Abstract
The visible-infrared self-amplified spontaneous emission amplifier (VISA) free electron laser (FEL) is an experimental device designed to show self-amplified spontaneous emission (SASE) to saturation in the near infrared to visible light energy range. It generates a resonant wavelength output from 800\char21{}600 nm, so that silicon detectors may be used to characterize the optical properties of the FEL radiation. VISA is designed to show how SASE FEL theory corresponds with experiment in this wavelength range, using an electron beam with emittance close to that planned for the future Linear Coherent Light Source at SLAC. VISA comprises a 4 m pure permanent magnet undulator with four 99 cm segments, each of 55 periods, 18 mm long. The undulator has distributed focusing built into it, to reduce the average beta function of the 70\char21{}85 MeV electron beam to about 30 cm. There are four FODO cells per segment. The permanent magnet focusing lattice consists of blocks mounted on either side of the electron beam, in the undulator gap. The most important undulator error parameter for a free electron laser is the trajectory walk-off, or lack of overlap of the photon and electron beams. Using pulsed wire magnet measurements and magnet shimming, we were able to control trajectory walk-off to less than $\ifmmode\pm\else\textpm\fi{}50\ensuremath{\mu}\mathrm{m}$ per field gain length.
Highlights
This work arose from a long-range goal to design and build an x-ray free electron laser (FEL) based on a linac electron source and a single pass undulator that will generate FEL radiation starting from noise
The eventual goal is the Linear Coherent Light Source (LCLS), a 1.5 Å selfamplified spontaneous emission (SASE) FEL based on a 15 GeV linac at SLAC [1]
The single pass design is required for a mirrorless x ray, and it must amplify noise because we have no coherent seed radiation at x-ray wavelengths
Summary
This work arose from a long-range goal to design and build an x-ray free electron laser (FEL) based on a linac electron source and a single pass undulator that will generate FEL radiation starting from noise. A preferable wavelength range would be in the visible or near infrared, where silicon detectors can be used and where Fourier transform methods are available to analyze the time structure of the FEL radiation. It was for this reason that we built the visible-infrared SASE amplifier (VISA) FEL. The TESLA test facility at DESY has achieved SASE lasing at 109 nm, with a gain of about 3000 They used three 4.5 m undulators of 27.3 mm period length with 12 Tm gradient distributed focusing [6]. A comparison of the three designs is presented in [7]
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