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Indus-2 |
ISUD |
MAASCD |
ACEFD
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Undulators for FEL and Light Sources |
Three-dimensional AUTOCAD drawing
of the undulator support structure |
The heart of
the FEL is a magnetic device called an undulator. The undulator produces
a static, but spatially periodic magnetic field. They are also used as insertion
devices in synchrotron light sources.
As part of the
design of a far-infrared FEL widely tunable around 80 mm, we have designed a
pure-permanent magnet undulator. The undulator period, i.e. the period of the
magnetic field, is 5 cm, and the undulator will be 50 periods, i.e. 2.5 m,
long. The undulator has been designed to be operated in two independent
sections, so that experiments on step-tapered undulators and multi-cavity FELs
can be performed.
The magnets
used are NdFeB magnets, which have the highest remnant field of around 1.2
Tesla. Each magnet has a cross-section of 12.5mm by 12.5 mm, and a length of 50
mm. The magnets are magnetized along one of the short dimensions. Four such
magnets, with the direction of magnetization rotated by 90 degrees from magnet
to magnet, form one period. When magnets are arranged in this fashion, in what
is called the Halbach configuration, one can show analytically that the
net magnetic field along the axis is sinusoidal.
Because of the
strong magnetic forces, actually holding the magnets in the required Halbach
configuration is difficult and requires ingenuity. We have designed and built a
mechanism for assembling and holding the magnets together. Further, in order to
build a good-quality undulator, it is necessary to control all lengths very
accurately. We have assembled the undulator to a tolerance of within 40 mm,
over the entire 1.25 m length of the undulator section. We have also devised a
mechanism for varying the undulator gap, which is needed for varying the
magnetic field seen by the electron beam, and thus tuning the wavelength of the
FEL.
The undulator
is presently under final assembly, and field mapping of the undulator will
start within a couple of weeks. This is third, and most sophisticated undulator
built in the country, and is also the only variable-gap undulator of the three.
We have also
proposed, in the Xth Plan, to build an infrared FEL at RRCAT, which
can be used in conjunction with the UV light from Indus-1.
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Related
References
(1) Plans for a far-infrared free-electron laser
in India, S. Krishnagopal,
V. Kumar and S. S. Ramamurthi, Nucl. Instr. and Meth. A 375 (1996) ABS 30.
(2) Design study of a
far-infrared free-electron laser, V. Kumar and S. Krishnagopal, Raja Ramanna Centre for Advanced Technology Internal Report,
RRCAT-94/6 (1994). |
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