Compensation of undulator focusing variation at FEL radiation wavelength tuning
Free-electron lasers (FELs) have a key advantage over other types of lasers: the possibility of an operative and smooth tuning of the wavelength of monochromatic radiation. However, this process causes changing of the focusing strength of the undulator and, consequently, requires the matching of the rest of the FEL magnetic system. As a solution to this problem, two approaches were proposed: 1) conservation of the transport matrix of the section that includes the undulator; 2) matching the input and the output Twiss parameters with the optimal ones at the center of the gap that depend on the current of the undulators. In this paper the process of radiation wavelength tuning of the first stage of the Novosibirsk FEL (the radiation wavelength range: 90-220 μm) is studied. The variation of the focusing strength of undulators was compensated by the tuning of the currents of the 8 quadrupoles at the undulators track. Two methods to find the rule for tuning the currents of quadrupoles were used: 1) numerical optimization of the transport matrix; 2) solution of the system of the ordinary differential equations for the quadrupole currents as functions of the undulator current. By these numerical methods some example regimes that enable to tune the radiation wavelength in the range 120-130 μm were found. The experimental verification confirmed the validity of our approach. During the experiments the conservation of the transport matrix was monitored by the response matrix diagnostic. The search of new regimes with the larger tunable radiation wavelength range is in progress now.