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Betatron
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== 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.{{r|kersthistory}} [[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.<ref>{{cite patent |country= USA |number=1645304 |title=X ray tube |pubdate= 1927 |inventor= Joseph Slepian }}</ref> However, he did not pursue the idea past the theoretical stage. In the late 1920s, [[Gregory Breit]] and [[Merle Tuve]] at the [[Carnegie Institution for Science|Bureau of Terrestrial Magnetism]] constructed a working device that used varying magnetic fields to accelerate electrons. Their device placed two [[solenoid|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.{{r|kersthistory}} In 1929, [[Rolf Wideroe]] made the next major contribution to the development of the theory by deriving the ''Wideroe Condition'' for stable orbits. He determined that in order for the orbit radius to remain constant, the field at the radius must be exactly half of the average field over the area of the magnet. This critical calculation allowed for the development of accelerators in which the particles orbited at a constant radius, rather than spiraling inward, as in the case of Breit and Tuve's machine, or outward, as in the case of the [[cyclotron]].<ref name="edwards">{{cite book |last1=Edwards |first1=D. A. |last2=Syphers |first2=M. J. |title=An introduction to the physics of high energy accelerators |date=1993 |publisher=Wiley |location=New York |isbn=978-0-471-55163-8 |pages=22β23}}</ref> Although Wideroe made valuable contributions to the development of the theory of the Betatron, he was unable to build a device in which the electrons orbited more than one and a half times, as his device had no mechanism to keep the beam focused.{{r|kersthistory}} 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.{{r|kersthistory}} These oscillations about a stable orbit in a circular accelerator are now referred to as ''[[betatron oscillations]]''.{{r|edwards}} 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.{{r|steenbeck}} He later claimed to have built a working machine, but this claim was disputed.{{r|kersthistory}} 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.{{r|kerst1941}}
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