“Necessary and sufficient”—and classic

The letter by Robert Hirsch and the response by Steven Cowley (Physics Today, October 2017, page 11) discuss a classic issue related to fusion research. In 1991, as chairman of the American Physical Society’s division of plasma physics, I met with US secretary of energy James Watkins and pointed out the problems a deuterium–tritium fusion reactor has with tritium fuel storage and with radioactive waste created by neutron-damaged reactor structure.

The debate between Hirsch and Cowley demonstrates that no progress has been made on the issue in the past quarter century. Hirsch advocates using an alternative advanced fuel, such as proton–boron, which produces no neutrons. Although p–B fuel in theory could be ideal, the excessively higher temperature and the necessary plasma confinement time make its use unworkable. In meeting with the energy secretary, I proposed deuterium–helium-3 fuel, which also produces no neutrons (although subsidiary deuterium–deuterium fusion produces neutrons but with much less energy and quantity than D–T reaction).

The D–³He reactor requires a container that can withstand an order-of-magnitude higher pressure than D–T requires, but still in a more feasible range than p–B. I also recommended the magnetic dipole container that allows much higher pressure than ITER standards for D–³He fuel.

I agree with Hirsch that the research goal of a fusion reactor should be based on what he calls the sufficient condition of being economically and environmentally acceptable. If the goal is right, the physics problem will eventually be solved.