FREE ELECTRON LASERS
Effect of gain on the optical mode structure of FELs
It is usually believed that a stable resonator cavity is needed
for low gain systems such as FELs. This leads to problems in high
power systems because the mode size at the mirrors is small.
Recent calculations indicate that an arbitrarily small gain
added to an unstable resonator system creates at least one real
eigenmode and stabilizes the system. Further theoretical and
experimental investigations are needed to assess the limits of
this approach and its potential advantages for high power FELs.
Thompson X-ray production and diagnostics
The Jefferson Lab FEL beam produces Thompson X-rays in the 5 to
250 keV range by scattering the light off subsequent electron
pulses. Development of a beamline for these X-rays would allow t
heir use to study materials in conjunction with the IR pulses
of the FEL.
High brightness injector development
The future of both high power FELs and of Energy Recovering
Linacs is dependent on the technological developments in the
injector area. Injectors with high average current, and low
emittance and energy spread, are needed for both these applications.
Development of high-performance injectors will require to
properly model the electron beam, including space charge
effects, in order to perform a system optimization, and then
to experimentally verify the performance of the design.
Effect of optical mode quality on the gain of an FEL
Ideally the optical mode in a free-electron laser is a lowest
order gaussian mode characterized by its Rayleigh range and
waist position. Oddly, there is still no good theory for the
dependence of the saturated gain on even the Rayleigh range
or waist position. A realistic resonator will also have
aberrations that increase as mirrors become thermally loaded.
We do not know what affect these aberrations have on the optical
mode. For a high gain laser the effect must be calculated
self-consistently since the gain medium affects the mode as well.
It is interesting that this most basic dependence of a free-electron
laser is still unknown.
Efficiency enhancement in FELs
Many schemes have been proposed to enhance the efficiency of
the free-electron laser. One can use linear of step tapers
in the wiggler, Modified optical modes, or short optical pulses.
Though many simulations have been done for each of these schemes
one cannot say that any of them are understood. The dependence
of the efficiency enhancement on even simple factors such as
resonator losses and gain has not been worked out. Understanding
these schemes would allow one to optimize the design of a FEL
using rational planning rather than the random walk now used.