Many scientific disciplines ranging from physics, chemistry and biology to material sciences, geophysics and medical diagnostics need a powerful X-ray source with pulse
Jefferson Laboratory's kW-level infrared free-electron laser utilizes a superconducting accelerator that recovers about 75% of the electron-beam power. In achieving first lasing, the accelerator operated "straight ahead" to deliver 38-MeV, 1.1-mA cw current for lasing near 5 &mgr;m. The waste beam was sent directly to a dump while producing stable operation at up to 311 W. Utilizing the recirculation loop to send the electron beam back to the linac for energy recovery, the machine has now recovered cw average currents up to 5 mA, and has lased cw with up to 1720 W output at 3.1 &mgr;m.
The ability to generate small transverse emittance is perhaps the main limiting factor for the performance of high-gain x-ray free-electron lasers (FELs). Noting that beams from an rf photocathode gun can have energy spread much smaller than required for efficient FEL interaction, we present a method to produce normalized transverse emittance at or below about 0:1 m, which will lead to a significantly shorter length undulator as well as a lower electron beam energy for an x-ray FEL project. The beam manipulation consists of producing an unequal partition of the initially equal emittances into two dissimilar emittances by a flat-beam technique and exchanging the larger transverse emittance with a smaller longitudinal emittance. We study various issues involved in the manipulation. In particular, a new emittance exchange optics we found enables an exact emittance exchange necessary for this scheme.
The generation of a flat electron beam directly from a photoinjector is an attractive alternative to the electron damping ring as envisioned for linear colliders. It also has potential applications to light sources such as the generation of ultra-short x-ray pulses or Smith-Purcell free electron lasers. In this Letter, we report on the experimental generation of a flat-beam with a measured transverse emittance ratio of 100 ± 20.2 for a bunch charge of ∼ 0.5 nC; the smaller measured normalized root-mean-square emittance is ∼ 0.4 µm and is limited by the resolution of our experimental setup. The experimental data, obtained at the Fermilab/NICADD Photoinjector Laboratory, are compared with numerical simulations and the expected scaling laws. 41.75.Fr Flat electron beams, e.g. beams with large transverse emittance ratios, have been proposed in the context of linear colliders and some novel electron-beambased light sources. In the case of a linear e + /e − collider, a flat beam at the interaction point reduces the luminosity disruption caused by beamsstrahlung [1]. In the case of light sources, such as the LUX project proposed at LBL [2], a flat beam with a smaller emittance of 0.3 µm and emittance ratio of 50 is needed to produce x-ray pulses that can be compressed to the order of femtoseconds via standard x-ray pulse compression techniques [3]. Another type of light source recently drawing attention is based on self-amplification of Smith-Purcell radiation [4]. Given one or two planar metal gratings, a flat beam could enhance the interaction between the electrons and metal grating surface, thus reducing the gain length associated with the Smith-Purcell free-electronlaser mechanism [5,6,7].In the proposed International Linear Collider (ILC) the needed flat-beam parameters (emittance ratio of 300) are foreseen to be achieved via radiation cooling in a damping ring [8]. Although the required transverse emittances for the ILC have been demonstrated at the ATF damping ring of KEK [9], ILC puts stringent requirements on the damping ring design, and the cost of the damping ring is a significant portion of the total collider cost. Therefore alternative ways of producing flat beams directly from an electron source have been explored by several groups [10]. In conjunction with the invention of a linear transformation capable of transforming an incoming flat beam into an angular-momentum-dominated (or "magnetized") beam [11], a scheme which inverses this * Electronic address: piot@fnal.gov † Electronic address: yinesun@uchicago.edu; now at Argonne National Laboratory.transformation was proposed to generate a flat beam directly out of a photoinjector [12]. The method consists of generating an magnetized beam by immersing the photocathode in an axial magnetic field. After acceleration, the beam is transformed into a flat beam using three skew quadrupoles [13]. This has been verified experimentally [14,15,16,17], and transverse emittance ratios of 40-50 were reported. Theoretical analysis of the conversion of a magnetized cyl...
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