## 2 Review of Efforts toward Attosecond Hard X-ray FEL
The inherent problem in the current enhancement scheme by a CEP-stabilized few-cycle laser is the possibility of the generation of a train of attosecond pulses. When a few-cycle laser interacts with an electron beam, the energy of the electron bunch is modulated to follow the oscillation of the electric field of the laser. When such a modulated electron bunch passes through a wiggler, the microbunching happens every cycle, leading to several sizable current spikes. These spikes strongly radiate in an undulator, resulting in several attosecond pulses or a train of attosecond pulses. The contrast between a major peak and secondary peaks is also poor. To avoid this problem and to obtain a dominant (or isolated) spike, special attention is demanded, either in the selection of the wavelength of laser or in the energy profile of the electron bunch interacting with a laser. As one way to improve this contrast, a two-color approach was proposed, as shown in Figure 4[36]. The electron beam enters two adjacent wiggler magnets, W1 and W2. Each magnet has just one wiggler period. Two co-propagating laser pulses (1200 nm, 0.2 nJ pulse energy and 7.5 fs FWHM; 1600 nm, 0.07 mJ and 10 fs FWHM) enter the wigglers at a small angle. The first laser is focused at the center of the first wiggler, and the second laser is focused at the center of the second wiggler. The phase of the carrier wave of the first laser pulse is adjusted so that when the peak of the envelope of the laser pulse reaches the center of the first wiggler, the electric field is zero. The phase of the carrier wave of the second laser pulse is also adjusted in a similar way. After passing through two consecutive modulation sections, the electron beam enters into a dispersive section for microbunching of the electrons and enhancement of the current peak. The enhancement produced in the central cycle of the two laser pulses is about two-times stronger than that produced during other laser cycles. However, after propagation through a long undulator, this current still emits a train of attosecond X-ray pulses of 250 attoseconds FWHM. Figure 3: (Color online). Double undulator ESASE scheme. **(a)** The layout of the scheme. (**b)** Temporal structure of the radiation pulse after monochromator [34]. The effect of a linear energy chirp on FEL amplification has been studied and a new proposal of using a tapered undulator was made, showing the generation of 200 attoseconds X-ray pulses with a power of 100 GW [37]. The angular modulation of electron's trajectories can also contribute towards an attosecond pulse generation. The modulation of electron's angles by a CEP laser was utilized for the creation of a 115 attoseconds X-ray pulse with a peak power of 100 GW [38]. Moreover, the angular modulation was combined with the energy modulation for the shortening and the improved selection of an attosecond pulse. The manipulation of the energy profile of an electron beam may lead to the generation of an isolated attosecond pulse of \(\sim\)100 attoseconds FWHM at 0.1 nm wavelength [39]. A Compton backscattering scheme can be utilized for the generation of an attosecond \(\gamma\)-ray pulse. Using Compton backscattering with an energy-modulated electron beam, it was shown in simulation that an isolated attosecond gamma-ray pulse at 64 keV with a pulse width of 550 attoseconds and a contrast ratio of 5.1:1 can be generated [40]. The spectral filtering (such as a crystal spectrometer) allows one to obtain an isolated attosecond gamma ray.