Reducing the beam impedance of the kicker at the 3-GeV rapid cycling synchrotron of the Japan Proton Accelerator Research Complex

Abstract

The present four-terminal kicker at the rapid cycling synchrotron (RCS) at the Japan Proton Accelerator Research Complex has the power-saving benefit that it allows beam extraction by doubling the excitation currents with two shorted ends. In this configuration, two terminals of the kicker are connected to the pulseforming line while the other two are terminated in a short circuit. On the other hand, beam instabilities are excited in the RCS by the kicker beam impedances, which result from the short-circuit termination of the kicker. In this paper, we describe a scheme to reduce the beam impedance of the kicker using diodes (nonlinear devices), while retaining the benefit of the doubled kicker excitation currents. We employ a simulation technique to determine the beam impedance of the kicker, even when such nonlinear devices and long cables are included. The characteristic of beam impedance measured using the accelerated beams is well explained by that obtained from the simulation.

Source of horizontal instability at the CERN Proton Synchrotron Booster

“…The diode-resistor network reduced the maximum induced voltage by a factor of approximately three, relative to an open circuit at this end of the cables. In the case of the PSB extraction kickers, the increase in rise time and field ripple as reported in [44] would be unacceptable. However, it is also shown in [44] that the impedances of the container, including the diodes and resistors, significantly contribute to the damped oscillation introduced at the top of the rising edge of the current pulse.…”

“…Nevertheless, if it is necessary to suppress the first KFA14L1 resonance at ∼1.72 MHz, this cannot be achieved by replacing the kicker magnet short-circuit terminations by a matched resistor: the short circuits are necessary to achieve the required deflection angle in the available space. Thus, several ideas are under consideration: (i) Incorporate a beam screen in the aperture of each kicker magnet; (ii) Incorporate serigraphy in the aperture of each kicker magnet; (iii) Install a matched resistor from the thyratron cathode to ground [43]; (iv) Install a diode and series resistor, connected between the output of the main switch on the transmission line side and ground [44]; (v) Install a saturating inductor between each 25 Ω transmission cable and its associated kicker magnet [45]. The pros and cons of each of these options are discussed below.…”

“…Shobuda [44] proposed, simulated, and implemented a diode and series resistor connected between the output of the main switch on the transmission line side and ground, to suppress a resonance in a kicker at the 3-GeV rapid cycling synchrotron of the Japan Proton Accelerator Research Complex (J-PARC). Similarly to the PSB, this kicker system also had a short-circuit termination.…”

“…However, it is also shown in [44] that the impedances of the container, including the diodes and resistors, significantly contribute to the damped oscillation introduced at the top of the rising edge of the current pulse. Therefore, it may be possible to reduce the magnitude of the oscillation by reducing the length of the cable that connects to the diode resistor network [44].…”