A. Doyuran

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.

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.

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.

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.

Very High Energy Gain at the Neptune Inverse Free Electron Laser Experiment

We report the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected in an undulator strongly tapered in period and field amplitude. The IFEL driver is a CO2 10.6 μm laser with power larger than 400 GW. The Rayleigh range of the laser, ∼ 1.8 cm, is much shorter than the undulator length so that the interaction is diffraction dominated. A few per cent of the injected particles are trapped in a stable accelerating bucket. Electrons with energies up to 35 MeV are measured by a magnetic spectrometer. Three‐dimensional simulations, in good agreement with the measured electron energy spectrum, indicate that most of the acceleration occurs in the first 25 cm of the undulator, corresponding to an energy gradient larger than 70 MeV/m. The measured energy spectrum also indicates that higher harmonic Inverse Free Electron Laser interaction takes place in the second section of the undulator.

Electro-optic longitudinal electron beam diagnostic at SDL

The linac at the NSLS Source Development Lab (SDL) provides a high brightness electron beam for the DUV-FEL project with subpicosecond bunch length and several hundred Amperes peak current by means of a photoinjector and a magnetic bunch compressor. Previous diagnostics of the longitudinal bunch dynamics relied on the rf zero-phasing method and measurements of CTR spectra. In order to have a fast and nonintercepting longitudinal diagnostic available, the electro-optic measurement technique has been implemented with its major component, a synchronized 100 fs Ti:sapphire laser coaligned with the electron beam, already in place as a seed for the FEL. The theoretical temporal resolution for a 100 /spl mu/m thick ZnTe crystal is limited to about 200 fs and the signal contrast to more than 1%. We present preliminary results of multishot scanning measurements and the single-shot diagnostics of the bunch shape as well as its application as a rf-laser jitter monitor.

Electron Bunch Compression and Coherent Effects at the SDL

The DUVFEL accelerator in the Source Development Lab of NSLS/BNL generates a high brightness electron beam from a laser driven electron source and a magnetic bunch compressor. This beam is used for different FEL experiments in SASE and future HGHG configurations. The compression of the electron beam to high peak current while preserving the transverse properties is of great importance to the performance goals of these FELs. In this paper we report on the experimental methods to characterize the longitudinal properties of the electron beam and the measured results for various settings of the DUVFEL accelerator. The observed effects on the electron beam spectra and time profiles during compression are most likely due to coherent effects while their exact origin is still subject of ongoing investigation.

High-gain harmonic generation free-electron laser at saturation

We report on observations of the output of a high-gain harmonic-generation (HGHG) free-electron laser (FEL) at saturation. A seed CO/sub 2/ laser at a wavelength of 10.6 /spl mu/m was used to generate amplified FEL output at 5.3 /spl mu/m. Measurement of the frequency spectrum, pulse duration and correlation length of the 5.3-/spl mu/m output verified the longitudinal coherence of the light. At the exit of the device, measurements of the energy distribution of the electron beam indicated that the HGHG process had reached saturation. By comparing the intensities of the higher harmonics (2.65 /spl mu/m and 1.77 /spl mu/m) relative to the 5.3-/spl mu/m fundamental, additional evidence confirming saturated operation was obtained. These results agree with theoretical predictions at saturation. We also briefly describe the status of a new HGHG experiment at the Source Development Laboratory (BNL), aimed at achieving generation of 100-nm radiation.

High Energy Gain IFEL at UCLA Neptune Laboratory

We report on the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected in a 50 cm long undulator strongly tapered both in period and field amplitude. A CO210.6 µ m laser with power > 400 GW is used as the IFEL driver. The Rayleigh range of the laser (∼ 1.8 cm) is shorter than the undulator length so that the interaction is diffraction dominated. Few per cent of the injected particles are trapped in stable accelerating buckets and electrons with energies up to 35 MeV are detected on the magnetic spectrometer. Three dimensional simulations are in good agreement with the electron energy spectrums observed in the experiment and indicate that substantial energy exchange between laser and electron beam only occurs in the first 25-30 cm of the undulator. An energy gradient of > 70 MeV is inferred. In the second section of the undulator higher harmonic IFEL interaction is observed.

XUV Generation at the BNL DUV-FEL Facility

The performance and future upgrade of the XUV at the Brookhaven DUV-FEL facility are presented in this report. The High Gain Harmonic Generation (HGHG) Free Electron Laser (FEL) at the DUV-FEL now routinely produces fully coherent 266 nm radiation with the 800 nm seed, and the output energy of the HGHG is about 100 uJ with 1 ps pulse duration (FWHM) [1], The first three harmonics of the HGHG from the nonlinear harmonic generation process were experimented characterized. The third harmonic (89 nm, 14 eV) of the HGHG has been explored for chemical science applications [2]. To further expand the DUV-FEL user base and explore its unique potential, it is proposed that the DUV-FEL to be upgraded to produce XUV (< 100 nm) at the HGHG fundamental. Several options are now under consideration to upgrade the DUV-FEL for XUV radiation generation; which including the electron beam energy upgrade to 300 MeV and installing the shorter period VISA undulator. PACS: PACS codes: 42.72.Bj, 41.60.Cr, 41.60-m