Van de Graaff accelerator
A Van de Graaff accelerator is a direct voltage particle accelerator based on the Van de Graaff generator . It was invented in 1930 by Robert Jemison Van de Graaff . It is almost always used to accelerate positively charged ions for physical experiments.
High voltage generation
The acceleration voltage is generated with a mechanically driven, electrically only very weakly conductive endless belt, onto which positive electrical charge is applied by a loading unit by means of a corona discharge and conveyed to a high-voltage terminal. There the tape is discharged at a discharge unit again by corona discharge and the transported charge flows off to a terminal electrode. This increases their electrical voltage , creating a positive potential to ground that can amount to several megavolts.
The maximum terminal voltage is primarily limited by the electric field strength and the associated dielectric strength of the surrounding medium . For this reason, the Van de Graaff generator is located in a gas- filled pressure vessel . A suitable gas, such as pure, dry nitrogen, carbon dioxide or sulfur hexafluoride SF 6 , under suitable pressure, can significantly increase the dielectric strength between the terminal and the environment and thus the voltage that can be achieved. The charging current is selected as large as is necessary to achieve the maximum terminal voltage.
Disturbances and fluctuations in the high voltage at the terminal directly influence the energy of the accelerated ions. This is why the high voltage is additionally stabilized by means of a corona triode by directing negative charge to the terminal as required. Fluctuations and alternating components of the high voltage are measured by means of a capacitive measuring electrode , a metallic band that is insulated on the inside of the pressure vessel near the terminal. This arrangement represents a capacitive voltage divider between the terminal, metal strip and container.
Accelerator part
The ions are generated by an ion source in the terminal electrode. This requires an electrical power supply inside the terminal. This can consist of a power generator that is driven by the roller of the charge conveyor belt.
The high voltage is divided by a chain of resistance along an ion beam tube (also called a column) in such a way that the partial voltages on the metal potential rings of the tube result in a uniform field distribution. The ions are accelerated in the evacuated column and after passing through a u. U. several meters long pipe system to the target . On the way there, the ion beam is focused by various electron-optical components, mostly electromagnets, and deflected as required. The achievable current strengths of the particle beam are in the microampere range.
Because of the possibility of keeping the acceleration voltage and thus the energy of the ions very precisely constant, the Van de Graaff accelerator has become a precision tool in low-energy nuclear physics .
The tandem accelerator is a further development of the Van de Graaff accelerator .
literature
- Ragnar Hellborg (Ed.): Electrostatic Accelerators . Springer Verlag, Berlin 2005, ISBN 3-540-23983-9
Individual evidence
- ^ EB Paul: Nuclear and Particle Physics. North Holland, 1969, p. 169.