Abstract
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.
Highlights
CLIC (Compact Linear Collider) is a possible generation e þ e À collider
The design relies upon a two-beam acceleration scheme, where one of the beams, referred to as the Drive Beam, is decelerated to create high fields used to accelerate the other beam, referred to as the Main Beam
The tight tolerance derives from the fact that any current variation of the Drive Beam translates into a change in accelerating gradient which in turn gives a variation of the energy of the Main Beam
Summary
CLIC (Compact Linear Collider) is a possible generation e þ e À collider. The design relies upon a two-beam acceleration scheme, where one of the beams, referred to as the Drive Beam, is decelerated to create high fields used to accelerate the other beam, referred to as the Main Beam. The tight tolerance derives from the fact that any current variation of the Drive Beam translates into a change in accelerating gradient which in turn gives a variation of the energy of the Main Beam. This leads to a decrease in luminosity mainly due to two mechanisms: First, the limited energy bandwidth of the Beam Delivery System and second, an increase of the emittance in the main linac. As the beam is transported from the linac to the combiner ring the beam current stability degrades due to losses related mainly due to the energy variations Controlling these losses is essential to reach the CLIC requirements.
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