Development of Very Thin Beam Chopper with High Field Gradient for High Intensity Accelerator

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Proton and ion Synchrotrons consist of an ion source, linear accelerator and synchrotron (s). Each accelerator has a different time structure and RF frequency. Usually, the beam from the ion source has the width of few hundred micro seconds and it is much longer than the time structure in the synchrotron (less than one micro second). Because of the difference on the time structure, the beam will be spread out in the synchrotron while the injection. To capture the beam for the acceleration, an adiabatic capture method is used and the beam will be bunched by the RF voltage. However, the beam loss is easily occurred during the process because of the space charge effects and others. On the high intensity proton accelerator, a beam chopper is employed to reduce the loss. So far, the beam is kicked to the transverse direction before the injection. A disadvantage of this method is the transverse halo formation by the slow rise and fall times of the chopper magnets. A new type of beam chopper using the longitudinal kick is proposed. The transverse halo formation can be avoidable by the method. The chopper is located between the RFQ and ion source. By the energy modulation of about 10%, kicked part of the beam will not be accelerated in the RFQ.We developed the chopper using a magnetic alloy which has very high permeability. The high power FET switch drives the chopper. The beam test has been performed and it was chopped with the repetition rate of 1 MHz. The chopped beam was injected and accelerated, successfully. By using the chopper, low emittance beam was also formed and it is very useful for the accelerator study.