A lost detail in D–T fusion history

In the summer of 1942, at J. Robert Oppenheimer’s “luminaries” meetings, Emil Konopinski proposed the introduction of tritium into deuterium fuel to enhance the feasibility of a fusion-driven super bomb. Several histories and memoirs by Manhattan Project scientists mention Konopinski’s insight, but none that we could find explain where it came from. In one interview, Konopinski stated, “I happened to know from prewar work that the reaction of deuterium with hydrogen-3 produces much more energy and has a larger cross section, so to speak—happens more easily—than deuterium with deuterium.” ¹ The recent ignition of a D–T mixture at the National Ignition Facility by inertial-confinement fusion (see “National Ignition Facility surpasses long-awaited fusion milestone ,” Physics Today online, 13 December 2022) prompts the question: How did Konopinski, and perhaps other physicists, know about the unexpectedly high D–T reaction rate?

Enter Arthur Ruhlig, who did graduate work at the University of Michigan in the 1930s with H. Richard Crane. They studied electron and positron propagation through lead. ² In the same year that Ruhlig submitted his dissertation, he published a letter in Physical Review assessing whether ³He possesses excited states.

Ruhlig did not detect any excited-state signal, consistent with present understanding. But the title of the letter, “Search for gamma-rays from the deuteron-deuteron reaction,” cloaks the tectonic finding of its penultimate paragraph: The reaction ³H + ²H → ⁴He + n¹ + 17.6 MeV “must be an exceedingly probable one.” ³ Ruhlig deduced the presence of energetic neutrons, correctly ascribing their production to secondary, in-flight D–T fusion reactions following the ²H + ²H → ³H + ¹H primary reactions and quantifying the rate of the D–T reaction relative to that of D–D. That remarkable observation had never been explicitly cited before.

While working at Michigan on his doctoral degree with George Uhlenbeck, Konopinski overlapped with Ruhlig. (Uhlenbeck is warmly cited in Ruhlig’s thesis acknowledgments.) Ruhlig’s proximity to Konopinski at Michigan and his inclusion in reference of a citation to a private communication from Hans Bethe—who worked with Konopinski at Cornell University—afford possible conduits for the key piece of surprising information on D–T fusion. A follow-up measurement confirmed the large cross section. ⁴

As the story of nuclear-reaction physics continues to unfold, we hope to uncover more of its historical details, ⁵ hopefully with input from readers of Physics Today.