JLab FEL User Meeting and
Laser Processing Consortium Workshop
January 17-18, 2002
The first phase of the Jefferson Lab FEL program, the IR Demo, is now complete, having delivered a total of 2600 hours of user beamtime in the course of 8 months of running that began in October 1999. The IR Demo initiated a new generation of FEL’s, incorporating for the first time critical fundamental advances: superconducting RF accelerator technology and same-cell energy recovery from the spent electron beam, enabling average output power to exceed the previous FEL record by a factor of more than 100. Further, the IR Demo lased at harmonics of the main frequency with record-setting power and delivered tens of watts in the terahertz range, the latter roughly 4 orders of magnitude more than its predecessors. All performance specifications were met or exceeded. A more solid basis for the development of future FEL’s can hardly be imagined. The user procedures functioned successfully beamtime assignment, safety in experimentation, and operations. They will continue to be refined.
The IR ugrade phase now under construction will deliver more than: 10 kW average power, 100 mJ/pulse, 1.5 to 15 mm wavelength, 0.2 – 2 ps pulse length, and 4.68 MHz to 74.85 MHz repetition rate. A further power upgrade based on cryogenically-cooled cavity mirrors is funded and a proposal for increasing power to beyond 100 kW is in preparation. UV light from a separate, dedicated wiggler is expected about a year after the IR upgrade starts up. The major output characteristics will be: 0.25 – 1 mm wavelength, more than 25 mJ/pulse and more than 1 kW average output power. The desire of users for shorter wavelengths and more power has been clearly communicated; the chief issue is absorption by the optical cavity mirrors. Plans also include a beamline for THz radiation, a beamline for Compton femtosecond x-rays and operation of the Helios compact synchrotron.
Results from using the FEL for basic research this year include:
- Wavelength tunability enables interrogation by transient bleaching experiments of designated H or D related defects in Si to measure (e.g.) lifetimes and determine dynamics. Three Physical Review Letters publications have resulted in the last 12 months from this on-going effort (G. Luepke, et al., College of William and Mary).
- A first effort toward PLD of Cr-doped ZnS and ZnSe resulted in apparently good films, despite experimental problems associated with the first run. Absorption measurements in Ga:La:S glasses were also successful, but need to be expanded (H. Rutt, University of Southampton).
- Second harmonic generation (SHG) alters the angular dependence of the intensity of light reflected from a buried Si-dielectric interface, but not in way amenable to interpretation before the advent of present model (D. Aspnes, NCSU).
- A metal AFM tip acts as an antenna to intensely localize light into the adjacent region of the surface, a dimension more than two orders of magnitude smaller than the diffraction limit. Such spatial resolution may be obtained with chemical specificity by using the FEL to provide a light of a wavelength that is strongly absorbed by the molecule of interest (E. Gillman, NSU).
- Intense, resonant IR excitation can selectively distort a molecular ground state, so that subsequent reaction chemistry may follow a different path, affording quantum control over chemical reactivity. The expected non-linear absorption has been demonstrated, but the characteristics of the upgraded FEL are needed to generate enough product to carefully document the reaction pathway (D. Ermer, MSU).
- The wide and growing range of IR FEL pulse profiles (and later in the UV) provides many opportunities to selectively excite molecular or cluster architecture for spectroscopy or transformation (T. Sears, BNL)
The applied science program strives to translate science into technology, and technology into applications that are ultimately deployed into manufacturing. One family of promising candidates is based on ablation – micromachining (drilling), specialized coatings, and production of single-walled carbon nanotubes. A second is based on rapid thermal processing – metal surface glazing, polymer surface amorphization. Earlier, we determined that the native FEL beam, even after the upgrade, does not offer the best parameter range for ablation studies. We therefore sought funding to achieve it by downstream beam conditioning equipment and (since the January meeting) we are successful.
Highlights of applied research results include:
- MEMS as most broadly understood offer a very wide range of opportunities for UV laser micromachining. A UV micromachining workstation is under development at Aerospace Corp. for eventual installation at the FEL (H. Helvajian, Aerospace Corp.)
- Computational modeling of ablation in the nanosecond and picosecond regimes continues to make progress so that most observations can already be understood (X. Xu, Purdue Univ.).
- By tuning to or way from IR wavelengths associated with strong absorption, molecularly-selective ablation is obtained, with benefits from analytic proteomics (MALDI) to organic thin film deposition (R. Haglund, Vanderbilt)
- By tuning to the most strongly absorbed wavelength (5.80 microns), PET film that is ordinarly semicrystalline can be amorphized to a depth that agrees with model predictions. This surface layer can be used to selectively take up entities ranging from molecules to nanoparticles (J. Kelley, JLab/CWM).
- Experimental results from welding, cladding and surface modification of metals provides a sufficient basis for models that show how to now (e.g.) create final-form parts by laser-driven 3-D deposition (J. Mazumder, Univ. of IL).
- FEL-driven synthesis of single-walled carbon nanotubes shows higher production of cleaner material than other methods. Reactor equipment improvements will make it possible to use more of the FEL’s power and increase output further (B. Holloway, CWM).
- The development work done here on drilling fuel injector orifices with solid-state and copper vapor lasers gave excellent results. Taking the next step by using ultrafast conditions with the FEL requires more intense, more widely spaced pulses (C. Hamann, Siemens).
- The ablative response of a series of aerospace defense related materials was consistent with previous results obtained with conventional lasers for the conditions that were obtained with the demo FEL (M. Shinn, JLab).
As always, continuing progress depends on a continuing flow of funds into the program. The FEL leadership team is hard at work. Progress also depends on users obtaining and communicating important results. A vigorous user effort during the shutdown to prepare new experiments is critical. Users are urged most strongly to exert themselves accordingly.