Page 123 - High Power Laser Handbook

92 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 93

4.4 Status

Presently there exist only three FELs in the world operating at pow-
ers above 10 W—the Japan Atomic Energy Agency’s (JAEA’s) FEL,
the recuperator FEL at the Budker Institute, and the Jefferson Lab’s
IR/UV Upgrade. These three FELs operate with energy recovery to
improve overall efficiency, reduce RF power costs, and lower back-
ground radiation. The JAEA’s system (Fig. 4.8) operates an FEL with
millisecond pulses in the kilowatt range. It is powered by an 8-mA,
30
17-MeV superconducting accelerator that produces 0.4-nC, 12-ps-
long pulses. It first lased in August 2002 at 22 µm and extracted
greater than 2.5 percent energy from the electron beam. CW opera-
tion was precluded by the capacity of the helium refrigerator.
The Budker system has energy recovered greater than 30 mA of
average current. It also recently achieved two-pass acceleration and is
on its way to a five-pass recirculation up to 80 MeV, followed by five
passes down in energy (see Fig. 4.9) with multiple wiggler systems.
31
The Budker system has produced more than 400 W of average power
at 60 µm. It uses 180-MHz copper-lined RF cavities. The system runs
1.5-nC, 70-ps-long pulses at 22.5 MHz.
The highest-power FEL in the world is Jefferson Lab’s IR Upgrade
33
FEL, which has produced 14 kW of average power at 1.6 µm (Fig. 4.10).
It is an upgrade of the IR Demo laser, which successfully demonstrated
2 kW of average power while energy recovering the electron beam ener-
gy. The upgrade produces up to 9.3 mA of average current in 130 pC
32
pulses at 75 MHz. The quality of the electron beam is sufficiently high

17 MeV loop First arc
Half chic
Undulator
2.5 MeV injector
Return Beam
230 kV arc dump
E-gun Merger 500 MHz SCA
(7.5 MV × 2)
500 MHz SCA 20 m
(1 MV × 2)
FEL wavelength is 22 µm and electron bunch charge is 0.5 nC.
The injector consists of 230 kV thermionic cathode DC gun, 83.3 MHz subharmonic
buncher and two single-cell 500 MHz SCAs.
17 MeV loop consists of a merger chicane, two five-cell 500 MHz SCAs, a triple-bend
achromat arc, half-chicane, undulator, return-arc, and beam dump.
First lasing in August 2002.
Figure 4.8 The JAEA FEL. The beam energy is 17 MeV with bunches of 0.4 nC at
20.8 MHz repetition rate. Light output is at 22 µm in 1 ms pulses at 10 Hz.
(Courtesy R. Hajima)