This project explored scenarios in which interaction of particles with FF pulses provides an advantage to stationary focus gaussian (SFG) setups. First, we have shown the ability of L=1 orbital angular momentum FF pulses to trap the electrons or positrons close to the beam axis for prolonged interaction lengths (see Fig. 2) using much less laser energy than other approaches. This can be beneficial for future laser-based electron or positron accelerators, by preventing the spreading of particle beams between acceleration stages. An experimental demonstration of this effect is required for further development.
By investigating collective effects in a classical interaction of electrons with laser pulses, we have shown that FF pulses can substantially reduce the energy required to produce coherent, narrowband, high-power X-rays in a laser-driven free-electron laser. In contrast to the static focal point of a conventional laser pulse, the dynamic focal point of a flying-focus pulse travels with the electron beam, ensuring a uniform undulator over the entire interaction length (Fig. 3). Coherent X-rays allow for imaging of molecules, cells, high-energy-density materials, and structural defects, absorption spectroscopy and Thomson scattering to probe the structure of matter, and the exploration of QED processes. If realized, our FF-based setup provides an extremely compact source with much lower laser pulse energy requirements than other optical undulator proposals.
In the interaction of high energy photons with vacuum polarized by a laser pulse the interaction time can be prolonged by many orders of magnitude by employing FF pulses. Figure 1 shows that while in the FF pulse case (a) the blue probe x-ray beam co-propagates with the peak intensity of the FF pulse (red) for the distance of many Rayleigh ranges, in the SFG case (b) the x-ray beam quickly passes through the focal region. We have demonstrated the possibility of measuring vacuum birefringence in low-power flying focus pulses by letting the effect on the probe x-ray beam phase shift accumulate. There is a concerted push to provide the all-optical proof of vacuum birefringence at XFEL facilities including European XFEL. Our proposal uses more accessible laser systems by lowering the requirements on the laser peak power from petawatts to terawatts and thus could prove viable for experimental implementation in the near future.
Similar tradeoff can be achieved in the interaction of a high energy laser pulse with ultra-relativistic electrons in which the electrons lose energy through radiation. With fixed-energy FF pulses we can again substantially lower the laser power and intensity in exchange for extending the interaction time. Moreover, we demonstrated that the electron energy loss in the quantum regime, and the photon yield in general, scale more favorably with the interaction time than the laser intensity. Therefore higher energy loss and photon yields can be achieved by employing a long FF pulse, rather than an equal energy short SFG pulse. In this way FF pulses create a more controllable environment with simpler diagnostics of the interaction field strength, while simultaneously providing an extremely high-brilliance source of gamma radiation. Planned future experiments will demonstrate this method's usefulness for producing gamma rays applicable in cancer treatment, nuclear waste disposal, and medical isotope production.
