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

通往高亮度的途径：用于新型加速器的太赫兹频率超短束序列

• 批准号: ST/X004090/1
• 负责人: Morgan Hibberd
• 金额: $ 76.72万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FX004090%2F1
• 关键词: route; luminosity; Terahertz; frequency; ultrashort

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.

粒子加速通常涉及将低能量带电粒子束（例如，电子）注入加速电场中以将束驱动到较高能量。传统的加速器使用几GHz的射频（RF）场，但这些场仅限于100 MV/m左右的最大加速梯度。为了达到更高的束流能量，同时降低未来粒子加速器的尺寸、成本和效率，新型高梯度（>GV/m）概念（如等离子体尾场）利用更高太赫兹（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，其使用质子束在大约0.25 THz处产生比常规RF加速场强100倍以上的强尾场。然而，从GHz跳到THz频率使得聚束注入过程更具挑战性，因为“加速桶”的尺寸明显更小。规模上的差异相当于飞越足球场，并将足球（注入的一束）投进足球场（RF加速桶）或罚球点（韦克菲尔德加速桶）。因此，产生更短的聚束与更好的定时精度是必不可少的控制注入到新型的高频粒子acceleration.The解决方案是一种新的聚束压缩方案由激光产生的太赫兹脉冲供电，使用“啁啾”电子聚束与较低的能量电子在开始增加到较高的能量在结束时。当啁啾聚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一起挤压成能量尖峰，其中取决于聚束长度和太赫兹频率，可以产生多达100个能量尖峰。然后可以使用磁弯道来及时分离和压缩能量尖峰，产生一系列超短微束，其间距由THz波的周期定义。激光产生的太赫兹脉冲是这种超短束流产生的关键。首先，太赫兹脉冲的波长和周期与典型的啁啾电子聚束理想匹配，从而能够有效地使用强太赫兹电场将聚束挤压成尖锐的能量尖峰，并允许产生非常短的微聚束（约10 fs）。其次，太赫兹驱动的压缩大大降低了微聚束的“定时抖动”。抖动描述了一束的到达时间可以变化多少，并且对于由RF机器提供的啁啾束，它们有时可以非常早或非常晚（高达100 fs）到达。THz驱动的压缩将超短束“锁定”到激光产生的THz脉冲的定时，这可以以极高的精度（<1 fs）同步。产生以太赫兹频率间隔开的多个微聚束（“聚束串”）的能力具有同时有效注入多个加速桶的巨大潜力。聚束列车的重复率可以完美地匹配到新的高梯度加速器概念，如等离子体尾场的频率，使高能量（GeV级）聚束列车成为可能。这对AWAKE等计划至关重要，在AWAKE中，可以同时加速多达100个束流，而不是单个束流，从而为高能粒子物理实验提供100倍的电荷，提高“亮度”，开辟新的探索领域。产生超短（10 fs）、超低抖动（<1 fs）的束团将有效地取代以前的-提到的足球与一粒米（注入束），并准确地把它用手内的点球点（加速桶）。结合产生太赫兹频率聚束串的能力，这种受控注入将改变新型高频粒子加速器的能力，并最终释放其全部潜力。