Terahertz Radiation
John Neumann, Patrick O'Shea

This research explores relativistic electron beams modulated at terahertz frequencies using laser driven photoemission and covers three distinct areas: laser beam modulation; electron beam dynamics; and an application of electron beam modulation, the generation of terahertz radiation. An interferometer is used to control the 266 nm drive laser modulation. Laser pulses are delivered to the photocathode of the accelerator and are used as a switch that induces an initial electron beam modulation at frequencies between 0.5 and 1.6 terahertz. The longitudinal distribution of the electron beam is measured after acceleration to relativistic energy and is compared to numerical simulations obtained using the code PARMELA. Both experimental and numerical results indicate that some of the initial density modulation is retained on the electron beam, although the density modulation that remains, and the frequency of the modulation, falls as a function of increasing charge. Electron beam modulation is achieved between 0.712 and 1.66 terahertz. One application of the deliberate modulation of an electron beam is the generation of coherent radiation, as seen in many devices ranging from the klystron to the free electron laser. A final focus of this experiment is the generation of terahertz light by transition radiation when a mirror intercepts the modulated electron beam. Transition radiation measured by a bolometric detector is compared to expected results based on the longitudinal electron beam distributions predicted by PARMELA simulations as well as measurements from the accelerator system. This work demonstrates that it is possible for an electron beam premodulated at the cathode on a sub-picosecond timescale to be accelerated to relativistic energy and used for the production of tunable terahertz radiation.

This research is supported by the Office of Naval Research and the Department of Defense Joint Technology Office.

Section of Picosecond Laser Beam-Line Including Q-Switch and Faraday Rotator

YLF Amplifying Crystal

We are currently in the final processes of upgrading a commercial, 4.1 picosecond micropulse, 148.2 MHz Mode-Locked laser/amplifier system. The immediate goal is to deliver a series of macro pulses, principally for investigating Cesium-enhanced, dispenser photocathode output. We are mainly interested in multi-optical frequency, laser-induced high-density photoelectron emission. Our ultimate objective is to improve and study the extraction mechanism from a thermally assisted, low energy-spread photoemission cathode for the purpose of improving the FEL average power output. The main features we attempt to provide and subsequently characterize with this drive laser are long-term spatial and temporal stability, and additionally, wavelength and beam profile agility. Such versatile beam requirements impose severe environmental conditions and accurate diagnostic demands, both on the optical amplifiers and on the subsequent driven nonlinear high harmonic generators. Initially, we will be investigating various drive laser space-time profiles, employing high nonlinear optical harmonic generation and their accompanying diagnostics.