The thing this department exists for is a discharge that looks nothing like a Tesla coil's. Straight, smooth, one channel, growing visibly rather than appearing. A sword instead of lightning.
Why it comes out straight
Not because of anything clever in the geometry. Because of the clock.
A DRSSTC's bang lasts about 300 microseconds and its channel gains twenty millimetres per RF cycle. That much charge arriving that fast means the channel can break out in several directions at once, and it does.
A QCW's ramp lasts 5 to 25 milliseconds and its channel gains 0.16 millimetres per cycle, a hundred and twenty five times slower. At that rate one channel grows, and it grows more or less continuously.
What the ramp has to do
The ideal growth is at a constant speed, and constant speed means a constant voltage at the tip relative to ground. Any jerk in the supply is a jerk at the tip, and the arc answers a jerk with a new branch.
Which is why the bus is ramped rather than switched, why the ramp is made by a converter rather than by skipping pulses, and why a corner anywhere in the curve shows up as a branch. The shape wanted is linear in the bridge's voltage, which gives power rising as the square of time because the load is resistive.
How long it gets
Length goes as the cube root of the energy in the bang. That is the single most important number in the department, because it is what makes upgrades disappointing:
L ~ E^n, n = 0.31 to 0.33Eight times the energy for twice the arc. Raising the bus from 325 to 440 V is 1.35 times the energy and about 18 to 20 centimetres on a 188 cm arc, not the 67 that a linear guess promises.
And the ratio it reaches, arc over the length of the winding that threw it, runs from about 7 to 25 across published builds on similar coils. That spread is technique, not size, which is what the records page is arranged to show.
What limits it
- The overcurrent detector, if the energy piles into the end of the ramp. The bang tears and you hear a pop instead of seeing a sword.
- The frequency. Below about 200 kHz the channel runs cooler and branches; a slow ramp does not compensate for that, which has been tested.
- The channel's own charge. Space charge repulsion in the channel is about 0.01 newtons per cubic centimetre against 10⁻⁵ for the buoyancy of hot air, three orders of magnitude apart, and that is what sets the useful upper limit on ramp time rather than anything in the electronics.
And the part nobody can explain
Whether the sword comes out straight or curls over depends on the room more than on the coil: temperature, humidity, atmospheric pressure, and what is standing nearby. Two people with the same machine get different results and go looking for a fault that is not there. Nobody has isolated a single factor, and that is worth its own piece.
The arc on the QCW department page is drawn to this: one channel, grown rather than thrown, with its length following the ramp slider at the cube root of the energy.