Question

In words, explain the Lawson criterion and its significance. a)One needs high densities for extended time periods to produce enough energy for fusion reactions to break even. b)One needs high densities for short time periods to produce enough energy for fusion reactions to break even. c)One needs low densities for extended time periods to produce enough energy for fusion reactions to break even. d)One needs low densities for short time periods to produce enough energy for fusion reactions to break even

Answer 1

Fusion requires two nuclei to come sufficiently close.

Since the two fusing nuclei have positive charge they are repelled by each other's Coloumb repulsion.

So both of them are heated to very temperature to produce energy to overcome the Coloumb repulsion.

At High temperatures which is produced in Plasma of ions , the Cross-section for nuclear fusion is high for energies greater than 10KeV and consequently high temperature is needed.

This High Temperature must be sustained for a time known as "confinement time" -

Further ion density (of fusing nuclei) also should be sufficiently high. - this density is denoted as - n

Lawson Criterion says that

n > or = 10^14 s/cm^3 for dueterium- tritium fusion

Similarly diffrent fusion reactions have diifrent minimum product of n for sustainability of reaction.

So we need sufficient n as well as sufficient to sustain the reaction - hence answer is choice (a)

At this stage the Output energy > Input Energy