UCLA

High average and peak power radiation sources across the entire range of the electromagnetic spectrum have the potential for breakthrough advances in many scientific and technology areas, ranging from single-molecule imaging in the X-rays, to EUV lithography to particle acceleration and space applications using VIS/IR lasers.

The inherent characteristics of Free-Electron Lasers (FELs) such as peak power, coherence, and wavelength adjustability coupled with high repetition rate / high efficiency electron accelerators make them nearly ideal radiation sources, operating in vacuum with essentially no mechanism for waste heat and material breakdown. On the other hand, the efficiency of the short wavelength FELs is typically limited to less than 1% by saturation effects.

A traditional method to increase the efficiency is to vary the undulator parameters with the electron beam energy by tapering. While FEL tapering research started decades ago, the UCLA group has been recently pointed out a novel approach (termed TESSA or tapering enhanced stimulated superradiant amplification) in which electron beams enter a strongly tapered undulator already pre-bunched along with an intense seed laser. In this case, the seed laser can efficiently decelerate the prebunched electrons converting large fraction of their kinetic energy into superradiant coherent emission, significantly increasing the output power of the system.

As a preliminary study, the Nocibur experiment at Brookhaven National Laboratory few years ago investigated the resonant interaction of a high intensity 10.3um 200 GW seed laser, a 65MeV electron beam, in a short 54cm long tapered helical undulator. While the experiment measured high deceleration efficiency of 30%, an accurate characterization of the radiation was not possible because the high seed power would damage the diagnostics. This document discusses the next generation experiment in this line of research (TESSA-266) aimed at demonstrating 10% conversion efficiency in the UV range of the electromagnetic spectrum. The name of the experiment is due to the original target wavelength (266nm), even though after reviewing the experimental parameter the target wavelength shifted to 257.5nm. The experiment was designed to use the Argonne National Laboratory APS linac electron beam at an energy 343MeV, and a seed power of 1GW. Due to the moderate amount of seed power, the TESSA-266 is in a high-gain regime in which the radiation field grows significantly along the undulator. This document discusses the magnetic design of the TESSA undulator and prebuncher section, the beamline design for the seed laser transport, the tapering optimization, start-to-end beam dynamics, and the post-undulator diagnostics for future experiments.

Due to delays associated with the 2020 COVID pandemic the experiment was delayed. This document also discusses a recent related experiment called TESSAtron, where the already built TESSA undulator was used at Pegasus Laboratory at UCLA to demonstrate the TESSA path to very high efficiency at longer wavelength. In the TESSAtron experiment, we combined the TESSA concept with the so called zero-slippage interaction where the group velocity of the radiation is matched with that of the electrons by using a waveguide in the undulator. A prebunched electron beam was generated using velocity bunching in the photoinjector and then decelerated in the strongly tapered undulator by around 10%. The experiment demonstrated the generation of THz radiation of around 50uJ at a frequency of 0.16~THz.

Even though the long range of the program is to push the TESSA concept to shorter wavelengths, these results are very important in their own right. The THz range is often called a "THz Gap" because conventional radiation sources such as the vacuum electronic sources and the solid-state lasers do not generate power efficiently in this range. Obtaining a higher average power radiation source is critical for many areas such as plasma ignition, nuclear power sources, and laser-based propulsion.