Betatron

File:Betatron 6MeV (1941).jpg

A 6 MeV betatron (1941)

A betatron is a cyclotron developed by Donald Kerst at the University of Illinois in 1940 to accelerate electrons, but the concepts ultimately originate from Rolf Widerøe and previous development occurred in Germany through Max Steenbeck in the 1930s. The betatron is essentially a transformer with a torus-shaped vacuum tube as its secondary coil. An alternating current in the primary coils accelerates electrons in the vacuum around a circular path.The betatron was the first important machine for producing high energy electrons. Astrophysicist Michio Kaku built a betatron in his garage while still in high school.

Operation principle

In a betatron, the changing magnetic field from the primary coil accelerates electrons injected into the vacuum torus, causing them to circle round the torus in the same manner as current is induced in the secondary of a transformer (Faraday's Law).

The stable orbit for the electrons satisfies $\theta_0=2\pi r_0^2 H_0$ where θ₀ is the flux with the orbit at r₀ is the radius and H₀ is the magnetic field at r₀. In other words, the magnetic field at the orbit must be half the average magnetic field over its circular cross section.

Etymology

The name "betatron" (a reference to the beta particle, a fast electron) was chosen during a departmental contest. Other proposals were rheotron, inductron, and even Ausserordentlichhochgeschwindigkeitelektronenentwickelndenschwerarbeitsbeigollitron, supposedly German for "extraordinarily high-speed electron producing hard work by golly-tron.".^{[citation needed]}

Applications

Betatrons were historically employed in particle physics experiments to provide high energy beams of electrons—up to about 300 MeV. If the electron beam is directed at a metal plate, the betatron can be used as a source of energetic x-rays or gamma rays; these x-rays may be used in industrial and medical applications (historically in radiation oncology). A small version of a Betatron was also used to provide electrons converted into neutrons by a target to provide prompt initiation of some nuclear weapons.

The Radiation Center, the first private medical center to treat cancer patients with a betatron, was opened by Dr. O. Arthur Stiennon in a suburb of Madison, Wisconsin in the late 1950s.

Limitations

The maximum energy that betatron can impart is limited by the strength of the magnetic field due to the saturation of iron and by practical size of the magnet core. The next generation of accelerators, the synchrotrons, overcome these limitations.

In popular culture

• In the Frank Black song "Los Angeles", the betatron is referenced in the line "I met a man; he was a good man, sailing and shoring; he got a betatron, man."
• The Rap artist 'Canibus' refers to betatrons in his song "Levitibus".

Further Reading

• Kerst, D. (1940). "Acceleration of Electrons by Magnetic Induction". Physical Review 58 (9): 841–841. Bibcode 1940PhRv...58..841K. doi:10.1103/PhysRev.58.841. 
• Kerst, D. (1941). "The Acceleration of Electrons by Magnetic Induction". Physical Review 60: 47–53. Bibcode 1941PhRv...60...47K. doi:10.1103/PhysRev.60.47. 
• Kerst, D.; Serber, R. (1941). "Electronic Orbits in the Induction Accelerator". Physical Review 60: 53–58. Bibcode 1941PhRv...60...53K. doi:10.1103/PhysRev.60.53. 

External links

• The Betatron at UIUC