6 MV RESONATORS

The UCSB MM-FEL, FIR-FEL, and 30 µm FEL all employ overmoded, parallel plate, waveguide resonators, terminated with cylindrical mirrors.

	length	gap	width	Rm1	Rm2
MM-FEL	5.93 m	1.9 cm	12.7 cm	2.97 m	3.60 m
FIR-FEL	5.36 m	1.05 cm	7.6 cm	3.0 m	3.0 m
30µm-FEL	5.27 m	5.0 mm	5.5 cm	2.74 m	2.68 m

These offer unique advantages for FELs that operate at long wavelength:

• Desirable Fabry-Perot characteristics

  

• Extremely low waveguide loss

  

• Small gap permits high undulator fields

The FIR-FEL decay time, measured with a Schottky detector and fast digitizer, is fitted to an exponential to measure loss.
The table shows estimated contributions to this loss, where r_c is coupling coefficient, l_c is coupling loss (backscattering to higher order modes), l_wg is resistive waveguide wall loss, l_d is mirror edge diffraction loss, l_m is mirror reflection loss, and l_t is total loss.