Saturday, 30 April 2016

Working Principle and construction of Cyclotron

Cyclotron is a device used to accelerate charged particles to high
energies. It  was  devised  by  Lawrence.
Principle
Cyclotron  works  on  the  principle  that  a  charged  particle  moving normal  to  a  magnetic  field  experiences  magnetic  lorentz  force  due  to
which  the  particle  moves  in  a  circular  path.
Construction
It   consists of  a hollow metal cylinder divided into two sections D1 and D2
 called Dees, enclosed in an evacuated chamber. The  Dees  are  kept separated and a source of ions is placed at  the  centre  in  the  gap between  the Dees. They  are  placed  between  the  pole pieces of a  strong  electromagnet.  The  magnetic field  acts  perpendicular  to  the  plane  of the  Dees.  The  Dees  are  connected  to a high frequency oscillator.

Working
When a positive ion of charge  q  and mass  m is emitted from the
source, it is accelerated towards the Dee having a negative potential at
that  instant  of  time.  Due  to  the  normal  magnetic  field,  the  ion
experiences magnetic lorentz force and moves in a circular path. By the
time  the  ion  arrives  at  the  gap  between  the  Dees,  the  polarity  of  the
Dees  gets  reversed.  Hence  the  particle  is  once  again  accelerated  and
moves into the other Dee with a greater velocity along a circle of greater
radius.  Thus  the  particle  moves  in  a  spiral  path  of  increasing  radius
and  when  it  comes  near  the  edge,  it  is  taken  out  with  the  help  of  a
deflector plate (D.P). The particle with high energy is now allowed to hit
the  target  T. When  the  particle  moves  along  a  circle  of  radius  r with  a
velocity v, the magnetic Lorentz force provides the necessary centripetal
force.
Limitations

  1.  Maintaining  a  uniform  magnetic  field  over  a  large  area  of  the Dees  is  difficult.
  2. At high velocities, relativistic variation of mass of the particle upsets  the  resonance  condition.
  3. At  high  frequencies,  relativistic  variation  of  mass  of  the electron  is  appreciable  and  hence  electrons  cannot  be  accelerated  by cyclotron.