A 10 Kw Irfel Design for Jefferson Lab*

Recent work at Jefferson Lab has demonstrated the viability of same-cell energy recovery as a basis for a high average power free-electron laser (FEL) [1]. We are now extending this technique to lase at average powers in excess of 10 kW in the infrared. This upgrade will also produce over 1 kW in the UV and generate high brightness Thomson back-scattered X-rays. The power increase will be achieved by increasing the electron beam energy by a factor of four, and the beam current and the FEL design efficiency by a factor of two. Utilization of a nearconcentric optical cavity is enabled by the use of very low loss state-of-the-art coatings. The FEL will be placed in the return leg of the electron beam transport, giving a machine footprint quite similar to that of the existing 1 kW IR device. Some features of the Upgrade are straightforward extensions of those in the present 1kW design; others break new ground and present new challenges. These will be described. The required electron beam parameters and the laser performance estimates will be summarized. Changes required in the electron beam transport will be outlined and the optical cavity design briefly reviewed. 1 EXPERIENCE GAINED FROM THE INFRARED DEMONSTRATION LASER The Jefferson Lab IR Upgrade FEL is an evolutionary derivative of the JLab IR Demo FEL. It thus retains the approach used in the earlier machine – that of a low peak, high average power wiggler-driven optical cavity resonator with an energy recovering SRF linear accelerator driver operating at high repetition rate. The 10 kW design goal will be achieved via an increase in both drive beam power (doubled current and quadrupled energy) and FEL extraction efficiency (from 0.5% to 1%). The design builds on experience gained with the Infrared Demonstration FEL (IR Demo FEL) now operating at Jefferson Lab. Primary electron beam specifications for the IR Demo and Upgrade are listed in Table 1. Several features of the IR Demo have influenced the design for the Upgrade: • The injector produces a beam with enough brightness at 135 pC to drive the FEL with adequate lifetime (nearly 700 C – over 40 hours – at high current [2]). Initial experience demonstrates that performance at this higher charge is acceptable. • Beam loss at full energy can be held to quite low values. Measured total losses for energies in excess of 15 MeV are less than 0.1% and single point losses inferred from beam line activation and radiation measurements near the wiggler are much less than 100 nA. The apertures in the Upgrade have been enlarged to accommodate larger beam envelope functions and emittance. We anticipate that total losses should be much less than 0.1%. • Operation at the peak of the power vs. cavity length curve is possible with no instabilities. This has been demonstrated in the IR Demo. Because the threshold for the RF/FEL instability with feedback on is on the order of 1 A of beam current, operation at 5 mA of current is quite stable [3]. • The near-concentric cavity design used in the IR Demo can be used at even higher power without power limitations. The power limit is now known to be due to heating-induced aberrations [4], which arise when the power absorbed in a sapphire output coupler exceeds 40 W at 3 μm. For the IR Demo, this occurs at over 4 kW of laser power. FEL efficiency limits the power before this limit is reached. When the laser efficiency vs. bunch charge is Table 1: IR Demo and Upgrade Parameters Parameters Demo Upgrade Achieved (6/2001) energy (MeV) 35-48 80-210 20-48