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The J-PARC Main Ring plans to increase its beam power from 830 kW to 1.3 MW for the Hyper-Kamiokande neutrino experiment by 2028. To enable this higher-power operation, the RF system has been upgraded by increasing both the number of cavities and the anode current of the tetrode tubes. However, the required anode current is now approaching the maximum capability of the tubes, and several associated power-supply systems are also nearing their operational limits.
Traditionally, the anode current has been estimated using a phasor diagram approach. As an alternative, we evaluate the current using LTspice simulations, which can incorporate the vacuum-tube characteristics, cavity impedance, and beam loading. We have constructed an LTspice model of the Main Ring RF system that includes the impedance of the magnetic alloy loaded RF cavity, two 600 kW tetrode tubes, and the beam current corresponding to eight bunches with a total intensity of 3.3 \times 10^{14} protons. This allows us to estimate not only the anode current but also the screen grid currents, and the simulation results have been compared with measurements.
In this paper, we present LTspice based RF system calculations for the Main Ring and evaluate the RF power requirements necessary to achieve 1.3 MW beam operation.
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