Illustrated Physics
physics, drawn
Concept

Thermal runaway — where it appears

A feedback in which heating raises the rate of whatever produces the heat, so the temperature climbs faster and faster. It appears in compost heaps, chemical reactors, frozen food and fusion plasmas, and it stops only when something new limits the rate.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

The product a fusion plasma has to beat. The density, temperature and confinement time multiplied together at which a deuterium–tritium plasma heats itself, against temperature. Multiplying by T turns the previous curve into a pressure times a time, which is what a magnet can actually hold, since the field limits the pressure nT and not the density alone. The curve has a clear minimum of 3.0 × 10²¹ keV s/m³ near 14 keV, and that is why magnetic machines are designed to run hot but not hotter. ITER's design point, a density of 10²⁰ per cubic metre at 8.9 keV and 3.7 s, gives 3.3 × 10²¹: a little under this line, because ITER is meant to be driven by external heating with a tenfold gain and not to ignite outright.

The product a fusion plasma has to beat

A star makes less power per litre than a compost heap, and it has burned for four and a half thousand million years. A fusion plasma makes thousands of times more and has to be held for seconds, and it has taken seventy years to get close. Both are obeying one inequality, written down in a classified report in 1955, in which density and time appear only as their product.

astrophysics · Plasma oscillation Where the heat would land. The average field strength squared on a horizontal plane half-way up the oven from 3 of the in-band modes taken together, 2,3,2; 3,2,2; 2,4,1, darker meaning stronger. The bright patches are spaced by about half a wavelength of the mode along each wall, some 7 to 15 cm for these indices, and between them are nodes where the heating falls to almost nothing. A plate of cheese on a stopped turntable shows this as melted spots, and a measurement of their spacing against the frequency gives the speed of light to within several per cent, a popular school experiment. Since many modes can be involved and the load disturbs them, the method is not as clean as it looks, and here the load's own effect is not drawn.

The oven that cooks in spots

A microwave oven is a metal box, and a metal box has only so many ways of ringing at a given frequency. At 2.45 GHz a kitchen oven allows a handful, and the field inside is the sum of those few standing patterns, with hot patches and nodes several centimetres apart. Food on a stopped plate shows them as a pattern of scorch and frost. A turntable can smear this only into rings, and frozen food has a further trap, because ice absorbs thousands of times less than water.

waves · Standing waves

Named alongside it

The objects these essays reach for when they reach for this one.

Alpha heatingBremsstrahlungCavity modeConfinement timeDebye relaxationDielectric lossFusion reactivityIgnitionLawson criterionMicrowave heatingPenetration depthStanding wave

All concepts