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Laser-wakefield accelerators generate femtosecond-duration electron bunches with energies from 10s of MeV to several GeV in millimetre distances by exploiting the large accelerating gradients created when a high-intensity laser pulse propagates in an underdense plasma. The process governing the formation of the accelerating structure ("bubble'') also causes the generation of sub-picosecond duration, 1-2~MeV nanocoulomb electron beams emitted obliquely into a hollow cone around the laser propagation axis. We present simulations showing that these wide-angle beams can be used to produce coherent transition radiation in the 0.1-5 THz frequency range with 10s~$\mu$J to mJ-level energy if passed through an inserted metal foil, or directly at the plasma-vacuum interface. We investigate how the properties of terahertz radiation change with foil size, position and orientation. The bunch length and size of wide-angle beams increase quickly as the electrons leave the accelerator, causing a shift of the radiation frequency peak from about 1 THz at a distance of 0.1~mm from the accelerator exit to 0.2 THz at 1~mm. If the foil size is reduced, for example to match the typical diameter of the plasma channel formed in a laser-wakefield accelerator, simulating the emission from the plasma-vacuum boundary, the low-frequency side of the spectrum is suppressed. The charge of wide-angle electron beams is expected to increase linearly with the laser intensity, with a corresponding quadratic increase of the terahertz radiation energy, potentially paving the way for mJ-level sources of coherent terahertz radiation.
|Title of host publication||Proceedings Relativistic Plasma Waves and Particle Beams as Coherent and Incoherent Radiation Sources III|
|Editors||Dino A. Jaroszynski, MinSup Hur|
|Place of Publication||Bellingham, WA|
|Number of pages||7|
|Publication status||Published - 24 Apr 2019|
|Event||SPIE Optics + Optoelectronics - Clarion Congress Hotel, Prague, Czech Republic|
Duration: 1 Apr 2019 → 4 Apr 2019
|Name||Proceedings of SPIE|
|Conference||SPIE Optics + Optoelectronics|
|Period||1/04/19 → 4/04/19|
- terahertz generation
- laser wakefield acceleration
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