DESIGN CONSIDERATION OF ONE METER LONG UNDULATOR*
Geetanjali Sharma, Roma Khullar, Mona Gehlot, G Mishra
School of physics, DAVV, Indore.
Abstract
One meter long planar undulator i.e. devu20.50p is under fabrication at DAVV, Indore for radiation measurements studies.
It will be of PPM type with twenty periods. Each period will be of 50mm. NdFeB magnets will be used with four magnet blocks
per period .The size of the NdFeB magnet is 12.5mm x 12.5mm x 50mm. It will be designed with variable gap and tapered gap as
well.
PPM undulator
4000
a)
3500
3000
2500
Magnetic field (in Gauss)
There exist interests in undulators [1] and wigglers for
free electron laser. It is proposed to develop a one meter long
PPM undulator using NdFeB magnets of 12.5mm × 12.5mm ×
50mm at DAVV, Indore. There will be four magnets in each
period of length 5cm. There will be twenty periods in the
undulator. We will also use half end magnet i.e.
12.5mm×6.25mm×50mm for end field correction. It will be
designed with variable uniform gap as well as tapered (Fig 1).
12.5 x 12.5 x 50 mm
1
2
3
4
5
2000
1500
1000
500
0
-500
3000
2500
b)
2000
1500
12.5 x 6.25 x 50 mm
1
2
3
1000
500
0
-500
-0.9
-0.6
-0.3
0.0
0.3
0.6
0.9
Position along the magnet length(in cm)
Fig 2
3000
Fig 1
Magnet size 12.5x12.5x50mm
1
2
3
4
5
b)
Magnet size 12.5x6.25x50mm
1
2
3
1000
0
Magnetic field (in Gauss)
The NdFeB magnets are procured from Shanghai
Strong Magnet Industrial & Commercial Co. Ltd, Shanghai,
China. The magnet specifications are Grade- N35, Remanence
1.1-1.28T. The measured characterstics of the vertical and
horizontal magnets are shown in Fig 2 and Fig 3 respectively.
It is proposed to fabricate the lower and upper jaw of
the undulaotr. Fig 4a- 4b shows the sample jaw with 5cm long
and four magnets are clamped with the tapered clamps. The
tapered clamps are preferred in this new design in comparison
to old T clamps [2-3]. The clamps for the magnets are tapered
at an angle of 15 degree to hold the magnets.The magnet
length is 50mm .The jaws are 112 mm in width.The tapered
clamps are screwed from the top. Clamp width are 17.7mm
each.
a)
2000
-1000
-2000
-3000
3000
2000
1000
0
-1000
-2000
-3000
-150
-100
-50
0
50
100
150
Position along the magnet (in mm)
Fig 3
--------------------------*The work is supported by DRDO, Delhi, India.
Magnet fixure
The magnets have to be fixed to the undulator jaws.For
this purpose we have developed a special magnet fixture. Fig 5
shows the schematic of magnet fixture. Part (1) is made of
brass with two magnetic points. Magnet is attached to part (1)
with two magnetic points. On both side of part (1) there are
two slots represented by point (2) which can be moved back
and forth. These slots are set in front first as shown in the
Figure. Now the whole assembly of jaw is placed across the
fixer who is designed according to the size of the jaw at point
(3). The first magnet is placed manually. By rotating the
handle at point (4) the magnet is placed in the jaw, now the
slots on either side of the magnet i.e.at point (2) are moved at
the back and the magnet is fixed by the clamp on either side of
the magnet. Similarly other magnets are placed in the whole
assembly.
Fig 4a
Fig 5
Fig 4b
Laser micrometer
Conclusion
The magnetic field in the undualtor gap is given by
𝐵(𝑇) = 𝐵𝑟 𝑒𝑥𝑝(−𝜋𝑔⁄𝜆𝑢 )
(1)
Where Br is the remnant field , g is the gap between
magnets and 𝜆𝑢 is one undulator period.Eq (1) tells that the
magnetic field inside undulator gap falls exponentially with
increased gap. In order to avoid beam wander along the length
of the undulator, the gap has to be uniform. It requires perfect
size magnets has to be used. We have developed a laser
micrometer for non – contact magnet size measurements. In
the laser micrometer setup , a laser light from the polygon
mirror falls on the magnet. The polygon mirror has eight
facets and it can rotate with fixed RPM between 1000-11000.
The magnet obstructs some of the light that is collimated at the
convex lens. The detector output is connected to a digital
storage oscilloscope to read the magnet size. The size of
magnets is shown in Fig 6 at a accuracy of 20µm (Fig 6).
12485
Magnet size (inm)
𝑔(𝑚𝑚) =
−50
𝐵
𝑙𝑛 (
𝜋
𝐵𝑟
× 10−4 )
(2)
B is in Gauss. From Eq. (2), it is predicted that the field
variations of 6% corresponds gap variation 1mm. The magnet
size are measured by a laser micrometer. Preliminary
measurements on some magnets show a size variation of
45µm. We aim to complete the fabrication by this year end.
References
12480
12475
a)
[1] James A. Clarke, ‘The Science and Technology of
Undulators and Wigglers’,Oxford Science Publications.
[2] S Tripathi et.al. , Optiocs Communication, 284(2011) 350357.
[3] S Tripathi et.al. , Nuclear Instruments and Methods in
Physics Research A, 635 (2011)121-126.
12470
12465
12460
12455
12450
RPM 79 = 7978
12445


12440
12435
12430
5
Variation in magnet size (inm)
We have completed the design of the undulator
jaws of one meter long undulator. The magnets have been
procured. The magnets have been measured by F W Bell
8010Tesla meter. The magnetic measurements are shown in
Fig 2. The magnets are chosen randomly. It is observed that the
magnets give 3564 Gauss and 3795 Gauss respectively,
measured at a distance of 1mm from the magnet surface. This
implies a variation of 6%. The end magnets from Fig 2 gives a
variation of fields around 2%. The variations in Fig 3 is around
7%. The gap can be calculated from Eq. (1) as for 𝜆𝑢 =
50𝑚𝑚 ,
0
-5
b)
-10
-15
-20
-25
RPM 79 = 7978
-30


-35
-40
-45
-50
0
2
4
6
8
10
12
14
16
18
Position along the magnet length (in mm)
Fig 6
20