Q. Years ago, hydrogen fusion was promised as the answer to all our energy problems. Are we any closer to using it?

A. There is a famous joke among scientists: The practical use of the fusion of hydrogen atoms to produce energy is only 20 or 30 years in the future — and always will be.

But it does seem progress is being made. The largest and most expensive research effort is the International Thermonuclear Experimental Reactor. A multinational effort headquartered in France, ITER has a doughnut-shaped chamber called a tokamak that will eventually hold a plasma of hot ionized atoms constrained by very strong magnetic fields.

The next major step will be the arrival in 2019 of the first of the project’s huge magnets, now being fabricated in Japan . The magnets are needed to generate and contain the extreme temperatures necessary to fuse atomic nuclei and to produce energy without the harmful environmental effects of today’s technologies.

A smaller fusion experiment, called Sparc, is being designed at the Massachusetts Institute of Technology. It will rely on smaller, stronger magnets, which in theory will reduce the amount of energy needed to produce short but powerful bursts of heat.