Michelle D. Shinn

Single-element laser beam shaper for uniform flat-top profiles

A novel design method is presented for a simple laser beam shaper. Unlike earlier reports and designs based on the 2-element model, we prove it is possible to convert a laser beam from a non-uniform profile to a uniform flat-top distribution with one single aspherical lens.

Production of high power femtosecond terahertz radiation

The terahertz (THz) region of the electromagnetic spectrum is attracting interest for a broad range of applications ranging from diagnosing electron beams to biological imaging. Most sources of short pulse THz radiation utilize excitation of biased semiconductors or electro-optic crystals by high peak power lasers. For example, this was done by using an un-doped InAs wafer irradiated by a femtosecond free-electron laser (FEL) at the Thomas Jefferson National Accelerator Facility. Microwatt levels of THz radiation were detected when excited with FEL pulses at 1.06 mm wavelength and 10W average power. Recently substantially higher powers of femtosecond THz pulses produced by synchrotron emission were extracted from the electron beamline. Calculations and measurements confirm the production of coherent broadband THz radiation from relativistic electrons with an average power of nearly 20W, a world record in this wavelength range by a factor of 10,000. We describe the source, presenting theoretical calculations and their experimental verification. Potential applications of this exciting new source include driving new non-linear phenomena, performing pump-probe studies of dynamical properties of novel materials, and studying molecular vibrations and rotations, low frequency protein motions, phonons, superconductor band gaps, electronic scattering, collective electronic excitations (e.g., charge density waves), and spintronics.

First Lasing of the Jefferson Lab IR Demo FEL

As reported previously [1], Jefferson Lab is building a free-electron laser capable of generating a continuous wave kilowatt laser beam. The driver-accelerator consists of a superconducting, energy-recovery accelerator. The initial stage of the program was to produce over 100 W of average power with no recirculation. In order to provide maximum gain the initial wavelength was chosen to be 5 mu-m and the initial beam energy was chosen to be 38.5 MeV. On June 17, 1998, the laser produced 155 Watts cw power at the laser output with a 98% reflective output coupler. On July 28th, 311 Watts cw power was obtained using a 90% reflective output coupler. A summary of the commissioning activities to date as well as some novel lasing results will be summarized in this paper. Present work is concentrated on optimizing lasing at 5 mu-m, obtaining lasing at 3 mu-m, and commissioning the recirculation transport in preparation for kilowatt lasing this fall.

Selective photothermolysis to target sebaceous glands: Theoretical estimation of parameters and preliminary results using a free electron laser†

The success of permanent laser hair removal suggests that selective photothermolysis (SP) of sebaceous glands, another part of hair follicles, may also have merit. About 30% of sebum consists of fats with copious CH2 bond content. SP was studied in vitro, using free electron laser (FEL) pulses at an infrared CH2 vibrational absorption wavelength band.

High power operation of the JLab IR FEL driver accelerator

Operation of the JLab IR Upgrade FEL at CW powers in excess of 10 kW requires sustained production of high electron beam powers by the driver ERL. This in turn demands attention to numerous issues and effects, including: cathode lifetime; control of beamline and RF system vacuum during high current operation; longitudinal space charge; longitudinal and transverse matching of irregular/large volume phase space distributions; halo management; management of remnant dispersive effects; resistive wall, wake-field, and RF heating of beam vacuum chambers; the beam break up instability; the impact of coherent synchrotron radiation (both on beam quality and the performance of laser optics); magnetic component stability and reproducibility; and RF stability and reproducibility. We discuss our experience with these issues and describe the modus vivendi that has evolved during prolonged high current, high power beam and laser operation.

Laser pulse structure dependent texture of FEL synthesized TiNx coatings

Titanium was laser nitrided by means of free electron laser (FEL) irradiation in pure nitrogen atmosphere. The variation of macropulse frequency and duration of the FEL micropulse trains resulted in the formation of δ-TiNx coatings with different thicknesses and different micro- and macroscopic morphologies. The coatings revealed characteristic values for hardness, roughness and crystallographic texture, which originate from the growth mechanism, the solid–liquid interface energy and the strain. Further investigations showed that the dendritic growth begins at the surface and the alignment of the dendrites is normal to the surface. A correlation of the texture with the time structure of the laser pulses was found. Combined numerical simulations of temperature evolution and nitrogen diffusion were performed and the results were compared with the experimental findings. The simulations can explain the experimental results to a great extent. (Some figures in this article are in colour only in the electronic version)

Self-trapped states in proteins?

We show here that the temperature dependence of the amide I band of myoglobin shows evidence for a low-lying self-trapped state at 6.15 µm. We have conducted a careful set of picosecond pump–probe experiments providing results as a function of temperature and wavelength and show that this low-lying state has a 30 ps lifetime at 50 K, much longer than the relaxation time of the main amide I band at 50 K. Fits of the temperature dependence of thermal occupation of this state yield the result that it lies 280 K below the main amide I band. Since the gap energy of this state is approximately equal to room temperature, this self-trapped state can act as a transient store of vibrational energy at physiological temperatures in biomolecules and can help to direct the path of energy flow in a biomolecule under biological conditions.

High-average-power free-electron lasers: a new laser source for materials processing

Material processing with lasers has grown greatly in the previous decade, with annual sales in excess of

A high-DC-voltage GaAs photoemission gun: Transverse emittance and momentum spread measurements

1 B (US). In general, the processing consists of material removal steps such as drilling, cutting, as well as joining. Here lasers that are either cw or pulsed with pulsewidths in the microsecond(s) time regime have done well. Some applications, such as the surface processing of polymers to improve look and feel, or treating metals to improve corrosion resistance, require the economical production of laser powers of the tens of kilowatts, and therefore are not yet commercial processes. The development of FELs based on superconducting RF (SRF) linac technology provides a scaleable path to laser outputs above 50 kW, rendering these applications economically viable, since the cost/photon drops as the output power increases. Such FELs will provide quasi-cw (PRFs in the tens of MHz), of ultrafast (pulsewidth approximately 1 ps) output with very high beam quality. The first example of such an FEL is the IR Demo FEL at the Thomas Jefferson National Accelerator Facility (Jefferson Lab), which produces nearly 2 kW of high average power on a routine basis. Housed in a multilaboratory user facility, we as well as members of our user community have started materials process studies in the areas mentioned earlier. I will present some of the first results of these studies. I will also briefly discuss the status of our DOD-funded project to upgrade the FEL to 10 kW in the mid IR.

A 10 kW IRFEL design for Jefferson Lab

We have built a high-DC-voltage photoemission gun and a diagnostic beamline permitting us to measure rms transverse emittance (/spl epsi//spl tilde//sub x/) and rms momentum spread (/spl delta/) of short-duration electron pulses produced by illuminating the cathode with light from a mode-locked, frequency-doubled Nd:YLF laser. The electron gun is a GaAs photocathode source designed to operate at 500 kV. We have measured /spl epsi//spl tilde//sub x/ and /spl delta/ for conditions ranging from emittance-dominated to space-charge-dominated. We report these measurements as functions of microbunch charge for different beam radii, pulse lengths, and voltages/field gradients at the cathode, and compare them with PARMELA calculations.