Betatron
A betatron is a type of cyclic
The concept of the betatron had been proposed as early as 1922 by
History
After the discovery in the 1800s of Faraday's law of induction, which showed that an electromotive force could be generated by a changing magnetic field, several scientists speculated that this effect could be used to accelerate charged particles to high energies.[2] Joseph Slepian proposed a device in 1922 that would use permanent magnets to steer the beam while it was accelerated by a changing magnetic field.[9] However, he did not pursue the idea past the theoretical stage.
In the late 1920s, Gregory Breit and Merle Tuve at the Bureau of Terrestrial Magnetism constructed a working device that used varying magnetic fields to accelerate electrons. Their device placed two solenoidal magnets next to one another and fired electrons from a gun at the outer edge of the magnetic field. As the field was increased, the electrons accelerated in to strike a target at the center of the field, producing X-rays. This device took a step towards the betatron concept by shaping the magnetic field to keep the particles focused in the plane of acceleration.[2]
In 1929,
Simultaneously with Wideroe's experiments, Ernest Walton analyzed the orbits of electrons in a magnetic field, and determined that it was possible to construct an orbit that was radially focused in the plane of the orbit. Particles in such an orbit which moved a small distance away from the orbital radius would experience a force pushing them back to the correct radius.[2] These oscillations about a stable orbit in a circular accelerator are now referred to as betatron oscillations.[10]
In 1935 Max Steenbeck applied in Germany for a patent on a device that would combine the radial focusing condition of Walton with the vertical focusing used in Breit and Tuve's machine.[5] He later claimed to have built a working machine, but this claim was disputed.[2]
The first team unequivocally acknowledged to have built a working betatron was led by Donald Kerst at the University of Illinois. The accelerator was completed on July 15, 1940.[7]
Operation principle
In a betatron, the changing magnetic field from the primary coil accelerates electrons injected into the vacuum torus, causing them to circle around the torus in the same manner as current is induced in the secondary coil of a transformer (Faraday's law).
The stable orbit for the electrons satisfies
where
- is the flux within the area enclosed by the electron orbit,
- is the radius of the electron orbit, and
- is the magnetic field at .
In other words, the magnetic field at the orbit must be half the average magnetic field over its circular cross section:
This condition is often called Widerøe's condition.[11]
Etymology
The name "betatron" (a reference to the beta particle, a fast electron) was chosen during a departmental contest. Other proposals were "rheotron", "induction accelerator", "induction electron accelerator",[12] and even "Außerordentlichehochgeschwindigkeitselektronenentwickelndesschwerarbeitsbeigollitron", a suggestion by a German associate, for "Hard working by golly machine for generating extraordinarily high velocity electrons"[13][14] or perhaps "Extraordinarily high velocity electron generator, high energy by golly-tron."[15]
Applications
Betatrons were historically employed in
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.[19]
Limitations
The maximum energy that a 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, overcame these limitations.
References
- ^ "Betatron | particle accelerator". Encyclopedia Britannica. Retrieved 2019-01-24.
- ^ S2CID 42153256.
- S2CID 109942448.
- ISBN 978-0-7503-0865-6.
- ^ S2CID 6832156.
- S2CID 120616002.
- ^ .
- .
- ^ USA 1645304, Joseph Slepian, "X ray tube", published 1927
- ^ ISBN 978-0-471-55163-8.
- ISBN 978-0-19-850549-5.
- ^ Science Service (1942). "Shall New Machine Be Named Betatron or Rheotron". The Chemistry Leaflet. 15 (7–12).
- University of Illinois. Archived from the originalon 15 April 2012. Retrieved 13 April 2012.
- ASIN B002V8WB8I.
- ^ "The Biggest Betatron in the World". Life. March 20, 1950. p. 131.
- ISBN 978-0-8047-1879-0.
- ^ Nuclear Weapons Archive, Tumbler shot series, item George.
- ^ Nuclear Weapons Archive, Elements of Fission Weapon Design, section 4.1.8.2.
- ^ Wisconsin alumnus, Volume 58, Number 15 (July 25, 1957).