THZ radiation source based on two-stream instability

Summary form only given. THz radiation straddles microwave and infrared bands (50 GHz-10 THz), thus combining the penetrating power of lower-frequency waves and imaging capabilities of higher- energy infrared radiation. Since THz radiation is not absorbed by most dry, non-polar materials, it can be used for imaging internal structures. Besides its military uses, THz radiation is employed in such important applications as spectroscopy, industrial bio-medical imaging, and scattering. What also makes THz radiation attractive is its non-ionizing photon energy, which is less than 0.1 eV at 1 THz. Several conventional devices are used presently to generate THz radiation. For example, slow-wave devices require very small structures (mm or sub-mm) in size. This complicates fabrication and alignment and results in merely milliwatts of average output power. Conventional FELs and synchrotrons are bulky and very expensive to operate. We propose a new approach for generating THz radiation that is compact, dispenses with complicated structures and relies on a well-known phenomenon called the "two-stream instability." The proposed configuration involves two low-energy electron beams that are merged by a dipole magnet into a single beam and interact unstably provided the velocity difference exceeds a threshold value. Although this instability is undesirable and is usually suppressed, it can also be exploited for efficient narrowband and coherent THz production. Using a small-signal analysis, the threshold velocity difference and velocity difference for maximum gain are calculated and derived for two electron beams in a beam pipe. The calculations show an excellent agreement with a 1-D simulation of two overlapping electron beams that fill a beam pipe while interacting at 100 GHz. Preliminary 2-D PIC simulation results appear to be very promising and agree well with the theory. More 2-D PIC simulations are under way (to be followed by 3-D simulations) in order to further test and validate the proposed configuration and underlying theory.