Assignment I1. Why is a Cockcroft-Walton circuit preferred for voltage multiplier circuits ? Explain itsworking with a schematic diagram.2. What is a esla coil? !ow are damped high fre"uency oscillations o#tained from a eslacoil? $. %ive the &arx circuit arrangement for multistage impulse generators. '. (erive the expression for ripple and regulation in voltage multiplier circuits. !ow are theripple and regulation minimi)ed? *. +a,. Explain clearly the #asic principle of operation of an electrostatic generator.+#,. (iscuss the advantages and limitations of -an de %raaf generator. .. Explain the different schemes for cascade connection of transformers for producing veryhigh a.c. voltages.Van De Graaf Generator DescriptionVan De Graaf generators are a common sight in many sciencelaboratories and for many people it is a device that looks like a large metalball on a pedestal and can make hair stand on its end literally. However,there is more to the Van De Graf generators than just deploying staticcharge.The Van De Graaf Generator is basically an electrostatic machine thatcan generate high voltages. A typical Van De Graaf Generator consists ofan insulating belt that transports electrical charge to a terminal. Thecharges that are sent on the belt are generated through a high voltage DCsupply. These charges are collected in the inside of the terminal andtransferred to its external surface.A Van De Graaf generator can be used to generator high amounts ofpotential diference to the order of about 5 Mega Volts. Generally used forscientifc experiments, the generated charges are used to speed particlessuch as ions. Let us now take a detailed look into the history, constructionand working of Van De Graaf Generators.HistoryThe frst Van de Graf Generator was invented by Dr. Robert J Van DeGraf in 1931 in the Unites States of America (USA) for the sole purpose ofgenerating and using high voltages for use in nuclear physicsexperiments. Dr. Robert J Van De Graf, a professor in the reputedMassachusetts Institute of Technology in USA, designed and built theworld's largest air-insulated Van de Graaf generator for use in X-rayexperiments and for research in atom-smashing. Later on, as diferentmethods to accelerate atoms became available, the original Van De Graafgenerator became used for academic and instructional purposes.The largest Van De Graf Generator was constructed in an unuseddock in South Dartmouth, Massachusetts. It was constructed on railroadtracks for easier mobility and access. The two colossal domes wereconnected via a tube. Each of the two domes had a laboratory wherescientists were able to carry out experiments and study the efects of largeamounts of electricity on particles in the connecting tube.In the 1950's, MIT generously donated the giant Van De Graafgenerator to the Museum of Science. In 1980, the Van De Graaf generatorwas featured in the newly completed Thomson Theatre of Electricity.Today, the colossal Van De Graaf generator is demonstrated twice or moretimes a day to enlighten school students and other academia on thetheories of electricity.How does a Van De Graf Generator Work?The Van De Graaf generator works simply on the principle of staticelectricity. All matter, as we know is made up of atoms which furtherconstituted of electrons, neutrons and protons. Electrons carry negativecharge whereas protons are considered to be positively charged. When thenumber of electrons and protons remain the same, the matter isconsidered to be neutral in charge. A negatively charged matter has morenumber of electrons than protons while the opposite holds true for apositively charged matter. Electrons can fow from one matter to another.When two materials are rubbed together, a fow of electrons can takeplace depending on the triboelectric properties. When such a transferoccurs, the material that lost electrons will become positively charged andthe one that gained electrons becomes negatively charged. This basicallyhow static electricity is generated.A Van de Graaf generator creates static electricity. The current generatedby a Van De Graaf generator remains the same, while the voltage changesaccording to the applied load. A very simple Van De Graaf generator is made of the following: A motor Rollers, two in number Insulated belt Brush assemblies, two in number Metal sphere as the output terminalThe motor is required to turn the belt at a constant speed of 1000 to2000 metres per minute around the two rollers. The potential of the highvoltage electrode above the earth at any instant is V = Q/C where Q is thecharge stored and C is the capacitance of the high voltage electrode to earth.The potential of the high voltage electrode rises at a rate dVdt =1C dQdt =ICwhere I is the net charging current. The lower roller is built of a material thathas a stronger triboelectric property. Now when the motor starts turning thebelt around the lower roller, electrons are captured from the insulated belt ontothe lower roller. Slowly more and more charge becomes concentrated on theroller. This phenomenon of concentration of charge results in repelling theelectrons from the tips of the brush assembly. It also starts to attract electronsfrom the air molecules between the lower roller and brush assembly. Due tothis phenomenon, the positively charged air molecules get carried on the beltaway from the negatively charged roller. The belt therefore gets chargedpositively and moves towards the upper rollers.The upper roller is made from or coated with a material that is higher upin the triboelectric series such as nylon due to which it tries to repel thepositive charge on the belt. The upper brush is directly connected to theinside of the output terminal or sphere at one end and almost touches theupper roller and belt at the other. The electrons in the brush becomeattracted to the positive charges on the belt. The air particles break down tooand the free electrons move towards the belt. The sphere takes up all of thecharge and the excess charge gets spread to the outside of the terminaloutput or sphere.It is this simple electrostatic efect that allows the Van De Graafgenerator to output very high voltages continuously. The charging current for unit surface area of the belt is given byI =bv, where b is the breadth of the belt in meters, v is the velocity of the beltin m/sec, and is the surface charge density in coulombs/m2. It is foundthat is 1.4 x 10-5 C/m2 to have safe electric feld intensity normal to thesurface. With b = 3 m and v = 3 m/sec, the charging current will beapproximately 125A. The generator is normally worked in a high pressuregaseous medium, the pressure ranging from 5 to 15 atm. The gas may benitrogen, air, air-freon (CCl2F2) mixture, or sulphur hexafuoride (SF6).Uses of Van De Graf GeneratorsVan de Graaf generators are useful for very high voltage and low current applications. The output voltage is easily controlled by controlling the corona source voltage and the rate of charging. The voltage can be stabilized to 0.01 %. These are extremely fexible and precise machines for voltage control.In modern times, the application of Van De Graf generators is largelylimited to academic purposes to demonstrate the practical aspects andconcepts of electrostatic behavior of particles. Primarily designed as aparticle accelerator, the Van De Graaf generators are used in laboratoriesfor demonstration purposes only. However, it must be noted that Van deGraaf generators were one of the frst methods used to study nuclearphysics before the advent of better methods to accelerate particles.Though the use of Van De Graaf generators are limited in today's world,they mark a very important milestone in the study of particles in thehistory of nuclear physics