Sources of transverse beam motion in the APS storage ring have been investigated for ground-motion- and water-system-induced vibrations of the magnet and vacuum systems, and for power supply ripple. The displacement of magnets in a bandwidth of 4-30 Hz have been reduced significantly by inserting viscoelastic damping pads between the girder supports and pedestals, and by welding the magnet cooling headers to the ceiling of the storage ring tunnel. Current ripple on magnet power supplies was identified as a source of horizontal beam motion. Beam motion was measured without the closed-orbit feedback system activated. At {beta}{sub x} = 15.4 m and {beta}{sub y} = 10.4 m the rms beam motion in the 0.02-30 Hz band was 22.7 {mu}m and 6.3 {mu}m in the horizontal and verticle planes, respectively. A few narrow-band structures of the horizontal beam motion spectrum in the 1-4 Hz band have to be investigated further to identify the sources

Decker, G.

Doose, C.

Kim, S. H.

Mangra, D.

Merl, R.

UNT Digital Library

I A y - o /  C P - q a a y &  INVESTIGATION OF OPEN-LOOP BEAM MOT ON AT LOW FREQUENCIES AT THE APS c- 9 7 0 50 3 - - S .  H. Kim, G. Decker, C. Doose, D. Mangra, and R. Mer1 Advanced Photon Source, Argonne National Laboratory 9700 South Cass Avenue, Argonne, Illinois 60439 USA Abstract Sources of transverse beam motion in the MS storage ring have been investigated for ground-motion- and water- system-induced vibrations of the magnet and vacuum sys- tems, and for power supply ripple. The displacement of magnets in a bandwidth of 4-30 Hz have been reduced sig- nificantly by inserting viscoelastic damping pads between the girder supports and pedestals, and by welding the mag- net cooling headers to the ceiling of the storage ring tun- nel. Current ripple on magnet power supplies was identified as a source of horizontal beam motion. Beam motion was measured without the closed-orbit feedback system activated. At p,=15.4 m and p,=10.4 m the rms beam motion in the 0.02-30 Hz band was 22.7 pm and 6.3 pm in the horizontal and vertical planes, respectively. A few narrow-band structures of the horizontal beam motion spectrum in the 1-4 Hz band have to be investigated further to identify the sources. 1 INTRODUCTION The tolerances of the rms beam motion in the storage ring (SR) of the Advanced Photon Source (APS) [ 13 are to be less than 4.4 pm and 17 pm in the vertical and horizon- tal planes, respectively, in a bandwidth of 650Hz. In the horizontal or vertical planes, the transverse beam motions or time-dependent closed-orbit distortions at position s in the SR due to a kick A.8(sk,t) at position sk may be expressed as [ 2 ]  k cos [ Iw( - w(sk)l - 9 where p(s) and p(sk) are the beta functions at s and sk, respectively, w is the betatron phase, u is the betatron tune of the SR. The kick at sk may be expressed in general as a Fourier series: Here Bp = 23.349 T.m is the beam rigidity corresponding to 7.000 GeV, and %j is the angular frequency of the j-th When the transverse beam motions are superimposed to the beam offset for a single bunch of a beam in the SR, the beam signal to an r f  beam position monitor (BPM) located at s is kick at Sk. m f(s, t )  = 6(r- nG)[Ax(s)  + Ax,(s, t)] , (3) where o, is the angular revolution frequency, Ax(s) is the offset of the transverse beam position, and Axc(s,t) is the time-dependent transverse beam motion of Eq. (1). The frequency spectrum of Eq. (3) is as follows: 0, n = - m  with IAx,(s,qj)l as the spectral amplitude of the beam motion, which will be shown as a sideband to the offsets of the beam position. Major sources of time-dependent kicks in the SR are the effective dipole field due to the displace- ment of quadrupole magnets (quads), power supply current ripple, induced field due to eddy currents in the vacuum chamber, and mechanical vibrations. Displacement of the quads for the strong-focusing lat- tice of the SR causes a relatively high amplification factor of the beam motion. Random uncorrelated quad motions of rms displacement Axq result in rms beam motion of amplitude 17.2P'nAxq, and the amplification for random device source point is 14.2 m [3]. girder motion is 7.4P 112 Axgirder, where p at the insertion 2 MECHANICAL VIBRATIONS The vibration modes of the quad girders are domi- nated by a horizontal bending mode with lowest resonance frequency of approximately 10-1 1 Hz. The quad motion had a peak displacement of 210 nm at 10.2 Hz and an inte- grated rms displacement of 320 nm in 4-54 Hz. In order to reduce the quad displacement, vibration damping pads were inserted between the wedged jack sup- ports and the pedestals [3]. The damping pads are made of two layers of 0.15-mm-thick Anatrol-217 visco-elastic material and three layers of 1.6-mm-thick stainless steel plates. The damping pads were easily inserted below the jack supports with minimal impact on the girder align- ment. The damping pads reduced the peak displacement by a factor of three and the resonance frequency was shifted downward a few Hz in general. The total r m s  dis- placement was reduced by a factor of two. After three suc- cessive simulated alignments to shear the pads up to 0.75 rnm. no appreciable changes in Q-value at the resonance The submitted manuscript has been created by the University of Chicago as Operator of  Argonne National Laboratory ("Argonne") under Contract No. W-31-109-€NG-38 wt!h the U.S. Department of Energy. The U.S. Government retains for iself. and others acting on is behalf, a paid-up. nonexclusive, irrevocable worldwide license in said miclc: (0 reproduce. prepare derivauve works. distribute copies to the public. and perform publicly and display publicly, by or on behalf of the Government. @q@ DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof. I frequ’ency have been observed. Besides the ground- motion-induced displacements, the cooling-water headers induced magnet displacements. Figure 1 shows measure- ments of the rms displacements for sector 10 after welding the water header to the ceiling of the SR tunnel and an unwelded sector 9. The vertical axis in  Fig. 1 denotes hori- zontal rms displacement calculated for the 4-50 Hz band. Along the horizontal axis the measurement times are shown at the bottom, and the onfoff status of the cooling water for the magnet and vacuum chamber are shown on the top. The data indicate that the impact of the vacuum chamber cooling water on the girder vibration is minimal, The magnet cooling water, on the other hand, increases the quad vibration by more than a factor of two, regardless of whether the headers were welded or not. The welds reduced the quad rms displacement from about 130 nm to 90 nm. The ratios of the displacements between the quads and pedestals, however, are not significantly different for the welded and unwelded headers. mag on mag on mag on m a g  off mag off vacon vacon vacoff v z c n  ai Y K  off .,scoff “aeon l a g  off  all m q  on mag on I60 E ;;: g EO c IC0 - = 80 5 J C  0 20 L? 2255 23:24 23:52 3 2 ’  0 5 0  : ‘ 3  1-48 2: .5  2 4 5  3.14 Time [hr:mi] Figure 1: Displacements (rms in 1-50 Hz) of two quads and two pedestals with and without welding of the magnet cooling water headers. The on/off conditions of the cooling water for the magnets and vacuum chambers are shown on top. To further investigate the impact of the welds when the cooling water was on, coherences of the vibrations between the quads and pedestals were measured; the data in the 6-1 1 Hz band are shown in Fig. 2(a). For the girder in the welded sector, the quad rrns displacement was 4 4  nm compared to 106 nm for the one in the unwelded sec- tor. At the same time the coherence for the unwelded case was lower, as shown in Fig. 2(b), which indicates that the water header contributed more than half’of the vibration when the headers were not welded. The headers were welded as a test in  two sectors: it was decided not to weld the whole ring due to cost vs. benefit. 3 MEASUREMENTS OF BEAM hf OTION Low-frequency beam motions in the SR were mea- sured using a Hewlett Packard 8944OA Vector Signal Ana- lyzer [4] which obtains the frequency spectrum of the beam signal from rf BPMs [ 5 ] .  The periodicity of the out- put signal from a BPM module is one-half the revolution frequency. w,/2. This is due to the fact that beam positions are measured with a 180“ phase shift OR alternate turns for removal of systematic offsets. The amplitude of the signal 6 8 9 10 1 1  Frequemy [H?] 0.8 0.6 f 0.4 f 0 0.2 0 e 6 7 8 9 i o  I 1  Frequency [Hz] Figure 2: Coherence of the vibrations between the quads and pedestals (a) with and (b) without the welds. Without the welds the coherence is low, but the rms displacement in 6-1 1 Hz is 106 nm compared to 44 nm for the welded case. at or /2 is the average position offset of the beam, A x ( s )  in Eq. (4), relative to the geometric center of the four buttons. Low-frequency beam motion corresponding to IAx,(s,qj)I produces an amplitude modulation of the position offset signal and appears as sideband to the a, /2 “carrier.” Figure 3 shows a typical horizontal and vertical beam motion spectra measured at a position of &=15.4 m and py=10.4 m in sector 12 in a bandwidth of 530 Hz with a frequency resolution of 0.1 Hz. The magnitude of the beam offset signal at 0,/2=135.774 kHz (f = 0 in Fig. 3) is not shown in the figure. The horizontal rms beam motion in a bandwidth of 1-30 Hz was 33.1 pm with its major con- tribution in 1-4 Hz. For the vertical beam motion the rms value in the band 1-30 Hz was less than 5 pm. Only a few of the narrow-band structures in the spectra could be reproduced. When averaged from the measurements of 40 BPMs (one in each sector) at the same px and By, the rms beam motions were 22.7 pm and 6.3 mm in the horizontal and vertical directions, respectively. In order to correlate the beam motion with that of a magnet, the beam motion was measured while a quad girder in sector 19 was being displaced with a “shaker.” The spectra of horizontal displacements for the AQ4 quad on the girder and the beam motion in an adjacent sector are shown in Fig. 4. The girder has a resonance frequency at approximately 10 Hz and the shaker has its own resonance frequency around 14 Hz. The amplification of beardquad displacements is approximately two. This data and Fig. 3 indicate that horizontal beam motion due to a I-pm quad displacement from a single girder vibration near 10 Hz would be difficult to detect. -:o -s 3 5 .. 15 ?o 25 31) 5eaU.r- M ,.- Figure 3: Horizontal and vertical beam motion spectra measured at a position of p,=15.4 m and py=10.4 m with a frequency resolution of 0.1 Hz. The magnitude of the beam offset at 135.774 lcHz (f=O in the figure) is not shown. 25 , 0 10 15 m 25 h e a r s 9  [LPI Figure 4: Correlation of a quad displacement in sector 19 and beam motion in sector 20 both in the horizontal direction. The quad girder was displaced using a shaker. 4 CURRENTRIPPLE Statistics of current fluctuations for quads, sextupoles, and correctors were computed from 100 current samples, and some of those power supplies were measured in the frequency domain to identify the ripple spectrum responsi- ble for the beam motion spectrum in certain bandwidths. A sextupole magnet power supply had current ripple of 0.2 A near 6.5 Hz. After modification of the power supply, the horizontal beam motion near that frequency was reduced from 21.9 pm to 8.5 pm [6] .  The direct vertical field-inte- ,oral for the magnet at the current ripple of 0.2 A should be less than 0.8 x T.m within 1 mm of the magnetic axis. However, the ripple sextupole field due to the current rip- ple induces a considerable amount of eddy current in the aluminum vacuum chamber, which in turn induces a verti- cal field inside the chamber. The main contribution of the eddy current comes from the higher field at the larger aper- ture of the magnet. It should be noted that the direct and induced fields are out of phase. At the ripple current the induced field-integral was approximately 0.25 x Tm, which is an expected field strength for a beam motion of 10-20 urn. 5 CONCLUSION By inser’:ng viscoelastic damping pads between the girder supports and pedestals, and by welding the magnet cooling headers to the ceiling of the storage ring tunnel, the dispIacements of magnets induced by the vibrations in the floor and water system were reduced by more than a factor 9f two. Current ripple of a sextupole magnet was identified as a source of beam motion. The average rms beam motion at 0,=15.4 m and &=10.4 m was 22.7 pm and 6.3 pm in th- horizontal and vertical planes. respec- tively, in the band 0.02-30 Hz. Narrow-band structures of the horizontal beam motion spectrum, most of them not reproducible, have to be further investigated, the 1-4 Hz band in particular. 6 ACKNOWLEDGMENTS The authors gratefully acknowledge J. Gaiayr‘a for his encouragement and direction of the work and L. Emery for his helpful discussions on beam motion induced by current ripple. This work was supported by the U.S. Department of Energy, Office Basic Energy Sciences, under Contract No. W-3 1-109-ENG-38. 7 REFERENCES J.  N. Galayda, “The Advanced Photon Source,” Proc. 1995 PAC, IEEE 95CH35843,4 (1996). M. Sands, “The Physics of Electron Storage Rings: An Introduc- tion,” SLAC-I21 (1970). G. Decker, Y.G. Kang, S. Kim. D. Manga, R. Merl, D. McGhee. and S. Sharma, “Reduction of Open-Loop Low Frequency Beam Motion at the APS,” Proc. 1995 PAC. IEEE 95CH3SM3, 303 (1996). The explicit reference to a particular product does not imply that other vendors might not be able to supply equivalent products. E. Kahana, Y. Chung, A.J. Votaw. and F. Lenkszus, in Beam Insrrumenrarion Workshop. Santa Fe, NM 1993, AIP Conference Proc. 319,201 (1994). S. Kim. G. Decker, C. Doose. R. Hogrefe, D. Mangra. R. Merl, and S. Milton, “Investigation of low-frequency beam motion at the Advanced Photon Source.” Rev. Sui. Insrrum.. 67(9) 1996 (CD- ROM). 

Investigation of open-loop beam motion at low frequencies at the APS

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Investigation of open-loop beam motion at low frequencies at the APS

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