Status of the development of insertion devices for ELETTRA
ABSTRACT The status of the development of insertion devices for the 1.5–2‐GeV synchrotron radiation source ELETTRA is described. Recent results achieved with a prototype pure permanent magnet undulator and the construction and performance of a prototype hybrid multipole wiggler are presented. The parameters and design of the first undulator that will be constructed for ELETTRA, U12.5, are also presented, and some details are given about the magnetic measurement system.
Status of Development of the Insertion Devices for ELETTRA
99, 34012 Trieste,
The status of development of ins&on devices for the 1.5-2
GeV synchrotron radiation source ELETTRA is described,
including details of the prototype undulator, multipole
wiggler, mechanical support structure and vacuum chamber.
Plans for a novel source of circularly polarized radiation are
A 1.5-2 GeV third generation synchrotron radiation source,
ELETTRA, is under construction in Trieste, Italy [1,2]. The
storage ring design has been optimized for the inclusion of up
to 11 insertion devices (IDS), which will provide high
brightness radiation from pure permanent magnet undulators
(U), hybrid multipole wigglers (W), and special sources of
circularly polarized radiation. Table 1 summarizes the main
panmcters of the devices presently forseen for the initial phase
Table 1. Preliminary ELECTRA insertion device parameters.
Type-Period (cm) Field (T) No. of Periods
Spectromicroscopy U 7.3
II. INSERTION DEVICE SUPPORT STRUCTURE
For reasons of ease of construction, flexibility in use and cost,
a standard mechanical support structure for both undulators and
wigglers will be used [3,4]. The structure is 1.5 m long, so
that each of the 11 ID straight sections in ELETTRA will
accommodate up to 3 such structures. Following a design
study made by Sincrouone Trieste [43, a prototype carriage
was L:onstructcd by CONTEK, Italy, and delivered in June
1990 (see fig. 1). Since then, a series of detailed measurements
has been carried out to assess the mechanical performance,
including the effect of a magnetic load, as well as further finite
elcmcnt structural calculations. The mechanical measurements
were made using a HP 5528A laser interferometer and two
Heidcnhain optical rulers. The following values, that arc
within the specification, have been measured indcpcndent of
magnetic load : gap setting accuracy +lO pm, gap
reproducibility .c 10 pm, flatness of the reference surfaces 30
urn, parallelism of the I-beams 60 pm (for gaps between the I-
beams I 5.50 mm).
The mcasurcd deformation of the structure due to the magnetic
load of the undulator prototype has been found to be
significantly larger (5 times) than the value foreseen with the
fist structural model used during the design stage (using beam
elements)  but in good agreement (7% of difference
including the load determination error) with a more detailed
finite element model prepared to take into account the true 3D
geometry of the structure . A gap variation along the beam
axis of 1.4 pm/kN and a parallelism error of 12.7 prad/kN in
the plane normal to the beam axis are induced by the magnetic
force. ‘Ibis leads to a total gap error of + 5.5 l.trn in the case of
the undulator prototype which is higher than the specified
value but is still acceptable. Calculations made with the
accurate structural model have shown that a vertical thermal
gradient of 1 “C/m and an ambient temperature stability of +
2.5 “C do not cause unacceptable deformations of the insertion
device support structure.
Figure 1. Prototype of ELETTRA insertion device support
stn~cture and undulator magnet.
III. UNDULATOR PROTOTYPE
Initial measurements of the NdFcB blocks for a 56 mm period
pure permanent magnet undulator, using a Hclmholtz coil
system to determine the total block magnetizations and a Hall
plate system to make point measurements, were described
previously . It was clear from the results of these and later
measurements that to the level of accuracy required the blocks
could not bc modelled as idcal CSEM material, even if
different magnetization values were used for the upper and
lower faces of the blocks. Instead a purely empirical approach
had to be adopted, but still relying on the fact that to a good
approximation the field from different blocks superimposes
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linearly. In order to allow the main field component in the
undulator to be calculated at the peaks and zeros each block has
been measured at 21 points above and below the block, with a
spacing of one quarter of the period along the beam direction,
at a distance corresponding to the minimum magnetic gap (20
mm). In addition a new stretched-wire flipping coil bench has
been used to measure the field integrals in bolh planes, above
(u) and below (d) the blocks, along the beam direction and
displaced transversely by + 15 mm. The average time required
per block was 14 min. for the Hall plate and 30 min. for the
flipping coil. Table 2 shows the results from the flipping coil
for “A” and “C” blocks (vertical magnetization) and for “B”
blocks (horizontal magnetization) .
Table 2. Results of measurements of the field integrals for the
prolotype undulator blocks, in units of PTm.
A (111 blocks) B (117 blocks) C (24 blocks)
mean rms mean rms
(Iy”+Iyd)/z 2956.1 28.5
A computer program has been written to arrange the blocks in
Ihe structure using a simulated annealing algorithm 
involving the following components to the “cost function”
that was minimized: (a) rms field error at the peaks (AB/B) and
crossing-points, (b) first and second field integrals, (c)
horizontal trajectory ‘straightness’ and (d) rms vertical
displacement. A total cpu time of 5 hours was required for
calculating the arrangement of the 222 blocks in the 1.5 m
The undulator was constructed according to the specified
arrangement and then measured using both benches. Figure 2
shows the measured field and trajectory at minimum gap, and
Table 3 summarizes various parameters as a function of
Figure 2. Measured magnetic field (upper) and trajectory
(lower) in the prototype undulator at minimum gap (2 GeV).
Compared to a random configuration of the blocks the
performance is very much better, at least an order of magnitude
for AB/B, but not as good as predicted, particularly for the
field integrals. A factor which probably contributed to this
error is the variation in temperature during the block
measurements (k4.5 “C). Attempts to measure and correct for
the temperature variation have been unsuccessful, possibly due
to the thermal time constant of the blocks, about 20 minutes,
which means that the surface temperature can be different from
the bulk. Use of a temperature stabilized laboratory in the
future should eliminate this error.
Table 3. Results of initial measurements of the ELETTRA
gap (mm> W73
AB/B rms % Ix (NT m) Iy (NT m)
To improve the quality of the undulator field experiments are
presently being carried out using shims made from Fe-Si
laminations, placed on the surface of the blocks. So far, very
good agreement has been obtained between measured changes
in both field amplitude and field integral and model
calculations . Efforts are presently being directed towards
compensation of residual errors over a range of operating gap.
Correction coils for fine adjustment of field integrals have also
been fabricated and will soon be tested.
IV. MULTIPOLE WIGGLER PROTOTYPE
The optimization of the wiggler parameters to give the
maximum field level with the given constraints on gap and
radiation opening angle resulted in a period of 125 mm, with a
predicted field of 1.48 T . Since then the plan to include an
additional experimental station on the wiggler beamline which
will take radiation from the outer part of the radiation fan has
necessitated a modification of the parameters, to take into
account the effect of the third harmonic field component on the
critical energy of the radiation emitted off-axis . Figure 3
shows the relation between field and angle, calculated using
POISSON, for the original design which has a third harmonic
component (B3/BO) of 12 %.
Figure 3. Field amplitude as a function of horizontal emission
angle for the previous multipole wiggler design (lower solid
curve), a sinusoidal model with the same field amplitude
(dotted) and the present design (upper solid curve).
It can be seen that the field (and hence critical energy) is
significantly reduced for angles above about 2.5 mrad, and that
the total radiation angle is reduced by a factor 0.87, compared
to the case of a purely sinusoidal field distribution. To
overcome these effects the period length has been increased to
140 mm and the field to 1.55 T, keeping the same transverse
block dimensions. Figure 4 shows that the modified design
gives significantly improved performance at off-axis angles.
The increase in field compensates the reduction in number of
full poles from 21 to 19 per section at energies above 10 keV.
Components to construct a 0.5 m prototype will be delivered
by the end of May, to test design calculations, assembly
procedures, and different end configurations.
V. CIRCULARLY POLARIZED RADIATION SOURCE
Various schemes have been studied for the production of
circularly polarized radiation [3,10], in particular those that
satisfy users’ demands for a relatively broad spectral range (200
cV - 1 kcV) with a rapid variation of the helicity (> 20 Hz).
The solution favoured at present is a novel electromagnetic
elliptical wiggler (see fig. 4), similar to the elliptical wiggler
of ref. [l 11, except that the horizontal field component is
gcncratcd by an a.c. electromagnet in order to modulate the
hclicity of the on-axis radiation.
Figure 4. Preliminary design of an electromagnetic elliptical
wiggler source of circularly polarized radiation.
The horizontal gap is necessarily large in order to allow space
for the pure permanent magnet structure which produces the
vertical wiggler field, and so a relatively large period has been
chosen (230 mm) to reduce power supply demands. With this
period a maximum field of 0.6 T, in order to restrict the
opening angle of the radiation, gives an acceptable critical
energy of up to 1.6 keV (2 GeV). Table 3 gives preliminary
parameters for the device, which using a special ID support
structure 3m long will give 25 times the flux of a bending
magnet source with a modulation rate up to 100 Hz.
POISSON calculations have been carried out to calculate field
amplitude and inductance. A prototype will be constructed in
the following months using existing permanent magnet
blocks and an a.c. power supply.
Table 3. Preliminary parameters for an electromagnetic
elliptical wiggler, at 100 Hz excitation.
Number of full poles
Ampere-turns per pole, peak
The effect of the device on the dynamic aperture of the storage
ring has been calculated and is tolerable [ 121.
VI. INSERTION DEVICE VACUUM CHAMBER
Following the design and specification of a narrow gap (15
mm internal, 20 mm extcmal) vacuum chamber employing
NEG pumping [4,13], an order for the construction of a 2.4 m
half-length prototype vacuum chamber has been placed with
E.ZANON, Italy, including a tapered section 240 mm long
which incorporates a pumping tee. The chamber will be
constructed from stainless steel 316 LN. The elliptical beam
chamber will be obtained by deformation of a 54 mm diameter
circular pipe, formed from 1.5 mm thick sheet material, with
machined pumping slots 16 mm long x 10 mm high spaced
every 40 mm. The ante-chamber will be formed from 3.5 mm
sheet, machined then TIG welded to the beam chamber. After
delivery in May 1991 and subsequent installation of the NEG
pumping strips it will then undergo a thorough series of tests
in the Vacuum Laboratory.
 ELETTRA Conceptual Design Report, April 1989,
 B.Diviacco and R.P.Walker, Proc. 2nd European Particle
Accelerator Conference, Nice, June 1990, p. 1359
 C.Poloni and R.P.Walker, ibid, p. 1362
[5j C.Poloni, Report in preparation
[6j DZangrando and R.P.Walker, ibid, p. 1365
[q A.D.Cox and B.P.Youngman, Proc. SPIE 582, p. 91
[S] J.Chavanne et. al., European Synchrotron Radiation
Facility Report ESRF-SR/ID-89-32, Nov. 1989.
 W.Hassenzahl, private communication
[lo] B.Diviacco and R.P.Walker, Nucl. Instr. Meth. Phys.
Res. A292 (1990) 517.
[ll] S.Yamamoto and H.Kitamura, Jpn. J. Appl. Phys. 26
 L.Tosi and R.Nagaoka, this conference
 T.Monaci and C.Poloni, Sincrotrone Trieste Report
ST/M-7X-90/18, Sep. 1990
A.Wrulich, this Conference