FEL/Duke University, Durham, North Carolina, USA
PKU/IHIP, Beijing, People's Republic of China
Recently, the Duke optical klystron FEL (OK-5 FEL) has been commissioned to produce lasing in the VUV region (191 - 193 nm), overcoming substantial laser cavity loss due to low reflectivity of the VUV FEL mirrors. With two OK-5 FEL wigglers separated by more than 20 meters in a non-optimal configuration, an adequate FEL gain was realized by operating the Duke storage ring with a high single-bunch current (30 to 50 mA). This VUV FEL has enabled us to produce circularly polarized Compton gamma-ray beams in the 70 to 100 MeV region at the High Intensity Gamma-ray Source (HIGS), Duke University. This high energy gamma-ray beam capability will create new opportunities for both fundamental and applied research at HIGS. In this work, we report our experience of VUV FEL lasing with a high single-bunch current and first production of gamma-ray beams in the 70 to 100 MeV region.
PKU/IHIP, Beijing, People's Republic of China
FEL/Duke University, Durham, North Carolina, USA
A well-calibrated spectrometer is critical for measuring the real spectra of spontaneous radiation of an electron beam in undulators (i.e. undulator radiation), which is important for FEL research. A calibration method of spectrometers based upon the known undulator radiation spectrum has been developed at Duke FEL Laboratory (DFELL). It has been used to provide a precise calibration for spectrometers from infrared (IR) to ultraviolet (UV). This calibration method is expected to be useful for the calibration of spectrometers working in the extreme ultraviolet (EUV) and X-ray region. In this work, we present the details of the calibration method and illustrate the usefulness of the method using a portable spectrometer in the visible region as an example.
FEL/Duke University, Durham, North Carolina, USA
PKU/IHIP, Beijing, People's Republic of China
The 53.73 meters long free-electron laser (FEL) resonator at Duke University consists of two concave mirrors with the similar radius of curvature. The downstream mirror receives not only the fundamental but also higher order harmonic radiation (typically in the UV and VUV range) emitted by relativistic electrons in the magnetic field of wigglers. The power load of wiggler radiation on this mirror can thermally deform and permanently damage the multi-layer coating of the mirror, therefore, limiting the maximum power of the FEL operation and reducing the mirror lifetime. To mitigate these problems, a water-cooled aperture system has been installed inside the FEL resonator. This aperture system has been used to prevent most of off-axis helical wiggler radiation from reaching the downstream FEL mirror. It has also been used to manipulate the FEL gain process by increasing the FEL beam diffraction loss inside the resonator. In principle, this aperture system can be used as an independent FEL gain control device for FEL operation. This paper reports our preliminary study of the FEL operation using the in-cavity apertures to manipulate the FEL gain process.