David Hardy

High power operation of the JLab IR FEL driver accelerator

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.

Xtreme optics: the behavior of cavity optics for the Jefferson Lab Free-Electron Laser

The cavity optics within high power free-electron lasers based on energy-recovering accelerators are subjected to extreme conditions associated with illumination from a broad spectrum of radiation, often at high irradiances. This is especially true for the output coupler, where absorption of radiation by both the mirror substrate and coating places significant design restrictions to properly manage heat load and prevent mirror distortion. Besides the fundamental lasing wavelength, the mirrors are irradiated with light at harmonics of the fundamental, THz radiation generated by the bending magnets downstream of the wiggler, and x-rays produced when the electron beam strikes accelerator diagnostic components (e.g., wire scanners and view screens) or from inadvertent beam loss. The optics must reside within high vacuum at ~ 10-8 Torr and this requirement introduces its own set of complications. This talk discusses the performance of numerous high reflector and output coupler optics assemblies and provides a detailed list of lessons learned gleaned from years of experience operating the Upgrade IR FEL, a 10 kW-class, sub-ps laser with output wavelength from 1 to 6 microns.

Multivideo source image processing for beam profile monitoring system

Some experiments at Jefferson Lab demand tight beam size (≈100 μm) and very low energy spread (<5×10−5). These experiments also require simultaneous and continuous monitoring of these quantities. This paper focuses on the development of the image processing aspects of the beam profile monitoring system. A pipelined image processor, Datacube’s MaxVideo MV200, calculates beam sizes and positions from two beam profile monitors simultaneously at 10 Hz rate. Multiplexing software in the EPICS environment allows a single digitizer to process several input channels at high speed. This system makes the profile monitors usable for tuning the accelerator, as well as delivering critical information to the end stations. This paper discusses the issues related to the daily operational use of the system. The availability and reliability of the monitoring system became acceptable only after the implementation of programs that automatically setup and periodically check the monitors and digitizer. The system permits addit...

Measurement of absorption levels in mid-infrared coated optics

The IR Upgrade FEL at the Thomas Jefferson National Accelerator Facility (JLab) was used to make measurements of the absorption in laser cavity mirrors, both high reflectors and outcouplers. Measurements were made at 10, 6, and 3 um, by determining the temperature rise of the cooling water of the FEL cavity mirrors while operating at high average power, and by using a laser vacuum calorimeter and interpreting the data using the ISO 11551 standard.