BEAM-BASED OPTICS CORRECTION FOR NEW BEAM TRANSPORTLINE FROM LINAC TO PHOTON FACTORY IN KEKN. Iida and M. Kikuchi, KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
Abstract
The 8-GeV electron Linac in KEK supplies fourstorage rings with requested beam having specificenergies, charges and bunch structures, changing
sequentially its operation mode. In order to shorten theswitch-time of Linac from KEKB to PF and vice versa, anew beam transport line for PF (PF/BT) has been
constructed [1]. In this paper we describe on the beamcommissioning and the operational status of the new PF
line, with emphasis on the beam-based improvement ofoptics.

charge and energy. Mode-change between KEKB and PF,
however, needs to turn off/on the first bend of ECS. Since
the ECS bends are heavily saturated it requires about
three minutes to recover the magnetic fields. In the newsystem, the PF beam is extracted at the upstream of ECS
through a new switch-bend (BM581), which is dedicatedfor PF, and guided to the existing line at the downstreamof BSY. BM581 is able to be set up the magnetic
condition within about a half a minute. The new line
rejoins the exiting line at a bend, BH12.

INTRODUCTION
The Linac in KEK provides beams for four rings, i.e.,
Low Energy Ring (LER: 3.5 GeV, e+) and High Energy
Ring (HER: 8.0 GeV, e-) of KEKB, Photon Factory (PF:2.5 GeV, e-) and Advanced Ring for pulse X-rays (PFAR: 3.0 GeV, e-). The Linac continuously injects beams
into LER and HER alternatively every about five minutes,and both of KEKB rings maintain almost full operating
currents. The PF or PF-AR, on the other hand, had needof about 20 minutes several times a day, for topping off
the current or from scratch, including 5 minutes forchanging the parameters of Linac. During this, the stored
currents in KEKB rings significantly decreased, and theoptimum point of luminosity tuning was lost. It had taken
more than two hours to recover the luminosity. Itis therefore important for maximizing luminosity ofKEKB to shorten the switch-time of Linac from KEKB toPF or PF-AR. In summer of 2005, a transport line fromLinac to PF were renewed, in which a new DC benddeflects the electron beam from the end of Linac to thenew line. We have succeeded in reducing the occupancy
time of PF injection to about five minutes, and there has
been almost no significant interference to the luminosity.
In the near future the DC bend is envisaged to be replacedwith a pulse-bend, which will make it possible to deliver
the beams to the PF and KEKB in pulse to pulse basis. To
this end the PF will also enjoy the top-up injection to keep
brilliant high. In this paper we describe on theoperational status of the new PF line, with emphasis on
the optics improvement based on the beam-based
measurements.

NEW PF/BT LINE
Fig. 1 shows a layout of the beam-switch yard (BSY) at
the end of Linac. The thick black(red) line designates theold(new) line to PF. In the old system all beams had
shared the first bend of Energy Compression System(ECS) of KEKB positron line as a common magnet and it
had played a role of switch-bend. Two beams for KEKB
are designed to be guided to their own transport lines
without changing the ECS-bend, owing to their different

Figure 1: Layout of Beam Transport Lines.
The new system has successfully commissioned
without major difficulties [2]. The elapsed time in modechanging from KEKB to PF, for go-and-return, was 2.6
minutes, while it was 5.3 minutes in the old system. Thus
the mode-switch time is successfully made almost a halfof that of the old system. Orbit deviations were 4 mm and
2 mm at maximum for horizontal and vertical plane,
respectively, which is small enough to accommodate thebeam (3 mm in horizontal and 1 mm in vertical beam
sizes in 1σ) in the chamber aperture of 57.2 mm. Theinjection rate for PF ring surpassed 2 mA/sec at arepetition rate of 25 Hz, which is very good values
compared to that in the old system.

LEAKAGE FIELD FROM ECS BENDS
The beam line is designed such that the beam is bent bybends BM581 and BM61F1 by the same quantity inopposite direction. In the initial stage of commissioning,
however, we could not steer the beam without changing
the strength of BM581 by 1.5 % higher than that of
BM61F1. It was found that this was due to the magnetic
leakage field originated from ECS-bends. The magnets ofECS consist of six bends in which the first and the sixth
magnets are H-type whereas the second to the fifth ones
are C-type. The third bend was closest to the PF beam anddirect field of their coils had caused sizable orbit change.

Measurement of Leakage Fields using Beam
On a profile monitor (SC61F2), the difference of thehorizontal position with the ECS on and off was measured
to be about 12 mm. Assuming that an hypotheticalmagnetic field exists at a point where the beam is closest
to ECS bends, kick angle and integrated field should beabout 0.80 mrad and 67 Gm. The quadrupole componentof the leakage field is estimated as follows; Variations ofthe horizontal positions on a BPM (SP61F1) changing
strength of BM581 are fitted to linear function to obtainthe slope or R12 component of transfer matrix. We
measured the R12 component under the conditions that
ECS bends were on and off. The difference of the two
R12’s is attributed the quadrupole component originatingfrom leakage field of ECS-bends. Measured value of B’L
is 0.0025 0.0060 T and the quadrupole component canbe ignored within an error.

Measurement of Leakage Field using Hallprobe

diameter of the pipe is 60.5 mm, we obtained 0.0060 T,
being consistent with the beam-based measurement.
We have measured transverse profile of the field at theclosest point to ECS. The results are shown at the inset ofFig. 2(b). The outside edge of the coil of magnet isdefined as X=0. The strength of magnetic field is zeroaround X=0, where the upward leakage-field from the gap
cancels the downward field arising from a coil of ECS. At
the region X>0, the gap field diminished and the coil fielddominates.

Design of Magnetic Shield

Figure 3: Calculations of magnetic fields of the ECS bend.
In order to design magnetic shielding, we made a fieldcalculation using code OPERA-2D. In the calculation ashield that is made of steel of 0.6 mm in thickness wasplaced at the closest position in the beam line. The field
inside the gap is shown in Fig. 3(a) while the field outsideof coils is shown in Fig. 3(b). The direction of X-axis istaken from gap to coil as positive. It is seen from Fig. 3(a)that the field change due to shield is 0.0036%, which does
not give any effects to the KEKB-beam line. Fig. 3(b)indicates that the direction of the field outside of coils is
opposite to that inside of coils and that the field at thePF/BT line decreases from 37 G to 0.25G with the ironshield.

Magnetic Shielding and its EffectFigure 2: Measurements of magnetic leak from ECS.
We measured leakage field from ECS bends by using aHall-probe. The positive field refers to the upward
direction. Fig. 2(a) shows the measured magnetic field inthe vicinity of ECS along the beam line. The red, yellow
and green lines indicate the measured values on the right,
upper and left sides of the beam pipe, respectively. The
leakage field from the third and fourth bends are observed.
The other source of leakage field can be seen around z=0,
which arises from direct field of BM61F1, though thisfield is contained in the strength of BM61F1. Integrated
fields along the beam line are 97.8, 78.9 and 61.1 Gm forthe right, upper and left sides of the pipe, respectively,
being consistent with the value obtained from beammeasurement.
We estimated an integrated quadrupole component of
the leakage field using difference of integrated fields at
the right and left sides of the beam pipe. With a outer

We have made a shield around the beam pipe in thewhole region from the vicinity of the third bend of ECS to
the BM61F1. The beam pipes are wrapped with double µmetal sheets of 0.35 mm thick and boarded up with
galvanized steel sheets whose cross sections are squarebracket shape of 0.6 mm thick. With this structure, weexpected that the leakage field greater than 10 G isshielded by steels and the minute fields like 0.25G which
remained in the Fig. 3(b) are shielded by µ-metals.
After installing the magnetic shield, we observed orbitchange in the profile monitor (SC61F2) with ECS on andoff, which was less than 1 mm and dramatically reduced,
compared with the value of 12 mm before the shielding.
The remnant orbit change could be attributed to leakage
from the first ECS bend because the same amount oforbit-change was observed when only the first bend wason and off. Although this orbit-change has practically noproblem, we added µ-metal shields to the PF/BT line in
the close vicinity to the first and second ECS bend. Aftermaking shields, no orbit distortion was observed evenwith the two upstream bends (BM581 and BM61F1) setto the design values.

DISPERSION CORRECTION

Figure 4: Design Optics Parameters of PF/BT.
All calculations for optics were done with SAD code[3]. The design optics of β-functions, dispersion functions
and beam sizes with an energy spread of 0.125% areshown in Fig. 4. The blue and red lines refer to thehorizontal and vertical plane, respectively. The dispersion
function at a profile monitor (SC61F2) is zero in design.
In the actual operation, however, with a beam energychanged, orbit-shift of a few mm was observed on
SC61F2. Observed dispersion error could be attributed tostrength error of quads in the region where the horizontal
dispersion functions are not zero. We made measurementsof the correction factors of quads using beam.
Measurements were divided into two groups of quads, i.e.,quads installed in the upstream of BH12, for which wecall Group-1 and the others (Group-2). Because thecurrents of quads in the two groups are set using thedifferent excitation curves, which are not cross-calibrated,
there should be at least two independent correction factors
for the quads.

made a least-square fit to minimize the difference ofmeasured dispersion and the design value, taking thecorrection factor of the quads as free parameters. Because
the three quads are cross-calibrated, correction factors areconsidered to be identical. We tried to make a fitting witha constraint of equal correction factors but we could not
find realistic solution. Note that the present method
depends on the BPM scaling error. We assumed that thethree quads have independent correction factors. Theresults are summarized in Table 1. We did not make any
corrections about quads downstream of QC3 this time,
because the dispersion function is zero in the region.
Table 1: Correction Factors of Quads
Correction Factors
Quads of Group-1
0.9536QC1
1.0127QC20.9845
QC31.0782
The measured dispersion function after the correction
factors are applied is shown in Fig. 5. The measured
horizontal dispersion functions (EX in the top row) aregood in agreement with the design values (ηx in the thirdrow).

Measurement for Group-1
There are four quads (QF61F1, QD61F1, QF61F3 andQD61F5) upstream of BH12. BPM’s are installedadjacent to the quads. To prevent scaling errors of BPM'sfrom affecting the results, we employed a special optics
where the horizontal dispersion function at a BPM(S861F3), which is just downstream of QF61F3, is zero.
The dispersion function at S861F3 depends on the threequads upstream of it. Varying a correction factor which iscommon to the three quads, we measured the dispersion
at the BPM. We obtained the correction factor of 0.9536
for the quads, with which measured dispersion is -0.0041
0.002. Without correction factor, it was -0.178 0.002.
The correction factor of QD61F5 was assumed to be thesame value as the others.

Measurement for Group-2
There are three quads (QC1, QC2, QC3) downstream of
BH12 in the region where the horizontal dispersion
function is not zero. Prior to the measurement we set up
the initial design-optics (Fig. 4) with a correction factor
applied to the quads of Group-1. Varying beam energy
within 0.2%, we measured BPM responses against thebeam energy, thus dispersion function at BPM's. We

Figure 5: Measured dispersion functions after setting thecorrection factors in Table 1 to the quads.

SUMMARY
We have made improvement of the optics of the newPF/BT transport line based on the measurements usingbeam. First we found imperfections originating fromleakage field from ECS magnets, and made magnetic
shielding. Second we decided correction factors for quads
using special optics and least-square fit. After these
corrections beam performance has been greatly improved.

REFERENCES[1] N.Iida, et al., “New Beam Transport Line from Linac
to Photon Factory in KEK”, Proceedings ofEPAC06.
[2] M.Satoh, el al., “The KEK Injector Upgrade for the
Fast Beam Mode Switch”, Proceedings of EPAC06.
[3] Strategic Accelerator Design (SAD), URL: http://accphysics.kek.jp/SAD/sad.html.