Improvements in the SPIDER RF system

In SPIDER the negative ions are extracted from a plasma generated with RadioFrequency drivers, each fed with a power up to 100 kW at the frequency of 1 MHz. One of its distinguishing characteristics is the source fully installed within a vacuum vessel and operated in the residual gas pressure, including the matching networks for the driver impedance installed on the source backside. These features lead to more challenging design and integration also of the RF system, and in fact during the experimentation some issues arose: the operation of the RF generators is affected by the magnetic coupling among the RF circuits on board the source; breakdowns are observed on the source backside while powering the RF circuit and common mode RF currents flow through the diagnostics and dc power circuits. An overall system design review was launched with the aim to identify the root cause of these issues, feasible solutions and a plan for their fix, which are presented and discussed in this paper.