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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TeraClear (Novel THz radiation source based on compact linear accelerator)

Teaser

Summary

There are more than thirty thousand accelerators in the world starting from small-scale linear accelerators used for medical applications such as cancer therapy and non-destructive welding and other joint diagnostics in industry, large-scale third and fourth generation light sources used to probe the molecular and atomic properties of matter, and ending with giant â€œatom-smashersâ€ such as Large Hadron Collider used to unlock the secrets of creation. Operation of these machines would simply be impossible without a comprehensive set of non-invasive diagnostics equipment revealing the properties of the beam and how it behaves in the machine. A vast majority of non-invasive diagnostics devices is based on electromagnetic (EM) radiation generated by charged particles passing by a condensed medium. On the other hand large-scale light sources utilising synchrotron radiation are very expensive and compete with compact accelerator based light generators affordable by a small industrial company or a university. The EM radiation is typically generated when a fast charged particle interacts with a condensed medium and generates radiation from keV to MeV region. The project is greatly motivated by a growing demand for powerful, compact, inexpensive and tuneable sources of EM radiation in the THz range for applications in research and medicine.

Polarisation Radiation appearing when a fast charged particle passes by a material is a well-recognised candidate for generating intense EM radiation beams with a very broad spectrum. Its characteristics are very sensitive to various beam parameters as well, which create an opportunity to develop non-invasive diagnostics. However, a simple tool capable of choosing an optimal radiator configuration, material, and observation geometry is still absent.

Research objectives:

- to investigate Cherenkov and Smith - Purcell mechanisms for THz radiation generation using EM simulation tools and propose the most efficient target configuration and an observation geometry;
- to build a radiation source that provides high peak power levels and based on a compact linear accelerator technology with a femtosecond duration beam;
- to evaluate the possible applications of the investigated radiation mechanisms for electron beam diagnostics and other promising applications;
- to provide the knowledge exchange between the partner organisations and other interested parties via seminars, satellite and progress meetings, conferences and workshops;
- to improve the public awareness about the ongoing research via outreach activities;
- to develop project management skills.

Work performed

We proposed a corrugated dielectric capillary as a source of intense Cherenkov Smith-Purcell (ChSPR) THz radiation. The capillary was experimentally tested at Laser Undulator Compact X-ray facility in Japan. We studied THz radiation produced by a femtosecond electron beam and the dielectric capillaries with and without corrugation. The measured radiation intensity from the corrugation was comparable to that of coherent Transition Radiation. The spectral sensitivity characteristics of the detector, electromagnetic simulations and the ChSPR dispersion relation confirmed that the measured radiation was THz ChSPR produced by the corrugation. The non-central propagation of the beam in the corrugated capillary yielded 10-fold increase in the radiation intensity. ChSPR was also investigated experimentally as a tool for electron beam position monitoring and electron beam bunch-to-bunch distance monitoring. Beam diagnostics capability could potentially be achieved by measuring and analysing the spectrum of ChSPR. Issues that remain to be considered in the future work are further investigations of how sensitive is the radiation directivity pattern to the transverse beam position; studies of the beam dynamics during and after propagation through the capillaries; and spectral measurements for single and multi-bunch beams.

In addition, ChSPR in the corrugated capillary was investigated as a mechanism for wakefield acceleration in the collinear scheme, when a so called witness bunch is accelerated by the accelerating electric field produced by a driver bunch. So far, we compared performance of a corrugated and a conventional cylindrical capillary with a constant inner radius (flat capillary) as accelerating structures in the driver-witness acceleration scheme and as devices capable to reduce energy spread of the witness bunch. The maximum acceleration of 170 kilo-electron-Volt-per-meter at 20 pico-Coulomb driver bunch charge was measured for the beam propagation with a transverse offset in both capillaries. The corrugated and the flat capillary demonstrated similar performance in reducing the energy spread of the witness bunch, while the measured and simulated energy spread of the driver bunch were only moderately higher for the corrugated capillary. We showed that the corrugation changed the phase velocity of the wakefield and shifted the frequency of the main accelerating mode from 50 to 55 Giga-Hertz. An optimisation of the corrugated capillary parameters is required to explore its capability to provide accelerating gradients higher than in the flat capillary.

The results were shared with the partner organisations and the international scientific community via the progress meetings and the following international conferences - Channeling 2016, IPAC17, RREPS18 and a topical workshop AGTaX17. All the major results were published in the conference proceedings and in the open access peer-reviewed publications.

Final results

THz radiation sources based on circular waveguides with dielectric loading or metallic corrugation have been extensively investigated theoretically and experimentally by the international scientific community. Together with high repetition short electron beams they can have a transformative impact for free electron lasers as well as for linear colliders. Further development in the field of novel non-invasive beam diagnostics is of utmost importance for established and future particle accelerators. In this project, we showed that Cherenkov Smith-Purcell (ChSPR) from mm-scale corrugated capillary can produce radiation intensities comparable to coherent Transition Radiation, and investigated ChSPR as a tool for electron beam position and bunch-to-bunch distance monitoring. THz ChSPR intensities can be considerably increased by using electron beams with high repetition frequency, by reducing the capillary inner radius, and by optimising the corrugation parameters.

In addition to THz radiation production, circular waveguides can also serve as simple, high gradient and ultra-compact accelerating structures. THz Cherenkov radiation (CR) is considered to be the main mechanism in dielectric wakefield acceleration (DWA). Passing through a dielectric capillary, relativistic electrons create CR fields which, once reflected, create an accelerating field on axis. Smith-Purcell radiation (SPR) is generated when a periodicity is introduced along the trajectory and can be more intense than CR. We proposed to apply SPR mechanism in DWA. A preliminary study with corrugated capillaries has shown some modification to the spectral content and amplitude of CR-generated wakefields, but the possibility of using SPR in DWA is still under consideration.