- Email: [email protected]
Performance of the modified Daresbury SRS
392
Nuclear Instruments and Methods m Physics Research A282 (1989) 392-397 North-Holland, Amsterdam
PERFORMANCE OF THE MODIFIED DARESBURY SRS E.A. HUGHES, J.N. CORLETT, D.M. DYKES, J.S. MACKAY, P.D. QUINN, V.P. SULLER, S.L. THOMSON and R.P. WALKER SERC Daresbury Laboratory, Warrington WA4 4AD, UK
In October 1986 the Daresbury synchrotron radiation source (SRS) was shut down for a period of five months during which major modifications were made . The primary purpose of the shutdown was to greatly reduce the emttance by the addition of extra quadrupoles which doubled the number of FODO lattice cells. Other changes included additional correction magnets and major revisions to the vacuum system . Recommissioning started in March 1997 and scheduled operation resumed in June with beams of 75 mA at 2.00 GeV. By June 1988 the standard operating current had been increased to 200 mA with a beam lifetime of 25 hours. Thus paper describes the installation of the high-brightness lattice, the recommissioning of the SRS and the performance of the machine m its new configuration. l . Introduction The Daresbury synchrotron radiation source (SRS) was designed in 1975 as the first dedicated source of synchrotron radiation. A simple, but flexible FODO lattice was used which permitted easy access to the synchrotron radiation . At the design energy of 2 GeV the horizontal electron beam emittance was rather large at 1.5 x 10 -6 mrad, but low coupling of less than 1% gave a reasonably high brilliance in the vertical plane. After the SRS had become operational in 1981, consideration was given to increasing the radiation brilliance through a reduction in the electron beam emittance by means of a suitable modification to the lattice, with the overall constraint that the dipole magnets and their attached beam lines must remain unaltered. Vanous schemes were studied; the one finally selected was again a FODO structure, but with the unit cell number increased from 8 to 16. The advantage of this system was that it allowed most of the equipment in the straight sections, such as injection and rf equipment, to remain unchanged, but still gave over an order of magnitude reduction in emittance. A description of the proposal was given by Saxon [1] and final approval to proceed was obtained in June 1983 . A comparison between the original-lattice SRS-1 and the modified-lattice SRS-2 is shown in fig. 1 and their parameters are listed in table 1. It is apparent that the major difference between the two structures is the doubling of the number of cells in SRS-2 by the installation of a new D-quadrupole magnet at the upstream end of each straight . This allows the storage ring to be oper-
* Present address: Trieste Laboratory, Trieste, Italy. 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
m
SRS-1
-
E_ :7
fu
FS ci
SRS-2
CD
DIPOLE 120m -
CF
~0
F9Cr
DIPOLE
m DIPOLE -6
~ CF
E3 v -
DSXr
0 m
Fig. 1 Comparison between old and new SRS lattices . ated at a much higher tune, 6.2, as compared with the original 3.2, and with a greatly reduced dispersion function and momentum compaction factor . The nett inTable 1 Lattice parameters Parameter Circumference [m] Lattice type Number of cells Maximum energy [GeV] Betatron tune, horizontal vertical Momentum compaction factor Uncorrected chromaticity, horizontal vertical Natural horizontal emittance at 2 GeV [mmmrad] Natural bunch length at 2 GeV [mm (FWHM)]
SRS-1
96 .0 FODO 8 2 .0 3.18 2.22 0.14 -9.0 +0.5 1.5 60
SRS-2
96 .0 FODO 16 2.0 6.18 3.22 0.03 -10.9 -4.1 0.11 35
E.A Hughes et al. / Performance of the modified Daresbury SRS Table 2 Brilliance comparison between SRS-1 and SRS-2 Parameter Horizontal beam size Vertical beam size Vertical electron divergence Photon emission angle Photon beam divergence Brilliance ratio
SRS-1 SRS-2 [mm FWHM] [mm FWHM] [mrad FWHM] [mrad FWHM] [mrad FWHM]
14 .4 0 .57 0 .19 0 .38 0 .42 1
2 .6 0.24 0.09 0.38 0.39 15
crease in photon beam brilliance expected is 15, as detailed in table 2 . The space into which the new quadrupoles had to fit was very restricted by the radiation beam pipes emerging from the dipole magnets and by the path of the injected electron beam . These quadrupoles have a physical length of 27 .5 cm, an inscribed circle of 35 mm radius and operate at a field gradient of up to 12 T/m . They have been described by Walker and Poole [2] . To achieve the required small cross section for the new D-quadrupoles it was necessary to make use of the smaller vacuum chamber which is allowed by the reduced beam size . However, this reduced the vacuum conductance between the major component of the gas load - which is where the synchrotron radiation hits the absorbed near the downstream end of the dipoles - and the ion pumps in the straights . To maintain an average vacuum pressure of the order 10 -9 Ton in the presence of several hundred milliamperes beam current, it was necessary to double the number of ion pumps in SRS-2, these being added to the straight sections. Other vacuum system changes made were to mount ion pumps directly on each of the 4 rf cavities and to install extra vacuum valves . The latter permit smaller sections of the storage ring to be vacuum-isolated, especially each of the rf cavities . The lattice chromaticity, arising both from the natural chromaticity and the sextupole errors in the dipoles, is corrected by two families of sextupoles . The F-family, adjacent to the F-quadrupoles, are the original 16 sextupoles designed for SRS-1 . The D-family consists of four new sextupoles uniformly distributed between the straights . There is insufficient space to include more than four D-sextupoles, which are 20 cm long and produce a sextupole field of 200 T/m2 within the inscribed radius of 64 mm . Orbit correction and control is extremely important for a synchrotron radiation source, and to maintain a good measurement capability of the orbit position with the increased tune, a new set of 16 horizontal and 16 vertical beam monitors was designed . The original monitors were quarter-wave transmission lines which proved to be inaccurate due to multiple modes within the beam vacuum chamber. The new monitors are but-
393
ton pickups which are flush with the vacuum chamber walls and are an integral part of the chambers . The horizontal part of the chambers . The horizontal monitors are within the F-quadrupoles, and the vertical monitors are adjacent to the D-quadrupoles. The existing SRS-1 orbit correction elements were retained, namely backleg windows on the dipoles and horizontal and vertical dipole trims in the multiple magnets. In addition, for SRS-2 a new set of vertical steering magnets are located adjacent to the D-quadrupoles. These 15 cm long C-shaped dipoles produce a field of 115 G m at maximum excitation of 5 A . By simultaneous computer control of the appropriate vertical correction elements it is now possible to produce vertical orbit adjustment localised to a single dipole, which is very useful for adjusting individual radiation beams . Also installed in the lattice are two 20 cm long octupoles which can generate fields of approximately 250 T/m3 . These will be used to study and control various instabilities . 2 . Installation and recommissioning Although approval for the lattice modification was given in 1983, it was intentionally delayed until October 1986 in order to complete the scientific programme already in place at the facility. Up to the start of the shutdown which then began, SRS-1 was regularly operating with 300 mA initial stored beam current and beam lifetimes of 10 hours . The modification programme was planned to take 42 months from October '86 to mid-February '87 . Since over 60% of the vacuum chambers would be replaced, it was possible to have much of the equipment ready as sub-assemblies at the start of the shutdown . However, it was still necessary to remove totally the equipment from each of the 16 straights, modify the components such as quadrupoles, sextupoles, kickers, etc., which were to be re-used, and combine them with the new subassemblies . The dipole magnets with their vacuum chambers and beam ports were left in place after being let up from vacuum to an atmosphere of dry nitrogen . Due to some unexpected problems in modifying the equipment for re-use, the shutdown took two weeks longer than planned and the first commissioning with injected beam started on 1 March '87 . SRS-2 commissioning took place rapidly despite the fact that major systems had been radically changed . Within two weeks a beam had been accumulated and ramped to over 1 GeV. By the end of March 75 mA had been ramped to 2 GeV, the beam orbit had been fully corrected and the beam size measured. During April and May one quarter of the vacuum chambers were given an in-situ bakeout and the radiation beam lines 1 . SOURCES
394
E.A . Hughes et al. / Performance of the modified Daresbury SRS
were brought back into operation . Further beam studies were made and the 5 T superconducting wiggler was recommissioned. Finally in mid-June the facility came back to routine 24 hours/day operation for experimental science .
3 . SRS-2 performance In the 11 months that the facility has been back in scheduled operation the beam current has increased as understanding of the electron beam behaviour has improved . The increase in average initial beam current at 2 GeV is shown in fig . 2, where the averaging is made on a monthly basis . At present this current is normally about 200 mA, although it is usually possible to accumulate more, up to about 300 mA, at the injection energy. However, as the energy is raised from 600 MeV to 2 GeV transverse beam instabilities occur intermittently which result in beam loss to about the 200 mA level . These instabilities are presently being studied [3] . SRS-2 also operates, according to schedule, in the single-bunch mode . This is normally performed with the lattice in a slightly detuned, lower-brilliance mode in order to increase in natural bunch volume and reduce the natural chromaticity. In this mode the radial betatron tune is decreased from 6 .2 to 4 .2, and the emittance increases from 0 .11 x 10 -6 to 0 .27 x 10 -6 mrad . The maximum current which has been accumulated into a single bunch is approximately 50 mA, but this limit has not been fully explored [3] . At the present, a limit of about 30 mA is experienced due to the control circuits of the injection kickers receiving interference from the orbit harmonics radiated by the single bunch . This results in kickers malfunctioning when the single-bunch current approaches this value .
250
a E m
U Em
m m m a
200
150
100
so
0
Fig . 2 . Improvement in SRS-2 beam current .
t m E ECu
m mm m a
Fig. 3 . Improvement in SRS-2 beam lifetime .
The beam lifetime, averaged over each beam fill and over each month, has shown a steady improvement, as indicated in fig . 3 . The two instances where the general trend for improvement was reversed happened when some of the distributed ion pumps were out of operation . The importance of these pumps in achieving good lifetime is demonstrated by the comparison between February and March. In March all pumps were active whilst in February two distributed pumps, representing 13% of the available distributed pumps and only 4% of the total pumping speed, were out of action and yet the beam lifetime decreased by 30% . Further information on the performance of the vacuum system is given by Reid [4] . In the single-bunch mode the beam lifetime is considerably reduced. A reduction is to be expected due to the Touschek effect and plot of inverse lifetime against single bunch current, shown in fig . 4, does show the expected linear variation . The zero-current lifetime is in good agreement with the vacuum lifetime measured in a multibunch, but the Touschek lifetime is 2 or 3 times shorter than predicted from the measured beam dimensions and lattice parameters . Complementary measurements, that is to say lifetime measured as a function of bunch volume at a constant single-bunch current, are shown in fig . 5 . In these measurements the beam dimensions in the three normal axes were measured as the horizontal to vertical emittance coupling ratio was varied . The Touschek lifetime is derived from the measured lifetime after correction for the zero-current lifetime obtained from fig . 4 . There appears to be agreement with the predicted Touschek lifetime when the bunch dimensions are much larger than normal, but for small coupling and bunch volume, again the Touschek lifetime is 2 to 3 times shorter than expected . The bunch length achieved in SRS-2 is of great
E.A . Hughes et al / Performance of the modified Daresbury SRS
395
020
015 N 0 G
m E
010
m
m m
005
c
20
sr«;Le
Bll4CH c,1
r MA
Fig. 4. Linear variation of inverse lifetime against current . The solid line shows the calculated Touschek lifetime . interest because of the information it can provide on the vacuum chamber impedance and any instability processes which are taking place, particularly at high bunch currents when operated in single-bunch mode . Comprehensive bunch length measurements have been made as a function of bunch current, rf voltage and energy. The results are reported in detail by Poole [3] so that it is sufficient merely to report here that, compared with SRS-1, SRS-2 no longer shows the clear signature of turbulent lengthening . Nevertheless, the bunch length is longer than the natural value, especially at low energy. For example, at 600 MeV the low-current bunch length has been measured as ranging between 55 and 65 ps (sigma) depending on the rf voltage. This is to be
Fig. 6 SRS-1 beam image. compared with the corresponding natural values of 11 and 20 ps . However, at 2 GeV the measured length of 68 ps agrees with the natural value. For a synchrotron radiation source such as SRS, the measurement of the electron beam size is of prime importance and has received much attention. The first rough measurements were made from a focussed image of the beam, using the visible component of the synchrotron radiation. This was viewed with a TV camera and figs . 6 and 7 show the comparison between SRS-1 and SRS-2 made with the same focussing system but with different screens. The small divisions in each case are millimeters and it is apparent immediately that in the SRS-2 the horizontal beam size has been reduced by about 5 times, as predicted. Precise measurements of the size of the focussed image have been made using photo diode arrays as described by Mackay [5]. At 2 GeV the horizontal beam
100
80 1
70-
O t
60-
m
Y
m
5040
L U
30 ,
H
2010 -
100
200
300
400
500
600
700
Bunch Volume (mm3)
Fig. 5. Variation of lifetime with bunch volume .
N MI. OEM No
Fig. 7. SRS-2 beam image. 1. SOURCES
396
E.A . Hughes et al / Performance of the modified Daresbury SRS diagnostic X-ray beam line at another location on the storage ring . Although it can be used only at higher beam energies, where there are enough X-rays, this camera has the advantage of producing a 4 times mag-
nified image of the electron beam which is particularly
useful in measuring the very small vertical beam size. The pinhole confirms that at 2 GeV the horizontal beam size agrees
e
to within 10% with the predicted
natural size and that the vertical emittance coupling is
E
4% .
É . 0 N
Equally important for a radiation source is the position and angle of the electron beam at the origins of the beam lines. The beam orbit in SRS-2 is measured with a set of button pickups which are multiplexed on demand
through the computer control system . This then displays a set of 16 horizontal and 16 vertical beam positions
Fig. 8. Horizontal beam size measured by a photodiode array during the energy ramp, at 0.1 and 1.0 mA per bunch.
which are fully corrected for pickup calibrations and surveyed offsets. Over the range 20-200 mA these monitors have demonstrated accuracies of +_ 0.25 mm in the horizontal and _+0.15 mm in the vertical . The beam orbit can be corrected to zero or any other
size agrees with that expected from the natural emittance to within the experimental uncertainty of ±5%,
and the vertical beam size corresponds to a horizontalvertical emittance coupling ratio of 4%, though this latter figure depends on the specific choice of betatron tunes. At 600 MeV the beam size is generally larger
than the natural beam size to an extent depending on the beam current. This is illustrated in fig. 8 which
shows typical examples of beam sizes measured with the photo diode arrays as the beam energy is increased from
injection to 2 GeV. The effect of instabilities and changes in the betatron tunes may be seen .
An independent check on these measurements has
been obtained using a simple X-ray pinhole camera on a
desired position by running a control program which accesses the monitor data and computes the optimum corrector settings using a theoretical model of the lattice.
Not only is this a very useful operational aid, but it is also a powerful diagnostic facility for locating equipment faults. There have been occasions when corrector faults, stray fields and shorted turns have been rapidly located using this program.
The operation of the orbit program in correcting the horizontal and vertical orbits is illustrating in fig. 9. In 5 iterations the vertical orbit is reduced from +_ 6 mm to
within a standard deviation of 0.05 mm about zero, and the horizontal from _+ 10 mm to a standard deviation of 0.2 mm about a constant offset of 2.4 mm . This offset is a real orbit effect due to the fringe fields of the dipole
a E E E
Ô
m
m
a
cC
C
O
N O
5
Iteration Number
8
6
8
Iteration Number
Fig. 9. Correction of the vertical (a) and horizontal (b) orbits, showing the average horizontal offset .
E.A . Hughes et al / Performance of the modified Daresbury SRS
magnets penetrating into the straight sections. It has not been corrected, for example by adjusting the rf frequency, to avoid disturbance to the alignment of the radiation beams. The directions of the radiation beams are adjusted using groups of correctors which are arranged under computer control to produce an orbit distortion localised to a single dipole . These orbit bumps are optimised for each beam line at the start of a user run using beam position detectors, located several meters along the lines . Over a typical user run of 12 hours these position monitors show beam movements of about 0.2 to 0.3 mm . Some of the movement has been identified as being due to drift in the corrector currents, and power supplies with better stability are being fitted. The reason for the remaining movement has not yet been identified. Other long-term movements are apparent, over periods of months, from changes required to the orbit bumps for optimised beam steering . This may result from gradual movements in the positions of the storage ring magnets. 4. Conclusions The lattice change from SRS-1 into SRS-2 represents one of the most major transformations inflicted on an
397
operational accelerator. The change has been completely successful with the main design goals having been achieved or surpassed. It is probable that the present level of performance will be improved further in terms of beam current and beam stability. A continuing programme of studies will attempt to understand the various beam instabilities which are experienced and useful comparisons will be drawn between the behaviour of SRS-1 and SRS-2.
References [1] G. Saxon, IEEE Trans. Nucl . Sci. NS-30 (1983) 3136 . [2] R.P . Walker and M.W . Poole, submitted for publication m Nucl. Instr. and Meth . [3] M.W . Poole et al., Proc . Eur. Accelerator Conf., Daresbury, 1988, preprint DL/SCI/P589 A. [4] R.J . Reid, Proc. Eur. Particle Accelerator Conf., Daresbury, 1988, DL/SCI/P593 A. [5] J.S. MacKay, Proc. Eur. Particle Accelerator Conf., Daresbury, 1988, prepnnt DL/SCI/P591 A.
1. SOURCES
Nuclear Instruments and Methods m Physics Research A282 (1989) 392-397 North-Holland, Amsterdam
PERFORMANCE OF THE MODIFIED DARESBURY SRS E.A. HUGHES, J.N. CORLETT, D.M. DYKES, J.S. MACKAY, P.D. QUINN, V.P. SULLER, S.L. THOMSON and R.P. WALKER SERC Daresbury Laboratory, Warrington WA4 4AD, UK
In October 1986 the Daresbury synchrotron radiation source (SRS) was shut down for a period of five months during which major modifications were made . The primary purpose of the shutdown was to greatly reduce the emttance by the addition of extra quadrupoles which doubled the number of FODO lattice cells. Other changes included additional correction magnets and major revisions to the vacuum system . Recommissioning started in March 1997 and scheduled operation resumed in June with beams of 75 mA at 2.00 GeV. By June 1988 the standard operating current had been increased to 200 mA with a beam lifetime of 25 hours. Thus paper describes the installation of the high-brightness lattice, the recommissioning of the SRS and the performance of the machine m its new configuration. l . Introduction The Daresbury synchrotron radiation source (SRS) was designed in 1975 as the first dedicated source of synchrotron radiation. A simple, but flexible FODO lattice was used which permitted easy access to the synchrotron radiation . At the design energy of 2 GeV the horizontal electron beam emittance was rather large at 1.5 x 10 -6 mrad, but low coupling of less than 1% gave a reasonably high brilliance in the vertical plane. After the SRS had become operational in 1981, consideration was given to increasing the radiation brilliance through a reduction in the electron beam emittance by means of a suitable modification to the lattice, with the overall constraint that the dipole magnets and their attached beam lines must remain unaltered. Vanous schemes were studied; the one finally selected was again a FODO structure, but with the unit cell number increased from 8 to 16. The advantage of this system was that it allowed most of the equipment in the straight sections, such as injection and rf equipment, to remain unchanged, but still gave over an order of magnitude reduction in emittance. A description of the proposal was given by Saxon [1] and final approval to proceed was obtained in June 1983 . A comparison between the original-lattice SRS-1 and the modified-lattice SRS-2 is shown in fig. 1 and their parameters are listed in table 1. It is apparent that the major difference between the two structures is the doubling of the number of cells in SRS-2 by the installation of a new D-quadrupole magnet at the upstream end of each straight . This allows the storage ring to be oper-
* Present address: Trieste Laboratory, Trieste, Italy. 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
m
SRS-1
-
E_ :7
fu
FS ci
SRS-2
CD
DIPOLE 120m -
CF
~0
F9Cr
DIPOLE
m DIPOLE -6
~ CF
E3 v -
DSXr
0 m
Fig. 1 Comparison between old and new SRS lattices . ated at a much higher tune, 6.2, as compared with the original 3.2, and with a greatly reduced dispersion function and momentum compaction factor . The nett inTable 1 Lattice parameters Parameter Circumference [m] Lattice type Number of cells Maximum energy [GeV] Betatron tune, horizontal vertical Momentum compaction factor Uncorrected chromaticity, horizontal vertical Natural horizontal emittance at 2 GeV [mmmrad] Natural bunch length at 2 GeV [mm (FWHM)]
SRS-1
96 .0 FODO 8 2 .0 3.18 2.22 0.14 -9.0 +0.5 1.5 60
SRS-2
96 .0 FODO 16 2.0 6.18 3.22 0.03 -10.9 -4.1 0.11 35
E.A Hughes et al. / Performance of the modified Daresbury SRS Table 2 Brilliance comparison between SRS-1 and SRS-2 Parameter Horizontal beam size Vertical beam size Vertical electron divergence Photon emission angle Photon beam divergence Brilliance ratio
SRS-1 SRS-2 [mm FWHM] [mm FWHM] [mrad FWHM] [mrad FWHM] [mrad FWHM]
14 .4 0 .57 0 .19 0 .38 0 .42 1
2 .6 0.24 0.09 0.38 0.39 15
crease in photon beam brilliance expected is 15, as detailed in table 2 . The space into which the new quadrupoles had to fit was very restricted by the radiation beam pipes emerging from the dipole magnets and by the path of the injected electron beam . These quadrupoles have a physical length of 27 .5 cm, an inscribed circle of 35 mm radius and operate at a field gradient of up to 12 T/m . They have been described by Walker and Poole [2] . To achieve the required small cross section for the new D-quadrupoles it was necessary to make use of the smaller vacuum chamber which is allowed by the reduced beam size . However, this reduced the vacuum conductance between the major component of the gas load - which is where the synchrotron radiation hits the absorbed near the downstream end of the dipoles - and the ion pumps in the straights . To maintain an average vacuum pressure of the order 10 -9 Ton in the presence of several hundred milliamperes beam current, it was necessary to double the number of ion pumps in SRS-2, these being added to the straight sections. Other vacuum system changes made were to mount ion pumps directly on each of the 4 rf cavities and to install extra vacuum valves . The latter permit smaller sections of the storage ring to be vacuum-isolated, especially each of the rf cavities . The lattice chromaticity, arising both from the natural chromaticity and the sextupole errors in the dipoles, is corrected by two families of sextupoles . The F-family, adjacent to the F-quadrupoles, are the original 16 sextupoles designed for SRS-1 . The D-family consists of four new sextupoles uniformly distributed between the straights . There is insufficient space to include more than four D-sextupoles, which are 20 cm long and produce a sextupole field of 200 T/m2 within the inscribed radius of 64 mm . Orbit correction and control is extremely important for a synchrotron radiation source, and to maintain a good measurement capability of the orbit position with the increased tune, a new set of 16 horizontal and 16 vertical beam monitors was designed . The original monitors were quarter-wave transmission lines which proved to be inaccurate due to multiple modes within the beam vacuum chamber. The new monitors are but-
393
ton pickups which are flush with the vacuum chamber walls and are an integral part of the chambers . The horizontal part of the chambers . The horizontal monitors are within the F-quadrupoles, and the vertical monitors are adjacent to the D-quadrupoles. The existing SRS-1 orbit correction elements were retained, namely backleg windows on the dipoles and horizontal and vertical dipole trims in the multiple magnets. In addition, for SRS-2 a new set of vertical steering magnets are located adjacent to the D-quadrupoles. These 15 cm long C-shaped dipoles produce a field of 115 G m at maximum excitation of 5 A . By simultaneous computer control of the appropriate vertical correction elements it is now possible to produce vertical orbit adjustment localised to a single dipole, which is very useful for adjusting individual radiation beams . Also installed in the lattice are two 20 cm long octupoles which can generate fields of approximately 250 T/m3 . These will be used to study and control various instabilities . 2 . Installation and recommissioning Although approval for the lattice modification was given in 1983, it was intentionally delayed until October 1986 in order to complete the scientific programme already in place at the facility. Up to the start of the shutdown which then began, SRS-1 was regularly operating with 300 mA initial stored beam current and beam lifetimes of 10 hours . The modification programme was planned to take 42 months from October '86 to mid-February '87 . Since over 60% of the vacuum chambers would be replaced, it was possible to have much of the equipment ready as sub-assemblies at the start of the shutdown . However, it was still necessary to remove totally the equipment from each of the 16 straights, modify the components such as quadrupoles, sextupoles, kickers, etc., which were to be re-used, and combine them with the new subassemblies . The dipole magnets with their vacuum chambers and beam ports were left in place after being let up from vacuum to an atmosphere of dry nitrogen . Due to some unexpected problems in modifying the equipment for re-use, the shutdown took two weeks longer than planned and the first commissioning with injected beam started on 1 March '87 . SRS-2 commissioning took place rapidly despite the fact that major systems had been radically changed . Within two weeks a beam had been accumulated and ramped to over 1 GeV. By the end of March 75 mA had been ramped to 2 GeV, the beam orbit had been fully corrected and the beam size measured. During April and May one quarter of the vacuum chambers were given an in-situ bakeout and the radiation beam lines 1 . SOURCES
394
E.A . Hughes et al. / Performance of the modified Daresbury SRS
were brought back into operation . Further beam studies were made and the 5 T superconducting wiggler was recommissioned. Finally in mid-June the facility came back to routine 24 hours/day operation for experimental science .
3 . SRS-2 performance In the 11 months that the facility has been back in scheduled operation the beam current has increased as understanding of the electron beam behaviour has improved . The increase in average initial beam current at 2 GeV is shown in fig . 2, where the averaging is made on a monthly basis . At present this current is normally about 200 mA, although it is usually possible to accumulate more, up to about 300 mA, at the injection energy. However, as the energy is raised from 600 MeV to 2 GeV transverse beam instabilities occur intermittently which result in beam loss to about the 200 mA level . These instabilities are presently being studied [3] . SRS-2 also operates, according to schedule, in the single-bunch mode . This is normally performed with the lattice in a slightly detuned, lower-brilliance mode in order to increase in natural bunch volume and reduce the natural chromaticity. In this mode the radial betatron tune is decreased from 6 .2 to 4 .2, and the emittance increases from 0 .11 x 10 -6 to 0 .27 x 10 -6 mrad . The maximum current which has been accumulated into a single bunch is approximately 50 mA, but this limit has not been fully explored [3] . At the present, a limit of about 30 mA is experienced due to the control circuits of the injection kickers receiving interference from the orbit harmonics radiated by the single bunch . This results in kickers malfunctioning when the single-bunch current approaches this value .
250
a E m
U Em
m m m a
200
150
100
so
0
Fig . 2 . Improvement in SRS-2 beam current .
t m E ECu
m mm m a
Fig. 3 . Improvement in SRS-2 beam lifetime .
The beam lifetime, averaged over each beam fill and over each month, has shown a steady improvement, as indicated in fig . 3 . The two instances where the general trend for improvement was reversed happened when some of the distributed ion pumps were out of operation . The importance of these pumps in achieving good lifetime is demonstrated by the comparison between February and March. In March all pumps were active whilst in February two distributed pumps, representing 13% of the available distributed pumps and only 4% of the total pumping speed, were out of action and yet the beam lifetime decreased by 30% . Further information on the performance of the vacuum system is given by Reid [4] . In the single-bunch mode the beam lifetime is considerably reduced. A reduction is to be expected due to the Touschek effect and plot of inverse lifetime against single bunch current, shown in fig . 4, does show the expected linear variation . The zero-current lifetime is in good agreement with the vacuum lifetime measured in a multibunch, but the Touschek lifetime is 2 or 3 times shorter than predicted from the measured beam dimensions and lattice parameters . Complementary measurements, that is to say lifetime measured as a function of bunch volume at a constant single-bunch current, are shown in fig . 5 . In these measurements the beam dimensions in the three normal axes were measured as the horizontal to vertical emittance coupling ratio was varied . The Touschek lifetime is derived from the measured lifetime after correction for the zero-current lifetime obtained from fig . 4 . There appears to be agreement with the predicted Touschek lifetime when the bunch dimensions are much larger than normal, but for small coupling and bunch volume, again the Touschek lifetime is 2 to 3 times shorter than expected . The bunch length achieved in SRS-2 is of great
E.A . Hughes et al / Performance of the modified Daresbury SRS
395
020
015 N 0 G
m E
010
m
m m
005
c
20
sr«;Le
Bll4CH c,1
r MA
Fig. 4. Linear variation of inverse lifetime against current . The solid line shows the calculated Touschek lifetime . interest because of the information it can provide on the vacuum chamber impedance and any instability processes which are taking place, particularly at high bunch currents when operated in single-bunch mode . Comprehensive bunch length measurements have been made as a function of bunch current, rf voltage and energy. The results are reported in detail by Poole [3] so that it is sufficient merely to report here that, compared with SRS-1, SRS-2 no longer shows the clear signature of turbulent lengthening . Nevertheless, the bunch length is longer than the natural value, especially at low energy. For example, at 600 MeV the low-current bunch length has been measured as ranging between 55 and 65 ps (sigma) depending on the rf voltage. This is to be
Fig. 6 SRS-1 beam image. compared with the corresponding natural values of 11 and 20 ps . However, at 2 GeV the measured length of 68 ps agrees with the natural value. For a synchrotron radiation source such as SRS, the measurement of the electron beam size is of prime importance and has received much attention. The first rough measurements were made from a focussed image of the beam, using the visible component of the synchrotron radiation. This was viewed with a TV camera and figs . 6 and 7 show the comparison between SRS-1 and SRS-2 made with the same focussing system but with different screens. The small divisions in each case are millimeters and it is apparent immediately that in the SRS-2 the horizontal beam size has been reduced by about 5 times, as predicted. Precise measurements of the size of the focussed image have been made using photo diode arrays as described by Mackay [5]. At 2 GeV the horizontal beam
100
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m
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2010 -
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Bunch Volume (mm3)
Fig. 5. Variation of lifetime with bunch volume .
N MI. OEM No
Fig. 7. SRS-2 beam image. 1. SOURCES
396
E.A . Hughes et al / Performance of the modified Daresbury SRS diagnostic X-ray beam line at another location on the storage ring . Although it can be used only at higher beam energies, where there are enough X-rays, this camera has the advantage of producing a 4 times mag-
nified image of the electron beam which is particularly
useful in measuring the very small vertical beam size. The pinhole confirms that at 2 GeV the horizontal beam size agrees
e
to within 10% with the predicted
natural size and that the vertical emittance coupling is
E
4% .
É . 0 N
Equally important for a radiation source is the position and angle of the electron beam at the origins of the beam lines. The beam orbit in SRS-2 is measured with a set of button pickups which are multiplexed on demand
through the computer control system . This then displays a set of 16 horizontal and 16 vertical beam positions
Fig. 8. Horizontal beam size measured by a photodiode array during the energy ramp, at 0.1 and 1.0 mA per bunch.
which are fully corrected for pickup calibrations and surveyed offsets. Over the range 20-200 mA these monitors have demonstrated accuracies of +_ 0.25 mm in the horizontal and _+0.15 mm in the vertical . The beam orbit can be corrected to zero or any other
size agrees with that expected from the natural emittance to within the experimental uncertainty of ±5%,
and the vertical beam size corresponds to a horizontalvertical emittance coupling ratio of 4%, though this latter figure depends on the specific choice of betatron tunes. At 600 MeV the beam size is generally larger
than the natural beam size to an extent depending on the beam current. This is illustrated in fig. 8 which
shows typical examples of beam sizes measured with the photo diode arrays as the beam energy is increased from
injection to 2 GeV. The effect of instabilities and changes in the betatron tunes may be seen .
An independent check on these measurements has
been obtained using a simple X-ray pinhole camera on a
desired position by running a control program which accesses the monitor data and computes the optimum corrector settings using a theoretical model of the lattice.
Not only is this a very useful operational aid, but it is also a powerful diagnostic facility for locating equipment faults. There have been occasions when corrector faults, stray fields and shorted turns have been rapidly located using this program.
The operation of the orbit program in correcting the horizontal and vertical orbits is illustrating in fig. 9. In 5 iterations the vertical orbit is reduced from +_ 6 mm to
within a standard deviation of 0.05 mm about zero, and the horizontal from _+ 10 mm to a standard deviation of 0.2 mm about a constant offset of 2.4 mm . This offset is a real orbit effect due to the fringe fields of the dipole
a E E E
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a
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Iteration Number
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Iteration Number
Fig. 9. Correction of the vertical (a) and horizontal (b) orbits, showing the average horizontal offset .
E.A . Hughes et al / Performance of the modified Daresbury SRS
magnets penetrating into the straight sections. It has not been corrected, for example by adjusting the rf frequency, to avoid disturbance to the alignment of the radiation beams. The directions of the radiation beams are adjusted using groups of correctors which are arranged under computer control to produce an orbit distortion localised to a single dipole . These orbit bumps are optimised for each beam line at the start of a user run using beam position detectors, located several meters along the lines . Over a typical user run of 12 hours these position monitors show beam movements of about 0.2 to 0.3 mm . Some of the movement has been identified as being due to drift in the corrector currents, and power supplies with better stability are being fitted. The reason for the remaining movement has not yet been identified. Other long-term movements are apparent, over periods of months, from changes required to the orbit bumps for optimised beam steering . This may result from gradual movements in the positions of the storage ring magnets. 4. Conclusions The lattice change from SRS-1 into SRS-2 represents one of the most major transformations inflicted on an
397
operational accelerator. The change has been completely successful with the main design goals having been achieved or surpassed. It is probable that the present level of performance will be improved further in terms of beam current and beam stability. A continuing programme of studies will attempt to understand the various beam instabilities which are experienced and useful comparisons will be drawn between the behaviour of SRS-1 and SRS-2.
References [1] G. Saxon, IEEE Trans. Nucl . Sci. NS-30 (1983) 3136 . [2] R.P . Walker and M.W . Poole, submitted for publication m Nucl. Instr. and Meth . [3] M.W . Poole et al., Proc . Eur. Accelerator Conf., Daresbury, 1988, preprint DL/SCI/P589 A. [4] R.J . Reid, Proc. Eur. Particle Accelerator Conf., Daresbury, 1988, DL/SCI/P593 A. [5] J.S. MacKay, Proc. Eur. Particle Accelerator Conf., Daresbury, 1988, prepnnt DL/SCI/P591 A.
1. SOURCES