Nuclear Physics Group

        Department of Physics & Astronomy
     
             
Accelerator Facilities
      
                         

The accelerator here at UK is a 7 megavolt machine modified from an original Van de Graaf design. Charged particles, usually protons, deuterons, or helium ions,can be accelerated through this or lower potentials, producing particles with energies up to 7 MeV (Mega electron-Volts). Because of the high voltages involved in acceleration, the charged dome, potential rings and charging belt are all housed in a large tank. This tank is then filled with extremely dry nitrogen and carbon dioxide. The gas is at high pressure, with much greater dielectric strength than air, thus preventing sparks. The photo to the right shows engineer Gene Baber operating the lift, raising the 17 ton tank (the blue, outer shell of the accelerator which can just be seen at the top) off of the accelerator (the silver cylinder). The very tip of the silver housing can be removed to expose the focusing assembly and the ion source (see below). The rings below it are the potential rings mentioned previously. These are all connected by resistances such that each one is at a lower voltage than the one above it, thus creating the electric field needed for acceleration. RAISING THE TANK
ION SOURCE The source bottle can be seen in this picture. This contains the gas which is ionised to produce our charged particles. To accelerate protons, the bottle contains hydrogen, for deuterons it is deuterium, and in the case of alpha particles or 3He ions, the gas is helium. The hydrogen isotopes leak into the bottle through palladium leaks. A radio-frequency electric field is then used to strip the electrons from the atoms in the gas (the field is at 134 MHz) creating a cloud of free electrons and ions. The positively charged plasma is contained within the source by a magnetic field. At the very top of the bottle is a probe wire. When accelerating, a positive potential is applied to it, setting up a field which forces the ions in the source downward, through a hole into the focusing assembly below. In the photo (left) the eerie purple glow due to ionisation and de-excitation in the source can be seen.
This wider shot shows Tyler, a mid-shipman from the US Naval Academy, standing beside the focusing assembly. The source bottle can still be seen at the top of the photo. Tyler is standing on a moveable platform used to carry out maintenance on the assembly and replace the source. Below the assembly, the first of the accelerating potential rings can be see at the very bottom of the picture. In the focusing assembly, the ions which are being forced out of the source bottle with a spread of angles are focused into a narrow beam ready for acceleration. The ions may also be pulsed. This procedure involves applying an rf sweep in the assembly, causing the beam to describe an elipse as it travels downward. Below the application of this potential is a chopping aperture, which only lets the beam through at one point on its eliptical projection, when it passes the aperture. This happens 1.875 million times per second. The rest of the time the beam cannot pass, thus resulting in a time pulsing of the beam with each pulse seperated by a time equal to the period of the beams elipse. Further refinement of the beam is achieved via bunching. That is the length of the beam-pulse in time is compressed such that all the ions in the pulse arrive at the target within one (1) nanosecond of each other. FOCUSING ASSEMBLY


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