Now that the beam has bean focused, pulsed and bunched it is ready for full acceleration. The ions leave the focusing assembly and travel an evacuated tube down the centre of the potential rings. The large potential required between the dome and ground is generated using the same principle as a basic Van de Graaf generator. Charge is deposited onto a rubber-coated cloth belt at the base of the tank by a power supply held at about thirty thousand volts. This belt carries the charge upward to the dome at the very top of the tank. Here, the charge is removed from the belt in similar manner to its deposition and flows onto the surface of the dome (effectively a hollow, metallic conductor). This charge builds up and can be large enough to create potentials as large as 7 megavolts. In theory charge can be placed onto the dome almost indefinitely, or until the dome reaches its breakdown point. In practice the limiting factor is the breakdown potential of the inert gas which fills the accelerator tank (the blue outer shell of the accelerator). This large potential between the dome and ground produces a huge electric field, accelerating the ions downward. Each of the potential rings is at a voltage intermediate between the dome and ground, with each consecutive ring at a lower voltage than the one above. Thus, the ions are accelerated further below each ring. Further focusing can be achieved after the potential rings by electrostatic quadrupole lenses. ACCELERATOR DIAGRAM
ANALYSING MAGNET Upon leaving the accelerating tube rings, the beam is focused, pulsed and bunched and the ions have been accelerated to the desired energy. The ions path must now be bent 90 degrees into the horizontal plane so that they can travel along the beamline to the sample at the far end. This is done with the analyzing magnet shown to the left. This is a powerful electromagnet, and is the large grey mass on the far left. The pipe leaving it is the beamline. The whole magnet is mounted on a turret (actually a converted gun platform from a WWII destroyer!) allowing it to be rotated and used in conjunction with several beamlines. The magnet strength required to bend the ions is dependent on the energy to which they have been accelerated. Higher energy ions require a larger magnetic field to be deflected, while lower energy ions require a smaller field. This principle can be used to select the desired energy of ion, as only those with particular energies make it through the 90 degree turn (others are deflected out of the beamlines path). Once the magnetic field is selected, a pair of detection slits (for the horizontal and vertical planes) allow the operator to determine if the ions are following the correct path. If the accelerator voltage is too high, the ions will not be deflected enough, and the slits will read a voltage on the "low-side". An electronic feedback system then uses this information to make fine adjustments to the accelerator voltage to stabilise the beam energy to within one part in 20,000. In the opposite case of too low a voltage, the feedback system increases it. The magnetic field of the magnet itself is measured accurately using an NMR (Nuclear Magnetic Resonance) probe.


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