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Register a new userProgress on Experiments towards LWFA-driven Transverse Gradient Undulator-Based FELs
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Free Electron Lasers (FEL) are commonly regarded as the potential key application of laser wakefield accelerators (LWFA). It has been found that electron bunches exiting from state-of-the-art LWFAs exhibit a normalized 6-dimensional beam brightness comparable to those in conventional linear accelerators. Effectively exploiting this beneficial beam property for LWFA-based FELs is challenging due to the extreme initial conditions particularly in terms of beam divergence and energy spread. Several different approaches for capturing, reshaping and matching LWFA beams to suited undulators, such as bunch decompression or transverse-gradient undulator schemes, are currently being explored. In this article the transverse gradient undulator concept will be discussed with a focus on recent experimental achievements.
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We show that a short relativistic electron beam propagating in a plasma with a density gradient perpendicular to the direction of motion generates a wakefield in which a witness bunch experiences a transverse force. A density gradient oscillating along the beam path would create a periodically varying force---an undulator, with an estimated strength of the equivalent magnetic field more than ten Tesla. This opens an avenue for creation of a high-strength, short-period undulators, which eventually may lead to all-plasma, free electron lasers where a plasma wakefield acceleration is naturally combined with a plasma undulator in a unifying, compact setup.
We study the tapering optimization scheme for a short period, less than two cm, superconducting undulator, and show that it can generate 4 keV X-ray pulses with peak power in excess of 1 terawatt, using LCLS electron beam parameters. We study the effect of undulator module length relative to the FEL gain length for continous and step-wise taper profiles. For the optimal section length of 1.5m we study the evolution of the FEL process for two different superconducting technologies NbTi and Nb3Sn. We discuss the major factors limiting the maximum output power, particle detrapping around the saturation location and time dependent detrapping due to generation and amplification of sideband modes.
Generally, turn-to-turn power fluctuations of incoherent spontaneous synchrotron radiation in a storage ring depend on the 6D phase-space distribution of the electron bunch. In some cases, if only one parameter of the distribution is unknown, this parameter can be determined from the measured magnitude of these power fluctuations. In this Letter, we report an absolute measurement (no free parameters or calibration) of a small vertical emittance (5--15 nm rms) of a flat beam by this method, under conditions, when it is unresolvable by a conventional synchrotron light beam size monitor.
This paper describes simulation analyses on beam and laser (X-ray)-driven accelerations in effective nanotube models obtained from Vsim and EPOCH codes. Experimental setups to detect wakefields are also outlined with accelerator facilities at Fermilab and NIU. In the FAST facility, the electron beamline was successfully commissioned at 50 MeV and it is being upgraded toward higher energies for electron accelerator R&D. The 50 MeV injector beamline of the facility is used for X-ray crystal-channeling radiation with a diamond target. It has been proposed to utilize the same diamond crystal for a channeling acceleration POC test. Another POC experiment is also designed for the NIU accelerator lab with time-resolved electron diffraction. Recently, a stable generation of single-cycle laser pulses with tens of Petawatt power based on thin film compression (TFC) technique has been investigated for target normal sheath acceleration (TNSA) and radiation pressure acceleration (RPA). The experimental plan with a nanometer foil is discussed with an available test facility such as Extreme Light Infrastructure - Nuclear Physics (ELI-NP).
Tabrizi et al. [physics/0701342] discuss the feasibility of an electron-based crystal undulator (e-CU) by planar channeling of 50 GeV electrons through a periodically bent crystal. We show that their scheme is not feasible. First, their undulator parameter is K >> 1 always, which destroys photon interference. Second, they overestimate the electron dechanneling length in e-CU by an order of magnitude, which shortens the number N of e-CU periods from 5-15 (as they hope) to just 1-2. This kills their e-CU concept again. We made first simulation of electron channeling in undulated crystal and conclude that an electron-based crystal wiggler is feasible with wiggler strength K=10 and number of periods N=2.
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