What do you mean by 'dees' which is used in cyclotron?

(N/A) In a cyclotron,'dees' are two hollow,semi-circular metal chambers shaped like the letter '$D$'.
They are placed in a vacuum chamber with a small gap between them.
$A$ high-frequency alternating voltage is applied across these dees.
The interior of the dees is a field-free region,meaning the electric field inside them is zero.
As a charged particle enters a dee,it moves in a semi-circular path due to the presence of a uniform perpendicular magnetic field.
When the particle reaches the gap,the electric field accelerates it,increasing its kinetic energy before it enters the next dee.