The Klystron

December 15, 2010

Aristotle wasn't much of an experimentalist, but he would feel somewhat at home in one of our laboratories, since there's Greek everywhere. Among the instruments are barometers and thermometers. Elementary particles are called electrons and mesons; and the corps of mesons includes the J/ψ, which still hasn't decided whether or not it would like to be Greek. Even the biologists have jumped on the bandwagon, giving things names like eukaryotes. Perhaps one reason for a scientist's love of Greek is that it sounds just a little strange to Latin ears, but it's still somewhat readable and pronounceable; and it's a significant portion of the western intellectual tradition.

That's why we have the klystron, the first useful microwave amplifier and oscillator. The word, klystron, supposedly has its root in the Greek stem κλυσ- (klys-), which refers to waves breaking against a shoreline, combined with electron. All very poetic, since microwaves are waves, and the klystron is an electron tube. That was obviously what was intended, but the etymologist in me (not to be confused with the entymologist - more Greek), has found a closer stem. That's the Greek word, κλυστηρ, which transliterates as klyster. In old medical parlance, a clyster is an enema. It makes perfect sense, since electron tubes are always evacuated.

My first experience with a klystron was for an undergraduate physics "senior project" on electron paramagnetic resonance (EPR, a.k.a., electron spin resonance, ESR) spectroscopy. The spin state of a free electron in a magnetic field will resonate at a radio frequency that depends linearly on the field strength. The proportionality constant for a free electron is 1.76 x 10⁷ rad/sec/gauss, or 2.8 MHz/gauss. For practical magnets of a few thousand gauss, this puts the radio frequency signal into the "X" band (8.0 - 12.0 GHz), which is a microwave frequency band, an ideal range for a klystron. In a simple EPR spectrometer, a klystron connects to a waveguide that couples to a resonant cavity (a shorted waveguide section) through an iris. The cavity, which contains the material under study, is placed in a highly homogeneous magnetic field (which translates to large pole pieces). There's a detector somewhere in the waveguide circuit to detect signal strength.

The utility of EPR spectroscopy lies in the fact that electrons are immersed in a local environment of a molecule, so the resonant condition will tell you something about the molecule. This only works when the electron is unpaired, which means it's part of a free radical, or a transition metal atom, usually associated with a dopant atom. The free radical, 2,2-diphenyl-1-picrylhydrazyl (DPPH), is typically used as a standard. In the environment of its host molecule, the unpaired electron can gain or lose angular momentum, so it has a g-factor that's slightly different than that of a free electron. This results in a resonance shift in magnetic field at a constant frequency. Other environmental factors will change the line shape of the resonance, which allows tracking the rate of some chemical reactions. The free electron g-factor, g_e = 2.002319, is known quite accurately from quantum electrodynamics.

Fig. 1 of US Patent No. 2,242,275, Russell H. Varian's klystron patent, May 20, 1941. The toroidal resonators are shown at 21 and 25.

The klystron was invented by Russell H. Varian in 1937.[1-3] This is especially interesting, since Varian was an undiagnosed dyslexic, and for that reason wasn't considered too bright. As if to prove everyone wrong, he earned both a BS and MS degree in physics from Stanford University. His younger brother, Sigurd, never went to college, and became a pilot. His aviation knowledge likely led him to the conclusion that the US would be unprepared for aerial bombing in the anticipated war, and he decided that a better means of aircraft detection was needed. Sigurd convinced his brother, Russell, who was working on television for Philo Farnsworth at the time, to jointly develop a radar system. One of the requirements for such a system was a powerful source of microwave energy.

Looking for ideas, Russell Varian contacted Bill Hansen, his former college roommate and a professor of physics at Stanford University. Stanford saw the potential of such a radar system, and allowed the Varian brothers to work in a campus laboratory. For use of this space and a $100 annual supplies budget, Stanford got a 50% cut of patent royalties on any invention.[3] When Varian's patent was issued, the assignment was to Stanford. Russell Varian's inventive idea was using the time of flight of the electrons to allow resonance of the current flow in a series of toroidal resonators (see figure).

The klystron served its intended radar purpose, and it assisted in the Allied war effort. Radar, however, wasn't the only application of microwaves, so the Varian brothers went on to found Varian Associates in 1948. Within ten years, the company employed more than 1,300 people and had twenty million dollars in annual sales. Businesses usually have a shorter lifetime than the technologies they create. In 1999, Varian Associates split into several companies, one of which, Varian, Inc., was acquired by Agilent Technologies in May, 2010. Klystrons, especially the high power versions, are still important devices.

References:

1. Russell H. Varian, "Electrical Translating System and Method," US Patent No. 2,242,275, May 20, 1941.
2. Russell H. Varian and Sigurd F. Varian, "A High Frequency Oscillator and Amplifier," Journal of Applied Physics, vol. 10, no. 15 (1939), pp. 321-327.
3. John Edwards, "Russell and Sigurd Varian: Inventing The Klystron And Saving Civilization," Electronic Design, November 22, 2010.
4. Dorothy Varian, "The Inventor and the Pilot: Russell and Sigurd Varian," (Pacific Book Pub, October 1983), 314 pp. (via Amazon).