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Why does fusion fuel need both high temperature and enough confinement time?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

To achieve practical energy yields from nuclear fusion, fuel must be maintained at high temperatures and confined for a sufficient duration. These requirements are essential for the following reasons:

Overcoming the Coulomb Barrier: Atomic nuclei are positively charged and experience a repulsive Coulomb force. To fuse, nuclei must approach each other closely enough for the strong force to dominate. This requires high kinetic energy, which is achieved by heating the fuel to millions of degrees, creating a plasma state.12
Maintaining Energy Balance: A plasma loses energy through processes like radiation, conduction, and convection. To become self-sustaining or "ignited," the energy released by fusion reactions must be captured and deposited back into the plasma to offset these losses.32
Confinement Quality: The ability to maintain this balance depends on the product of plasma density and energy confinement time ( $n τ$ ). If the confinement time is too short, the plasma loses heat faster than fusion can replenish it, causing the reaction to quench.32