A few turns of heavy copper around the base of the secondary. It is not connected to the secondary anywhere, and it is not trying to be.
What it is and why
The bridge pushes current back and forth through those turns at the coil's resonant frequency. That current makes a magnetic field, the field passes through the secondary, and because the field is changing it induces a voltage there. Only part of the field reaches the secondary at all, so what a single cycle induces there is a good deal smaller than the two windings' turns on their own would suggest.
So this is not a step-up transformer, and the high voltage on the secondary is not a turns ratio. The secondary is a resonator: each cycle's small push arrives in step with the one before, and the voltage builds up cycle after cycle instead of being stepped up once. That build-up is the whole mechanism and it belongs to the secondary's own page. What the primary has to do is put a field through it, over and over, in time.
Why it is thick. The whole tank current runs through it, and it is a large current. How large is not something anybody chooses: it falls out of the impedance of the tank. Copper tube or wide strip: at these frequencies the current only travels on the surface, so the middle of a round wire is not doing anything. What follows is not that thin beats thick, it is that hollow beats solid, and the reason is where the copper sits rather than how deep the current goes. A bundle of bare parallel wires does not help, because one magnetic field surrounds the lot and they behave as a single conductor; strands only win when each is insulated from its neighbours. The figures, and the tube against solid wire comparison that makes the case, are on the design page.
Why it is short, and what it is tuned against. The primary is only half a circuit. The capacitor in series with it is the other half, and the two together are what anybody means by the tank. What that pair does is ring, at
f = 1 / (2 · pi · sqrt(L · C))and the number that comes out has to land on the coil's working frequency, because the secondary only builds voltage from pushes that arrive in step.
So turns and nanofarads are not two decisions. At a chosen frequency their product is fixed, and you are only choosing how to split it: more turns and less capacitance is a higher impedance and a smaller current, fewer turns and more capacitance is the reverse. That is the one lever, and everything else on the design page is what it costs in each direction.
Why it stands beside the secondary rather than around it. Coupling is how much of the primary's field actually reaches the secondary, and it depends on geometry: how tall the primary is relative to the secondary, its diameter, and where the secondary sits inside it. More coupling means more of the field is shared. What that buys, and what it does not, is an article of its own.
What one actually looks like
Three shapes, and the names people use for them. A flat spiral, wound outwards on a disc like a pancake, with the secondary standing in the middle of it. A helix, wound up a cylinder around the secondary. And the cone between the two, leaning outwards as it rises.
The choice is not one argument, it is two that disagree. A cylinder runs at lower current, which is easier on the bridge, on the copper and on the bank's RMS rating. A cone puts its top turn further down the secondary and lowers the flashover risk. Both are real and they pull opposite ways, so what decides is the current: whichever of the two you can afford to lose. That is worked on the design page.
Why lower is safer, since the page above states it and does not say why. The secondary's potential rises from its earthed base upwards, so the part of the winding beside a low primary is the least stressed part there is. Keep the primary's turns low and they face volts rather than kilovolts. That single fact sets the shape, the height and the reason there is a gap at all.
Six to twelve turns is the usual range on a DRSSTC. This site's own coil taps between 9.4 and 12.4; Shane Colton calculated six for his and wound seven and a half so there was somewhere to move. An SSTC's primary sits higher, eight to nineteen turns on the builds this corpus has read, and the reason is the whole of an SSTC primary is a different part: without a tank to resonate with, the winding has to do the work by turns instead.
The turns are spaced, not laid tight. Adjacent turns of a primary sit at nearly the same potential so the spacing is not about insulating them from each other. It is geometry: the spacing is what sets the primary's height, and the height against the secondary's is a large part of the coupling.
You tune it by moving a tap. A clip or a bolted block on one turn, so the number of turns in circuit can be changed without rewinding. That is why taps are cut generously in both directions and why an extra turn or two gets wound in the first place: until the coil is running, the right number is not known. The tap is also the highest voltage point on the winding, which is not obvious and matters when you mount one.
And the grounded ring above it is not part of it. That is the strike rail, there to catch an arc that comes down before it reaches the primary. What it is for is with the things that will kill you, because its real job is the mains, not the copper; how to earth it, on its own wire and at one end only, is with the RF ground.
Which way round to wind it, which is asked more than it is answered
It does not matter, and the reason is worth having because it also tells you what to do after you change it. The winding direction sets the sign of the mutual inductance between primary and secondary, and nothing else: not the coupling's size, not the frequency, not the impedance above. A sign is not a quantity.
And at the driver a sign flip on the primary is indistinguishable from a sign flip on the feedback transformer, because both invert the same signal. That is why boards ship a phase switch for the current transformer at all, and Eastern Voltage Research says as much about theirs, that it "has the same effect as reversing the wires (or direction of the primary lead) of the feedback current transformer". So the two errors cancel, and a primary wound either way runs identically once the feedback phase matches it.
What follows is the practical half. If you rewind, re-tap or reverse the primary, the feedback phase is now a fresh question, and a coil that started yesterday will refuse to start today. That is a sign error, it is ten seconds to test by turning the ring over, and no amount of phase lead corrects it, which is with the two rings. The reasoning here is ours; what is quoted is the board's own manual.
The one decision this page is enough to make
Not the inductance. The impedance, which is the number a designer actually sets and which the primary current then falls out of rather than being chosen. That is the primary is an impedance, and it is where turns and tank capacitance get their values.
On this site's own coil the two halves are 16.96 µH of primary and 12.2 nF of tank. Put them through the ring above and the primary sits at 349.9 kHz; put them through sqrt(L/C) and the impedance is 37.3 Ω [derived, both]. The frequency is the constraint and the impedance is the choice, which is the whole of why the second number gets a page and the first does not.
What will get you
And bringing the two coils too close breaks the secondary across its own winding. That is the ceiling on coupling, and it is set by the winding's geometry rather than by any calculation you can do in advance.
The ceiling is a voltage on that winding, though, and not a coupling number, and there is a machine on file that proves it. A three coil magnifier is wound at about 0.5 between primary and secondary, twice what any two coil machine dares, and lives there, because in that topology the secondary is not the high voltage coil at all: the volts appear further along the chain. Tight coupling is dangerous only where the winding it is tight against is the one holding the kilovolts. The whole of that is a DRSSTC magnifier is not just a longer coil.
What goes wrong
- The copper gets hot enough to soften. High bangs per second. Air and cross-section, both taken seriously.
- A flashover across the secondary's winding, low down. Racing sparks. Too much coupling, and the ceiling is lower on the lower pole than on the upper.
- The frame draws kilowatts and the bolts melt. The shorted turn above.
- The current is nothing like the calculation. The calculation probably chose turns. The impedance chooses the current.
Where next
- The primary is an impedance, the design number.
- Sizing a tank capacitor by RMS current, the part in series with it.
- An SSTC primary is a different part, where the same component is wound the opposite way for the opposite reason.
On the department diagram the primary is the only part drawn touching nothing. That is the point of it: everything else on the page is wired to its neighbours, and this one does its whole job across a gap.