What are cross field device? What role does the dc magnetic field play in their normal operation?
The cavity magnetron is a cross field device. Here the electric field between anode and cathode is radial whereas the magnetic field produced by a permanent magnet is axial. The permanent magnet is placed such that the magnetic lines are parallel to the vertical cathode and perpendicular to the electric field between cathode and anode.
The cathode is accelerated by the relative strength of the electric field and emitted electron from it towards anode with a velocity. The emitted electrons are travel throw the interaction space. In the absent of d.c magnetic field (B = 0), the electron travels straight from the cathode to the anode due to the radial electric field force acting on it. If the d.c magnetic field strength is increased slightly it will exert a lateral force bending the patch of the electron. The radius of the path is given by R=mv/eB, that varies directly with electron velocity and inversely as the magnetic field strength. Here R is the radius of the electron path, m is the mass of the electron, e is the charge of the electron, v is the velocity of accelerated electron and B is the d.c cross magnetic field which is perpendicular to the electric field. If the strength of magnetic field is made sufficient high so as to prevent the electros from reaching the anode, therefore anode current becomes zero. The magnetic field required to return electron back to cathode just grazing the surface of the anode is called the critical magnetic field of cut-off magnetic field. If the magnetic field is made larger, then the electrons come back to the cathode very faster. The electron is experienced a greater force causes heating the cathode. If the RF field is applied to the anode circuit, those electron entering the circuit during the retiring field are decelerated and give up some of their energy to the RF field. Thus their velocity is decreased and these slower electrons travel the d.c. electric field far enough to regain essentially the same velocity.
What are cross field device? What role does the dc magnetic field play in their normal operation?
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Fig.22(a).png)
Fig. 18(a)