Implementation and effects of undulator scanning at Aladdin

Nuclear Instruments and Methods in Physics Research A291 (1990) 413-418 North-Holland

413

IMPLEMENTATION AND EFFECTS OF UNDULATOR SCANNING AT ALADDIN Walter S. TRZECIAK, Roger W .C. HANSEN, Michael A. GREEN, Mark BISSEN and Dave EISERT University of Wisconsin-Madison Synchrotron Radration Center 3731 Schneider Drive, Stoughton, Wisconsin 53589, USA

The stability of the Aladdin closed orbit while scanning the U2 undulator gap is reported. Over the range of undulator gap settings the entire optimum closed orbit has been corrected to 20 pin rms vertically and 40 Win rms honzontally. The correction scheme utilizes computer control of the two undulator end correctors for the horizontal orbit and two of the regular ring correctors for the vertical orbit. Data showing the output of several other beam lines while the undulator is scanned over its entire range is presented. Small fluctuations m the intensity observed in some user beamlines has been attributed to the discrete steps used by the orbit correctors. A change m vertical beam size with undulator gap is shown to be related to a sextupole component in the undulator field. Possible remedies for these problems are discussed

1 . Introduction

0.46 to 1 .72. Details concerning the initial operation of

The Aladdin electron storage ring has been to oper-

ation for users since 1984 . Late in 1986 an undulator built at Lawrence Berkeley Laboratory for the Stanford

Synchrotron Radiation Laboratory [1] was installed in

long straight section 2; this undulator is now called U2 . The undulator is of the planar type, mounted coplanar

with the median plane of the ring . The undulator has 30 periods,

a

= 6.1 cm, and a range of K values from

* This work is supported under National Science Foundation Grant No . DMR 8601349.

the undulator are reported by Green et al . [2]. Fig. 1

shows the mechanical structure of the undulator, and fig. 2 shows the undulator to position over the ring vacuum chamber. A plan view of the ring showing the location of the undulator and its beamline is shown in fig. 3.

The Aladdin storage ring can store electrons at any from 100 MeV to 1 GeV [3]. In particular,

energy

running at 170 MeV allows the undulator to radiate visible light, which is very useful for alignment. Normal operating energies are 800 MeV or 1 GeV. Under these

conditions the undulator generates first harmonic light

MAGNET GAP ADJUSTING MECHANISM

END BLOCK ROT A, OR

BLOCK MAGNET'-'..- TATION

Fig 1 . Drawing of the SSRL/LBL undulator showing the permanent-magnet blocks, the vacuum chamber, and the gap and end corrector drive mechanisms . 0168-9002/90/$03 50 © 1990 - Elsevier Science Publishers B.V . (North-Holland)

111. INSERTION DEVICES

414

W S Trzeciak et al / Undulator scanning at Aladdin

Fig. 2 Photo showing the undulator in position over the vacuum chamber from 38 eV (800-MeV ring operation) to 135 eV (1-GeV ring operation) . 2. Beam position monitoring The electron beam position monitoring (BPM) system consists of striplines at the quadrupoles feeding a tuned receiver, which is sampled by an analog/digital (A/D) converter through a chain of rf relays . Details can be found in ref. [4]. The resolution of the system is ±25 lm, limited by noise generated by the stored electron beam . The reproducibility of the synchrotron light source positions to the dipoles is actually better than this because of a small demagnification which results from the larger ß values at the quadrupoles compared to the ß function at the source points to the dipoles. The motions of two of the synchrotron light source pants are monitored by photodiode arrays through lens systems that magnify the vertical image and demagnify the horizontal image [4]. These two monitoring stations are separated by about 90' in betatron phase, thus providing an independent optical beam position monitoring system . The resolution of these optical monitors is 5 [,m at the source. These photodiode arrays also provide a measurement of the source size. Fig. 4 shows the vertical source distribution with and without the undulator inserted over the stored beam at 800 MeV. The normal vertical beam size as measured at diagnostic port 022 is a, = 71 ~Lm (FWHM = 168 ~ m) . With the undulator inserted at

a small gap (corresponding to 50 eV photons) the vertical beam size becomes a,, = 93 l..m (FWHM = 218 gm). The vertical beam size varies continuously from large values at small gaps to being the same size as without undulator for the largest gap. This change to beam size has a large effect on beamline performance and will be discussed more fully below. 3. Correcting the horizontal closed orbit The undulator is equipped with rotatable permanent magnet correctors at each end (fig . 1) . These correctors provide a means to adjust fB d/ in a symmetrical fashion so as to minimize the dipole error caused by the undulator. Fig. 5 shows the shaft encoder values for the end correctors, when properly adjusted, as a function of the undulator gap. The values were obtained by varying the end correctors at different undulator gap settings so as to keep the horizontal closed orbit invariant, at least to the resolution of the BPM system . These corrector values were then incorporated into the undulator control program so that any undulator gap uniquely determines the values for each of the two end corrector magnets [5]. During a scan over the full range of undulator gap settings the horizontal closed orbit is corrected to 40 lLm rms. 4. Correcting the vertical closed orbit Closed orbit correction in Aladdin is accomplished by powering back-leg windings on the sextupoles in the

W S. Trzeciak et al / Undulator scanning at Aladdin

41 5

PORT 041 UNDULATOR BEAMLINE 0

PORT

m

02 2(3 LSS-2

o

i~

c

wr

_

s

30 PERIOD UNDULATOR

,J Ilv

\-?02B

2D?A

2\

PORT 022 DIAGNOSTICS

111

PORT 042 6M TGM

0PORT 052 DIAGNOSTIC S

B

1

100

~1

LSS-3

~

0

MeV 111CROTRON

L55-í

SRC 1-Geil

s

STORAGE RING 0 R.F. SYSTEM

8

OL1-L~

72

METERS 11

10 LSS-C

PORT 081 SRC 4M NIM

E&PERIMENTAL

ELECTRONICS AREA

remti

PREPARATIDN AREA

OPTICS I CLEAN ROOM

Fig. 3 Diagram of Aladdin highlighting the undulator, its beamllne, and the places where the beam was monitored during undulator tracking experiments . arcs and back-leg windings on the quadrupoles to the long straight sections . In particular, any vertical closed orbit error coming from the presence of the undulator can be corrected by adjusting the two closest vertical correctors, one on each side of the undulator (at 2Q2A and 2Q2B in fig. 3) . To lowest order, there should not be any vertical closed orbit error due to a horizontal planar undulator. For the LBL/SSRL undulator, however, the vertical closed orbit error, at small gap settings (large K values) must be corrected. Without correction, the vertical orbit changes by 360 pin rms over the full range of gap settings . With correction, the vertical orbit is held constant to within 20 win rms, a value of the same order as

the resolution of the BPM system . Fig. 6 shows the correction current necessary in each of the two quadrupole back leg windings, as a function of undulator gap, to correct changes to the vertical closed orbit. 5. Scanning the undulator in user mode The opportunity to scan the undulator while several beamlines were active presented itself during recent machine studies. During the scanning of the undulator, the photodiode arrays at ports 022 and 052 were also monitored . The maximum excursion on either port was 20 p m. Fig. 7 shows the photodiode position output III. INSERTION DEVICES

416

W. S . Trzeciak et al. / Undulator scanning at Aladdin

E C

Port 052

zo

.l ..rl f."rrr^

ü.

-

nJV^"fJbnM~rtMMJ~^f~^y

0

-40

Fig. 4. Vertical beam size at diagnostic port 022 with (a) the undulator withdrawn from the beam and (b) with the undulator inserted at a gap corresponding to 50 eV photons .

9370

04

9365 03

9360 9355 9350 9345 9340

00

Fig. 5 . Graph showing the experimental values of the end corrector counter (proportional to end corrector field) as a function of undulator gap Also included is the gap field as a function of the gap and/or gap counter.

Fig. 7. Data taken on photodiode arrays at diagnostic ports 022 and 052 during undulator scanning. from each of the ports as the undulator was scanned from 40 eV to 90 eV . These beam position monitors are independent of beam size over a large range and accurately show how the beam position changes as the undulator is scanned. While many other beamlines were monitored, only three that seem to characterize the results are included here : a 4-m NIM (normal incidence monochromator), an ERG (extended range grasshopper) monochromator, and a 6-m TGM (toroidal grating monochromator) . Their locations are shown to fig. 3. During undulator scanning from 40 eV (gap fully closed) to 90 eV (gap fully open) and back to 40 eV, flux through the 4-m NIM monochromator was monitored. The data (fig. 8) clearly shows that the beamline output changes as the gap is scanned. However, beamline alignment was tested at several points during the scan, and it was concluded that the synchrotron light was centered on the monochromator entrance slit. This means that the stored beam was changing in size while the undulator was 100

Gap Counter 10000

15000

000

5000

95

90

005

65

-0 10

60 -0 15 75 -020

20

30

40 Gap (m .)

50

60

Fig. 6. Graph showing the necessary current adjustments m the nearest vertical steerers to maintain the vertical orbit during undulator scanning .

70 T-

Fig. 8. Data taken on the 4-m NIM monochromator during undulator scanning .

41 7

W S. Trzeciak et al. / Undulator scanning at Aladdin 90 e0 70

Start AL 50eV

Scan t o 50eV

At 40eV

e0 100 90 00 70

At 50

V

AL 60eV

Scan to 60eV

110

90 00

Scan to 70eV

At 70eV

Scan=77

120

Scan to 90eV

At 90eV

90 i

Fig. 9 Data taken on the 6-m TGM monochromator during undulator scanning . scanned, even though the center of mass of the beam was not moving. Beam alignment in the ERG beamline was also tested at various points during the undulator scan, and the synchrotron source again remained centered on the entrance slit . This beamline also reported a 5% decrease in signal intensity in going from a large undulator gap to a small gap . This is consistent with the beam size change mentioned above . Output from the third beamline (fig. 9) shows several interesting details. First, the variation to intensity with undulator scanning is again observed . Second, the beamline output shows the discrete stepping of the power supply currents in the vertical orbit correctors as the vertical closed orbit is adjusted . When the undulator gap is full open (high photon energy), there is little need for orbit correction since the undulator fields are minimum. As the gap is closed (heading toward lower photon energy), the higher undulator fields cause an increasing vertical orbit perturbation . This orbit error is corrected by making minimal corrector changes, but at an ever increasing rate . Hence the period of the small power supply steps gets shorter as the undulator scans toward a smaller gap.

usage. The vertical closed orbit is maintained to within 20 ltm rms. The beam vertical distribution is shown to fig. 4. The peak of this signal is used as the photodiode beam position monitor and it does not move more than +10 [t m during the entire scan on either of the two optical monitoring systems (fig . 7) . Second, the small pulses that appear to the output of other beamlines as the undulator is scanned need to be eliminated . This can be done by increasing the resolution of the vertical correctors and then incorporating more steps for a given correction . Third, the change in beam size during the scan cannot be tolerated. This leads to a 5 to 10% change to the apparent beam intensity at other beamlines during the undulator scanning . Similar changes in beam size have been observed at low currents when the sextupole excitation has been varied . Fig. 10 shows vertical beam size as a function of the defocusing sextupole strength . The smallest beam size occurs at normal sextupole strengths for 800-MeV operation. These observations may be the result of a strong sextupole component in the undulator field, and this sextupole component could be responsible for the changes to the beam size . With the increase in vertical beam size, the bunch volume increases, and the stored beam lifetime increases by 33% (Touschek effect). The contribution to the sextupole component is also borne out by chromaticity measurements with and without the undulator inserted . With the undulator gap set to give 50 eV photons at 800 MeV, the vertical beam size was measured at the diagnostic port 022 (see fig. 3) . At the same time the horizontal and vertical chromaticities were measured using the betatron tune shift with varying energy method . The measurements were repeated at 1 GeV with the same undulator gap. All other machine parameters were kept constant, especially the closed orbit. The results are shown in table 1. Note that 200

1 75

1 50 b 1 25

1 00

6. Discussion and conclusions Three aspects of undulator scanning need to be discussed. First, the closed orbit corrections in both planes are more than adequate for present beamline

Fig. 10 Change m vertical beam size (as measured at diagnos tic ports 022 and 052) as a function of defocusing sextupole strength . III.

INSERTION DEVICES

41 8

W. S Trzeciak et aL / Undulator scanning at Aladdin

Table 1 Change of beam size and chromaticity (undulator out/in) Energy [GeV] 0.8 1

Gy

[Wm]

72/93 169.3/1172

1 .97/2.34 2.06/2.91

1.74/2.43 2.98/2.61

at 1 GeV the insertion of the undulator has the opposite effect on the beam size, decreasing from 169.3 lim to 117.2 . For both energies ~ x increases when the undulator is inserted, but ~,, increases at 0.8 GeV and decreases at 1 GeV. While these experiments are evidence of a sextupole error m the undulator, another experiment failed to yield further confirmation . A spare sextupole was installed next to the undulator and powered to try to offset any error of the undulator. Only a small ( - 10%) change in vertical beam size could be obtained using this spare sextupole. A whole series of dynamical and mechanical experiments were carried out over a long period of time. Betatron functions were compared, the undulator was moved and rotated, and vertical dispersion was carefully measured . Only the chromaticity measurements

yielded hard evidence of a connection between beam size and sextupole fields . The fact that the dispersion is large and negative (-0.7 m) at the undulator probably plays an important role to this problem. More study and experiments are being carried out. Perhaps this problem goes away in lattices that have zero dispersion at the locations of insertion devices. Acknowledgement The authors wish to thank the entire SRC staff. Without their dedication this work would not have been possible . References [1] K. Halbach et al ., IEEE Trans. Nucl . Sci. NS-28, (1981) 3136 . [2] M.A . Green et al., Nucl . Instr and Meth A266 (1988) 91 . [3] E.M . Rowe, 1987 IEEE Particle Accelerator Conf . Proc, p. 391. [4] K J Kleman, Nucl . Instr. and Meth . A266 (1988) 172. [5] R.W .C . Hansen et al ., these Proceedings (6th Nat. Conf . on Synchrotron Radiation Instrumentation, Berkeley, CA, USA, 1989) Nucl . Instr. and Meth . A291 (1990) 162