Design of a Resonator for the CSU THz FEL

A 6-MeV L-band linac will be used to drive a planar, fixed gap, 2.5-cm period, hybrid undulator with parabolic pole faces. Consequently, this system is capable of generating wavelengths from 160 to 600 μm. In this paper we discuss the design of an optical resonator for this system. The resonator uses hole-coupled mirrors to allow for a straight electron beam line. The Rayleigh length, the position of the waist of the cold-cavity mode and the hole radii will be investigated to optimize the performance of the FEL

Spectral control of high-harmonic generation via drive laser pulse shaping in a wide-diameter capillary

We experimentally investigate spectral control of high-harmonic generation in a wide-diameter (508 μm) capillary that allows using significantly lower gas pressures coupled with elevated drive laser energies to achieve higher harmonic energies. Using phase shaping to change the linear chirp of the drive laser pulses, we observe wavelength tuning of the high-harmonic output to both larger and smaller values. Comparing tuning via the gas pressure with the amount of blue shift in the transmitted drive laser spectrum, we conclude that both adiabatic and non-adiabatic effects cause pulse-shaping induced tuning of high harmonics. We obtain a fractional wavelength tuning, Δλ/λ, in the range from −0.007 to + 0.01, which is comparable to what is achieved with standard capillaries of smaller diameter and higher pressures

Coherent control of high harmonic generation in a large-volume capillary for seeding of free-electron lasers

FEL-1 at FERMI@Elettra is a seeded free-electron laser using sub-harmonic seeding to generate soft x-rays down to 10 nm. The current seed laser, a standard solid-state laser followed by frequency quadrupling in nonlinear crystals, has a minimum wavelength of 200 nm. Injecting much shorter seed-laser wavelengths, for which high-harmonic generation (HHG) is of high promise, can shorten the laser output wavelength significantly. However, the minimum seed pulse energy required is not readily available with standard approaches to HHG. To increase the energy available in a particular harmonic for seeding, we use a gas-filled capillary with a large diameter (500 μm), pumped by an 8 mJ, 35 fs Ti:Sapphire laser. A wide capillary allows a large gas volume for HHG, thereby increasing the output energy. We also investigate the coherent control of HHG by shaping the spectral phase of the drive laser using an acousto-optic programmable dispersive filter. Here, we use a learning algorithm with the objective to simultaneously tune and selectively enhance an individual harmonic order. We present first results including pressure dependent harmonic output energy, spectrum and beam stability, as these are important for seeding of FELs. Further, we discuss initial experiments with coherent control that has shown selective enhancement up to a factor of 10

Size and density redistribution by a rod obstacle in a cluster jet for quasi-phase matching of high harmonic generation

We investigate the the possibility to realize a fully coherent XUV light source generating wavelengths down to 4 nm by using high-order harmonic generation (HHG) in an ionized medium. Due to the strong ionization, current p We investigate the possibility to realize a fully coherent XUV light source generating wavelengths down to 4 nm by using high-order harmonic generation (HHG) in an ionized medium. Due to the strong ionization, current phase-matching techniques for HHG are not suitable. Instead, we will investigate quasi-phase matching (QPM) over an extended interaction length to increase the output pulse energy. For this, we will prepare a cluster jet from a 5 mm long supersonic nozzle operated at high backing pressure (up to 75 bar). The modulation for QPM is obtained by placing either an array of wires or slits on top of the exit of the nozzle. Here, we report on the characterization of the modulated argon cluster jet. We apply Rayleigh scattering imaging and interferometry to infer the cluster size and total atomic number density distribution in the jet. Initial experiments concern the modulation of the jet by placing a 2 mm rod above the nozzle. The rst results on the cluster size and density distribution will be compared with the simulation results from our 2D fluid dynamics model

The CSU Accelerator and FEL Facility

The Colorado State University (CSU) Accelerator Facility will include a 6-MeV L-Band electron linear accelerator (linac) with a free-electron laser (FEL) system capable of producing Terahertz (THz) radiation, a laser laboratory, a microwave test stand, and a magnetic test stand. The photocathode drive linac will be used in conjunction with a hybrid undulator capable of producing THz radiation. Details of the systems used in CSU Accelerator Facility are discusse

Comparison of terahertz technologies for detection and identification of explosives

We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection. © 2014 SPIE

Using a Neural Network Control Policy for Rapid Switching Between Beam Parameters in an FEL

FEL user facilities often must accommodate requests for a variety of beam parameters. This usually requires skilled operators to tune the machine, reducing the amount of available time for users. In principle, a neural network control policy that is trained on a broad range of operating states could be used to quickly switch between these requests without substantial need for human intervention. We present preliminary results from an ongoing study in which a neural network control policy is investigated for rapid switching between beam parameters in a compact THz FEL