Alignment of the optical feedback system of VUV regenerative FEL amplifier at the TESLA test facility at DESY

TO this paper, we describe optical feedback system of VUV Regenerative FEE Amplifier (RAFEL) at the TESLA test facility at DESY. The aim of the RAFEL experiment is to construct fully coherent, tunable VUV radiation source by means of applying narrow-band optical feedback in the VUV SASE FEL operating Currently at DESY. One of the problem of the realization of the RAFEL is severe requirements for the angular stability of the optical elements (about few microradians). This problem has been solved by means of installation of active alignment system with reference laser. Another problem is alignment of optical elements separated by 65 in within complicated experimental conditions connected with aperture limitations (down to 6 min). This problem has been solved in two steps. Preliminary alignment with an accuracy of about 80 murad has been performed with laser alignment system and OTR screens used at the TTF accelerator for electron beam diagnostics. Final alignment has been performed with VUV SASE FEE radiation. Measured feedback coefficient is about 1 percent and is in agreement with the designed value. (C) 2002 Elsevier Science B.V. All rights reserved.