Fig. 4-1 Beam collision allowance time in the J-PARC linac (worst case) The proton beam produced by the ion source is first accelerated in a linear accelerator (linac). When certain errors occur in the linac, a beam collides with linac components, and the beam energy is converted to heat. Even at lower energy levels, this heat is deposited near the surfaces of the linac structure. The permissible exposure time to this heat is 40 to 60 microsecond in ACS. However, the colliding beams cause physical damage to the RFQ within a few microsecond.

Fig. 4-2 The fast-response interlock system for J-PARC When the beam loss monitor detects a collision, the beam is interrupted by the RFQ fast-response interrupting semiconductor switch. This response is accomplished within a few microsecond.

Fig. 4-3 Example of surge noise measurement Surge noise occurred as a result of inserting and removing an electrical appliance plug. The average surge noise detected in the fast-response interrupting module is 360 mV and 250 ns. However, the maximum surge noise voltage may reach 3 V. The module control voltage was set to 2.4 V. It must be lower than this surge noise lebel.

The beam power of the high intensity proton accelerator at J-PARC is 1 MW, which is one order of magnitude higher than any other pulse-driven proton accelerator now in operation. When certain errors occur in the accelerator, the beam is diverted from its ideal orbit and collides with the components of accelerator. The diverted beam produces intense heat damage within a few microsecond, as shown in Fig. 4-1. In addition, components are to be activated by the beam collision, which will delay maintenance service. Therefore, when beam collisions occur, an interruption of beams within a few microsecond is necessary. Beam interrupting modules having such response times were developed at other institutes. However, many modules malfunctioned due to the electromagnetic noise emitted by the accelerator itself, and these modules have not been suitable for practical use. Additionally, the causes of these malfunctions were not analyzed yet. The fast-response beam interrupting system was designed for the usage at J-PARC (Fig. 4-2). The prototype module for this system was used to investigate its performance. It was found that the malfunctions of modules were caused by normal operation because noises were detected through the power supply sysytem. The characteristics of noises were measured (Fig. 4-3). Using these results, the fast-response and high-reliability module employing a programmable LSI, Field Programmable Gate Array, was developed.

Reference H. Sakaki et al, Performance Test of the Prototype-Unit for J-PARC Machine Protection System, JAERI-Tech 2004-021 (2004)  (in Japanese). (http://jolisf.tokai.jaeri.go.jp/pdf/tec/JAERI-Tech-2004-021.pdf)

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