# Synchronised TeraHertz Radiation and Soft X-rays Produced in a FEL Oscillator
## 1 Introduction  
Fundamental and applied studies at the science frontiers need tuneable, brilliant and coherent radiation pulses. The synchronised radiation of two different wavelengths is required for testing phenomena developing on different time-scales or for pump and probe experiments, where one of the two pulses excites microscopic processes and the other one detects them. The combination of strong THz excitation sources with the most advanced X-ray light sources enables the study of long-range order at atomic length scales and ultrafast time-scales [1, 2]. The expectation of experiments of this kind is so promising that almost all the laboratories allocating the most brilliant X-ray sources, namely synchrotrons and free-electron lasers (FELs), are also endowed with THz sources to be coupled with the X-rays [3, 4]. However, when these experiments involve Synchrotrons and FELs as sources of X-rays, they can be exclusively carried out in a poorly sustainable way in few huge laboratories, limiting the widespread diffusion of this research technique. Regarding the generation of THz radiation, considerable efforts are currently being made to develop novel high-power radiation sources able to compensate for the scarce availability in this frequency range. Much interest has been focused on FELs, because they are widely tuneable in the THz range and deliver high-quality pulses that satisfy the requirements of energy stability, polarisation, spectral and spatial distribution posed by the most advanced applications. Far-infrared and THz FELs mainly operate as oscillators, i.e., they are equipped with resonators confined by mirrors. The main advantages of this operational mode, with respect to other FEL configurations, are compactness, relaxed requirements of the electron bunch quality and the fact that oscillators are suitable for super conducting (SC) linacs, allowing the generation of powerful quasi-cw light. Such a class of FELs includes UCSB-FEL [5], CLIO [6], FELX [7], FELBE [8], NovoFEL [9], KAERI-FEL [10] and ISIR FEL [11]. Among them, FELBE, which relies on a superconducting RF linac operated in the cw mode,and UCSB-FEL, based on an electrostatic [PERSON] accelerator with energy recovery operated in a long pulse mode, are characterised by the highest power yield. CLIO, FELIX, and ISIR FEL generate maximum micropulse energies between 100 and 40 \(\upmu\)J in the mid-infrared region, while their energies in the THz region decrease to values between 20 and 5 \(\upmu\)J. NovoFEL consists of a series of 180 MHz normal conducting RF cavities and an energy recovery system operating in the cw mode that can generate an average power of 400 W and a micropulse energy of 50 \(\upmu\)J. Various new projects [12; 13; 14] or studies involving THz FELs have been proposed [15; 16]. The dual source (THz plus X-rays) that we expose in this paper exploits the fact that the THz radiation generated by the passage of successive electron bunches in an FEL undulator, driven by a super-conducting ERL, propagates at each round trip inside the cavity, first forward towards the front mirror and then backward to the rear mirror. After being reflected by a tilted front mirror, the radiation is sent obliquely off-axis with respect to the electron beam train and hits a successive electron bunch in a condition suitable for Thomson/Compton back-scattering. THz FEL intracavity pulses with mJ-class energy at 15-50 microns of wavelength, driven by 20-100 MeV energy electron bunches, can deliver up to few \(10^{5}\) soft X-ray photons per shot by Compton back-scattering at a rate of 10-100 MHz and synchronised with the THz radiation. The total of \(10^{12}\)-\(10^{13}\) X photons generated per second can be useful in many imaging fields (see Ref. [17]). Even taking into account size and cost of the cryogenic plant, this source is more compact (hundreds instead of tens of thousands square meters) and less expensive (less than one hundredth of the costs) than synchrotrons and soft X-ray FELs.