OPTICS CORRECTION FOR KLYSTRON SWITCHING
AT THE KEKB INJECTOR LINAC
T. Kamitani, Y. Onishi, N. Iida, N. Akasaka, A. Enomoto, J. Flanagan, H. Fukuma,
Y. Funakoshi, K. Furukawa, T. Ieiri, H. Koiso, S. Michizono, T. Nakamura, Y. Ogawa,S. Ohsawa, K. Oide, K. Satoh, M. Suetake, And T. Suwada,
KEK, Tsukuba, Japan
Abstract
For the high luminosity operation of the KEKB-factory,
the beam injection time from the linac to the storage rings
is desired to be short. However, the injection is sometimes
interrupted by a temporary rf-trip of the klystron. In caseof a serious failure, the klystron is switched to a spare one
to compensate for the acceleration energy. After theswitching, the injection is often degraded because of thebeam optical mis-matching due to the change in the beamenergy at each quadrupole. We have developed software
which can estimate the beam energy at each quadrupole,calculate the optics so as to achieve the desired matching
and change the settings of the relevant quadrupoles inshort time. The experimental result of the recovery of the
optical matching is also shown.

To maintain a good injection condition, even in the caseof klystron switching, a correction of the optics according
to the change in the acceleration energy is necessary. We
have developed software which can evaluate the beamenergy at each quadrupole, and calculate the optics so as
to achieve the desired matching condition and change thesettings of the relevant quadrupole magnets. (See thefigure 1 as for the user interface.)

1 INTRODUCTION
The KEK B-factory (KEKB) is an asymmetric-energy
electron-positron collider for the research of B-meson
physics, especially CP violation. To obtain a sufficientnumber of useful collision events, not only achieving highluminosity during beam collision but also maintaining agood condition of the whole accelerator system, is
important.
In KEKB operation, the linac injects beams to thestorage rings approximately every 2 hours to compensate
for any decrease of the stored beam currents. One of thefrequent problems which prevent injection is a temporary
rf-trip of the klystrons in the linac. In most cases, becausea tripped klystron recovers as it is in several seconds, it is
not a serious problem. However, in some cases, theklystron repeats discharging frequently, and requires a rfconditioning time for recovery. In that case, the operator
decides to switch the troubled klystron to a spare one tocompensate for the acceleration energy.
In the beginning of a beam tuning, the field strengths of
the quadrupole magnets are set to achieve the desired
optical matching according to the beam energy in eachposition. However, when switching of the klystron has
occurred, because of a change in the beam energy in theregion between these switched klystrons, opticalmismatching arises. This cause a degradation of theinjection efficiency to the storage rings, in some cases,
serious beam loss in the injector linac.

Figure 1: User Interface of the software for the beamenergy estimation, the optics correction and thequadrupole setting

2 BEAM-ENERGY ESTIMATIONIn this section we describe the various factors which areused in estimating the beam energy at each quadrupole
magnet. Before mentioning each factor, the componentsof the KEKB injector linac are briefly reviewed. The mostfundamental block forming the linac is an "acceleration
unit", which is comprised of an S-band klystron and four2m-long accelerating structures. These four structures
have a common phase of the microwaves supplied fromthe klystron with respect to beam passage. This phase canbe changed with the phase-shifter of each klystron andwith that of the sub-booster, which supplies microwaves
to a set of the eight klystrons, typically. The structures insame unit have almost equal field gradients, except for aslight difference in the shunt impedances. From theviewpoint of beam focusing, in most regions of the linac,a doublet or a triplet of the quadrupoles is placed in every
unit. Thus a unit forms a cell of a periodic lattice.
The beam energy at each quadrupole magnets is
estimated based on the following factors: (1) theacceleration field, (2) the acceleration phase, (3) the
klystron status, (4) the longitudinal wake-field effect and(5) the normalization factor.

(1) The field gradient in each accelerating structure isestimated from the rf-power supplied from the klystronand the shunt impedance of the structure. The rf-powerfrom the klystron is estimated based on the characteristic
curve describing the relation between the supplied highvoltage and the output power. The curve for each klystronis obtained from a measurement in the test stand. The
shunt impedances are measured for each types of theaccelerating structure. In the KEKB linac, at least fivedifferent types of the structure are used. They are allquasi-constant gradient type of structures, but haveslightly different dimensions to spread out the resonant
frequencies of the harmful wake-field component.
(2) The acceleration phase for each unit is given by thedifference between the present rf-phase setting and thecrest phase for maximum acceleration. The rf-phase isdetermined from the setting of the klystron phase-shifterand the sub-booster phase-shifter. The crest phase foreach unit is determined by measuring the beam energy
while changing the rf-phase at the 1.7-GeV 180-deg arc orat the end of the linac.
(3) The klystrons used for beam acceleration are
triggered in synchronous to the beam arrival timing (theacceleration state). On the contrary, the spare klystrons
are triggered out of the timing (the stand-by state) so as
not to affect the beam. In switching the klystrons, thetrigger timing of a failed klystron is changed to the standby state, and one of the spare klystron’s to the acceleration
state. In calculating the beam energy, the status of the
klystron is checked so as to decide whether it contributes
to acceleration or not.
(4) A single-bunch beam loading effect in theaccelerating structure due to the longitudinal wake field istaken into account. The wake function of the accelerating
structure has been measured for some typical structures.
Since the effect is charge-dependent, the charge intensity
at each position, which is measured with a strip-line typebeam-position monitor, is used for the calculation.
(5) The estimated beam energy includes errors in thefactors. Therefore, the estimated value, e.g. at the end of
the linac, may be slightly inconsistent with the energy
defined for the injection. To match the difference, thebeam energy at each position is re-scaled so as to achieve
the following conditions: the energy at the 180-deg arc is
1.7 GeV, the final energy of the positrons is 3.5 GeV andthe final energy of the electrons is 8.0 GeV. For the caseof injection to the Photon Factory ring and to the
Accumulator Ring for SR applications, the beam energy isnormalized to be 2.5 GeV.

3 OPTICS CORRECTION
Here, we describe the procedure for beam opticaltuning of the focusing quadrupoles and their correction for
klystron switching, taking the first 1.7-GeV linac before
the 180-degree arc as an example. A typical result of the
improvement of the optical parameters by the correction isshown.

The 1.7-GeV linac has two important matching
sections. One is at the pre-injector, for matching the beamfrom the rf-bunching section focused by the solenoids tothe quadrupole focusing system. The other is for the beamat the end of this linac to match to the special optics of the
180-degree arc, which was designed to be achromatic andisochronous. The details concerning the optical tuning of
the arc itself, are described elsewhere [1].
At the pre-injector, the beam optical parameters, likethe emittance and the Twiss parameters, are evaluatedfrom the measured beam size on a screen monitor bychanging the focusing strength of a quadrupole magnet.
These measured parameters are used as the initialcondition for the beam optics calculation performed with
the SAD code [2] to achieve matching to the downstream
focusing system. The 1.7-GeV linac can be divided into
three parts which have different lattices. The quadrupole
triplets are placed after every accelerating structure in thefirst two acceleration units, and every two or every four
structures in successive two and eight units. The betatronphase advance is generally designed to be 90-degree percell. Matching between the different lattices is performed
only by a calculation. Matching at the end of the linac is
performed using the measured optical parameters alongwith wire scanners [3] to accommodate any deviationfrom the design optics. This is because precise matchingis required to avoid beam loss in the arc, which has alimited acceptance in the energy and in the transverse
phase space.
For a beam-optics calculation with the SAD code andfor translating the calculated k-values of the quadrupoles
into the field strength of the magnet, information
concerning the energy gain in each accelerating structure
and the beam energy at each quadrupole is necessary. It isestimated as described in the previous section. The supply
current of the magnet is determined from the fieldstrength with an excitation curve which has beenmeasured for each type of magnet.
During normal operation, the beam optical parameters
are well matched to the designed optics, resulting in
negligible beam loss. When one of the klystron fails
because of frequent rf-trips, it is changed to the stand-by
state. Instead, one of the spare klystrons is set to theacceleration state to compensate for the accelerationenergy. In some cases, the energy is compensated just bychanging the phase of the klystrons used for adjusting thebeam energy to fit with that of the arc, instead of using aspare klystron. After switching the klystrons, the beamenergy is considerably changed at each quadrupole andthe focusing strength is deviated from the matched
condition. To correct this optical mismatching, the beamenergy for each quadrupole magnet is recalculated basedon the new status of the klystrons. Based on this beamenergy, the focusing strength of the quadrupoles iscalculated so as to achieve the desired matching. The
supply current settings of the quadrupoles are changed
according to the result of the calculation.

4 EXPERIMENTAL RESULTS

0
-5
-10
-15
-20
-25
-30
0

8

16

24

32

40

Distance (by No. of ACC structures)

Figure 3: Change in beam-energy due to the klystronswitching (ratio)
At first, the optical parameters with the normal
operation states were measured. (data (1) in Table. 1 andFigure. 4) Then, the data (2) with the klystrons switchedwere taken. With this beam-energy condition, two kindsof the optics correction were applied. As a simple andquick correction, all the quadrupole settings were changed
to have the same k-value as before the klystron switching.
Finally, the full correction which re-calculated the opticalmatching all through the beam-line was applied.
Table 1: Measured Optical parameters
Twiss parameter Emittance[10^-6 m]α,β [m](1) Normal state
3.4,
3.0
184
(2) KLY down
7.0,
9.0
96
(3) Simple Corr.
6.5,
5.9
107
(4) Full Corr.
3.2,
2.7
165
Figure 4: Measured emittance ellipses
Clear deviation of the optical parameters was observedby the klystron switching (data (2)). It was improved tosome extent with the simple correction and improved
much better with the full correction.

4 CONCLUSION
We have developed software which can evaluate thebeam energy at each quadrupole and calculate its focusing
strength so as to achieve the desired matching conditions.
In the case of klystron switching due to rf-trips, it is used
to correct for any mismatching of the optics. The
experimental results show that it can well recover the
optical matching.

2000

Beam Energy (MeV)

Change in
Beam Energy (%)

In the beam operation for the commissioning of the
KEKB, the software has been proved to be very useful.
Most successful improvement was in the case of theserious failure of the klystron which supply rf-power into
the second acceleration unit after the positron capture
section. Since this unit resided in quite upstream in thepositron accelerator, the change in the beam energy wasvery large (67% at maximum). Most of the beam charge
was lost in this unit due to the enormous optical mismatching. In this occasion, the beam commissioning
members have soon decided to correct the matching byusing this software. Soon after finishing the calculation,
the quadrupole settings were changed and resulted inalmost full recovery of the beam intensity.
Recently, we have performed an experiment which
demonstrate the improvement of the optical matching bymeasuring the beam optical parameters, like the emittanceand the Twiss parameters. It is performed at the 1.7-GeV
linac which included the pre-injector and the elevenacceleration units before the 180-degree arc. The opticalparameters were measured just before the arc. In order to
see the change in the optical parameters by a klystronswitching and also by a correction of the optical matching,
the klystron for the third acceleration unit after the preinjector was set to the stand-by state on purpose. Instead,the rf-phases of the eighth and ninth units were adjusted
so as to compensate for the acceleration energy. The
estimated beam-energy in each accelerating structure isshown in Figure 2 and 3. There is no acceleration in the
11-th to 14-th accelerating structures which corresponds
to the klystron turned off. The acceleration energy iscompensated in the 31-st to 38-th structures, so as toobtain same energy at the arc. The ratio of a change in the
beam-energy is around 28 %.

5

Normal Operation
After Klystron-Switching
1500

1000

500

0

0

8

16
24
32
Distance (by No. of ACC structures)

40

Figure 2: Change in beam-energy due to the klystronswitching (absolute value)

REFERENCES[1] T. Kamitani et al.: "Beam optics matching in the
KEKB injector linac", APAC98 (Tsukuba), 1997[2] SAD Home Page,
http://www-acc-theory.kek.jp/SAD/sad.html[3] N. Iida et al.: “Optics Matching System using Wire
Scanners for KEKB Injector LINAC and BeamTransport Lines”, in this conference