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

粒子加速通常涉及将低能量带电粒子束（如电子）注入加速电场，以驱动粒子束达到更高的能量。传统的加速器使用几ghz的射频（RF）场，但这些被限制在100 MV/m左右的最大加速梯度。为了达到更高的束流能量，同时减少未来粒子加速器的尺寸、成本和效率，新的高梯度（>GV/m）概念，如等离子尾流场，利用了更高太赫兹（THz）频率范围内的加速场。一个完美的例子是欧洲核子研究中心的高级韦克菲尔德（AWAKE）实验，该实验使用质子束产生约0.25太赫兹的强尾流场，比传统的射频加速场强100倍以上。然而，从GHz频率跳到太赫兹频率使得束注入过程更具挑战性，因为“加速桶”的尺寸要小得多。规模的差异相当于飞越足球场并将足球（注入的一束）扔进足球场（RF加速桶）或罚球点（wakefield加速桶）。因此，产生更短的束和更好的定时精度对于控制注入新型高频粒子加速器至关重要。解决方案是一种新颖的束压缩方案，由激光产生的太赫兹脉冲提供动力，使用“啁啾”电子束，开始时能量较低的电子在最后增加到能量较高的电子。当啁啾束与太赫兹脉冲相互作用时，振荡的加速和减速太赫兹电场将较低和较高的电子能量挤压在一起，形成能量尖峰，根据束的长度和太赫兹频率，可以产生多达100个能量尖峰。然后可以使用磁弯来及时分离和压缩能量尖峰，产生一系列超短微束，其皮秒间隔由太赫兹波的周期定义。激光产生的太赫兹脉冲对于这种超短束列的产生是必不可少的。首先，太赫兹脉冲的波长和周期与典型的啁啾电子束理想匹配，从而能够有效地利用强太赫兹电场将电子束挤压成尖锐的能量尖峰，并允许产生非常短的微束（约10秒）。其次，太赫兹驱动的压缩极大地减少了微束的“时序抖动”。抖动描述了一束到达时间的变化程度，并且通过射频机器发送的啁啾束，它们有时可以很早或很晚到达（高达100秒）。太赫兹驱动的压缩将超短束“锁定”到激光产生的太赫兹脉冲的时间，这可以以极高的精度（< 1fs）同步。在太赫兹频率上产生多个微束（“束列”）的能力具有巨大的潜力，可以同时有效地注入多个加速桶。束列重复率可以与新型高梯度加速器概念（如等离子尾流场）的频率完美匹配，使高能（gev规模）束列成为可能。这对于像AWAKE这样的计划来说是至关重要的，在这个计划中，一列多达100束的粒子束可以同时加速，而不是单一的束，为高能粒子物理实验提供100倍的电荷，提高“亮度”，开启新的探索机制。产生具有超低抖动（<1 fs）的超短（10 fs）束，将有效地用一粒米（注入束）代替前面提到的足球，并精确地将其放在罚球点（加速桶）内。结合产生太赫兹频率束列的能力，这种受控注入将改变新型高频粒子加速器的能力，并最终释放它们的全部潜力。

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.