A non-intercepting beam current monitor for the ISAC-II SC-linac

A personnel protection system will monitor the ion beam current into the experimental hall from the ISAC-II SC-linac. It will have continuous self-test and redundancy and an accuracy of +10%plusmn0.5 nA from 1 to 200 enA. The system, based on an Atlas design, will use capacitive pickups with RF resonators and buffer amplifiers. Ion charge, velocity and bunch width will affect the sensitivity so periodic calibration with dc Faraday cups will be needed. The signal from each 13 cm long, 5 cm diameter pickup tube will pass through a vacuum feedthrough to a helical resonator. An AD8075 IC with an input impedance of 40 kOmega at 35 MHz will allow a high coil tap. The ISAC beam, bunched at 11.8 MHz, is injected into the ISAC-II SC-linac via a 25 m long transfer line. The 35 MHz and 70 MHz coils (3rd and 6th harmonic) have loaded Q's of -675. A test in the transfer line of the 35 MHz coil gave a sensitivity 93 muV/enA from the unity gain buffer using ²⁰Ne⁺⁵ ions at 1.5 MeV/u. Another test downstream of the linac before the experimental hall yielded a sensitivity of 60 muV/enA for a beam of ²²Ne+⁴ ions at 4.97 MeV/u. The backgrounds were equivalent to 1 enA or less. System design and test data will be presented.