Harmonic lasing of X-ray free electron laser： on the way to smaller and cheaper

By utilizing higher harmonics of undulator radiation, harmonic lasing is helpful in the development of compact X-ray free electron lasers （FELs）, i.e. reducing their cost and size. Harmonic lasing of FELs has been experimentMly demonstrated in the low-gain FEL oscillators from terahertz, infrared to ultraviolet spectral range. Based on the current status and future directions of short-wavelength FELs worldwide, this paper reviews the progress on harmonic lasing of X-ray FELs, mainly concentrating on the recently proposed harmonic lasing of X-ray FEL oscillators and further ideas on harmonic lasing of single pass X-ray FEL amplifiers.