For each complex notion of fusion physics ITER Director-General Pietro Barabaschi proposed an analogy. A pile of manure for the Sun's power density; blankets on a cold winter night for energy confinement time (but that's without counting on turbulence); a wood-fired pizza oven to illustrate the issue of size vs power density.
In terms of "volumetric power density," a key concept when dealing with energy issues, the Sun and Sun-like stars are very poor, very slow, and very inefficient systems. One cubic metre of the Sun's core generates about as much power as the chemical reactions inside an equivalent volume of manure—something in the range of 250 to 300 watts per cubic metre (W/m3), or much less than a human body (1 kW/m3). Hummingbirds, which hold the record of power density among living organisms, generate 50 kW/m3—that is, 200 times the performance of the proton-proton reactions inside the Sun. However, poor performance as a fusion reactor combined with giant size (333,000 Earth masses) is what makes the Sun what it is: a giant furnace that has been dispensing light and heat to for 4.6 billion years and will continue for another 5 billion years.
