Cyclotron - An Introduction

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Cyclotron is a Particle Accelerator, which propels charged subatomic particles along a spiral path in the presence of an alternating electric field and a uniform magnetic field.

The spiral path enables the charged particles travel more distance and thus gain more acceleration compared to a linear path. Due to this, the cyclotron is more advantageous over Linear Particle Accelerators in many applications. It is capable of producing a continuous stream of radiation at a fairly constant speed.

This particle accelerator was first conceptualized by Austro-Hungarian scientist Leo Szilard. The device came into existence and started to be used in 1932 through the efforts of American physicist Ernest Lawrence. 

Following are the main components of a cyclotron

  • High frequency AC voltage supply in the order of kilo-volts. This is essential to achieve high rate of acceleration.
  • A permanent magnet which is positioned perpendicular to the electric field.
  • Two ‘D’ shaped electrodes, enclosed in a vacuum chamber, across which the high voltage is applied.

When the electrodes are powered on, positive or negative charged particles start emanating from them. The charges are propelled at a speed proportional to the magnitude of input voltage. Due to the effect of electric and magnetic fields, the charged particles are subjected to Lorentz force as well as a centripetal force which push them outward in a circular path, gradually increasing in radius until they reach the periphery of the vacuum chamber where they can be utilized as an output.

The accelerated particles can be made to collide on a target to observe secondary emissions for analysis in nuclear and particle physics. Alternatively they can be channelled into a beam for applications like therapeutic radiation for destroying malignant tumours.

The magnetic field strength should be such that the particles attain a frequency (cyclotron resonance) that is equal to the input alternating frequency and in the same phase. The cyclotron frequency is given by

Frequency(f) = particle charge(q) * magnetic field strength(B) / 2p * relativistic mass of particle(m)

There is one notable disadvantage to the cyclotron due to which very high speeds cannot be achieved. When a particle starts nearing the speed of light, its relativistic mass tends to increase. Consequently its frequency goes out of synch with the input AC power. As a result the particle will no longer be accelerated and the output will be affected. Hence the device is tuned to achieve speeds only up to one-tenth of the speed of light.

 To mitigate the relativistic effects, some variations can be made to the standard cyclotron. When the input frequency is synchronised with the cyclotron frequency, the device is known as synchrocyclotron. When the magnetic field is varied to adjust to the mass change of the particle to keep the frequency constant, the device is termed as isochronous cyclotron.

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Cyclotron - An Introduction

This article was published on 2012/03/26