The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

IMPROVEMENT OF BEAM LIFE TIME DUE TO OPTIMIZATION OF
VERTICAL ORBIT AND CORRECTION OF MODULATION OF
VERTICAL BETA FUNCTION AT NEWSUBARU
Y. Shoji, LASTI, Himeji Institute of Technology, 678-1205, Japan,Abstract
An optimization of the vertical beam orbit improved thebeam life time of NewSUBARU by about 40%. An
additional 5% improvement was obtained by a correctionof a modulation of the vertical beta function. Theyenlarged the effective acceptance in vertical direction andreduced a beam loss due to the Rutherford scattering withthe residual gas.

1 INTRODUCTION
The synchrotron radiation facility NewSUBARU [1] isan EUV and Soft X-Ray light source at the SPring-8 site.
The Laboratory of Advanced Science and Technology forIndustry (LASTI) at the Himeji Institute of Technology isin charge of its operation collaborating with SPring-8.
The ring has two operation modes: In the 1.5 GeV mode,the beam is accelerated to 1.5 GeV and stored. In the 1.0
GeV top-up mode, the beam current is kept at
250±0.15mA by an occasional injection with theundulator gap closed. The beam life and the number ofelectrons allowed by the radiation safety rule limit thestored beam current.
At the energy of 1GeV, with stored beam current of250mA, mainly two kinds of beam lives contribute to thetotal life. One is the beam life came from the Rutherford
scattering of beam electrons with the residual gas
molecules that is estimated to be about 6 hrs. Another wasTouschek beam life, estimated to be 12 hrs [2]. We paid
great efforts to improve the vacuum pressure in thesesome years. It helped to push up the former beam life tothe present level. However we also used our time toenlarge the effective beam acceptance, which also
improved the former beam life. Here we report theoptimization of the vertical orbit (COD) and thecorrection of vertical beta function, which enlarged thebeam acceptance and improved the beam life by about
40% and 5%, respectively.

Here r0, Z and γ are classical radius of electron, nuclear
number of gas nucleus and the Lorentz factor,respectively. The critical angle is roughly given by

φC= (aYC /√βYC) /√βYAV(2)Here aYC and βYC are vertical mechanical acceptance andthe vertical beta function at where the effective
acceptance (aYC /√βYC) takes the minimum. The βYAVE isthe average of vertical beta function. Fig.1 shows thedesign values of aYC and √βYC. It is clear that the edges ofbending magnets facing to the dispersion free sections are
the critical locations, where the effective acceptance wasminimum. At these locations the designed values are, aY=aYC =10 mm and βY =βYC=24 m. The lifetime by theRutherford scattering τR is proportional to 1/σR. Thismeans that

τR ∝ aYC2/βYC.

(3)s(m)Figure 1; Vertical mechanical acceptance aY (solid line inmm) and the beam envelope (2√βY; dots in √m). The large
and small white boxes represent locations and lengths ofnormal bending magnets and inverse bending magnets.
The shaded blocks are quadrupole magnets.

3 OPTIMIZATION OF BEAM ORBIT
2 RUTHERFORD SCATTERING

If the beam center is displaced at the critical location by

The Rutherford scattering in vertical direction mainly
determines the present beam lifetime, where an electron
scattered with residual gas molecules is lost at thelocation of the minimum aperture. The cross section ofRutherford scattering σR with vertical critical angle φC isapproximated by the equation∆Y in vertical direction, the aYC is reduced to aYC= aYC0 ∆Y . Here aYC0 is the ideal aYC when ∆Y =0. The beam life

σR = 2π r02( Z /γ φC)2.

(1)

becameτR /τR0 ∝ (aYC0 - ∆Y )2/aYC02 ≈ 1-2∆Y/aYC0for a small ∆Y.

(3)

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

BPM16 900

I τ0Using the new value Iτ0 =895 mAhr, τ0/τR0 was calculated
to be τ0/τR0=0.64. The other kind of the lifetime was about
twice of τR0.
We optimized the beam orbit at the critical locations
and the total beam lifetime was improved from 2.74 hrs to3.80 hrs (about 40% increase) with the stored beam
current of 250mA. The required orbit displacements areshown in Fig.3
2.5
2
1.5
1
0.5
0-0.5
-1
12

3

4

800

5678

9 10 11 12 13 14 15 16 17 18

location (BPM#)

Figure 3: Optimized vertical displacement at the criticallocations (at the edge of bending magnets).

750

4 MODULATION OF BETA FUNCTION

The flat area at the top of Fig.2 meant that after theoptimization of the orbit at this location (BPM16) thereappeared a tolerance in vertical aperture because of thedisplacement at the other critical location. The slope atthe top was thought to be an effect of the spill of the local
bump orbit.
The expected slope calculated from Eq.(3) isd(IτR)/d∆YBPM = ±2 (IτR0)(∆Y/∆YBPM)/ aYC0= ±220 mAhr/mm(4)

here we used I τ R0 =895 mAhr, ∆ Y/ ∆ Y BPM =1.25, anda Y C 0 = 10mm . Th e τ R 0 i s a l i fet im e wi th per fect
optimization, represented by the peak of the triangle madeof extrapolation lines shown in Fig. 2. On the other handthe observed slope was d(Iτ)/d∆YBPM = ±140 mAh/mm,smaller than the expected. This difference suggests acontribution of other kind of lifetime than τR. In that case

2.5

BPM18

BPM14
BPM15

3

BPM13

3.5

BPM12

Table 1: Relation between the displacement at the BPM:
∆YBPM and at the critical location: ∆Y, produced by thelocal bump orbit.
Location (BPM#) ∆Y/∆YBPM straight section
6, 9, ,15, 18
1.4
SSS
1, 7, 10, 16
1.25
SSS
3, 12
1.0
LSS4, 13
1.0
LSS

2
1.5
10.5

BPM9
BPM10

Figure 2: A typical result of the vertical aperture survey.
Vertical axis is the beam current times the lifetime.

After the optimisation of the beam orbit we expect that
there would be no flattop at any critical location because
the aY and βY are almost the same at these locations.
However there still exists a flattop at many of locations.
Fig.4 shows a plot of the flattop width with respect to thevertical betatron phase. The flattop width propagatedaround the ring like a modulation of beta function withharmonic number of 4, roughly the twice of the betatrontune (νY =2.2).
BPM6
BPM7

2.5

BPM4

∆YBPM (mm)

2BPM3

1 1.5

BPM1

700
- 0.5 0 0.5

flat top width (mm)I τ (mA h)

850d(Iτ)/d∆YBPM = (dτ/dτR) [d(IτR)/d∆YBPM]= ±2 (τ0/τR0) (Iτ0)(∆Y/∆YBPM) aYC0 . (5)optimized ∆Y (mm)We produced a vertical local bump, that is anintentional ∆Y, at the edges of bending magnets andmeasured the lifetime with respect to ∆Y. The height ofthe bump was monitored at the beam position monitor(BPM) nearby. There are 12 critical locations in thewhole ring, 8 face to the short straight section (SSS) and 4
face to the long straight section (LSS). Each of them isreferred by a number of BPM nearby, for example as‘BPM16’. Fig.2 shows a typical result of themeasurement of beam life varying ∆Y, in other wards anaperture survey. About 15% improvement of the lifetime
was obtained by the 1.2mm displacement of ∆YBPM. Here∆YBPM is a displacement at the BPM. The relation
between ∆YBPM and ∆Y is calculated from the linear optic
model and listed in Table 1.

0-50

510

1520

vertical betatron phase (rad)Figure 4: Flattop width of the aperture survey vs. thevertical betatron phase. The line is a 4th harmonicsinusoidal wave. The vertical betatron phase is defined tobe zero on one of the four-fold symmetric line of the ring.
The flattop widths are the widths at the critical locations,
not at the BPMs nearby.
The estimated modulation amplitude of the beta
function was °∆βY/βY°max = 15%. We applied a correctionof asymmetric modulation of the beta function (amplitudeof 7%) using trim windings on the quadrupole magnets.
Finally we obtained about more 5% improvement of thetotal lifetime.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

5 DISCUSSION
The required displacements shown in Fig.3 was larger
than the expected from the measurements of heights ofthe BPMs and the vacuum chambers. We do not havegood explanation for this inconsistency.
The optimization of the orbit and the correction of themodulation of the beta function changed the distributionof the radiation level around the ring. The highest
radiation level had been observed at BPM6 since the firstcommissioning in 1998. Investigation of the mechanicalstructure around this area did not give any explanation ofit. After the optimization and the adjustment the radiationlevel at BPM6 became low and now there exists nolocation of high radiation level.
The other lifetime than IτR0 is thought to be mainly aTouschek lifetime. An intentional expansion of the beamemittance improved the total lifetime by about 30%. This
support our hypothesis.

6 SUMMARY
The vertical orbit optimisation improved the beam
lifetime by about 40%. More 5% improvement wasobtained by the correction of the non-symmetricmodulation of the vertical beta function.

7 REFERENCES[1] A. Ando et al., J. Synchrotron Rad. 5, 342-344 (1998).
[2] S. Hashimoto et al., Presentation at the Meeting ofJapanese Physics Society, Sendai, March, 2003.