G. L. Carr

Ultrashort electron bunch length measurements at DUVFEL

The DUVFEL electron linac is designed to produce sub-picosecond, high brightness electron bunches for driving a short wavelength FEL. Four experiments have been commissioned to address the challenge of accurately measuring bunch lengths on this timescale. In the frequency domain, a short 12 period undulator is used to produce both off-axis coherent emission and on-axis incoherent single-shot spectra. The total coherent infrared power scales inversely with the bunch length and the spectral cutoff is an indication of bunch length. The density of the power spikes in the single-shot visible spectrum may also be used to estimate the bunch length. In the time domain, the linac accelerating sections and a bending magnet are used to implement the RF-zero phasing method, and a subpicosecond streak camera is also installed. The beam measurements with comparisons of these methods are reported.

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.

Measured properties of the DUVFEL high brightness, ultrashort electron beam

The DUVFEL electron linac is designed to produce sub-picosecond, high brightness electron bunches to drive an ultraviolet FEL. The accelerator consists of a 1.6 cell S-band photoinjector, variable pulse length Ti:Sapp laser, 4 SLAC-type S-band accelerating sections, and 4-dipole chicane bunch compressor. In preparation for FEL operation, the compressed electron beam has been fully characterized. Measurement of the beam parameters and simulation of the beam are presented.

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.

Coherent radiation measurements at the NSLS Source Development Lab

The far-infrared coherent emission from short electron bunches has been detected and partially characterized for two source locations at the NSLS Source Development Lab (SDL) accelerator system. One location is designed to extract dipole edge radiation while the other extracts transition radiation produced a short distance downstream of the SDL linac. The coherently radiated energy per pulse is 10s of nJ, and depends quadratically on bunch charge. The spectral content agrees with the longitudinal bunch density as determined from other measurements.

Measurement of coherent off-axis undulator radiation as a beam diagnostic

A diagnostic undulator is used to measure relative electron bunch lengths. The radiation produced by the short undulator is observed off-axis so that the beam is not interrupted. Wavelengths radiated which are longer than the bunch length are coherent and may be more intense than the shorter wavelengths by a factor of up to 10/sup 9/. The total energy radiated thus changes as the bunch length changes, and a measurement of the radiated energy provides a robust beam diagnostic. This non-disturbing diagnostic would be useful in applications where the length of short electron bunches is critical, such as single-pass FELs and linear colliders. For example, real-time bunch length measurements could be used to optimize performance of a single-pass FEL. The experiment is performed at the Source Development Lab (SDL) at Brookhaven National Laboratory. The theory and description of the experiment are reviewed, and the first experimental results are presented.

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.

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.

A deep ultraviolet ultrafast source driven by high gain harmonic generation in a free electron laser

High-gain harmonic generation has been demonstrated at the shortest wavelength to date (266 nm). Megawatt levels of 89 nm radiation are also produced. Experimental applications of the deep ultraviolet light and FEL issues are discussed.