A route to high luminosity: Terahertz-frequency ultrashort bunch trains for novel accelerators

Particle acceleration typically involves injecting low-energy charged particle bunches (e.g. electrons) into an accelerating electric field to drive the bunch to higher energy. Conventional accelerators use few-GHz radio-frequency (RF) fields but these are limited to maximum accelerating gradients around 100 MV/m. To reach higher beam energies while reducing the size, cost and efficiency of future particle accelerators, novel high-gradient (>GV/m) concepts such as plasma-wakefields exploit accelerating fields in the higher terahertz (THz) frequency range.

A perfect example is the Advanced Wakefield (AWAKE) experiment at CERN, which uses proton beams to create intense wakefields at around 0.25 THz that are over 100x stronger than conventional RF accelerating fields. However, jumping from GHz to THz frequencies makes the bunch injection process much more challenging, as the size of the "accelerating bucket" is significantly smaller.

The difference in scale is equivalent to flying over a football stadium and dropping a football (the injected bunch) either inside the football pitch (RF accelerating bucket) or inside the penalty spot (wakefield accelerating bucket). Therefore, generating shorter bunches with better timing precision is essential for controlled injection into novel high-frequency particle accelerators.

The solution is a novel bunch compression scheme powered by laser-generated THz pulses, using "chirped" electron bunches with lower energy electrons at the start increasing to higher energies at the end. When the chirped bunch interacts with the THz pulse, the oscillating accelerating and decelerating THz electric fields squeeze the lower and higher electron energies together into energy spikes, where depending on the bunch length and THz frequency, up to 100 energy spikes can be produced.

A magnetic chicane can then be used to separate and compress the energy spikes in time, generating a train of ultrashort micro-bunches with picosecond spacing defined by the period of the THz wave.

Laser-generated THz pulses are essential for this ultrashort bunch train generation. Firstly, the wavelength and period of THz pulses are ideally-matched to typical chirped electron bunches, enabling efficient use of the strong THz electric fields to squeeze the bunch into sharp energy spikes and allow very short micro-bunches (around 10 fs) to be produced.

Secondly, the THz-driven compression drastically reduces the "timing jitter" of the micro-bunches. The jitter describes how much the arrival time of a bunch can vary, and with the chirped bunches delivered by an RF machine, they can sometimes arrive quite early or quite late (up to 100 fs). The THz-driven compression "locks" the ultrashort bunches to the timing of the laser-generated THz pulse instead, which can be synchronised with extreme precision (<1 fs).

The ability to produce multiple micro-bunches spaced at THz frequencies (a "bunch train") has huge potential for efficient injection into multiple accelerating buckets at the same time. The bunch train repetition rates can be perfectly matched to the frequency of novel high-gradient accelerator concepts such as plasma-wakefields, making high-energy (GeV-scale) bunch trains a possibility.

This will be critical to schemes such as AWAKE, where rather than a single bunch, a train of up to 100 bunches can be accelerated at once, delivering 100x more charge to high-energy particle physics experiments, boosting "luminosity" and opening up new regimes of exploration.

Generating ultrashort (10 fs) bunches with ultralow jitter (<1 fs) will effectively replace the previously-mentioned football with a grain of rice (injected bunch) and precisely place it by hand inside the penalty spot (accelerating bucket). Combined with the ability to generate THz-frequency bunch trains, this controlled injection will transform the capabilities of novel high-frequency particle accelerators and finally unlock their full potential.