It is a device developed by Lawrence and Livingstone and is used to accelerate charged particles like protons and deuterons. This results in the production of high energy beam which is then used for artificial disintegration, etc.
It consists of two D shaped metal boxes (about 21″ in diameter) with their straight sides facing each other. There is a very small gap between these Dee’s faces across which a high potential difference (of order of 105 volts) is applied by a high frequency (10-15 mega cycles per second) oscillator. Thus an alternating electric field is established between Dee’s during one half cycle, one dee is positive and the other is negative while during the next half cycle, the other is positive and one is negative. A source of particle (heavy hydrogen for producing deutrons) is placed between the faces. The whole apparatus is placed between the pole pieces NS of a very strong magnet.