Effects of rf breakdown on the beam in the Compact Linear Collider prototype accelerator structure

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.

Figure 1

Sketch of the CLIC Test Facility 3 at CERN. An electron drive beam is accelerated in a normal-conducting linac and its bunch frequency and current multiplied by means of a delay loop and a combiner ring. The beam is then sent to the Two-beam Test Stand experimental hall where the CALIFES linac is also installed to provide a second electron beam that is as well sent to the Two-beam Test Stand probe beam line for two-beam acceleration experiments.

Figure 2

Sketch of the probe beam line of the Two-beam Test Stand at the CLIC Test Facility 3. The CLIC prototype accelerator structure is fed with rf power produced in the power extraction and transfer structure installed in the drive beam line. The probe beam line is equipped with two quadrupole triplets, steering magnets, inductive and cavity beam position monitors, removable imaging screens and directional couplers at the input and output ports of the accelerator structure. A spectrometer line is used to measure the beam energy just before the beam dump.

Figure 3

Longitudinal cross section of a 3D model of the CLIC prototype accelerating structure installed in the Two-beam Test Stand in 2012.

Figure 4

Collection of measurements from three consecutive bunch trains and prediction of the beam spot in case of rf breakdown: (a) accelerated bunch train; (b) nonaccelerated bunch train; (c) bunch train corresponding to a rf breakdown in the accelerator structure; (d) prediction of the beam spot in case of rf breakdown; (e) beam orbit measurements; (f) power measurements corresponding to a rf breakdown in the accelerator structure.

Figure 5

Collection of measurements from three consecutive bunch trains and prediction of the beam spot in case of rf breakdown: (a) accelerated bunch train; (b) nonaccelerated bunch train; (c) bunch train corresponding to a rf breakdown in the accelerator structure; (d) prediction of the beam spot in case of rf breakdown; (e) beam position measurements; (f) power measurements corresponding to a rf breakdown in the accelerator structure.

Figure 6

Vertical beam position measurements corresponding to a nonaccelerated bunch train at the top, an accelerated one in the middle, and one corresponding to a rf breakdown during the acceleration at the bottom. The bleep in the bottom signal corresponds to the effects of a short breakdown that displaces downwards the beam position.

Figure 7

Distribution of (a) accelerating gradient and (b) missing energy for the 246 breakdown events considered in our analysis.

Figure 8

Distribution of the total magnitude of rf breakdown kicks to the beam orbit.