Discussion
For a successful operation of a thermonuclear reactor, Lawson's criterion is given by
Answer: C
The keyed answer is \(n\tau T \ge 50 \times 10^{20}\) keVs/m\(^{3}\).
Lawson's criterion is a lower limit: for net energy from a thermonuclear plasma the product of density n, confinement time \(\tau\) and temperature T must be at least a certain value, so the sign must be \(\ge\), which removes options A and B (those give an upper limit).
The widely quoted threshold for deuterium-tritium fusion is a triple product of about \(3 \times 10^{21}\) keV s/m\(^{3}\), that is \(30 \times 10^{20}\). The keyed \(50 \times 10^{20}\) is the same order of magnitude and a stricter figure, and the value of \(150 \times 10^{20}\) in option D is five times the usual threshold. So among the four, option C is the only one with the right inequality and a size near the standard value. What could not be confirmed is a source quoting exactly \(50 \times 10^{20}\), so the figure rests on the key.
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