Thursday, January 05, 2006

Cherenkov Free Electron Laser


Cherenkov Free electron laser (CFEL) generates coherent high power radiation. The laser uses a relativistic electron beam as the lasing medium, and hence it is called a "free electron laser".

Components and functioning of the CFEL

Design of the Cherenkov Free Electron Laser

The CFEL consists of an electron gun which uses high voltage to accelerate the electrons to relativistic speeds. The beam passes through a cylindrical waveguide which is filled with a dielectric near its boundary and has vacuum near the axis. This is done to make the velocity of electromagnetic waves inside the waveguide less than c, the speed in vacuum. At this decreased speed of the electromagnetic wave, the electron beam can interact with it. This interaction causes the electromagnetic wave to grow in amplitude and is responsible for the lasing action. The mechanism for this process is described later.

Inside the waveguide, a strong magnetic field is produced by wounding solenoids outside the cylindrical waveguide and passing high current though them. This magnetic field does not let the electron beam diverge radially. This is required to keep the electron beam traveling in the axial direction so that it can interact with the electromagnetic wave, which has the electric field in the axial direction. Note that the wave is also traveling in the axial direction. Such electromagnetic waves, which have the electric field in the same direction as the direction of propagation are said to be propagating in a Transverse Magnetic mode, or a TM mode.

A necessary condition for the functioning of the Cherenkov Free Electron Laser is that the phase velocity of the electromagnetic wave should be nearly equal to the velocity of the electron beam in magnitude. This makes the frequency of the electromagnetic wave, as seen by the electrons, nearly zero (Doppler Shift in frequency). Hence the electric field of the wave appears like an electrostatic field to the electrons, and hence can continuously accelerate or decelerate electrons.

Growth of the Wave

For our purpose, we need to decelerate the electrons, so that the energy is transferred to the Electromagnetic wave, so that it grows. In a case where the phase velocity of the wave is exactly equal to the electron beam velocity, half of the electrons see the electric field in the direction of their motion, and half see it opposite to it. However, for a net retardation of the electrons, there should be more number of electrons in the retarding zones. This is achieved by keeping the electron beam velocity slightly higher than the phase velocity of the wave.

In this condition, the electrons in the accelerating zones are accelerated over to the decelerating zones while the electrons in the decelerating zones are slowed down and remain in the decelerating zones. In this way, the number of electrons in the retarding zones is increased. This leads to a net retardation of the electrons and in turn to the growth of the wave.

After emerging out of the waveguide, the electron beam is sent to a beam dump.

Cherenkov free electron laser is sometime mentioned as Cerenkov Free electron laser.

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