The IR FEL and driver discussed in this site is the first of several FELs under development at Jefferson Lab. The goal of this process is to build a pair of industrially and militarily useful FELs - a 100 kW UV device for photo-processing and a 1 MW IR device for Naval defense applications. An evolutionary pathway to these machines is presented in the following figure.

This evolution requires, globally,

• source technology development
• RF window development
• SRF development - cavities with

  - lower frequency/lower HOM impedance cavities

  - higher gradient

Along this path, we first move from the 42 MeV/3 - 6.6 micron 1 kW machine to a 75 MeV/1-6.6 micron 1-2 kW machine. This requires 1) adding a second cryomodule to raise the energy and 2) adding a new optical cavity/wiggler to reach 1 micron. If the long-wavelength IR output is to be retained (left schematic), the original FEL should be kept in the system; if the upgrade is to simply move to shorter wavelength (right schematic), the FEL can be replaced by an optical cavity (such as an R5) and wiggler that can manage the optical beam power for 1 micron output. This upgrade is the subject of a subsequent detailed discussion . It is explicitly assumed that the time scale for this upgrade (planned in 1998-9) will require a design solution in late 1997, prior to definitive conclusions about the impact of CSR. The schematic on the right therefore assures the success of machine operation at 1 micron regardless of the results of the CSR experiment (assuming, of course, CSR doesn't overwhelm operation of the 42 MeV machine!); it does not insure that the "broad reach" 1-6.6 micron machine on the left will be successful.

The next upgrade is to a 100 MeV/~1 - few (10?) kW IR machine. This will, in general, require addition of a third cryomodule, further development of the FEL(s) proper, and source technology improvements to provide an injected beam current giving the desired power reach. Both the gun and the RF windows must provide performance beyond that presently available to move beyond ~1 kW at this stage. Machine performance in the presence of CSR effects and characterization of those effects must be demonstrated, as this device is constrained (by available space and the desire for future upgrades to a compact Naval device and a multipass industrial demo) to using a backleg FEL. In principle, the machine length could be extended, but this would require significant civil construction and would violate the Navy paradigm of a "small" machine.

Once 100 MeV/~1 kW is obtained in the IR, the family tree branches. One path is motivated by industrial interests, the other (primarily) by potential military applications. Given the 100 MeV/1 kW machine and a demonstration that CSR and space charge are not prohibitive effects, addition of a second recirculation and an appropriate optical cavity/wiggler system will provide a 200 MeV/1 kw UV demo system. This machine requires only modest improvements in source parameters beyond those demonstrated in the original 42 MeV/1 kw IR demo, and serves as test bed for concepts to be used in future industrial machines. This is shown as the left branch of the family tree.

As injector and source development continues, both the 100 MeV and 200 MeV machines evolve. Improvements in gun technology and RF windows allow generation and acceleration of much higher current beams than those that are presently manageable. This, in turn, allows driving the laser at much higher power - the goals being 100 kW in the UV and 1 MW in the IR. We note that the required higher beam currents are to be achieved by increasing the repetition rate from 37.5 MHz to, perhaps, as high as 750 MHz, NOT by increasing the single bunch charge! Single bunch collective effects issues then remain identical to those in the original 42 MeV driver - only coupled bunch effects become more serious. To this end, SRF development must provide cavities with lower HOM impedances that those in the present 1500 MHz CEBAF cavities; this will probably be accomplished at least in part by moving to lower frequencies such as 750 (or even 500) MHz. Coupled to this is the desire for a smaller machine footprint for production versions of either an industrial or a military device. This will require improvement in cavity gradients so as to reduce machine size.

Last modified: 10 March 1997
http://www.jlab.org/~douglas/ is maintained by: douglas@jlab.org