Improved vacuum system for high-power proton beam operation of the rapid cycling synchrotron

The vacuum system in the rapid-cycling synchrotron of the Japan Proton Accelerator Research Complex has been operated for more than 10 years. It becomes evident that the high-power beam operation has more powerful effects on the vacuum system than expected at the time of the design. Those effects of the high-power beam are categorized into two types of events, the malfunction of vacuum equipment and the large pressure rise. A specific example of the former event is the failure of the turbomolecular pump (TMP) controller by the full loss of a single-shot high-intensity beam. The TMP itself is also damaged by a bearing crush due to a touch down without braking the rotor. We have developed a TMP controller that can connect with long power and control cables of more than 200 m lengths. This length makes the controller be installed in a control room where there is no radiation influence. The TMP with high-strength bearing has been also developed. The TMP system becomes tolerant to a large loss of the high-intensity beam by using such a controller and a TMP. The latter event is an extreme pressure rise with increasing the beam power. The dynamic pressure behaviors during the beam operations are investigated to understand the outgassing mechanism by the beam. It is indicated that the pressure rise mechanism is a result of the ion-stimulated gas desorption. The analytical calculation based on the ion-stimulated gas desorption model well reproduces the measured dynamic pressure. The calculation also shows that, among the parameters, a larger pumping speed per unit length s and a smaller initial surface density of the adsorbed molecules q_{0} are required to suppress the extreme pressure rise. The nonevaporable getter (NEG) pumps are additionally installed to obtain the larger s and smaller q_{0}. It is confirmed by simulating the beam line pressure distribution that the additional NEG pumps are effective to obtain the larger s in every position along the beam line. The ability of the NEG pump to keep a low pressure during the vacuum system shutdown, which can contribute to maintain the small q_{0}, is examined by the buildup test. It is finally confirmed that the dynamic pressure during the high-power beam is effectually suppressed