Abstract
Understanding the effects of rf breakdown in high-gradient accelerator structures on the accelerated beam is an extremely relevant aspect in the development of the Compact Linear Collider (CLIC) and is one of the main issues addressed at the Two-beam Test Stand at the CLIC Test Facility 3 at CERN. During a rf breakdown high currents are generated causing parasitic magnetic fields that interact with the accelerated beam affecting its orbit. The beam energy is also affected because the power is partly reflected and partly absorbed thus reducing the available energy to accelerate the beam. We discuss here measurements of such effects observed on an electron beam accelerated in a CLIC prototype structure. Measurements of the trajectory of bunch trains on a nanosecond time scale showed fast changes in correspondence of breakdown that we compare with measurements of the relative beam spots on a scintillating screen. We identify different breakdown scenarios for which we offer an explanation based also on measurements of the power at the input and output ports of the accelerator structure. Finally we present the distribution of the magnitude of the observed changes in the beam position and we discuss its correlation with rf power and breakdown location in the accelerator structure.
Highlights
The Compact Linear Collider (CLIC) study aims to develop a linear accelerator to accelerate electrons and positrons to TeV scale energies [1]
In this paper we present the first study of the effect of discharges on an accelerated beam in the Two-beam Test Stand [9] in the CLIC Test Facility CTF3 at CERN [10]
We focus on the measurements of transverse kicks to the beam due to rf breakdown in a CLIC prototype accelerator structure, carried on during the CTF3 2012 run
Summary
The Compact Linear Collider (CLIC) study aims to develop a linear accelerator to accelerate electrons and positrons to TeV scale energies [1]. The field emission is triggered from nanometric imperfections on the structure walls which is observed as dark current during normal operation It can randomly initiate an avalanche process which results in a rf breakdown. A second effect is that the strong currents flowing during the discharge cause fields that deflect the beam These processes were investigated earlier at SLAC where dark currents and breakdown in both waveguides and accelerator structures have been simulated [4,5,6]. We focus on the measurements of transverse kicks to the beam due to rf breakdown in a CLIC prototype accelerator structure, carried on during the CTF3 2012 run. Afterwards we present evidence for rf breakdown effects on the beam, showing and discussing measurements of a few selected examples of bunch trains accelerated in a CLIC prototype accelerator structure while a breakdown occurs. Kick magnitude and its correlation with rf power and breakdown location in the accelerator structure
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