Design of the optical system for the gamma factory proof of principle experiment at the CERN Super Proton Synchrotron

The Gamma Factory proof of principle experiment aims at colliding laser pulses with ultrarelativistic partially stripped ion beams at the CERN Super Proton Synchrotron. Its goals include the first demonstration of fast cooling of ultrarelativistic ion beams and opening up many possibilities for new physics measurements in various domains from atomic physics to particle physics. A high average-power, pulsed laser system delivering approximately 200 kW needs to be implemented for this aim. This is possible thanks to state-of-the-art optical systems that recently demonstrated similar performances in the laboratory environment. Challenges lie in the implementation of this kind of laser system in the harsh environment of hadronic machines including their robust and fully remote operation. The design of this laser system, involving a high quality factor enhancement cavity, is drawn and described in this article. Mitigation procedures are proposed to overcome limitations imposed by the occurrence of degenerate high-order mode at high average power in such optical resonators. We show that the operation at average power above 200 kW is feasible.

Figure 1

Model of the location of the Gamma Factory POP experiment in the SPS showing the TI18 tunnel on the top, the SPS ring in the middle, and the TT60 extraction line on the bottom with the partially stripped ion beam traveling from the right to the left. The model shows the foreseen location of the Fabry-Perot cavity in the SPS ring (green dashed box), the location of the optical room (red dashed box), and the laser transport line in between the two.

Figure 2

Layout (top), betatron and dispersion functions (middle), and horizontal and vertical beam sizes as a function of the coordinate of the SPS ring (bottom). The beam goes from left to right. The layout clearly exhibits from left to right, a dipole magnet, in green the location of the Fabry-Perot cavity described in this paper, a corrector, an octupole, a BPM, another quadrupole, and a dipole. The betatron and dispersion functions for the horizontal and vertical directions are shown in red and blue, respectively. Note that there is no vertical dispersion while the horizontal dispersion is about 2 m at the interaction point between ions and the laser, approximately identified with the green dashed line. The vertical gray dashed line represents the location of an x-ray detector that will be installed to measure the scattered photons. The rms beam size is depicted by the plain double red (horizontal) and blue (vertical line) of the bottom plots. Black boxes represent the aperture of the magnetic elements. The dashed red and blue lines represent the stay-clear regions described in the text.

Figure 3

Top: power spectral density of phase noise measured from self heterodyne beating for two cw reference oscillators and heterodyne beating with the HighQ OEwaves cw reference oscillator for three commercial femtosecond mode-lock oscillators. The phase noise PSD is shown for a carrier frequency of 40 MHz that corresponds to the repetition frequency required for the Gamma Factory POP experiment.

Figure 4

Top: mechanical frame of the FPC including the vacuum chambers, the pumping system, and the granite support. The holes in the granite allow placing foreseen laser beam diagnostics within the frame itself. These holes will be closed with interlocked doors for usual operation to ensure the following laser safety regulation. The schematic shows the transport system based on air cushion systems. Bottom: inside of the vacuum chamber showing the shield of the electron beam (pink and brown) partly supported by the granite girder. The cavity and injection mirror are supported by cantilevered support mechanically decoupled from the vacuum chamber but supported by the granite frame. The thin red line represents the laser path. Mirrors outside the main vacuum chamber are not represented to ease the reading of the figure.

Figure 5

The transverse size at $1/e^2$ of the intensity distribution of the laser beam inside the optical cavity. The position $z=0(z\simeq 3.75\mathrm{m})$ corresponds to that of the plane (spherical) mirror. The geometrical center of the FPC is represented by the vertical dashed line.

Figure 6

The value of power at which degeneracy occurs as a function of the radius of curvature of the output coupler for mode orders from $n=1$ to 50. Each line corresponds to a given degenerate high-order mode. The presence of degenerate high-order mode is nearly dense. Only narrow ranges of mirror radius of curvature exhibit the possibility to reach several tens of kW of average intracavity power without the occurrence of mode degeneracy.

Figure 7

The value of power at which degeneracy occurs as a function of the radius of curvature of the output coupler for mode orders from $m+n=1$ to 18.