Free-electron lasers have been successfully operated with ultrahigh brightness and excellent transverse coherence at X-ray wavelengths1,2,3,4. One of the next goals for further improvements is full coherence5,6. An obvious approach is to seed the free-electron laser interaction using a conventional source that has good temporal coherence7,8,9,10,11,12. Here, we show the first lasing of a free-electron laser with an echo-enabled harmonic generation scheme11, which shows great promise for producing coherent lasing at short wavelengths, even in the X-ray regime. The experiment was conducted at a test facility13,14 that combines a 135.4 MeV electron accelerator with an amplifier consisting of a series of undulator magnets. Lasing was achieved at the third harmonic of the seed with a gain of ∼100,000 over spontaneous radiation. The measurements show typical exponential growth and excellent spectral characteristics, as well as good intensity stability.
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The authors thank the operation staff in the Shanghai Institute of Applied Physics for excellent support during the SDUV-FEL experiments, as well as the many people at NSRL Hefei, the Accelerator Laboratory of THU, and IHEP Beijing for their help and support. Thanks also go to L.H. Yu, Z. Huang, G. Stupakov and A.W. Chao for useful discussions and their continuous encouragement. The authors acknowledge support from the Chinese Academy of Sciences (grant no. KJCX2-YW-N2), the Ministry of Science and Technology of China (grant no. 2011CB808300) and the National Natural Science Foundation of China (grants nos 10935011 and 11075199).
The authors declare no competing financial interests.
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