B. Sheehy

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.

Measurement of thermal emittance for a copper photocathode

Measurements of the thermal emittance of an electron beam produced by photoemission from the copper cathode of a high power RF cavity are presented. The RMS normalized emittance has been measured as a function of laser spot size, applied surface field, and polarization of the laser beam at normal incidence. Local field enhancement due to surface effects is found to increase the emittance substantially beyond that expected for a perfect planar surface.

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.

Electron beam modulation using a laser-driven photocathode

Coherent synchrotron radiation may lead to a microwave instability on an electron bunch at wavelengths much smaller than the bunch length. It is possible that ripples (prebunching) on the electron bunch distribution may seed this instability. We report on research exploring this effect using a longitudinally modulated drive laser to generate a modulated electron beam. Our first step is to develop simulations that will help us study the beam generation process using PARMELA. Preliminary experiments on laser beam and electron beam modulation, conducted at the Source Development Laboratory at the National Synchrotron Light Source, show modulation at frequencies in the terahertz regime is attainable. Longitudinal prebunching may enhance the performance of FEL or other radiative devices in the terahertz regime. Alternatively, longitudinal control over the electron beam might be an effective method of suppressing coherent synchrotron radiation instabilities that cause beam break-up.

Photoinjected energy recovery linac upgrade for the National Synchrotron Light Source

We describe a major paradygm shift in in the approach to the production of synchrotron radiation This change will considerably improve the scientific capabilities of synchrotron light sources. We introduce plans for an upgrade of the National Synchrotron Light Source (NSLS). This upgrade will be based on the Photoinjected Energy Recovering Linac (PERL). This machine emerges from the union of two technologies, the laser-photocathode RF gun (photoinjector) and superconducting linear accelerators with beam energy recovery (Energy Recovering Linac). The upgrade will bring the NSLS users many new insertion device beam lines, brightness greater than 3rd generation light-sources and ultra-short pulse capabilities, not possible with storage ring light sources.

The DUV-FEL development program

We discuss the design and output radiation parameters for the Deep Ultra-violet Free Electron Laser at BNL, which will generate coherent output down to 100 nm using high gain harmonic generation. The result of the FEL calculation and the status of the experiment are presented.

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.

DUVFEL photoinjector dynamics: measurement and, simulation

The DUVFEL photoinjector consists of a 1.6 cell BNL gun IV with copper cathode, variable pulse length Ti:Sapp laser, and solenoid magnet. The beam dynamics and the electromagnetic fields in the photoinjector have been characterized by producing a short electron beam with very low charge that is used as a field probe. The transverse beam size and divergence are measured as a function of the initial RF phase and the initial spot size and compared with simulations using the code HOMDYN. The electromagnetic fields used in the simulations are produced by SUPERFISH, and have been verified with RF measurements. The simulations and measurements of beam dynamics are presented.

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.

Scaled intense laser-atom physics: The long wavelength regime

A systematic study of the temporal and spectral properties of high-harmonic generation was explored using a low-binding energy atom (caesium) excited by an intense mid-infrared (3–4 mm) laser pulse. The longwavelength fundamental field produces odd-order high harmonic radiation, which is experimentally accessible with standard temporal and spectral metrology. We present measurements of the pulse width and coherence times of the 5th through 9th harmonic orders with their corresponding bandwidths. Our study demonstrates that the strategy of using a scaled laser–atom interaction can offer not only a practical solution for direct probing of the properties of high harmonic radiation but also a means for exploring the physics in novel conditions. We present results showing the enhanced harmonic emission from an ‘incoherent’ target of ground and excited state atoms.