Optimising a High-Current Injector to Incorporate a Fast Beam Kicker
In addition to the requirement that the beam current limit imposed by space charge forces be as high as possible, provision should also be made in the injector for fast switching of the high-power beam. The intolerableness of beam losses in a high-current linear accelerator, such as proposed for the German spallation neutron source (SNQ) project, requires that the beam switching occurs cleanly between the beam bunches. The method of accomplishing this technically demanding beam switching is limited by the constraints on the injector chain such as the final energy and operating frequency. These boundary conditions, plus the choice of ion source voltage also come to bear on the current limit in the injector. In the SNQ the injector is asked to supply 200 mA of protons at 200 MHz at an energy of 2 MeV, while the ion source voltage has been chosen to be 50 keV. In order to satisfy all the constraints on the injector and at the same time deliver the maximum current, a frequency jump becomes necessary. The requirements for rf power efficiency in the high-energy linac also demand that all rf buckets be filled. A funneling scheme has been proposed for the SNQ wheremore » this can be achieved by doubling the current and bunch frequency from a pair of injectors operating at half the frequency. A new technique is also put forward in this paper for debunching a beam at an intermediate energy between two RFQ accelerators allowing an arbitrarily large frequency transition to be made between the two RFQ's and raising the current limit even further.« less