Cockcroft Institute Capital bid 2018

The expected size and costs of future HEP particle accelerators, such as CERN's proposed greater than 40 km long Compact Linear Collider or the similarly scaled International Linear Collider, stimulates demand for methods for higher-gradient more compact and cost effective particle acceleration. Current microwave structures, with decades of development, are limited to acceleration gradients of around 100 MV/m by electric-field breakdown. This breakdown limit increases with frequency, and an accepted route to higher gradients is through development of sources and structures at significantly higher frequencies. The challenges in delivering further major advances in radio-frequency (RF) acceleration suggests the need for a breakthrough disruptive technology to deliver extremely high accelerating gradients and significant cost and scale reductions.

Direct, in-vacuum, particle acceleration with laser derived terahertz (THz) frequency sources has the potential for GV/m acceleration gradients, and for delivering high quality, highly-controlled particle beams. Terahertz frequencies occupy a middle ground in frequency and length scale between optical and RF, with frequencies 2 orders of magnitude higher than used in current RF acceleration. With wavelengths of 300 micron at 1 THz, terahertz driven acceleration is possible with mm-scale structure apertures, offering tractably smaller and more compact acceleration. While direct (optical)-laser acceleration concepts are also candidate for high-gradient acceleration, the 1-micron scale apertures of optical acceleration are an obstacle to the high-current high-energy beams ultimately required for particle-physics luminosity. In comparison to the many-Joule low-repetition rate lasers required for plasma-based novel acceleration approaches, direct in-vacuum THz acceleration require modest lasers of order of 10mJ pulses. THz driven acceleration is a relative new-comer to the field of novel particle acceleration, and offers benefits to beam quality and control over competing optical and plasma novel acceleration. In the last 5 years of GV/m field strengths have been demonstrated, along with and acceleration and control of low-energy beams. Yet, to date there has been no demonstrations of high-gradient THz acceleration with relativistic beams. Such demonstrations are a necessary next step towards developing this disruptive approach to future compact high-energy particle physics accelerators.

Through this capital call, we seek equipment to take the demonstration of terahertz driven acceleration to a new and internationally leading level. The Cockcroft Institute has an established a programme in Dielectric and THz driven particle acceleration, and has developed leading approaches to THz dielectric acceleration structures, and in generating exotic polarisation states of THz electro-magnetic pulses. We seek a new laser system that will be capable of generating high-gradient THz fields at repetition rates of 100 Hz or greater while being synchronised to a particle accelerator for combined THz- relativistic electron beam experiments. The laser will be sited adjacent and linked to STFC Daresbury test accelerator CLARA. It will be utilised in international-first demonstrations of THz driven relativistic beam acceleration, and will form a core part of the Cockcroft Institute Novel Acceleration programme. We will establish the capability of the laser as a centre-piece for UK and international collaboration in the area of dielectric laser and dielectric terahertz acceleration.

The research into particle acceleration with terahertz frequency sources is scientifically motivated by need for more compact and more efficient national-laboratory scale particle accelerators. However, while such large science facility accelerators are few in number, particle accelerators on a much smaller physical scale are widespread in applications of x-ray security screening, industrial inspection x-ray imaging, electron-beam food irradiation for sterilisation, and in waste water treatment. In the industrial applications, our research will provide underpinning opportunities for more compact and energy efficient accelerators. With size and efficiency improvements would come reduction in capital and operational costs.

Proton, ion and electron beam accelerators are also very widely employed in medical applications, ranging from x-ray production for imaging and cancer therapy, to direct particle therapies with electron, proton and heavy ion particle beams. Currently proton and ion-beam therapies have very large infrastructure requirements, and the need for dedicated purpose-built buildings to house this infrastructure. The techniques developed here will enable significant reduction in the scale of infrastructure need for proton accelerators for cancer therapy, offering the potential for much wider availability of such therapy. In electron accelerators, new patient treatment approaches could be enabled through a reduction in the size of particle accelerators from a small room to the size of a pen, potentially leading to particle-beams being directly utilised in surgery. With compact means of acceleration and particle beam delivery, the beam may be more effectively directed at the target tissue.

The THz frequency acceleration concepts are capable of manipulation of particle beams on the time scale of femtoseconds, a time scale below which atomic motion in solid material and single molecules occurs. These ultrashort electron beams can be used to probe the atomic scale structure through a process of time-resolved electron diffraction, revealing the structure and dynamics of molecules and condensed matter. Through enabling development of scientific instruments for ultrafast molecular structure science, compact THz accelerators could underpinning capability for advances in molecular biology, and in condensed matter research.

The team in this proposal have string links to industry and hospitals involved in radiotherapy, and security scanning, and are well placed to ensure that any technological developments can be fully exploited.