A D Brynes, P Smorenburg, I Akkermans, E Allaria, L Badano, S Brussaard, M Danailov, A Demidovich, G De Ninno, D Gauthier, G Gaio, S B van der Geer, L Giannessi, M J de Loos, N S Mirian, G Penco, P Rebernik, F Rossi, I Setija, S Spampinati, C Spezzani, M Trovò, PHWilliams and S DiMitri
An understanding of collective effects is of fundamental importance for the design and optimisation of the performance of modern accelerators. In particular, the design of an accelerator with strict requirements on the beam quality, such as a free electron laser (FEL), is highly dependent on a correspondence between simulation, theory and experiments in order to correctly account for the effect of coherent synchrotron radiation (CSR), and other collective effects. A traditional approach in accelerator simulation codes is to utilise an analytic one-dimensional approximation to the CSR force. We present an extension of the 1D CSR theory in order to correctly account for the CSR force at the entrance and exit of a bending magnet. A limited range of applicability to this solution—in particular, in bunches with a large transverse spot size or offset from the nominal axis—is recognised. More recently developed codes calculate the CSR effect in dispersive regions directly from the Liénard– Wiechert potentials, albeit with approximations to improve the computational time. A new module of the General Particle Tracer (GPT) code was developed for simulating the effects of CSR, and benchmarked against other codes. We experimentally demonstrate departure from the commonly used 1D CSR theory for more extreme bunch length compression scenarios at the FERMI FEL facility. Better agreement is found between experimental data and the codes which account for the transverse extent of the bunch, particularly in more extreme compression scenarios.Space-charge effects in ultrafast electron diffraction patterns from single crystals
Robert P.Chatelain, Vance Morrison, Chris Godbout, Bas van der Geer, Marieke de Loos, Bradley J.Siwick
The impact of electron–electron interactions in the post-specimen region of ultrafast electron diffraction and dynamic transmission electronmicroscopy instruments has been studied. Specifically, space-charge induced distortions of ultrafast electron diffraction patterns from single crystal specimens and their dependence on electron bunch-charge, beamenergy, energyspread, focusing conditions and speciment hickness have been investigated using the General Particle Tracer code. We have found that these space-charge interactions lead to significant broadening and displacement of the Bragg spots at currently realizable electron beam illumination conditions. These impacts increase in severity with beam brightness and are reduced with increasing (relativistic) beam energies. The primary mechanism for the distortions has been determined to bespace-charge interactions between the scattered beamlets and the main unscattered beam. Overall, these results suggest that creative post-specimen electron optical design, relativistic beam energies and post-processing of diffraction patterns to correct for space-charge distortions hould be explored as routes to make good use of any futuree nhancements to beam brightness in UED and DTEM instruments.Compression of subrelativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffractionon
Van Oudheusden, T., Pasmans, P.L.E.M., Van Der Geer, S.B., De Loos, M.J., Van Der Wiel, M.J., Luiten, O.J.
We demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive spacecharge- induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs.