First Lasing of the Jefferson Lab IR Demo FEL Page: 2 of 10

Introduction

At the 18th International FEL in Rome we introduced the design of a free-electron
laser (FEL) driven by a recirculating, energy-recovered continuous electron beam
accelerator to demonstrate scalability to higher powers for industrial applications [1].
The design of this accelerator is such that the full current of 5 mA can only be produced
with energy recovery. Without energy recovery only 1.1 mA of current can be produced
due to limits on available rf power. Since the repetition rate of the accelerator can be
reduced by a factor of two, and since the electron beam quality improves as the charge is
lowered, the gain is not very much lower than that at full beam current when the laser is
operated with 1.1 mA. This makes it possible to lase in "first light" mode without energy
recovery and optimize the laser before energy recovery is attempted. Simulations of the
beam using PARMELA indicated that we should be able to achieve an emittance of 5 mm-
mrad with an energy spread of 50 keV and a bunch length of 370 fsec (all quantities rms).
Using these numbers the predicted gain is well over 100% indicating a good margin for
lasing. This paper is a description of the first lasing process and some of the interesting
results we have gained since achieving first light.
Driver Accelerator
The driver accelerator is shown in figure 1. The design and measured performance
are shown in Table 1. Microbunches with an rms bunch length of 20 psec are produced in
a DC photocathode gun and accelerated to 350 keV [2]. The bunches are compressed by
a copper buncher cavity operating at the fundamental accelerating frequency of 1.497
GHz. They are then injected into a high performance superconducting rf (SRF) cavity
pair operating at a mean gradient of 10 MV/m. The output beam from this is transported
through an achromat, injected into an eight cavity SRF cryomodule, and accelerated up to
-38 MeV. The electron beam is then bent around the two optical cavity mirrors passing
through the wiggler along the way and captured in a water cooled copper dump.
The program schedule was very aggressive with first-light hardware installed 15
months after first funding. Commissioning of the injector was started with the wiggler
removed so that the radiation dose to the NdFeB permanent magnets could be minimized.
The design current of 1.1 mA cw was achieved 21 months after first funding. The beam
quality at 60 pC (as shown in Table 1) was found to be more than sufficient for lasing. In
fact, the bunch length and vertical emittance are very close to the values predicted by
PARMELA. The horizontal emittance is larger than PARMELA either due to wakefield

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Benson, Stephen; Biallas, George; Bohn, Court; Douglas, David; Dylla, H.F.; Evans, R. et al. First Lasing of the Jefferson Lab IR Demo FEL, article, May 1, 1999; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc742655/m1/2/ocr/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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