Electrical Dost

Van de Graaff generator

Van de Graaff generator, Electrical Dost

Van de Graaff generator is an electrostatic generator, that can produce high potential of the order of millions of volt. It is used to accelerate charged particles such as protons, deuterons, particles, which are then further used for artificial transmutation. Artificial transmutation is the process
in which bombardment of highly energetic particles on nucleus of an element causes it to be transformed into some other element.

Van de Graaff generator, Electrical Dost
Fig.12.20 Van de Graaff generator

Construction:

P1 = Pulley rotated with motor

P2 = Pulley

B = An endless belt made up of insulating material

C1 = Comb connected to high voltage supply and is used to spray positive charges

C = Comb connected to dome

S = Metallic dome

D = Gas discharge tube

T = Target

van de Graaff generator consists of a hollow
metal polished sphere S, called dome which is supported on an insulating column. An endless insulated fabric belt is made to run over two pulleys P1 and P2. Pulley P1, is rotated using an electric motor (not shown in diagram). C1 is metal comb with sharp metal spikes and is connected to high voltage supply. Comb C2 is called as collecting comb which is connected to dome. A discharge tube D, acts as a source of positive ions such as protons, deuterons.
Target nucleus is kept at other end of discharge tube. The generator is enclosed in a steel chamber filled with nitrogen or methane at high pressure. (Fig.12.20)

Working:

When an electric motor is switched on, endless belt wound over pulley P and P2 starts rotating. A spray comb C is given positive potential of the order of 104 volt with respect to earth by high voltage rectifier (H.V.R.) supply. Charge from metal comb is
transferred to belt. The charge is continuously carried upto large hollow sphere. A negative charges are induced on the sharp ends of comb C2 and an equal
positive charges are induced on farther end of C2 where comb C2 collects these charges, which are spread on the outer surface of dome. The potential of dome continuously increases. The charges are crowded on outer surface of dome which leaks to the surrounding in the form of spark. To avoid such a leakage, van de Graaff generator is enclosed in earth connected steel tank filled with air under pressure.

The positive ions are introduced from ion source in the upper part of the evacuated accelerator tube. These ions are accelerated in downward direction Due to very high fall of potential the positive acquire high energy. These charged particles directed towards target (T).

Uses:

(1) It is used to produce high potential of the order of few millions of volt.

(2) It is used to accelerate charged particles such as protons, deuterons, a- particles.

3) It is used in nuclear physics studies involving collision experiments for probing nuclear producing different structure, radioactive elements/ isotopes for carrying out nuclear reactions, nuclear disintegration.

(4) In medicine such beams are used to treat cancer.

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