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Operation of the JLab IR Upgrade FEL at CW powers in excess of 10 kW requires sustained production of high electron beam powers by the driver ERL. This in turn demands attention to numerous issues and effects, including: cathode lifetime; control of beamline and RF system vacuum during high current operation; longitudinal space charge; longitudinal and transverse matching of irregular/large volume phase space distributions; halo management; management of remnant dispersive effects; resistive wall, wake-field, and RF heating of beam vacuum chambers; the beam break up instability; the impact of coherent synchrotron radiation (both on beam quality and the performance of laser optics); magnetic component stability and reproducibility; and RF stability and reproducibility. We discuss our experience with these issues and describe the modus vivendi that has evolved during prolonged high current, high power beam and laser operation.

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A gain-switched diode laser was used to injection modelock a Ti-sapphire laser. The pulse repetition rate of the modelocked Ti-sapphire laser was varied by setting the diode laser pulse repetition rate equal to different multiples of the Ti-sapphire laser cavity fundamental frequency. Pulse repetition rates from 223 MHz (fundamental) to 1.56 GHz (seventh harmonic) were observed. No intracavity modelocking device was required and the Ti-sapphire laser cavity length was not changed. Maximum average output power at 854 nm was 700 mW for all pulse repetition rates when pumped with 6 W from an argon-ion laser. Pulsewidths ranged from 21 to 39 ps (FWHM). Phase noise measurements indicate that timing jitter was 2.5 ps at a pulse repetition rate of 223 MHz. Pulse-to-pulse amplitude fluctuations however, were significant and suggest that active cavity length stabilization may be required for reliable photoinjector applications.1998 Elsevier Science B.V. All rights reserved.At many electron accelerators throughout the world, there is great interest in developing high duty factor, high average power ('1 W) laser systems with picosecond pulsewidths and repetition rates synchronized to the accelerating cavity RF frequency [1]. Such a laser system can be used to extract electrons from a high-voltage semiconductor-photocathode electron gun during the portion of the RF cycle when electrons are accelerated into the machine. In this way, all of the photoemitted electrons are accelerated and delivered to the ultimate user of the beam; none are wasted as is the case when a DC laser light source is used to create