal no closer than half the secondary's diameter anywhere. Both are about the same thing, which is the toroid heating inductively in the primary's field.

Slot the toroid so it is not a shorted turn. Steve Ward slots his.

What will get you

This is an argument against the calculator, not against the stack. Stacks work and are common. Measure the assembled secondary's resonance instead, and expect the calculator to read high, the more so the closer the bodies are.

And the skin effect argument about seams is right for the wrong reason. It is widely repeated that the seams of a home-made toroid need not be properly joined, because of skin effect. What actually carries the current across a seam is the capacitance between the overlapping sections. While that is large compared with the topload's total capacitance the voltage across the seam is small; when it is not, you get arcs between the sections and less stability than you had. In practice, fold one or two millimetres of tape over the edges so foil meets foil directly rather than through the adhesive.

The numbers

• The trade: Z = sqrt(L/C), volts as sqrt(P·Z).
• Two equal touching spheres: 1.386 times one, not twice.
• Breakout capacitance in the failed case: 12.6 to 20 pF, against a topload of the same order. It wants to be far larger.
• Primary to toroid gap: about the secondary's radius.
• Metal clearance: further than half the secondary's diameter.

The measurements here are other people's, not ours.

What goes wrong

• Arcing inside the structure rather than off the point. The breakout has too little capacitance to the topload. Measure it.
• The top turns of the secondary are burnt. It was run with no breakout point.
• The measured resonance is well below the calculated one on a stack. The calculator added the capacitances. They do not add.
• The toroid gets hot. Induction heating from the primary. Increase the gap and slot it.
• Fitting a bigger toroid made the arc shorter. It bought stability you did not need. On a coil that was not detuning, the volts are the whole loss.
• Branching and a drifting frequency after removing the toroid. Put it back; that coil needed it.

Where next

• Winding and measuring a secondary, where the other half of sqrt(L/C) is set, and why capacitance won in the winding is worse than capacitance won here.
• What a breakout point is for.
• Is it CW or not, the criterion that decides whether a topload earns its keep at all.

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The topload on the department diagrams answers for itself if you point at it.