1. Describe the process of charge transportation in the Van de Graaff accelerato
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Question
1. Describe the process of charge transportation in the Van de Graaff accelerator. What polarity has the high voltage electrode? 2. A proton cyclotron has a circumference of 100 m and can accelerate protons to a maximum energy of 500 MeV. If a magnetic field of 3.0 T is applied, what is the frequency of rotation (cycles/second) at the maximum energy? 3. What is the radius of circular arc for electron with energy 1.5 keV moving in 5 T magnetic field? Provide response for two cases: a) the electron moves perpendicularly to the magnetic field; b) an angle between the electron movement direction and magnetic field is 30 degrees. Should we take into consideration the relativistic effect and why? 4. What happens with a luminosity of a colliding beam accelerator if the numbers of particles in each bunch will be increased by twice in both beams? 5. A beam of alpha particles with 10 MeV energy is injected between two parallel plates separated by a distance of 5 cm and 2 cm long. Simulate the beam deflection (in degrees or radians) if the voltage difference between two plates is 3 kV. 6. (3 points) Mo-99 is the parent of the widely used medical diagnostic radioisotope Tc-99m. What material (isotope) will produce highest quantity of the Mo-99 radioasotope after irradiation by DD neutrons (En=2.5 MeV) and DT neutrons (En= 14 MeV)? What type of neutrons (DD or DT) is better for the Mo-99 radioisotope production? How isotope abundance can effect on your results? Just the following reactions have to be considered: (n,n
Explanation / Answer
Van de Graaff generators are described as "constant current" electrostatic devices. When you put a load on a Van de Graaff generator, the current (amperage) remains the same. It's the voltage that varies with the load. In the case of the Van de Graaff generator, as you approach the output terminal (sphere) with a grounded object, the voltage will decrease, but the current will remain the same. Conversely, batteries are known as "constant voltage" devices because when you put a load on them, the voltage remains the same. A good example is your car battery. A fully charged car battery will produce about 12.75 volts. If you turn on your headlights and then check your battery voltage, you will see that it remains relatively unchanged (providing your battery is healthy). At the same time, the current will vary with the load. For example, your headlights may require 10 amps, but your windshield wipers may only require 4 amps. Regardless of which one you turn on, the voltage will remain the same.
There are two types of Van de Graaff generators: one that uses a high-voltage power supply for charging and one that uses belts and rollers for charging. Here we will discuss the belts-and-rollers type.
This kind of Van de Graaff generator is made up of:
When the motor is turned on, the lower roller (charger) begins turning the belt. Since the belt is made of rubber and the lower roller is covered in silicon tape, the lower roller begins to build a negative charge and the belt builds a positive charge. You can understand why this charge imbalance occurs by looking at the triboelectric series: Silicon is more negative than rubber; therefore, the lower roller is capturing electrons from the belt as it passes over the roller.
The experiments were performed both with and without preheating of wires and at different polarities of the high-voltage electrode. The effect of plasma production at the electrodes on the initiation of breakdown along the exploding wire was investigated by using a frame camera. It is shown that, when the polarity of the high-voltage electrode is positive, breakdown begins with the formation of a bright spot on the wire surface near the cathode, whereas at the negative polarity, breakdown begins with the formation of bright spots on the cathode surface. A comparative analysis of the main characteristics of wire explosions is performed. It is shown that preheating of the conductor increases the resistive-heating time and, accordingly, the energy deposited in the wire core. This effect takes place during explosions of both single wires and wire arrays. The evolution of the state of a metal during the explosion (including melting and evaporation) is studied by one-dimensional simulations by using a semiempirical equation of state describing the properties of tungsten over a wide range of parameters.
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