The PEGASUS Waveguide FEL
Motivation
In an IR Free-Electron Laser, driven by a high-brightness beam, diffraction is the limiting effect for the FEL performance. The radiation size cannot be confined to the size of the electron beam and escapes transversely. As a result the field strength of the radiation field at the electron beam location is reduced and the FEL amplification is inhibited. In the equilibrium state of gain guiding the radiation beam size is with 1 mm rms about 10 times larger than the electron beam size for the PEGASUS beam parameters.
Waveguides for the PEGASUS FEL
The radiation wavelength of 12 microns requires a small waveguide size to become efficient. Normal waveguides out of metal do not work at this size, because the transmission losses are too high. The new design of a Hollow Glass Waveguide has overcome this obstacle. Fig. 1 shows the schematic cross section of the waveguide. A dielectric and a metal layer are deposited on a glass tube. The thickness of the dielectric layer is a couple of hundreds of nanometer to optimize the transmission. The waveguide under consideration has a AgI/Ag layer and it reduces transmission losses to 0.2 dB/m.
Fig.1: Hollow Glass Waveguide
Simulated Performance
The wave guide size can be optimized for best performance, however it has to guarantee a transport of the electron beam without losses. With the given beta function of the undulator and a normalized emittance of 2 mm mrad the rms beam size is 100 microns.
Fig.2: Waveguide FEL performance vs waveguide size
The improvement for smaller waveguides is significant (see Fig.2). The simulations for 5 mm are close to the free space results. Using a waveguide of 1 mm reduced the free-space gain length by 33 percent and brings the amplification close to saturation within the 2 m of undulator length. The performance at 0.8 mm is even better but reduces the tolerances for larger emittances and beam offsets.
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Fig.3: Mode decomposition for aligned and misaligned (250 microns) beam (left and right plot, respectively)
For a diameter of 1 mm, the waveguide is strongly overmoded at 13 microns. The waveguide mode TE01 has the strongest coupling to the electron beam, because it is constant in the x-direction and reasonable wide in the y-direction (half a period of a cosine function). For an aligned beam, only odd modes (n+m=odd) couple to the electron beam. However the next highest mode is the TM21, which contributes only to a 1 percent level to the total radiation field (Fig.3 - left plot).
The FEL performance is insensitive to a mis-steering in the x-direction, because the profile of the fundamental mode is constant in that direction and the coupling remains the same. But the offset allows the coupling to even modes (Fig.3 - right plot) in particular to the next higher modes TM11 and TE11. These modes are sensitive to the betatron oscillation, resulting in a dip in the amplitude, whenever the beam crosses the undulator axis.
Experimental Result
Unfortunately there are no experimental data yet, because the PEGASUS injector is still in its commissioning phase. The initial stage is to calibrate the diagnostic with the performance of the FEL before putting the waveguide in the undulator. The next step is to study the performance of the waveguide in particular with regard of radiation damage and heat problems due to the large peak power load from the FEL. The diagnostic to measure the mode contribution is still in its design phase.