Hansen went on to become interested in the problem of accelerating electrons for X-ray experiments, using oscillating fields, rather than large static voltages. At the University of California, Berkeley, Ernest Lawrence and his assistant David H. Sloan, had worked on an accelerator driven by a resonant coil. Hansen proposed replacing the coil with a cavity resonator. In 1937, brothers Russel H. Varian and Sigurd F. Varian came to Stanford to work on the foundations of what was to become radar. Hansen exploited some of the Varian's work to develop the klystron and during the years 1937 to 1940, along with collaborators such as John R. Woodyard, founded the field of microwave electronics.