The acceleration of electrons to approximately 0.8 GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5x10(18) cm(-3) by a 10 J, 55 fs, 800 nm laser ...pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10zR), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a0>6, thus driving a highly nonlinear plasma wave.
Accurate knowledge of the charged particle bunch longitudinal (time) profile is important in the context of wakefield accelerators, Compton Light sources, x-ray SASE FELs and THz radiation sources. ...However, it is still a challenge to obtain this information for subpicosecond long bunches and microbunched beams with the required femtosecond (fs) resolution and nondestructively. Apart from determining the profile in a nondestructive manner, the ideal bunch diagnostic would enable extraction of all required information in a single shot, have a sufficiently high repetition rate to monitor each bunch, small footprint, good cost efficiency and reliability. In this paper we present the design of a longitudinal bunch profile monitor that can determine the charged particle bunch profile with femtosecond resolution, nondestructively and in a single shot via the spectral analysis of coherent Smith-Purcell radiation (cSPr). It is based on the simultaneous deployment of three gratings with different periodicities, each with its own set of detectors. The number of the frequency sampling points is equal to the number of the optical channels and the discrimination against background radiation is based on the different polarization properties of cSPr and background radiation. The rationales for the choices made to optimize the monitor operation will be presented and the criteria for determining the number of frequency sampling points will be discussed. The possible future developments of the monitor are also presented and discussed.
We have used coherent Smith-Purcell radiation (cSPr) in order to determine the temporal profile of sub-ps long electron bunches at the Facility for Advanced Accelerator Experimental Tests, at SLAC. ...The measurements reported here were carried out in June 2012 and April 2013. The rms values for the bunch length varied between 356 to 604 fs, depending on the accelerator settings. The resolution of the system was limited by the range of detectable wavelengths which was, in turn, determined by the choice of the grating periods used in these experiments and the achievable beam-grating separation. The paper gives the details of the various steps in the reconstruction of the time profile and discusses possible improvements to the resolution. We also present initial measurements of the polarization properties of cSPr and of the background radiation.
Longitudinal profile monitors using Coherent Smith–Purcell radiation Andrews, H.L.; Bakkali Taheri, F.; Barros, J. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2014, Letnik:
740
Journal Article
Recenzirano
Odprti dostop
Coherent Smith–Purcell radiation has the potential of providing information on the longitudinal profile of an electron bunch. The E-203 experiment at the FACET User Facility measures bunch profiles ...from the SLAC linac in the hundreds of femtoseconds range and the SPESO collaboration at Synchrotron SOLEIL is planning to make an accurate 2D map of the Coherent Smith–Purcell Radiation emission.
•Coherent Smith–Purcell radiation can be used to measure longitudinal profiles in the hundred femtoseconds range.•The current setup used by the E-203 collaboration require integration over several shots and gratings.•Reducing the integration over a single shot and a single grating still yields a meaning full result.•The SPESO experiment at SOLEIL will make a systematic study of Coherent Smith–Purcell radiation.
Coherent Smith-Purcell (SP) radiation originating from three different gratings has been measured at End Station A, SLAC, and has been used to reconstruct the time profile of the electron bunches. ...The beam energy during these experiments was 28.5 GeV (γ≅55773 ) and the number of electrons in the bunch was 0.9–1.4×1010 . The spectral distribution of the radiated energy was measured by means of an array of 11 pyroelectric detectors. Typical values of the FWHM of the bunch length are about 2.5 ps, but sharper peaks with FWHM less than 2.0 ps have also been observed. The longitudinal profile also varies with accelerator conditions and can best be approximated by a superposition of 3–4 Gaussian curves. Some typical profiles are presented, together with a discussion of the limitations and strengths of coherent SP radiation as a diagnostic tool. It is concluded that SP radiation offers excellent prospects in this respect, not only in the picosecond range, but potentially in the femtosecond range as well.
The phase retrieval problem occurs in a number of areas in physics and is the subject of continuing investigation. The one-dimensional case, e.g., the reconstruction of the temporal profile of a ...charged particle bunch, is particularly challenging and important for particle accelerators. Accurate knowledge of the longitudinal (time) profile of the bunch is important in the context of linear colliders, wakefield accelerators and for the next generation of light sources, including x-ray SASE FELs. Frequently applied methods, e.g., minimal phase retrieval or other iterative algorithms, are reliable if the Blaschke phase contribution is negligible. This, however, is neither known a priori nor can it be assumed to apply to an arbitrary bunch profile. We present a novel approach which gives reproducible, most-probable and stable reconstructions for bunch profiles (both artificial and experimental) that would otherwise remain unresolved by the existing techniques.
The generation of quasimonoenergetic electron beams, with energies greater than 500 MeV, in a laser-plasma accelerator driven by 2.5 J, 80 fs laser pulses guided in a low density plasma channel, is ...investigated. The laser energy required to achieve electron injection is found to depend strongly on the quality of the input laser focal spot. Simulations show that, although the matched spot size of the plasma channel is greater than the self-focused spot size, the channel assists relativistic self-focusing and enables electron injection to occur at lower plasma densities and laser powers than would be possible without a waveguide.
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