Philip Burrows

Abstract:

During 2023, examination of the action dependence of sextupolar resonance driving terms (RDT) in the LHC at injection, as measured with an AC-dipole, demonstrated that a robust measurement of the RDTs could still be achieved even with very small amplitude kicks, typically used for linear optics studies. Consequently, analysis of optics measurements from 2022 to 2024 during the LHC energy ramp allowed a first measurement of the sextupole resonance evolution. A large asymmetry was observed between the two LHC beams, with the clockwise circulating beam (LHCB1) being significantly worse than the counter-clockwise circulating beam (LHCB2), and a clear increase in the RDT strength during the ramp was observed. During 2024 commissioning, a first attempt was made to correct the 𝑓1020 RDT of LHCB1 at injection. Results are presented in this report.

Abstract:

The status of commissioning of the electron injector intended for the next phase of the proton driven wakefield experiment, Advanced Wakefield Experiment (AWAKE), is presented, showing first results from operating the brazingfree electron gun. To provide a high-quality electron beam, the UV laser was centered on the copper cathode, and a novel beam-based alignment of the focusing solenoid was performed. Measurements of the beam parameters and working points are addressed. The electron gun is shown to provide a high quality, stable and reproducible beam.

Abstract:

To reduce filling times and enable advanced injection schemes, it is desirable for the Diamond-II booster to provide high charge in both single and multi-bunch modes. The single bunch charge will be limited by short range wakefields in the booster, and long-range wakefields limit the charge for the multi-bunch trains. Due to the relatively low 100 MeV injection energy into the booster, the injected beam is susceptible to instabilities due to the very weak synchrotron radiation damping. In this paper, we present the simulation results carried out to estimate the single and multi-bunch charge thresholds in the Diamond-II booster including short and long range wakefields, RF cavity HOMs, and with physical apertures applied. Simulations results will also be presented that demonstrate the extracted multi-bunch charge could be increased by installing a transverse multi-bunch feedback (TMBF).

Abstract:

We show in experiments that a long, underdense, relativistic proton bunch propagating in plasma undergoes the oblique instability, which we observe as filamentation. We determine a threshold value for the ratio between the bunch transverse size and plasma skin depth for the instability to occur. At the threshold, the outcome of the experiment alternates between filamentation and self-modulation instability (evidenced by longitudinal modulation into microbunches). Time-resolved images of the bunch density distribution reveal that filamentation grows to an observable level late along the bunch, confirming the spatiotemporal nature of the instability. We provide a rough estimate of the amplitude of the magnetic field generated in the plasma by the instability and show that the associated magnetic energy increases with plasma density.

Abstract:

Experimental results show that hosing of a long particle bunch in plasma can be induced by wakefields driven by a short, misaligned preceding bunch. Hosing develops in the plane of misalignment, self-modulation in the perpendicular plane, at frequencies close to the plasma electron frequency, and are reproducible. Development of hosing depends on misalignment direction, its growth on misalignment extent and on proton bunch charge. Results have the main characteristics of a theoretical model, are relevant to other plasma-based accelerators and represent the first characterization of hosing.