From Wikipedia, the free encyclopedia
A cyclotron is a type of particle accelerator. Cyclotrons accelerate charged particles using a high-frequency, alternating voltage (potential difference). A perpendicular magnetic field causes the particles to go almost in a circle so that they re-encounter the accelerating voltage many times.
Ernest O. Lawrence, of the University of California, Berkeley, is credited with the invention of the cyclotron in 1929. It is less known outside Hungary that Hungarian Sándor Gaál may have described the workings of a cyclotron at about the same time during the spring of 1929 as Lawrence; although almost all reputable international sources give credit to Lawrence for the invention and construction of the first cyclotron. He used it in experiments that required particles with energy of up to 1 MeV.
 How the cyclotron works
The electrodes shown at the right would be in the vacuum chamber, which is flat, in a narrow gap between the two poles of a large magnet.
In the cyclotron, a high-frequency alternating voltage applied across the "D" electrodes (also called "dees") alternately attracts and repels charged particles. The particles accelerate only when passing through the gap between the electrodes. The perpendicular magnetic field (passing vertically through the "D" electrodes) forces the particles to travel in a circular path.
The particles move in a circle, because a current of electrons or ions, flowing perpendicular to a magnetic field, experiences a perpendicular force. The charged particles move freely in a vacuum, so the particles follow a circular path.
 Uses of the Cyclotron
For several decades, cyclotrons were the best source of high-energy beams for nuclear physics experiments; several cyclotrons are still in use for this type of research.
Cyclotrons can be used to treat cancer. Ion beams from cyclotrons can be used, as in proton therapy, to penetrate the body and kill tumors by radiation damage, while minimizing damage to healthy tissue along their path.
 Problems solved by the cyclotron
The cyclotron is an improvement over the linear accelerators available when it was invented. A linear accelerator accelerates particles in a straight line, through evacuated tubes. A series of cylindrical electrodes in the tubes switch from positive to negative voltage. In the 1920's, it was not possible to get high frequency radio waves at high power, so the stages of acceleration had to be far apart, to accommodate the low frequency, or more stages were required to compensate for the low power at each stage.
Faster particles required longer accelerators than scientists could afford. Later linear accelerators could use high power Klystrons and other devices imparting much more power at higher frequencies, but before these devices existed, the cyclotron was cheaper.
Cyclotrons accelerate particles in a circular path. Therefore, a compact accelerator can contain much more distance than a linear accelerator, with more opportunities to accelerate the particles.
 Advantages of the Cyclotron
- Cyclotrons have a single electrical driver, which saves both money and power, since more expense may be allocated to increasing efficiency.
- Cyclotrons produce a continuous stream of particle pulses at the target, so the average power is relatively high.
- The compactness of the device reduces other costs, such as its foundations, radiation shielding, and the enclosing building.
 Limitations of the cyclotron
The spiral path of the cyclotron beam can only "synch up" with klystron-type (constant frequency) voltage sources if the accelerated particles are approximately obeying Newton's Laws of Motion. If the particles become fast enough that relativistic effects become important, the beam gets out of phase with the oscillating electric field, and cannot receive any additional acceleration. The cyclotron is therefore only capable of accelerating particles up to a few percent of the speed of light; higher velocity beams require a synchrocyclotron or a more complex synchrotron or linear accelerator.
 Mathematics of the cyclotron
The centripetal force is provided by the transverse magnetic field B, and the force on a particle travelling in a magnetic field (which causes it to curve) is equal to Bqv. So,
(Where m is the mass of the particle, q is its charge, v is its velocity and r is the radius of its path.)
v/r is equal to angular speed, ω, so
And, the frequency
This shows that for a particle of constant mass, the frequency does not depend upon the radius of the particle's orbit. As the beam spirals out, its frequency does not decrease, and it must continue to accelerate, as it is travelling more distance in the same time. As particles approach the speed of light, they acquire additional mass, requiring modifications to the frequency, or the magnetic field during the acceleration. This is accomplished in the synchrocyclotron.
The relativistic cyclotron frequency is
where fc is the classical frequency, given above, of a charged particle with kinetic energy T and rest mass m0 circling in a magnetic field.
The rest mass of an electron is 511 keV, so the frequency correction is 1% for a magnetic vacuum tube with a 5.11 kV direct current accelerating voltage. The proton mass is nearly two thousand times the electron mass, so the 1% correction energy is about 9 MeV, which is sufficient to induce nuclear reactions.
An alternative to the synchrocyclotron is the isochronous cyclotron, which has a magnetic field that increases with radius, rather than with time. The de-focusing effect of this radial field gradient is compensated by ridges on the magnet faces which vary the field azimuthally as well. This allows particles to be accelerated continuously, on every period of the radio frequency, rather than in bursts as in most other accelerator types.
 Related technologies
The Synchrotron moves the particles through a path of constant radius, allowing it to be made as a pipe and so of much larger radius than is practical with the cyclotron and synchrocyclotron. The larger radius allows the use of numerous magnets, each of which imparts angular momentum and so allowing particles of higher velocity (mass) to be kept within the bounds of the evacuated pipe.
 See also
- cyclotron radiation, synchrotron light or its close relative, bremsstrahlung radiation.
- electron cyclotron resonance
- Linear accelerator
- Particle accelerator
- Storage ring
- TRIUMF - largest cyclotron in the world
- Radiation reaction
 External links
- U.S. Patent 1948384 -- Method and apparatus for the acceleration of ions
- "The 88-Inch Cyclotron at LBNL"
- "The NSCL at Michigan State University" Home of coupled K500 and K1200 cyclotrons; the K500, being the first superconducting cyclotron and the K1200 currently the most powerful in the world.
- Rutgers Cyclotron and "Building a Cyclotron on a Shoestring" Tim Koeth, now a graduate student at Rutgers University, built a 12-inch 1 MeV cyclotron as an undergraduate project, which is now used for a senior-level undergraduate and a graduate lab course.
- "Cyclotron java applet"
- "Resonance Spectral Analysis with a Homebuilt Cyclotron" an experiment done by Fred M. Niell, III his senior year of high school (1994-95) with which he won the overall grand prize in the ISEF.
- Relativistic accelerator physics PDF
- Wired news article about a neighborhood cyclotron in Anchorage, Alaska