The Munich Compact Light Source: Flux Doubling and Source Position Stabilization At a Compact Inverse-Compton Synchrotron X-ray Source.

Abstract

Brilliant compact X-ray/XUV sources are mainly based on inverse-Compton scattering (ICS) or highharmonic generation (HHG) and enable the transfer of techniques developed at synchrotrons into a laboratory environment. Highly coherent XUV from HHG sources is used for, e.g., coherent diffractive imaging [1] and time-resolved photoelectron spectroscopy [2], while ICS sources are employed for, e.g., phase-contrast [3,4], dynamic imaging [5] and micro-beam radiation therapy [6].The Munich Compact Light Source (MuCLS) [7] belongs to the latter group and consists of an electron storage ring and a passive bow-tie laser resonator. The circulation direction in the laser resonator is opposite to the one in the electron storage ring generating a head-on collision of relativistic electrons with laser photons. This process of inverse-Compton scattering creates quasi-monochromatic X-ray radiation emitted into a small cone angle of ~4 mrad with a cut-off energy at E cut−off = 4γ E laser . Here, γ is the Lorentz factor of the electrons, and E laser the energy of a laser photon. At the MuCLS, the electron energy is varied in order to adjust the X-ray energy between 15 keV and 35 keV. Depending on the Xray energy, an X-ray flux up to 1.5⋅10 10 ph/s was available on a daily basis after commissioning of the source in 2015. Corresponding source characterization is given in [8].With this source we could demonstrate advantages in biomedical imaging arising from the brilliance of the MuCLS [3,4,6]. Nevertheless, these studies also motivated us to pursue a further increase of X-ray flux, especially for time-sequence studies and micro-beam radiation therapy. Accordingly, we installed a new laser system which more than doubled the available laser power from 14 W to 30 W and thus increases the power stored in the laser resonator from ~120 kW to ~300 kW. This value is directly proportional to the number of laser photons, i.e. number of scatterers. As a result, the X-ray flux at 35 keV is promoted above 3.0 ⋅10 10 ph/s, more than doubling the initial value. Figure 1 displays the corresponding X-ray source parameters over a period of 10 min. They were determined in the same way as in [8]. After initial optimization of the overlap of the laser and electron beam, the X-ray source position remains stable over short periods, but starts drifting on longer time scales typical for experiments, as depicted in Figure 2. This is due to relative shifts of electron and laser beam and causes a decline of the X-ray flux. The reason for the occurring shifts is attributed to temporal thermal gradients slightly affecting the laser resonator optics and shifting the orbit of the optical beam. In Figure 2, we

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“…The produced X-rays are emitted into an opening angle of 4 mrad, the X-ray beam is partially coherent and quasi-monochromatic. For an energy of 35 keV, the flux of the MuCLS—after a laser upgrade in spring 2017—is for example 3.0 ⋅ 10 10 photons per second with a source size of less than 50 x 50 μ m 2 [27]. In the experimental hutch, 16 m away from the interaction point, the beam has an elliptic shape of 62 mm x 74 mm (horizontal x vertical) [24].…”