A collaboration is underway to develop and demonstrate GW-level power generation in the sub-THz frequency range. Two key components--wakefield structure and electron bunch train--were prepared for the demonstration. A 5-cm long corrugated structure was fabricated using two thin metallic plates with through-holes of different inner diameters. The plates were fabricated by lithography and bonded by high-pressure, high-temperature boding process. The fabricated structure, whose fundamental mode frequency is 0.4 THz, was powered by an electron bunch train with 16 bunches (1 nC per bunch). Efforts are ongoing to achieve results comparable to simulations, targeting a peak power of 3 GW and a maximum deceleration field of 700 MV/m within the bunch. We present the most recent experimental results.

The Advanced Wakefield (AWAKE) experiment is a proof-of-principle accelerator facility at CERN (Geneva, Switzerland). Proton bunches from the CERN Super Proton Synchrotron are used to drive wakefields in 10 metres of laser-ionised rubidium plasma, over which externally injected 19 MeV electrons are accelerated. Run 1 of AWAKE successfully demonstrated the self-modulation of the long proton bunch, and the acceleration of electrons to 2 GeV. Upgrades to the rubidium vapour source during Run 2 have enabled the use of a plasma density step, and variation of the plasma length through the insertion of foils along the source to dump the laser pulse. When placed suitably within the development of self-modulation, the density step is expected to preserve the wakefield amplitude, and therefore accelerating gradient, over longer distances than with uniform plasma. This work presents the first measurements of electron acceleration with a density step, studied as a function of the plasma length.

A ultra-low emittance electron source utilizing laser-cooled neutral gas as the photo-ionization source has been developed in several laboratories in the accelerator community. We have started the development aiming for using the electron source for the injector of a THz accelerator which has a small aperture and requires a high quality beam. We have developed the electron source system, i.e. the laser system for laser-cooling, the vacuum chamber equipped with the acceleration electrodes and necessary viewing ports for laser access, the ionization laser system, and the diagnostic system for the electron beam. We will show the present status of the work and discuss the future prospect.