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D Acronyms and Glossary amplifier In the case of the FEL amplifier, there is no optical resonator; a seed laser sends optical pulses synchronized to overlap the electron pulses as they enter the undulator to ensure longitudinal coherence. ANL Argonne National Laboratory BAA Broad Agency Announcement BBU beam breakup beam dump located at the end of the electron trajectory, its purpose is to stop the electron beam after it has been decelerated by the energy recovery linac BINP Budker Institute of Nuclear Physics BNL Brookhaven National Laboratory booster high-current, non-energy-recovered linac section, which boosts the energy of the electron gun for acceleration by the ERL cathode robustness A robust cathode is one that operates without degradation of quantum efficiency for an extended (cathode lifetime) time in an electron gun. The quantum efficiency of the cathode can be degraded either through adsorption of foreign materials onto the surface or through desorption of cathode materials. Cathodes with higher quantum efficiency tend to degrade more quickly than those with lower quantum efficiency. The quality of the vacuum in the gun is critical to cathode robustness. Electrical breakdown (arcing) can lead to poor vacuum quality and damage the surface of the cathodes. 50

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APPENDIX D 51 COIL Chemical Oxygen Iodine Laser; operates at 1.315 µm cryomodule a cryostat containing accelerating cavities and ancillary equipment such as tuners, couplers, and HOM loads CSR coherent synchrotron radiation; coherent long-wavelength emission from the beam end that can cause emittance growth cw continuous wave; an electromagnetic wave of constant amplitude and frequency DC HV gun electron gun that relies on a direct current (DC) and high voltage (HV) applied across plates as the accelerating gradient for the electrons extracted from the cathode surface; a typical accelerating voltage is 300-500 kV over about 12-14 cm until the gun exit DF deuterium fluoride; these lasers operate at a wavelength over a series of lines from 3.6 µm to 3.9 µm DOE Department of Energy emittance measure of beam quality that is related to the product of beam divergence and spot size ERL energy recovery linac FEL free-electron laser field emission the emission of electrons from the solid-state surface caused by applying high electric fields perpendicular to the surface FWHM full width at half maximum FPC fundamental power coupler “generation” The synchrotron radiation sources of the past and present can be defined as follows: nomenclature • First-generation machines are electron synchrotrons and storage rings that were built for other purposes—for example, high-energy and nuclear physics—but whose bending magnet radiation was parasitically used by synchrotron radiation “users.” This radiation covered many wavelength regimes due to the nature of the bending magnet emission. In addition, the machines produced rather large photon source sizes as the electron beam emit- tance was large and neither intended for nor ideal for synchrotron radiation applications. • Second-generation machines are machines dedicated for synchrotron radiation users that employ bending magnets as the primary source of synchrotron radiation. The beam emit- tances were designed by the machine architects to be smaller in order to provide users with a smaller source size and greater brilliance. • Third-generation machines are also dedicated for synchrotron radiation users and were designed to accommodate many so-called insertion device magnets, such as undulator and

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52 SCIENTIFIC ASSESSMENT OF HIGH-POWER FREE-ELECTRON LASER TECHNOLOGY wiggler magnets. Undulator magnets generate narrow spectral lines, and this enhances the overall photon brilliance. • Next-generation light sources involve an optical gain mechanism, with the goal of trans- verse and longitudinal optical coherence such as in a free-electron laser. halo “spreading” of the beam in linacs; it is a consequence of filamentation caused by nonlinear and time-dependent forces, and it increases the risk of beam losses HEL high-energy laser HGHG high-gain harmonic generation HOM higher-order mode; a cavity mode in the accelerator other than the desired acceleration mode IBSD ion-beam sputtered deposition JAERI Japan Atomic Energy Research Institute JLab Thomas Jefferson National Accelerator Facility LANL Los Alamos National Laboratory LCLS Linac Coherent Light Source (at the Stanford Linear Accelerator Center) LEDA Low Energy Demonstrator Accelerator (at LANL) linac linear accelerator; an electrical device for the acceleration of subatomic particles such as electrons merger electron beam optical device composed of magnet beam optical elements; it merges the low- energy beam from the injector with the high-energy beam returning from the FEL, such that both will be directed along the axis of the energy recovery linac’s accelerating and decelerating cavities microphonics mechanical vibrations and helium pressure changes and noise that can change the resonant frequency of FEL cavities up to a few hertz MOPA master oscillator power amplifier NC RF gun electron gun that relies on a radio-frequency (RF) resonant cavity made from a normal- conducting (NC), low-resistance material, such as copper, to form the electric field gradient necessary to accelerate the electrons extracted from the cathode surface NRC National Research Council

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APPENDIX D 53 ONR Office of Naval Research oscillator In the case of the FEL oscillator, the optical pulses are bouncing between the cavity mirrors of an open optical resonator. Care must be taken to synchronize the sequence of electron pulses triggered by the cathode drive laser into the correct phase of the RF cycles, and to overlap with the stored optical pulses at the entrance to an undulator. photoemission emission of electrons from the solid state through the absorption of incident photons prf pulse repetition frequency Q Q factor or value is a dimensionless parameter that compares the frequency at which a system oscillates to the rate at which it dissipates its energy; the higher the Q value, the lower the rate of energy dissipation relative to the oscillation frequency (i.e., the oscillations diminish more slowly). quantum the number of electrons released compared to the number of photons absorbed efficiency RAFEL regenerative amplifier FEL; a hybrid FEL configuration with the combined features of an oscillator and a high-gain amplifier RF radio frequency rms root mean square SASE self-amplified spontaneous emission SDI Strategic Defense Initiative SLAC Stanford Linear Accelerator Center SRF gun superconducting RF gun, an electron gun that relies on an RF resonant cavity made from a superconducting material such as niobium cooled to a few degrees Kelvin, for example, to form the electric field gradient necessary to accelerate the electrons extracted from the cathode surface SSL solid-state laser thermal blooming atmospheric effect encountered by high-energy laser beams, which is the result of the nonlinear interaction of laser radiation with the propagation medium (typically air), which is heated by the absorption of a fraction of the radiation. The amount of energy absorbed depends on the laser wavelength; the term is frequently used to describe any type of self- induced thermal distortion of laser radiation.

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54 SCIENTIFIC ASSESSMENT OF HIGH-POWER FREE-ELECTRON LASER TECHNOLOGY thermionic charge emission process excited or induced by heating a cathode emission TJNAF Thomas Jefferson National Accelerator Facility undulator array of magnets with alternating poles along the beam path in a laser cavity. It produces a (or wiggler) ­ periodic transverse magnetic field causing the electrons in the beam to follow a sinusoidal path. UV ultraviolet VUV vacuum ultraviolet wavelength distance between repeating units of a propagating wave of a given frequency wiggler see undulator above (or undulator)