The Van de Graaff particle accelerator
About the same time that Cockcroft and Walton were building their proton accelerator at Cambridge, a physicist in America, named Van de Graaff, was developing an accelerator of a different kind. This one has an electrostatic generator which employs point action and the principle that the charge on a hollow conductor resides on the outside.
fig, 47.6 shows how it works. A transformer-rectifier circuit provides an initial high potential to a spraycomb consisting of a series of sharp points adjacent to a long moving belt made of special insulating paper. Electric charge is sprayed off the comb and onto the belt by point action (page 376) and is carried up inside a large hollow terminal at the top of an insulating column. Here it is removed from the belt by another spraycomb connected to the inside of the terminal. Thence the charge passes to the outside surface of the terminal and cumulatively builds up a very high poten
accelerator which injects protons into NIMROD, the 7 GeV proton synchrotron at the Rutherford Laboratory, Chilton Berkshire tial, limited only by the breakdown voltage of the surrounding atmosphere. The whole apparatus is therefore enclosed in a pressure vessel containing a gas such as nitrogen or freon, which considerably raises the breakdown voltage (Fig. 47.7). Potentials up to 10 million volts can be obtained with Van de Graaff machines and are used for producing beams of high-energy ions down the accelerating tube shown in the diagram.
The ion source used is a discharge tube containing hydrogen, deuterium .or helium, depending on whether protons, deuterons or ex-particles are required. At the bottom of the accelerating tube the particle beam passes through a magnetic field at right angles to its direction. This bends the beam by different amounts according to the particle velocities. The beam then impinges on a narrow.
Fig. 47.9. Proton linear accelerator with vacuum tank cover removed showing drift tubes. This accelerator was in use at the Rutherford Laboratory, Chilton until the middle 60s. At present, accelerators like this are used for injecting protons into synchrotrons
slit which allows only particles of uniform energy to enter the flight tube on their way to the target.