Use of inverse tapering to optimize efficiency and suppress energyspread in an RF-linac free-electron laser oscillator

We have studied the operation of tapered undulator free-electron lasers using a realistic numerical model which accurately accounts for short-pulse effects, mode pulling, and coupled electron-optical beam instabilities. Our simulations are based on the Maxwell-Lorentz equations of motion, incorporating realistic optical resonator modes and electron density fluctuations, and accurately track the phase and energy of the electrons throughout their entire interaction with the optical pulse. The studies assume a 2-m taperable undulator with a normalized vector potential of roughly unity, driven by an electron beam from either a thermionic or photocathode microwave gun. Inverse tapering was found to provide greater extraction efficiency and optical power than conventional tapering in moderate gain systems using thermionic injector technology, and yielded over four times the extraction efficiency of an untapered undulator with minimal effect on the energy spread of the electron beam. In contrast, little improvement in efficiency or power output was observed using a photocathode injector due to loss of coherence at high gain. The remarkable spectral stability, laser power output, and reduced energy spread achievable using inverse tapering in moderate gain systems are discussed with respect to applications in remote sensing and spectroscopy     

Emissivity of Lanthanum Hexaboride Thermionic Electron Gun Cathode

The emissivity of the thermionic electron gun cathode material lanthanum hexaboride (LaB \(_6\) ) at an operating temperature of 1622 K has been measured for wavelengths from 550 nm to 2400 nm. The emissivity is calculated from scanning monochromator spectral measurements calibrated with a tungsten lamp and the published emissivity of 0.82 at 1600 K for LaB \(_6\) at 650 nm. The results show a higher emissivity at shorter wavelengths (up to 0.86 at 729 nm) and a lower emissivity in the near-IR to mid-IR (down to 0.41 at 2146 nm). These measurements were motivated by the need to make fast, accurate optical measurements of the temperature of the LaB \(_6\) cathode in our thermionic microwave gun, and to select the wavelength most readily absorbed by the cathode for manipulation of its surface temperature as a means of counteracting back-bombardment heating.     

The Michelson resonator free-electron laser. II. Supermodestructure and mirror detuning effects

For pt.I, see ibid., vol.29, no.2, p.452-64 (1993). Conventional mode-locked laser theory is applied to the longitudinal mode structure in an RF linac-driven Michelson resonator free-electron laser (FEL). A greatly simplified derivation of the small-signal small-gain FEL coupled mode equations is obtained. These equations are solved numerically in the frequency domain to study the supermode evolution in the presence of mode dependent cavity losses. The results are compared with simulations of the detuned Michelson resonator FEL using a pulse propagation code based on the Maxwell-Lorentz equations of motion. Increasing the interferometer detuning broadens and shifts the cavity detuning curve, narrows the supermode spectrum, and decreases the hypermode decay rates. The practical consequences of each of these effects are discussed. A simple theory describing the dependence of the hypermode decay rates on interferometer detuning, in which the decay rates are abruptly decreased beyond a critical detuning that depends primarily on the slippage length, is outlined     

The Michelson resonator free-electron laser. I. Passive modestructure and mode decay

A general analysis of the evolution of phase locking in an RF linac-driven Michelson resonator free-electron laser (FEL) is presented. By providing a delay of one RF period in the secondary arm of the interferometer, successive optical pulses can be coupled at the beamsplitter, so that they build up from noise with a definite phase relationship. Phase locking is described in terms of longitudinal mode decay by extracting the mode losses directly from the passive frequency response of the resonator. The analyses predict significant mode structure simplification in microsecond macropulses, so that high-resolution spectroscopy is feasible on RF linac-driven FELs. Simulations of the perfectly tuned Michelson resonator FEL are described from spontaneous radiation to saturation, using a one-dimensional pulse propagation code. Excellent agreement with the analytical results in the small-signal regime is demonstrated     

Birefringent beam splitter for intracavity mode selection in high-power multimirror lasers

The design and application of an uncoated sapphire plate that acts as both the beam splitter and the output coupler of an interferometric laser resonator are described. Output coupling is provided at one of the surfaces by p-polarized (TM) reflection near the Brewster angle, and axial-mode selection is enforced at the other surface by s-polarized (TE) reflection at the same angle of incidence. The design is discussed in the context of the phase-locked, rf linac free-electron laser, in which the coupling of adjacent optical pulses at the beam splitter induces temporal phase coherence among all the pulses in the output beam; this coherence is manifested in the frequency domain as a reduction in the number of axial modes per rf frequency interval. The Michelson and Fox-Smith resonator designs are compared, and applications to cavity dumping are discussed.