Coherence and wavelength limits of free-electron lasers

6 problems

• Highest harmonic reachable by echo-enabled harmonic generation seeding

  Open

  Seeding an FEL with an external laser gives clean, stable pulses, but the seed frequency must be multiplied by large factors, and small random energy kicks to the electrons wash out the high harmonics.

• Observing the quantum regime of a free-electron laser

  Open

  If one photon's recoil is enough to knock an electron out of resonance, the laser behaves like a two-level quantum system with a much narrower spectrum; this regime has not been reached.

• Shortest wavelength at which a self-amplified FEL can saturate

  Open

  Lasing at shorter wavelengths requires more energetic electrons, but these radiate randomly in the magnets, which spreads their energies and stops the lasing.

• Suppressing electron-beam shot noise below the random level

  Partially resolved

  Electrons arrive at random, which seeds noisy x-ray output and harmful instabilities; collective forces can make them more evenly spaced than random, but only at long wavelengths so far.

• What sets the uncorrelated energy spread of high-brightness electron beams

  Partially resolved

  Lasing needs electrons of nearly identical energy, and measured energy spreads in x-ray FEL electron beams have come out larger than models predicted.

• Exponential growth to saturation in a hard x-ray FEL oscillator

  Partially resolved

  An x-ray FEL oscillator traps x-rays between diamond crystal mirrors and amplifies them on every pass, which would give a fully coherent, very narrow-band beam; amplification over a few passes has been seen, but steady growth to a stable, saturated output has not.