Multiturn injection techniques employed in the MURA SO-MeV electron accelerator are described, together with an evaluation of their performance. These include shifting of the equilibrium orbit by a time-varying bump, damping of betatron oscillation amplitudes by an rf voltage, and damping of amplitUdes by space-charge effects. Multiturn injection is achieved by filling the accelerator radial phase space with injected beam. The capture of injectedcharge approaches 100% when the programmed field bump is used. This high efficiency has been achieved for about 15 turns of total beam from the inflector and for about 40 turns of collimated beam. Injection at the higher beam intensities gives rise to spontaneous longitudinal bunching of the circulating beam for all energies from injection to above transition with the magnitude generally greatest just above transition.
The design and construction of probes to measure beam size, orbit shape, betatron oscillation frequencies, and beam intensity are described, together with the mechanical and electrical aspects of the probe drives which move the probes.
An electron gun and an inflection system which satisfy the injection requirements of the MURA 50-MeV electron accelerator are described. Electrons emitted from a tungsten ribbon filament are accelerated to 100 keV by a converging radial field of cylindrical geometry and deflected parallel to the equilibrium orbit of the accelerator by a 60° electrostatic inflector. The inflector also provides focusing of both the radial and vertical motion of the beam. The gun and inflector are located within the guide field region of the magnets and are adjustable in position as a unit. The emittances of the system are 0.5 π and 1.0 π mrad-cm for the horizontal and vertical motions, respectively. Although the injector is capable of delivering several hundred millamperes, it is normally operated at much lower currents since fewer than 20 mA are required to reach the space-charge limit of the accelerator when multiturn injection techniques are employed.
Four transformer cores symmetrically placed around the FFAG electron accelerator are used to accelerate particles from an injection energy of approximately 100 keV to about 2 MeV, which is well above the transition energy of 1.13 MeV. This type of acceleration during the early part of the operating cycle provides flexibility and simplification for the injection process, eliminates the complications required by an rf program suitable for accelerating particles through the transition energy, and is a useful analyzing tool for certain types of experiments. Detailed operation of the betatron system and its associated components is described. The construction parameters of the cores and a description of the associated pulsed power supply and integrator circuits are also discussed.
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