BEAM OPTICS MATCHING IN THE KEKB INJECTOR LINAC
T. Kamitani, H. Koiso, N. Akasaka, A. Enomoto, J. Flanagan, H. Fukuma,Y. Funakoshi, K. Furukawa, N. Iida, T. Ieiri, T. Nakamura,Y. Ogawa, S. Ohsawa, K. Oide, K. Satoh, M. Suetake, and T. Suwada
KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan
Abstract
The upgrade of the KEK electron/positron linac for
the KEK B-factory project is proceeding. The beamcommissioning of the first 1.5-GeV section of the linac
and the 180-degree arc section has been performed. This
paper reports on the beam optical matching in the linac
and the optical parameter tuning of the arc for theachromatic and isochronous conditions.
INTRODUCTION
The KEK B-factory is an asymmetric collider of 8.0GeV electrons and 3.5-GeV positrons. For full-energy
injection into the storage rings, the original 0.25-GeV
positron generator linac and 2.5-GeV electron/positron
injector linac were upgraded to 1.5-GeV and 6.5-GeV
energies respectively and connected by a 180-degree arc[1]. The positron production target has been moved to aposition where the electron beam energy is 3.7 GeV. The
designed beam intensities are 1.2 nC/pulse for injection
into the electron storage ring and 10 nC/pulse for positron
production. One of the most important issues in thebeam commissioning of the KEKB injector linac is thetransport of the intense electron beam from the bunching
section through the linac and the 180-degree arc to thetarget. In the fall of 1997, the beam commissioning of
the 1.5-GeV linac commenced. The beam opticalmatching in the 1.5-GeV linac has been performed. We
measured the emittances at three points along the linac.
Though the observed Twiss parameters agreed well withthe designed values, a large emittance growth wasobserved. The measured data were compared with thepreliminary results of a particle-tracking simulation. For
beam transport through the arc with minimum degradationof beam quality, achromatic and isochronous conditions
have to be satisfied. An extensive survey of the beam
optical parameters tuning of the arc was performed. For
the calculation of beam optics, the TRANSPORT [2] andSAD [3] codes were used.
1.5-GEV LINAC OPTICS
The 1.5-GeV linac (130 m long) is composed of apre-injector and 11 accelerator units. Each unit has four2-m accelerating structures (21 MV/m) fed by a 50 MW
S-band klystron. In the pre-injector, the electrons fromthe 200-kV electron gun are compressed to a singlebunched beam by two sub-harmonic bunchers, a prebuncher and a buncher [4]. During rf-bunching, the beam
is focused by a 1.0-kG solenoidal field supplied by

Helmholtz coils. At the exit of the bunching section, it
has an energy of 19 MeV. The latter part of the preinjector is used for acceleration up to 70 MeV, for beamdiagnostics (emittance, energy distribution, bunch timestructure) and for matching to the downstream lattice.
Quadrupoles are placed every 3 to 6 meters in the first
three units and every 10 meters in the subsequent eightunits. Focusing strengths were set to achieve almost 90deg betatron phase advance per cell. The calculated beta
functions and beam envelopes are shown in Fig.1.

Figure: 1 Calculated beta functions and beam envelopes of
the 1.5 GeV linac. (Solid: horizontal, Dashed: vertical)

For matching of the beam from the solenoidal
focusing system to the periodic quadrupole cells, theemittance just after the buncher was measured by scanningthe quadrupole strength and observing the beam spot sizeon the screen monitor. To reduce errors in the beam size
measurement, we used a CCD camera which can betriggered in synchronization with the beam arrival [5].
The data points for the different quadrupole strengths werefitted to evaluate the emittance and the Twiss parameters.
Based on the measured results, the quadrupole strengths
were calculated to achieve matching. To compare thebeam emittances along the linac with the designed optics,
we measured them at the 0.5-GeV point and at the end of
the 1.5-GeV linac. An example of the measured data isshown in Fig.2. Though the result of the matching wasgood, the emittance was observed to grow along thelength of the linac, as shown in Fig.3. The effects of thechromaticity and of wake fields were expected as causes of
the growth. To evaluate the effect of chromaticity,
particle tracking simulations were performed. One
thousand macro particles were tracked along the linac
with the designed optics in six dimensional phase space.
The variation of the r.m.s. emittance calculated by thetransverse distribution of the macro-particles are plotted inFig.4. A large emittance growth occurred when a large
energy-spread at the pre-injector was assumed. However,
the expected emittance growths were much smaller thanthe measurements. Further simulation studies including
wake field effects is necessary and will be performed later.

composed of six 30-degree bending magnets, sevenquadrupoles and six sextupoles as shown in Fig.5. Stripline type beam position monitors (BPMs) (resolution~100 microns) [6] are set in most of the quadrupoles fororbit and dispersion measurement. A bunch monitor
using optical transition radiation (OTR) viewed by astreak camera is placed just after the arc. It measures thebunch time structure and arrival time at 2-psec resolution
[7].
BM1

QF1

QD-m3 QF-m2

QD-m1

SF1
BM2

180-deg ARC Layout
QD2
QF3

E-beam = 1.5 GeV

15.0 meter

SF2

BM3
SD3

Figure: 2 Emittance estimated from the measured beam sizes
while scanning quadrupole strengths. Observed phase ellipse(solid) is compared with the designed optics (dashed).

QF4SD4BM4400

at 1.5 GeV,500

at 0.5 GeV

at 19 MeVy600

x700

measured emittance[10 –6 m]

800

10 nCxy

QF5
QD6SF5BM5SF6
QF7

QD-m4

QF-m5

QD-m6

BM6300Figure: 5 Layout of the 180-deg arc.
200

1 nC
100xy

0020

40

60

80

100120Distance from the pre-injector [m]Figure: 3 Observed emittance growth for 10 nC and 1 nCbeams.

Figure: 4 Simulated emittance growth (Solid:dE/E = +/-30%,Dashed:dE/E = +/-20% after the buncher. Initial emittance =85 m)

ARC BEAM OPTICS TUNING
To preserve the emittance and bunch time structureduring passage through the arc, the transport system wasdesigned to be achromatic and isochronous. The arc is

At the start of optical parameter tuning of the arc,
the designed values of quadrupole strengths to fulfill
the first order achromatic and isochronous conditions
were set as shown in Fig.6. The sextupoles were notused at that time.

Figure: 6 First order achromatic and isochronous opticsof the arc.

To evaluate the dispersion functions in the arc, weused the BPMs to measure the change of the beam
position due to the deviation of the beam energy from thenominal value as shown in Fig. 7. The energy waschanged by shifting the accelerating rf-phases of the last
two accelerator units; the phases were changed in opposite

directions from the crest to keep the energy spread small.

BPM:SPR062

4the dispersion measurement. The arrival times weredetermined from the center-of-mass values of the gaussianfitted bunch time structures. The correction factors of thequadrupole strengths for fulfilling both the achromatic andthe isochronous conditions were calculated from the dataand set. As seen in Fig. 9, the correction was successful.

2176.546x (mm)

after correction
0176.544

delay (nsec)-2

before correction
-4

-6
-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

176.542176.540

E / E 0 (%)

Figure: 7 Dependence of the horizontal orbit on theenergy deviation. (White: before, Black: after opticalparameter tuning)

176.538
1.495

1.500

1.505

1.510

energy (GeV)

By fitting a second-order polynomial to the data, thefirst- and second-order transverse components of thetransfermatrix( R 1 6 = x / ( E / E 0 ) and
R166= x/( E/E 0 )/( E/E 0 )) were obtained. Verticalcomponents R36, R366 were also obtained. The
measured dispersion functions are shown in Fig.8.

Figure: 9 Measured isochronicity just after the arc.
(White: before, Black: after optical parameter tuning)

SUMMARY
In the beam optical matching in the 1.5-GeV linac,
the measured Twiss parameters agreed well with thedesign, however large emittance growths were observed.
The measured data and the preliminary results ofsimulation were shown. The results of the opticalparameter tuning for the achromatic and isochronous
conditions of the arc were also presented. The leakage
dispersions and the non-isochronicity which observed in
the first measurement were well suppressed by fine tuningof the quadrupole and sextupole strengths.
REFERENCES

Figure: 8 Measured dispersion function around the arc.
(Dashed: before, Solid: after optical parameter tuning)

Though the designed dispersions after the arc werezero, finite values were observed in the first measurement.
The correction factors of the quadrupole strengths forfulfilling the achromatic condition were calculated fromthe observed R16 components. The quadrupoles wereadjusted with these factors and the dispersions weremeasured again. First order dispersions were greatlyreduced, however there were still large second ordercomponents. For suppressing them, the desired values ofthe sextupole strengths were calculated and set. The
results of the correction of the sextupoles weresatisfactory as shown in Figs .7 and 8.
With this dispersion-free beam condition, weproceeded to the measurement of the isochronicity. Thechanges of the beam arrival times were measured using
the OTR monitors while deviating the beam energy as in[1] A. Enomoto et. al. "Construction status of the KEKB 8GeV injector linac", proceedings of APAC98
S. Ohsawa et. al. "Commissioning of the extended partof the KEKB 8-GeV linac", proceedings of APAC98[2] K. L. Brown et al., CERN 73-16 (1973)[3] http://www-acc-theory.kek.jp/SAD/sad.html
Akasaka et. al. "Operation software for commissioningof KEKB linac programmed with SAD", proceedings ofAPAC98[4] S. Ohsawa et. al. "Pre-injector of the KEK 2.5-GEV linac
and high-current single-bunch acceleration", KEKPreprint 96-118 (1996)[5] T. Suwada et. al. "Emittance measurements of a highcurrent single-bunch electron beam at the pre-injector of
the KEK 2.5-GeV linac", KEK Preprint 94-97 (1994)[6] T. Suwada et. al. "Development of a stripline-type
position monitor for the KEK electron / positron linac",
KEK Preprint 93-166 (1993)[7] Y. Ogawa et. al. "New streak camera system for theKEKB linac", proceedings of APAC98