High brightness beams are streams of high intensity charged particles of low emittance. Emittance is the figure-of-merit of a charged particle beam in a particle accelerator. It is a measure for the average spread of particle coordinates in six-dimensional position-momentum phase space. It has the dimension of length times angle (i.e. meter-radians). A beam of smaller emittance implies a beam of better quality. The beam emittance produced by the current advanced high-brightness beam technology can be much smaller than 10 mm-mrad. High brightness electron beam injector is the key technology for next generation light sources such as X-ray free electron lasers and inverse Compton scattering etc. In-house development of a laser-driven photocathode microwave electron gun system was started in 2006. In the past several years, we have accumulated experiences on high energy ultrafast lasers, high power microwaves, low emittance electron sources and high-brightness electron beam design. Operation of the first photocathode rf gun system in Taiwan has been delivering high brightness electron beam since March 2013 successfully. The NSRRC High Brightness Injector Group has been established in April 2014. Our efforts have been focused on the continuous improvement of the photocathode rf gun system performance. We also pursue for a high brightness linac system that can be used for novel light source R&D aggressively. This linac system is now being installed in the accelerator test area (ATA) at NSRRC.
Coherent THz radiations driven by the high brightness photoinjector
Generation of a low-emittance electron beam from the laser-driven photoinjector has been carried out by the High Brightness Injector Group. An ultrashort electron bunch of ~ 500 fs bunch length and few tens MeV beam energy has been produced from the photoinjector linac in which the beam is accelerated and compressed simultaneously via the velocity bunching mechanism in the rf linac. Tunable, high-energy and narrow-band coherent THz radiation has been be generated from a U100 planar undulator when it is driven by such beam. In the meantime, broadband, single-cycle coherent THz transition radiation can also be generated by passing this beam through an Al foil.
Table 1 Specifications of the high brightness photoinjector
| beam charge (pC) |
100 |
| beam energy (MeV) |
62 |
| bunch length (ps, 1σ) |
1.27 |
| normalized emittance (mm-mrad) |
10 |
| repetition rate (Hz) |
10 |
Table 2 Specifications of coherent THz sources
| parameters |
coherent transition radiation |
coherent undulator radiation |
| beam charge (pC) |
210 |
280 |
| beam energy (MeV) |
17.7 |
17.7 |
| bunch length (fs, rms) |
490 |
490 |
| central frequency (THz) |
-- |
0.62 |
| bandwidth |
-- |
15% |
| output energy (μJ) |
6.7 |
26.4 |
| repetition rate (Hz) |
10 |
10 |
| average power |
67 μW |
264 μW |
| peak power |
9.4 MW |
530 kW |