Page under constructionPage under constructionA cyclotron is an accelerator for charged particles. In our case, it accelerates one of the four lightest nuclei, H, D, 3He or 4He, to energies from 5 MeV to 45 MeV depending on the settings.
The operating principle is illustrated below.
The ions (charged particles) are injected by an ion-source near
the center of the machine. They immediately start going in circular orbits
in a magnetic field inside the cyclotron, according to the formula:
F = qv x B (1)where F is the force vector, q is the charge, v is the velocity vector and B is the magnetic flux density vector.
Most of their orbit, the particles move within a hollow D-shaped
metal electrode, called a "D" (or "dee"). Here they only experience the
magnetic field, as they are screened from any electric field inside
the cyclotron by what is known as Faraday
Cage Effect. Only when the particles move in the small gap
between the dees, they are influenced by an electric field applied
from one dee to the other. We will then get an acceleration of the
particles, according to:
F = qE (2)where E is the electric field vector.
Of course, there will only be an acceleration if the electric force has the same direction as the velocity. Therefore, the electric field must change its direction while the particles are screened inside the dees. This means that the electric field has to oscillate with a frequency corresponding to the particles' revolution frequency.
If this condition is met, and we assume that the magnetic force
is equal to the centripetal force of the circular movement, we get
the cyclotron equation:
r = mv/qB (3)wher r is the radius of the particle orbit.
As we can see from this, the particles will increase their energy (velocity) proportional to the radius of their orbit, until they after typically a few hundred orbits reach the energy desired. They can then be extracted by a new electric field combined with a magnetic field, and be transported through a beam tube to the point where they are to be used to initiate nuclear reactions.
The observant reader will notice that (3) above requires the mass m to be constant for v to be directly proportional to r. Relativistically, the mass is not constant, but increases with v. This is highly relavant in our context, since for instance 35 MeV protons have a velocity of about 0.3 c.
Physically, the cyclotron weighs 55 tons, and is located inside an inner vault with walls and doors of about 2 m concrete, to shield the surroundings from the nuclear radiation which is present when the machine runs. Fortunately, most of this radiation has a half-life of only seconds to minutes, so there are no long-term waste disposal problems.
The cyclotron was manufactured by Scanditronix AB in Uppsala, Sweden. It has type code MC35, and was installed in 1978.
To see what it looks like, click here. You can see the yellow iron frame and the round poles of the main magnet, and the brown "towers" that contain the end amplifiers and tuning cavities for the RF electrical field that is applied to the dees to accelerate the particles. We are talking about frequencies in the range 12 - 24 MHz, in the middle of the shortwave band, and each end stage corresponds to a shortwave transmitter of ~100 kW. The dee-voltage is typically ~ 100 kV pp. In the front, left part of the picture you can se the beam tube emerging, and a quadrupole magnet (blue) which is part of the beam optics.
