Announcement

**Dr Mordrid** · 25 March 2010, 10:37

Just for the record the Navy Lab has also been working on the FEL with the Thomas Jefferson National Accelerator Facility for several years. A good discussion of FEL can be read at the National Academies Press here., and a summary of its advantages follows;

PROPERTIES

Many of the properties of the FEL can be inferred from the above discussion. These properties are summarized below.

1. Tunability. Because the FEL uses a single gain medium, the relativistic electron beam, and because the resonant condition can be easily tuned by changing either the electron beam energy or the magnetic field strength, FELs are broadly and easily tuned. A factor-of-10 tunable frequency range has already been demonstrated with the same accelerator and undulator.

2. High peak power. Because waste energy is carried away at nearly the speed of light and because the lasing medium cannot be damaged by high optical fields, FELs can produce very high peak powers. Gigawatt peak powers have been demonstrated.

3. Flexible pulse structure. Because the pulse structure of the radiation follows the pulse structure of the electron beam, the mature RF technology of linear accelerators can be used to manipulate and control the FEL pulse structure. Picosecond pulses with sub-picosecond jitter can be produced, the interval between pulses can be varied, and there is the possibility of producing complicated pulse structures.

4. Good laser characteristics. FELs easily achieve desirable properties associated with conventional lasers, such as a single transverse mode, high spatial and temporal coherence, and flexible polarization properties.

5. Broad wavelength coverage. Because the gain medium is transparent at all wavelengths, FELs in principle can produce radiation at any wavelength. In practice, electron-beam energy, current, emittance, and energy spread requirements become more stringent as the wavelength decreases, and the cost, size, and complexity of the FEL are therefore higher at shorter wavelengths. FELs can have significant emission at harmonics of the fundamental frequency given by the resonance condition, and harmonics can be used to extend operation to shorter wavelengths than would be practical using only the fundamental frequency. At present, the shortest wavelength that has been achieved in an FEL is 240 nm, and significant use of FELs for scientific research has been restricted to the infrared. There are proposals to build vacuum ultraviolet and x-ray FELs. (hmmm....a visible light to UV or even X-RAY laser for use in a vacuum whose phase properties can be adjusted? Another Star Trek device come true?)

6. Size and cost. Because it requires an electron accelerator with its associated shielding, the FEL has not been a device that can be placed in an individual investigator's laboratory and be operated and maintained by graduate students whose primary expertise is in other areas of science. FELs have been used principally in central facilities, where their utilization in scientific research involves associated costs of maintaining and operating the facility in addition to the cost of the device itself. FELs have been used by one user, or at most a few users, at any one time, while synchrotrons can be used simultaneously by many users. For example, the APS now under construction at Argonne National Laboratory can have 34 insertion device beamlines and 35 bending magnet beamlines, all of which can be used independently by different users at the same time. The basic science of electron accelerators is well known, but virtually all the effort spent on accelerator physics has been to reach higher energies with bigger and more costly machines. Relatively little effort has been devoted to producing the smaller and less expensive machines that would be most useful for an FEL. (apparently until Boeing's efforts, that is....)

**Mehen** · 25 March 2010, 12:58

YES - I was starting to miss these threads Doc!

Announcement

Navy's ship laser

Navy's ship laser

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