Dr James Chappell

Abstract:

We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400 GeV proton bunch. Results show that a positive or negative gradient increases or decreases the number of microbunches and the relative charge per microbunch observed after 10 m of plasma. The measured modulation frequency also increases or decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields’ phase velocity due to plasma density gradients adding to the slow wakefields’ phase velocity during self-modulation growth predicted by linear theory.

Abstract:

An electron beam derived from stripping of ultrarelativistic lead ions has been used to perform calibration measurements on the electron spectrometer of the Advanced Wakefield experiment at CERN. As part of this study, new measurements of the stripping cross-section for ultrarelativistic hydrogen-like lead ions passing through aluminium and silicon have been obtained which demonstrate good agreement with existing measurements and theory. Improvements in terms of electron beam quality and ion beam diagnostic capability, as well as further applications of such an electron beam, are discussed.

Abstract:

Plasma Physics and Controlled Fusion PAPER Proton beam defocusing in AWAKE: comparison of simulations and measurements A A Gorn1,2, M Turner3, E Adli4, R Agnello5, M Aladi6, Y Andrebe5, O Apsimon7,8, R Apsimon7,8, A-M Bachmann3,9,10, M A Baistrukov1,2Show full author list Published 6 November 2020 • © 2020 IOP Publishing Ltd Plasma Physics and Controlled Fusion, Volume 62, Number 12 Citation A A Gorn et al 2020 Plasma Phys. Control. Fusion 62 125023 80 Total downloads Turn on MathJax Get permission to re-use this article Share this article Share this content via email Share on Facebook Share on Twitter Share on Google+ Share on Mendeley Article information Abstract In 2017, AWAKE demonstrated the seeded self-modulation (SSM) of a 400 GeV proton beam from the Super Proton Synchrotron at CERN. The angular distribution of the protons deflected due to SSM is a quantitative measure of the process, which agrees with simulations by the two-dimensional (axisymmetric) particle-in-cell code LCODE to about 5%. The agreement is achieved in beam population scans at two selected plasma densities and in the scan of longitudinal plasma density gradient. The agreement is reached only in the case of a wide enough simulation box (several plasma wavelengths) that is closer to experimental conditions, but requires more computational power. Therefore, particle-in-cell codes can be used to interpret the SSM physics underlying the experimental data.

Abstract:

We study experimentally the longitudinal and transverse wakefields driven by a highly relativistic proton bunch during self-modulation in plasma. We show that the wakefields’ growth and amplitude increase with increasing seed amplitude as well as with the proton bunch charge in the plasma. We study transverse wakefields using the maximum radius of the proton bunch distribution measured on a screen downstream from the plasma. We study longitudinal wakefields by externally injecting electrons and measuring their final energy. Measurements agree with trends predicted by theory and numerical simulations and validate our understanding of the development of self-modulation. Experiments were performed in the context of the Advanced Wakefield Experiment (AWAKE).