ISR Clearing Current Monitoring System

Clearing electrodes remove electrons produced by ionisation of the residual gas in the ISR. The clearing current from a section of length L is related to the beam current I and the absolute pressure P by IC = 0.76 I L P. The described monitoring system measures clearing currents from over 300 electrodes with a sensitivity of 20 pA, adequate to observe pressure changes of 5 10-14 Torr. As compared with conventional UHV gauges which measure only locally, clearing currents yield the integrated pressure along the beam. For maximum flexibility and to cope with future extensions, the control and data acquisition system has been designed around a microprocessor unit connected through an existing CAMAC link to the central ISR computer. Measurements have been performed of the pressure stability in the presence of high intensity beams and of the secondary ionisation rate by electrons trapped in local neutralization pockets. At several occasions, large clearing currents were observed which have been explained tentatively by ionisation of metal atoms sputtered from the vacuum chamber.