Echo-Enabled Harmonic Generation (EEHG)
Echo-Enabled Harmonic Generation (EEHG) is a laser assisted electron beam manipulation scheme designed to produce high-harmonics in the e-beam density distribution for the generation of short-wavelength radiation [1,2]. As illustrated, a laser first modulates the electron beam energy. A strong chicane then folds the phase space, turning the initial sinusoidal modulation into striated energy bands with a fine-scale, highly non-linear structure. After modulation by the second laser, the final chicane then stands these bands upright to produce a density modulation at a high-harmonic of the laser frequencies. A major advantage of EEHG is the supreme upconversion efficiency for a small increase in the slice energy spread.
The EEHG concept was first demonstrated experimentally at NLCTA in 2010 (See experimental details here
), where the third and fourth harmonics of the 1600 nm laser were observed in the coherent radiation spectrum (3). These harmonics were distinguished from those generated by each laser individually (high gain harmonic generation - HGHG) by chirping the electron beam distribution. The chirp spectrally separates the EEHG and HGHG signals, and we were able to confirm that the harmonics were generated through the interplay of the two lasers via the echo effect.
In 2011, we experimentally observed the 7th harmonic produced by EEHG
(4). Further, it was verified that the harmonic appeared with laser modulation amplitudes only 2-3 times larger than the slice energy spread. In this regime high-harmonics of HGHG are strongly suppressed, and the small energy spread of the beam is preserved. Prepared in this way, the beam can then serve as a high-quality seed in a downstream free-electron laser (FEL) for the emission of fully coherent light at short wavelengths.
Recently, we experimentally observed the 15th EEHG harmonic at 160 nm
(5). Harmonics at this level are typically inaccessible to HGHG techniques, because the required energy modulation is too large to support high-gain FEL amplification. From these studies, we find that the EEHG signal is highly robust to distortions in the electron beam phase space, such as linear and quadratic energy-time correlations. Both the signal central wavelength and bandwidth are therefore more stable to imperfections in the e-beam, which may be important for using EEHG as an external seeding method on beams that suffer from CSR and microbunching instabilities.