Potential Applications

DLAs leverage well-established industrial fabrication capabilities and the commercial availability of fibre lasers to reduce cost effectively, while offering significantly higher accelerating gradients and hence a smaller footprint. Such DLAs have many potential applications beyond energy frontier science, including medical therapy, x-ray light sources and high-energy linear colliders.

(1) Medical Therapy

Medically, electron irradiation is used only for skin or superficial treatment due to the energy deposition along the path from the surface, while protons can be used to irradiate tumours deep inside the body. Typical radiotherapy devices operate at electron energies of 6- 20 MeV. The generation of such an electron beam usually requires a RF accelerator with a metre-long accelerating structure. The accelerating length is determined by the accelerating gradient, which is limited to 10-20 MeV/m. Furthermore, a metre-long delivery system is used to localize and position the electron beam onto the tumour. The irradiation volume and dose should be tightly controlled, and exposure is typically from multiple directions to avoid damage to normal organs and structures in the body. In order to improve the treatment quality, a series of technological improvements, from intensity-modulated radiation therapy to image-guided radiation therapy, have been developed [96]. However, existing medical electron radiation therapy still suffers from damage to healthy tissue and high operating costs.

With a DLA and an expected accelerating gradient of several GV/m, an accelerating length in the range of millimetres to centimetres can be expected to generate multi-MeV electron beams. In this case one may envision a multi-MeV electron beam generation device of the size of a pen tip. Such a compact source with micron-scale beams could also remove the complicated beam delivery system. Therefore, a whole accelerator system on a millimetre to centimetre scale would enable a DLA-based fibre endoscope [97] for the purpose of tumour irradiation, as illustrated schematically in Figure 25. Such a DLA-based endoscope offers clinicians new forms of minimally-invasive cancer treatment with increased flexibility of use. It could be placed within a tumour site using standard endoscopic methods, allowing it to deliver the same radiation dose as provided by existing medical electron radiation therapy without damage to surrounding tissue. Due to the small exposed volume, an electron current in the range of nA is sufficient to deliver a single massive dose

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over an exposure time of the order of 1s, which enables efficient intraoperative electron radiation therapy. The manufacturing and operating costs of such radiation therapy are anticipated to be much lower than those for conventional medical electron radiation therapy.

A DLA-based endoscope therefore has the potential to provide more effective and affordable radiation treatment with fewer side effects and better patient quality of life than current practice. Radiation energy and dose can be electronically controlled by the laser source, which enables the surgeon to treat a wide range of tumour morphologies. Such treatment is ideally suited to the precise and remote control afforded by robotic surgery systems.

Figure 25. Schematic of a DLA-based endoscope for the purpose of tumour irradiation [97].

(2) X-ray Light Sources

A dielectric laser-driven microstructure can be used to deflect the electron beam to generate x-ray radiation [98]–[102]. As shown in Figure 26, the period of the vacuum channel grooves, denoted by λp, is chosen such that its projection onto the electron beam

propagation axis equals the laser wavelength, 𝜆_p= 𝜆 cos 𝛼. Due to the tilt of the periodic grooves, the electrons travelling along the vacuum channel centre experience a nonzero deflection field from the laser electromagnetic wave. Such a deflecting field should be synchronous with the electron beam. Through appropriate geometry optimizations, a maximum deflecting field can be generated in the channel centre.

Many stages of dielectric deflecting structures can form an undulator, as shown in Figure 27, which can be specifically designed to generate soft and hard x-ray radiation [98], [99]. This allows for an undulator period which is much bigger than the laser wavelength. Numerical studies have demonstrated the generation of 0.01 nm radiation from an undulator

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period of 300 μm, with deflecting fields exceeding 1.3 GV/m, equivalent to a magnetic field of ∼4 T [99] on highly relativistic (2 GeV) electrons. In addition, such an undulator can be integrated with other DLA components onto an integrated chip, through well-established lithography techniques.

Figure 26. Schematic of a deflecting structure with a groove tilt angle α [99].

Figure 27. Schematic of an undulator consisting of many stages of laser-driven deflecting structures [98].

(3) High-energy Linear Collider

DLAs are a strong potential candidate for a future electron-positron collider due to their high accelerating gradients in the GV/m range. To reach 10 TeV centre-of-mass energies, a linear accelerator (linac) over 100 km in length is required, which would cost tens of billions of dollars to build. However, a DLA linac of length of 10 km operating at an accelerating gradient of 1 GV/m could reach the same energy. Considering well-established nanofabrication technology, the manufacturing cost would be much lower than that of a

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conventional TeV collider. As for the required luminosity of 1036 _cm-2_s-1 _{for such a collider,}

the DLA linac can operate at low bunch charge but at a high repetition rate to avoid bremsstrahlung energy spread at the interaction point [97].

The wall-plug efficiency is another key factor for development of a linear collider. An efficient integrated power coupler is needed to deliver the laser power into various DLA components. From the DLA calculations in Ref. [103], laser power to electron coupling efficiencies of 40% can be achieved. Given that a laser wall-plug efficiency above 30% has been obtained using current solid-state thulium-doped fiber laser technology, an electrical wall-plug efficiency for a DLA-based linac exceeding 10% is possible, which is desirable for a future DLA-based collider [46], [104].