Page 118 - High Power Laser Handbook

88 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s High-Power Fr ee-Electr on Lasers 89

Figure 4.5 An aerial view of the Continuous Electron Beam Accelerator at
Jefferson Lab. The FEL facility building is top center and CEBA is below
ground in a 7/8 mile circumference oval. The nuclear physics end stations
are in the three grass-covered domes at lower right. The cryogenic helium
refrigerator is housed in the building group at the center of the oval.

low dissipation of niobium operated at 2 K allows CW operation at
high gradients, though with the complication of a requirement for
helium refrigeration.
The SRF linac structure, typified by the Continuous Electron Beam
19
Accelerator (CEBA) at Jefferson Lab (Fig. 4.5), produces 6-GeV elec-
tron beams for nuclear physics research using 1497-MHz cavities oper-
ated at 2 K. Ohmic losses are reduced to negligible levels by using SRF
structures (6 W per cavity at typical gradients), while maintaining high-
20
acceleration gradients (5 to 18 MV/m). Among many additional factors,
the gradient achievable depends on frequency, with higher frequencies
producing higher gradients because of the reduced likelihood of a defect
occurring over the cavity surface. As with copper accelerators, higher
average current can be transported in lower-frequency cavities; for
100 mA and above, frequencies below 1500 MHz are desirable. It is
worth noting that the first FEL, the first tapered wiggler oscillator,
22
21
and the first visible lasing on a linac-based FEL operated using the
23
Stanford Superconducting Accelerator. Since its original demonstration,
this linac has been a workhorse, serving several generations of FELs,
because the CW beam yields high stability of power, wavelength, phase,