The Jefferson Lab 1 Kw Ir Fel 1 System Requirements 2 System Configuration 2.1 Overview

The Jefferson Lab (JLab) IR Demo Free Electron Laser (FEL) has completed commissioning and is initiating user service. The FEL — a high repetition rate, low extraction efficiency wiggler-driven optical cavity resona-tor — produces over 1 kW of tuneable light on intervals in a 3–6 µm wavelength range. It is driven by a 35–48 MeV, 5 mA superconducting RF (SRF) based energy-recovering continuous wave (CW) electron linac. The driver accelerator meets requirements imposed by low energy, high current, and a demand for stringent beam control at the wiggler and during energy recovery. These constraints are driven by the need for six-dimensional phase space management, the existence of deleterious collective phenomena (space charge, wake-fields, beam break-up, and coherent synchrotron radiation), and interactions between the FEL and the accelerator RF. We will detail the system design, relate commissioning highlights, and discuss present performance. The FEL is a wiggler-driven laser with an 8 m long optical cavity resonator [1]. It uses moderate gain and output coupling, low extraction efficiency and micro-pulse energy, and high repetition rate to avoid high single bunch charge while producing high average power. This paradigm leads to the use of SRF technology, allowing CW operation, and motivates use of energy recovery to alleviate RF system demands. The system architecture thus imposes two requirements on the driver accelerator: • delivery to the wiggler of an electron beam with properties suitable for the FEL interaction, and • recovery of the drive beam energy after the FEL. The first requirement reflects the needs of the FEL system itself. Optimized beam parameters are given in Table 1. We note the nominal FEL extraction efficiency produced with these parameters is >½%. The micropulse energy is modest; high output power is achieved through the use of very high repetition rate (20 th subharmonic of the RF fundamental) and CW operation. The energy recovery requirement reduces RF system demands (both installed klystron power and RF window tolerances), cost, and radiation effects by decelerating the beam after the FEL so as to drive the RF cavities. As the full energy spread after the wiggler exceeds 5%, this creates a need for a large acceptance transport system. The above system requirements couple to many phenomena and constraints. Phase space requirements at the FEL demand transverse matching and longitudinal phase space management during acceleration and transport to the wiggler. Similarly, the machine must provide adequate transverse beam size control …