User laboratory capabilities are provided to permit testing of various materials. Since many of the targets have not yet been determined, maximum flexibility in configuration of the facilities is desired. In each lab control of the laser is to be provided. Required light characteristics for each lab are as stated below.

Polarization

Although most applications have no dependence on polarization, in some cases it may be desired to have linearly polarized light for best optics performance. Beam polarization from the wiggler is linear and polarized to better than 1000:1. Some polarization mixing occurs in the resonator due to nonlinear mirror coating response. With standard coatings, maintaining greater than 100:1 polarization at the output should be straightforward.

Wavelength Control

It may be desirable to have control of the wavelength in the user lab to permit study of wavelength dependencies or to lock on to a particular absorption or transmission feature. Wavelength control of the FEL can be by wiggler field strength or electron beam energy. Limitations in range result due to bandwidths of mirror coatings. At lower powers very wide bandwidth coatings can be used, but at the highest average powers narrow band coatings (BW = ~10%) must be used to limit mirror distortion. Ultimately it would be desirable to allow 2x change in wavelength over 1 sec at full power. For initial operation a more modest set of goals has been adopted: In the IR the hybrid wiggler and wide-band metal-coated mirrors should permit 10%/sec change with a range of 10.

Power Control

The power output will require user control in order to optimize the interaction. Initially this will be achieved by varying the PRF of the photoinjector laser consistent with maintaining the round-trip time of the optical cavity. Small changes around the operating power on the order of 20% can be controlled by varying the cavity length. For tune-up a low-power pick-off of the high-power beam will be provided.

Pulse Length

Pulse length control in the FEL would be advantageous to control the depth of the interaction with surfaces and to optimize the Fourier-limited bandwidth. It is difficult to vary the pulse length because of the interdependence of gain and peak current. Initially a factor of 2 variation will probably be possible. Approaches to achieve wider ranges by optical means in the resonator cavity or in the optical transport are under investigation

Pulse Selection

Pulse selection is a technique for separating out a macropulse of limited duration so that single-shot or variable-shot exposures are possible. Microsecond selectability at full power would be desirable, but the means to achieve this has not been identified. Rotating prisms should be able to achieve msec or less exposures at high power, and the laser itself can be turned on and off in 100 usec time scales, although some slew of wavelength and pulse length can be expected.

Rastering

It is desirable to sweep the FEL output at a high velocity over a material surface in order to achieve a uniform exposure of the proper fluence. DuPont is presently designing such a system for installation in the user labs. Initial operation will be at 100 m/s with a 25 cm sweep width.

Focal Spot Minimum

The F number of the available optics and the desired focal depth of field are coupled and wavelength-dependent. The distance from the target to the lens sets restrictions on what is practical and will need to be optimized for best performance in a given application.