Cavity magnetron

The cavity magnetron emerged because German bombers were terrorizing England and existing radar systems required antennas 110 meters high. To fit radar on ships and aircraft, engineers needed to generate high-power microwaves at wavelengths short enough for small antennas—and in 1939, no one knew how.

The adjacent possible aligned at the University of Birmingham in September 1939. Physicists John Randall and Henry Boot, working under Mark Oliphant who advised the classified British radar program, began exploring a new magnetron design. Within two months they had a basic concept: a cylindrical metal piece with a cathode through a central hole, surrounded by an anode with symmetrical cavities arranged in a circle. The cross-section resembled a Colt revolver chamber—which actually served as a template for early prototypes.

On February 21, 1940, Randall and Boot tested their first working cavity magnetron. They were astonished: it produced over 400 watts at 9.8 centimeters wavelength—three orders of magnitude more power than klystrons at five times the frequency. The device could make radar portable.

Convergent evolution had produced similar devices elsewhere. Arthur Samuel at Bell Labs filed a four-cavity magnetron patent in 1934, though impractical. Soviet scientists Alekseev and Malairov developed a successful multicavity magnetron in 1937. Japanese researchers achieved an eight-cavity magnetron by 1939 but lacked materials for manufacture. The conditions were aligning globally, but Britain's wartime urgency accelerated development.

Britain couldn't mass-produce the devices. Winston Churchill agreed that Sir Henry Tizard should offer the technology to America in exchange for industrial help. In September 1940, the Tizard Mission carried several 10-kilowatt magnetrons across the Atlantic. James Phinney Baxter's Pulitzer Prize-winning book described it as 'the most valuable cargo ever brought to our shores.'

Bell Labs testing showed the British design produced 10 times the output at 5 times the frequency of American triodes. Before year's end, MIT's Radiation Laboratory was established to develop radar systems. By war's end, the cavity magnetron was the heart of over 150 new radar types designed between 1941 and 1944. Allies maintained a radar lead that Germany and Japan could never close.

The peacetime legacy proved equally significant. Percy Spencer at Raytheon noticed a magnetron melting a chocolate bar in his pocket, leading to the microwave oven. Today's microwave ovens still use the same cavity magnetron technology that Randall and Boot tested in 1940—warfare's tool becoming the kitchen's convenience.