Great questions.

The difference between energy harvested and the energy necessary to maintain confinement is the difference in denominators of Qscientific and Qengineering. Q is power out / power in.

Qscientific is a figure of merit used to know close to a burning plasma a machine is (how many fusion reactions it can do vs. how many it would need to do to be a working reactor).

Qengineering is power put on the grid / parasitic power needed to keep the machine running. Every electrical power source has an analogous concept (keep the lights on, fuel pumped, inverters operating, etc.) There are some noisy non-experts who claim that focusing on Qplasma is deceitful, but it's akin to complaining that engineers are focusing on engine efficiency instead of car efficiency before the engineers have finished making the engine. At the end of the day the scale of parasitic loads scales much less than the power output of a reactor, so the reactor size chosen will be at the economic minimum between "bigger machine is more expensive to make" and "smaller machine produces less power / lower Qengineering / other difficult scaling law things like neutron bombardment on plasma facing components (maintenance schedule)".

https://x.com/JB_Fusion/status/1506964692627034118

Yes, to have a real measure of Q you need to be doing fusion. In many research cases not a lot of fusion is happening and the neutrons are not actively being measured. What is typically done is to measure plasma performance metrics with protium or deuterium then say what the Q would have been if they used deuterium-tritium based on known plasma-performance to Q conversions (Lawson criterion).

https://en.wikipedia.org/wiki/Lawson_criterion

https://x.com/swurzel/status/1534556521744457731

Heat collection is done via neutrons. In D-T fusion 80% of the energy is released as a 14.1 MeV (17% speed of light, like a bat out of hell). The remaining 20% of energy is an acceleration of a He4 nucleus (fused byproduct). This He4 nucleus is a charged particle, so it stays in magnetic confinement and imparts its energy on fuel via collisions, helping to self sustain the reaction. The neutron has no charge so it flys straight out of the machine. You can model this as a small ring on the innermost core of the donut shooting neutrons in all directions. So you wrap a neutron-absorbing blanket around the vacuum vessel to slow these neutrons down via collision and heat up coolant in the blanket. You run this coolant through a heat exchanger to make pressurized steam to spin a turbine to... you get the idea.

https://en.wikipedia.org/w/index.php?title=Deuterium%E2%80%9...