Y. Shen

Terahertz laser modulation of electron beams

The study of modulated electron beams is important because they can be used to produce coherent radiation, but the modulations can cause unwanted instabilities in some devices. Specifically, in a free electron laser, proper prebunching at the desired emission frequency can enhance performance, while bunching resulting from instabilities and bunch compression schemes can degrade performance. In a photoinjector accelerator, tailoring the shape of the drive laser pulse could be used as a technique to either enhance or mitigate the effect of these modulations. This work explores the possibility of creating deeply modulated electron beams at the photocathode by using a modified drive laser designed to produce multiple subpicosecond pulses repeated at terahertz frequencies. Longitudinal space charge forces can strongly influence the evolution of modulations by converting density modulations to energy modulations. Experiments at the Source Development Laboratory electron accelerator at Brookhaven National Laboratory and PARMELA simulations are employed to explore the dynamics of electron beams with varying charge and with varying initial modulation. Finally, terahertz light generated by a transition radiator is used to confirm the structure of the electron beam.

Surface photoemission in a high-brightness electron beam radio frequency gun

We report the experimental characterization of high-brightness electron beam generation from a magnesium (Mg) photocathode. Both the quantum efficiency (QE) and the thermal emittance of an Mg cathode are experimentally investigated. The measured QE∼0.2% is the highest reported for a metal cathode. We observed no change in the Mg cathode thermal emittance as the QE varies from 0.015% to 0.15%. The upper-limit of the thermal emittance, 0.5 mm mrad, is about 50% lower than the theoretical prediction. Our results demonstrated the feasibility of having a high QE and a low thermal emittance simultaneously for a robust metal photocathode.

Efficiency enhancement using electron energy detuning in a laser seeded free electron laser amplifier

We report the experimental characterization of efficiency enhancement in a single-pass seeded free-electron laser (FEL) where the electron energy is detuned from resonance. Experiments show a doubling of the efficiency for beam energies above the resonant energy. Measurements of the FEL spectra versus energy detuning shows that the wavelength is governed by the seed laser. The variation in the gain length with beam energy was also observed. Good agreement is found between the experiment and numerical simulations using the MEDUSA simulation code.

Simulations and Experiments of Electron Beams Pre-Modulated at the Photocathode

The University of Maryland and the Source Development Laboratory at Brookhaven National Laboratory have been collaborating on a project that explores the use of electron beam pre-modulation at the cathode to control the longitudinal structure of the electron beam. This technique could be applied to creating deliberate modulations which can lead to the generation of terahertz radiation, or creating a smooth profile in order to suppress radiation. This paper focuses on simulations that explore some of the pre-modulated cases achieved experimentally.

Electro-optic cross phase modulation with an accelerator source of intense coherent THz pulses

We have used electro-optic methods to investigate single-cycle THz pulses produced as coherent transition radiation from relativistic electron bunches. The pulses have energies up to 100 muJ such that electric fields approaching 100 MV/m should be attainable. We find that the combination of these large fields and their rapid variation (on a sub-ps time scale) results in significant time-dependent phase modulation of a laser pulse when co-propagated in a non-linear medium such as ZnTe.

Electron bunch length monitors using spatially encoded electro-optical technique in an orthogonal configuration

A single-shot, nondestructive, electro-optical, electron bunch length monitor is experimentally verified by encoding the Coulomb field of the bunch profile on the spatial intensity distribution of an unchirped femtosecond laser pulse in an orthogonal geometry, hence a temporal-to-spatial transformation. This electron bunch measurement scheme can simultaneously measure large timing jitter (approximately in picoseconds) with a wide measurement time span covering picosecond to subpicosecond ranges.

Saturation of the NSLS DUV-FEL at BNL

The Deep Ultra Violet Free Electron Laser (DUV-FEL) experiment has reached a milestone by saturating the Free Electron Laser using High Gain Harmonic Generation (HGHG) method in the Source Development Laboratory (SDL) at NSLS. It utilizes the 10 m long, 3.9 cm period NISUS wiggler. The goal of the project is to produce radiation at a wavelength less than 100 nm, utilizing the HGHG method. HGHG at 266 nm has been accomplished by seeding with the 800 nm Ti:Sapphire laser and observed saturation. The third harmonic at 88 nm accompanying the 266 nm is being used in an ion pair imaging experiment in chemistry. We discuss the measurement of energy, gain, spectrum, and pulse length of the FEL output. We also describe the diagnostics used for operating the FEL.

Temporal and Spatial Lensing with an Intense Single-Cycle Terahertz Pulse

We demonstrate that an intense subpicosecond single-cycle terahertz pulse in an electro-optic medium can act as a temporal and spatial lens to phase modulate and focus a co-propagating ultrashort laser pulse.

Efficiency enhancement experimentwith a tapered undulator in a single-pass seeded fel at the NSLS SDL

The experimental observation of FEL efficiency enhancement using a tapered undulator in a single-pass seeded FEL at the NSLS SDL is reported. The last 2.5 m of the 10 m NISUS undulator was linearly tapered so that the magnetic field strength at the end of the undulator was reduced by 5%. The FEL energy gain along the undulator was measured for both the tapered and un-tapered undulators. We observed that the FEL efficiency was more than doubled by applying the taper. The experimental results are compared with the numerical simulation code,GENE- SIS1.3 [1].

Strong THz-Field-Induced Nonlinear Optical Effects in Electro-Optical Crystals

We demonstrate that time-dependent electric field associated with intense single-cycle THz pulses can induce nonlinear phase modulation in electro-optical crystals, leading to spectral shift, broadening and modulation of co-propagating laser pulses.