Cm. Fortgang, A PURE PERMANENT-MAGNET 2 PLANE FOCUSING TAPERED WIGGLER FOR A HIGH AVERAGE POWER FEL, Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment, 393(1-3), 1997, pp. 385-388
A high-average power FEL is under construction at Los Alamos. The FEL
will have aspects of both an oscillator and a SASE (self-amplified spo
ntaneous emission) device. That is, a high-gain and high-extraction ef
ficiency wiggler will be used with a very low-Q optical resonator. FEL
simulations reveal that a tapered wiggler with two-plane focusing is
required to obtain desired performance. The wiggler is comprised of a
1 m long untapered section followed by a Im tapered section. The taper
is achieved with the magnetic gap and not the wiggler period which is
constant at 2 cm. The gap is tapered from 5.9 to 8.8 mm. The gap, rat
her than the period, is tapered to avoid vignetting of the 16 mu m opt
ical beam. Two-plane focusing is necessary to maintain high current de
nsity and thus high gain through out the 2 m long wiggler. Several mag
netic designs have been considered for the wiggler. The leading candid
ate approach is a pure permanent wiggler with pole faces that are cut
to roughly approximate the classical parabolic pole face design. Focus
ing is provided by the sextupole component of the wiggler magnetic fie
ld and is often called ''natural'' or ''betatron'' focusing. Details o
f the design will be presented.