Why must helium ash be removed from a continuously operating fusion plasma?

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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.

In a continuously operating fusion reactor, the ash produced by the fusion process—specifically helium—must be removed to prevent it from quenching the plasma.1

If this byproduct is not effectively exhausted and the fuel is not replenished, the accumulation of helium interferes with the conditions necessary to maintain the plasma's energy balance and temperature, ultimately halting the fusion reaction.1