Electron beam measurements with beam loss monitors in AWAKE

In this thesis I study experimentally electron beam loss signals in AWAKE, the Advanced WAKEfield experiment at CERN. In AWAKE, electrons are accelerated by obliquely injecting them into the plasma wakefields driven by a self-modulating relativistic proton bunch. Due to the complexity of the 10 meter long vapor source that provides the plasma, we have to transport and inject the electrons through a 10 mm diameter entrance aperture. I designed, simulated and implemented a diagnostic system to study physics properties of the external injection of the 18MeV/c electron bunch into the plasma. We have installed seven scintillating detectors along the plasma as electron beam loss monitors. Each detector measures the secondary particles produced when the electron bunch interacts with material. To prove the feasibility of the system and to support understanding of the results, I run FLUKA simulations of the setup. According to simulations, secondary particles can exit the vapor source and their energy deposition in the detectors is above the detection threshold of 100 keV. The spatial resolution, determined by the distance between individual detectors, allows to estimate where the beam is lost and whether it interacts with the material surrounding the entrance aperture. We measured the electron transverse beam size at the aperture location, for different beam focus positions and beam charges, scanning the electron beam across the vapor source entrance aperture, while recording the beam loss monitor signals. For the 200 pC electron bunch, the r.m.s. transverse beam size (σx, σy) at the entrance increases from (0.45 ± 0.02, 0.33 ± 0.04) mm to (2.6 ± 0.4, 0.9 ± 0.1) mm as the beam is focused further inside the plasma. Furthermore, I observed the beam size to increase with the charge as σ600 ∼√ (2)σ200 (where σ600 and σ200 are the r.m.s. beam sizes for the 600 and 200 pC beams respectively), as expected from theoretical predictions. Spatial electron, proton and laser beam alignment is one of the crucial issues of the AWAKE experiment; therefore, we were interested in quantifying the deviation caused by the earth magnetic field on the electron beam trajectory in order to precisely overlap the beams. Aligning the electron beam onto a proton reference trajectory and scanning both beams across the aperture, I estimated the deflection from the straight trajectory to be: (1.2±0.1) mm in the horizontal plane (bending to the right) and (0.4 ± 0.1) mm in the vertical plane (bending downward). Beam loss detection gives also information on the beam propagation along the vapor source. I estimated electron beam losses at the entrance for different beam focusing optics and studied the propagation of electrons in vacuum and within the plasma channel. During the acceleration experiment, at the presence of proton driven wakefields, I observed an increase of electron losses downstream the injection point. This may be explained considering defocusing wakefields acting on part of the injected electron bunch. Additionally, studying the background generated by the proton beam on the beam loss monitors, I observed satellite pre-bunches ahead of the main proton bunch delivered by the CERN Super Proton Synchrotron.