Drive beam stability studies and stabilization algorithms in CLIC Test Facility 3

In this paper the study of the mechanisms responsible for the time varying beam current losses in CTF3 (CLIC Test Facility 3), together with the feedbacks implemented to mitigate them, is presented. The study shows that the losses were linked mainly to the energy variation induced by the RF (Radio frequency) amplitude fluctuations. The RF-amplitude instability sources were identified. A feedback developed to mitigate this instability, acting on the RF-compression system by controlling the phase program of the klystron, is described in detail. The result is a significant improvement of the overall stability in the machine. The energy variation is reduced further with the use of an energy feedback operating on the RF-amplitude of the last klystron in the CTF3 linac. This feedback loop closes on the energy measured from a dispersive pickup after the linac. With the energy stabilized a beam current stability close to the CLIC (Compact Linear Collider) specification for a factor 4 combined beam was achieved.