SRS BEHAVIOUR WITH A SUPERCONDUCTING $-TESLA WIGGLER ... shield. Each race-track coil is made from separate inner and oitter windings. By using.
© 1983 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.
IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, August 1983
3127
SRS BEHAVIOUR WITH A SUPERCONDUCTING $-TESLA WIGGLER INSERTION
SERC,
V.P. SULLER, N. MARKS, M.W. POOLE ahd R.P. Dares-bury Laboratory, naresbury, Warrington
Abstract A 5 Tesla superconducting wavelength shifting wiggler magnet has been inserted into the SRS lattice. Observations have been made of the behaviour of the Betastored electron beam with the magnet powered. tron tune shifts and modulation of the betatron function have been measured and good agreement obtained Closed orbit changes have been examined with theory. and the stored beam lifetime optimised. The magnet is fully operational and is producing intense x-ray beams for users. 1.
Introduction
The SRS wiggler magnet provides short wavelength x-rays, whilst minimising the beam disturbance to other users. The magnet has a single central dipole to produce the highest practical field, with two coinpensating half amplitude reverse polarity dipoles at the ends of the magnet to return the circulating electron beam to its original closed orbit. The magnet usee superconducting technology to generate a peak field of 5 T in the central dipole, and was designed and constructed at the SERC's Rutherford Appleton Laboratory'. 2.
Wi ggler
WALKER WA4 4AD,
and does not reduce the storage ring's available aperTbe three dipoles are constructed from race ture. track coils, the central dipole using a double vertical layer to. produce the high field region. The 2.2 mm by 1 .l mm niobium-titanium superconductor is stabilised by copper in a ratio 2:l copper to superconductor. A steel yoke surrounds the coil assanbly, which enhances the central field by 0.5 T and provides screening to minimise the external field. The complete assembly is immersed in a liquid helium cryostat at 4.35OK which is insulated by a liquid nitrogen radiation shield. Each race-track coil is made from separate inner and oitter windings. By using a graded currant operating mode the current density is reduced in the inner coils where the highest field levels occur, with a corresponding increase in the outer coil current. This leads to greater safety margins with respect to short sample limits. Adopting this mode resulted in an immediate field increase in excess of 0.5 T under test. At any field level the integrated dipole field length of the wiggler may be set to zero by current flowing in a trim coil wound on the central dipole.
Description
3.
A sectional view of the wiggler is shown in fig.1. The integral warm bare vacuum vessel has internal apertures of 32 mm vertically and 144 mm horizontally
--Liquid Heliunl Inkt/Cuurrent LOsda
UK
Field
Measurements
The wiggler was initially cooled to 4.2 K in a test cryostat where it was confirmed that higher fields were atihiwed with different currents in the inner and outer coils. Optimum conditions were obtained with approximately 25% greater current in the outer coil. A field of 5.5 T in the central dipole was promaximum duced after six quenches. This over-excitation during tests was considered to be necessary because the wiggler kuld operate at a slightly higher temperature, 4.35 K, when in its working cryostat in the storage ring. When mounted in its working cryostat, the trim current required to give zero integrated field length was This measured for a range of central wiggler fields. is shown in fig.2 together with the currents in the inner and outer coils.
,Iron Shielding Radiation Shield ‘Nltrogan
Radiation Shiali Bore Tube
Peak field
Figure Figure
1. Cut-away
view
of
the
superconducting
wiggler.
trim
2. coils
Calibrated currents for optimum wiggler
0018-9499/83/0800-3127501 JO@1983 IEEE
CT)
in the fields.
inner,
outer
and
3128 predicted
4.
Effects
on the
Ring
Storage
11 = [j,
Behaviour and similarly
The wiggler will produce no net deflection in the horizontal plane and no change to the horizontal lattice functions provided that the field is symmetric about the mid-point of the magnet and the field integThe magnet design ensures the first conral is zero. dition and use of the trim coil ensures the second. In the vertical plane it is well known that wigglers produce a focusing force due to edge field effects. Hence orbit shifts can be expected when the wiggler is energised unless the beam is correctly aligned to the wiggler axis.
for
the
B dl12/1,
B2 dll
end poles.
The resulting model is shown by dotted lines in Inserting such elements in a standard lattice fig.4. program results in a reduction in the transverse and longitudinal damping times, an increase in energy These calculaspread and a small enittance increase. tions are suusnarised in table 1.
Other effects on the storage ring optics can be calculated simply in terms of a focusing parameter, K , derived from the mean square value of the magnetic fYeld*:0.09 K =Y Y
< B *>
By vertical
field
E*
E
energy
electron
(Tesla) (GeV)
length L are In the present case K and the wiggler matrix can be sufficiently small thgt the transfer Such reduced to that of a thin lens of strength K L. a lens produces a vertical betatron tune shr .y t. ._
-10.
-2.0.
B
AQV =sKL and a modulation ring:-
of
the
Bv, &r(s) = 2 sin 2nQv
ABy
p,
-3.o* 0
Y
function
K L cos y
around
2(~(s)
the
100
200 300 Dakmx from magnetcentre (mm)
fi
and n refer vo 0
to values
Measured Figure 4. effective poles for dotted.
- No - r Q,)
For the SRS wiggler at at 2 GeV is +0.038 for A tribution4, is shown in fig.3.
at the
5 T the and the
wiggler
calculated expected
fields along the wiggler the hard-edged model are
Vertical
tune
Maximun
vertical
Snergy
4 "nut cell
5
6
7
No
and measured values Figure 3. Calculated cal beta function around the 'storage ring
The wiggler will also make small changes to the energy spread and damping time of the elecemittance, A method for modelling the contribution of tron beam. the pggler to the synchrotron radiation integrals exapproach has been adopted ists . For the SRS a simple by approximating to rectangular field distributions. The field strength, Bl, and pole length, 11, for the central pole are defined by Bill and
= I,
B dl
time
ty(ms)
t,(ms)
oE/E X 10m4
Horizontal
anittance
Overvoltage
factor
Synchrotron
frequency
6(mm-mrad) g fs( kHz)
U+(ps)
5.
6
tx(ms)
time
Measured
Wiggler On 0.038
21.5
f&(m)
time
damping
spread
5.1 Orbit of the verticircumference.
L?Q+, beta
damping
Bunch length
3
shift
damping
Longitudinal
2
1 Wiggler off
value new fi, dis-
Vertical
1
axis. The shown
centre.
Horizontal
0
500
storage
Table where
400
e. 1
7.1
5.0
4.6
2.1
2.0
6.7
7.1
1.51
1.53
6.96
7.30
172 84.1
Storage
26.6
184
Sh.4
Fling Behaviour
Shifts
The trim current data necessary to produce zero integrated field length is stored in the computer control system and applied automatically for any demanded At 2 GeV no horizontal orbit shift can central field. be detected by the beam position monitors to an accuracy of t1.5 nun as the wiggler field is increasd to 5 T. Vertical orbit shifts have been observed when the storage ring orbit has not been well corrected to coincide with the axis of the wiggler. It has been shown experimentally that it is easy to restore the orbit at the wiggler using dipole fields in the multipole correction magnets and at the same time to align correctly the external radiation beam.
3129 5.2
2 GeV, lifetimes of greater than 10 hours recorded with the wiggler at 5 T.
Tune Shift
In the SRS the fractional part of the betatron tune is measured by resonant excitation of the beam Coherent signals are with electrostatic deflectors. detected using position sensitive pick-up electrodes. As expected a vertical tune shift occurs when the wiggler is energised whilst no change in the horizontal tune can be detected to an accuracy of f.002. The vertical. tune shifts have been measured over the range 1.0 to 2.0 GeV for wiggler fields up to 5 T. In all cases the tune shift exhibits the expected dependence on the,square of the wiggler field. An example for 2 GeV is shown in fig.5, from which the amount of shift at 5 T can be seen to be 0.038, agreeing exactly with the calculated value.
0.05 1
As previously mentioned the tune shift can result in the vertical tune lying exactly on the quarter inwhere the lifetime is less than an teger resonance, Moving the tune away by adjusting the lattice hour. guadrupoles immediately removes this effect. The quantum lifetime is restored to the 100 hour level by increasing the applied r.f. voltage by about 140, as predicted. This compensates for the extra radiation (22 keV) emitted in the wiggler and for the increased energy spread. The vacuum deteriorates because the radiation from the wiggler is intense and energetic and if it contacts any new surface within the vacuum chamber outgassing is produced which leads to a temporarily reduced lifetime. It is therefore necessary to steer the wiggler beam with care and to use weak beams durThe lifetime recovers once the suring setting up. faces have become conditioned. 5.5
ob 0
1
Figure solid
5. line
Measured vertical is the calculated
field (TI tune shifts effect.
at
2 GeV. The
The normal working point for the vertical betatron tune at 2 GeV is close to 2.21. Thus the wiggler at 5 T can shift the vertical working point exactly onto a quarter integer resonance where the beam lifetime is as short as 30 mins. It is normal practice to restore the lifetime by moving the tune point downwards through adjustment of the lattice quadrupoles; 5.3
Beta
Ramp Rates
The wiggler is fully computer controlled, with the computer applying the previously calibrated currents to the outer, inner, and trim coils to generate a de manded field. 'Do produce a new field setting the computer changes all currents in a linear ramp. Ekperiments have shown that this ramp can be made to increase the field at a rate of 1 T/min. Much faster rates cause beam losses due probably to eddy currents in the vacuum chamber.
2 Wiggler
When ramping downwards the rate can be made no faster than 0.3 T/min otherwise the quench protection diodes take effect and uncalibrated currents flow in which causes beam loss. the various coils, The usual operating procedure is to inject at 600 MeV and ramp to 2 GeV with the wiggler off. The wiggler is only powered up to 5 T when the beam is stably stored at 2 GeV.
Modulation
6.
The SRS contains 16 multipole magnets which can apply a range of harmonic correction fields. The vertical beta values have been measured by energising the multipoles individually with quadrupole fields and measuring the tune shift. The beta value is then derived from the simple relationship 6 = 4xf
AQ
where AQ is the tune shift, 8 is the lattice beta value and f is the focal length of the multipole as a quadrupole. The strength of these quadrupoles is such that the betatron frequency shifts produced were in the range 20-40 kHz which were measurable to a precision of rl kHz. Figure 3 shows the measured cal beta values with the wiggler agreement is evident.
have been
and calculated vertipowered and good
Conclusions
The wiggler has produced no unexpected behaviour in the storage ring up to its maximum field of 5 T. All measured effects have been in good agreement with predictions based on the r.m.s. field in the wiggler and a simple hard-edged pole model. The wiggler is has been completed stations. 7.
now operational recently into
the
and its first
beam line user
Acknowledgements
The SRS wiggler was built to a Daresbury specification by the Science and Engineering Research Council's Rutherford Appleton Laboratory. Magnetic measuranents were made by D.E. Baynham of the Rutherford Appleton Laboratory. References
5.4
Beam Lifetime 1.
The most easily observed effect of the wiggler is to reduce the beam lifetime. This is due to a combination of three main effects; tune shift, quantum lifetime, and vacuum deterioration. All these can be easily overcome and the lifetime restored to a satisWith beam currents of over 100 mA at factory value.
2. 3.
D.E. Baynham and B.E. Wyborn, IEEE Trans. Magnetics, MAG-17, 5 (1981) p1595 R.P. Walker, Daresbury Iaboratory Preprint, ~L/zxI/P366A (1983). Submitted to Nucl. Instrum. Meth. Nucl. Sci. NS-26, (1979) R.H. Helm, IEEE Trans. ~3824.
