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

OBSERVATION OF THE LONGITUDINAL BEAM OSCILLATION AT ATF-DR
T. Naito, H. Hayano, E. S. Kim, K. Kubo, N. Terunuma, J. Urakawa,KEK, High Energy Accelerator Research Organization
1-1 Oho, Tsukuba, Ibaraki, Japan
Abstract
The longitudinal beam oscillation was observed byusing a streak camera at the high intensity multi-bunch
operation of the ATF-DR. Two mode measurements wereused for observing the behaviour with bunch by bunchmeasurement basis. The measurement results showed thepossibility of the HOM of the RF cavity exciting theinstability.
This paper describes the measurement
technique and discusses about the cure method of theoscillation.

Observation with a fast single scan mode of theINTRODUCTION
The longitudinal beam oscillation was observed at thehigh intensity multi-bunch operation of the Accelerator
Test Facility Damping Ring (ATF-DR) in KEK. The
multi-bunch operation at the ATF-DR was started since
2000. However, the beam storage was not so stablebecause of the beam loss at the injection. The injection
loss was improved by adopting the RF gun, since Autumn
2002. The single train consists of twenty bunches with
2.8ns spacing, which can be changeable from one totwenty. The ATF-DR was designed for a maximum of
five trains operation. The three trains of the multi-bunch
beam can be stored stably. Four trains and five trains are
not yet stored because of the effect of the pulse tail of theinjection kicker. [1] The stored current already reached
60mA at the three trains multi-bunch, which is limited by
the radiation safety regulation of the maximum power ofthe stored beam until Summer 2003. The limitation wasrefined from Autumn 2003. Minimizing the longitudinalbeam oscillation is a significant issue for the ATF-DR at
the high intensity multi-bunch operation.
The ceramic chamber of the injection kicker wassuspected as the source of the oscillation, which had anarrow beam aperture, 14 mm diameter, and the shapeformed a cavity like a dip. The temperature of the ceramic
was heated up according to the stored current. [2] A
conductor was inserted at the inside of the ceramicchamber to avoid exciting the electric field. However,there was no clear difference even after inserting theconductor. The simulation predicts that the cavity or thechamber which has a cavity like shape excite theoscillation. The estimated Q-value is not so high. [3]

MEASUREMENT
The hardware of the SR monitor including the streak
camera is described in reference [4]. The streak
camera(Hamamatsu 5650) is a two-dimensional streak
camera which can observe the bunch length and the timetrend.

Fig. 1 bunch shape of the stored beam by a wall
current monitor

streak camera
Fig.1 shows the bunch current by a wall currentmonitor. Seventeen bunches were stored at this time. The
total charge was 3.7x 10^10 electrons and the bunchcharge was about 2.5x10^9 electrons. The streak camera
image and the profile are shown in Fig. 2. Each image
shows the same bunch with 8 ms interval. Fig. 2 (A) is the
1st bunch and (B) is the 4th bunch. The streak timing wassynchronized with the revolution. If the beam circulates
stably, the streak image should be shown at the sameposition. The 4th bunch was clearly unstable compare tothe 1st bunch. The statistical data of the oscillation
amplitude, 1σ of the amplitude distribution, of eachbunch is plotted in Fig. 3. The amplitude of the oscillationof the succeeding bunches were clearly increased. The
amplitude was also increased according to the beam
intensity. The amplitude at the case of the 5x10^9corresponded to 1.5x10^-4 of the energy spread. The
oscillation seemed to come from the beam-induced
instability from these results. The longitudinal oscillation
makes the energy spread. Fig. 4 shows the energy spread,which is measured by the profile monitor at the extractionline. The scatter area assumed come from the longitudinaloscillation. The measurement agreed with the streak
camera measurement.
The fast scan mode of the streak camera can observe
the single bunch behaviour of the multi-bunch, however
the sweep interval is limited up to ~1ms. The frequency
component of the oscillation could not be observed in thismode.

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

Observation with a synchro-scan mode of thestreak camera
The streak camera can change the sweep mode bychanging the sweep unit. The synchro-scan mode is swept
by the divided RF frequency, 89.25MHz, which is 1/8 of
the 714MHz RF accelerating frequency. The beam profileis the projection of all of the bunches with this mode.
Some of the examples are shown in Fig. 5. The two lines
are mirror images of the same beam profile with theinversed form.
Fig. 2(A) The profile of the 1st bunch of the 17
bunchesFrequency
The frequency of the longitudinal oscillation was thesynchrotron frequency. The synchrotron frequency waswritten by the following equation,·§12 hηc eV 0 cosϕ s .
f = 2π ¨ω rev¸2πβcp¹©
012sThe synchrotron frequency changes with the squareroot of the accelerating voltage. Figs 5 A) and B) show
the oscillations. The frequencies were changed according
to the accelerating voltage. The measurements agreed
with the calculation.
Fig. 2(B) The profile of the 4th bunch of the 17bunches

The streak camera could observe the damping
phenomena of the oscillation. Figs 6(A) and (B) show thebunch profile in each time from the injection. The big
dipole oscillation was damped with the time, however thesmall oscillation existed even after 400ms from injection.

Longitudinal Oscillation @multibunch20021120
0.06
8e10
3.7e102e10
0.6e10

Oscillation(ns)0.05

Damping phenomena

0.04

Temperature of the cavity
0.03

0.02

0.01

05

10

15

20

BunchNumber

Fig. 3 The amplitude of the longitudinal oscillation
at each bunch
0.001

0.0008dp/p

0.00060.00040.0002005 1081 10

99

1.5 102 1092.5 10

9Bunch Intensity

Fig. 4 The energy spread: measured at theextraction line beam profile monitor

The amplitude of the oscillation was changed with theseason. We suspected the temperature of the RF cavity. Ifthe dumping of the higher order mode (HOM) was notenough, there is a possibility of the oscillation, which isinduced by the HOM of the RF cavity. The dumped
cavity was designed for dumping the HOM in the cavity,
however the data measured the dumping of the frequency
up to a few GHz. There is a possibility that the HOM
exists at more than a few GHz. [5]We carried out the experiment at different temperaturesof the cavity. The present system could not change thetemperature of the cooling water of the cavity, arbitrarily.
So we reduced the water flow of the cooling water andcontrolled the temperature by changing the RF power.
The sweep frequency was changed in this measurement toobserve the oscillation of each bunch. Figs 7(A) and (B)show the oscillation at the case of 24.3°C and 29.1°C,respectively. Each line shows the single-bunch profile ofthe multi-bunch. We could clearly observe the differenceof the amplitude of the oscillation. The sweep frequency
was changed from 89.25MHz to 95.2MHz in thismeasurement in order to observe individual bunch
oscillation. The different amplitude was observed at eachbunch and the phase was the same in Fig. 7(B). The
bunch number was not yet identified.

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

Improving the water cooling system
From these results, we are now preparing the newcooling water system for the RF cavity, which can control

the temperature from 15°C to 35°C. If the HOM of the RF
cavity produce the oscillation, the dependence will beobserved by the function of the cavity temperature. The
more precise data will be taken by using the system

SUMMARY
The longitudinal beam oscillation was observed byusing a streak camera. The measurement result showed
the possibility of the HOM of the RF cavity exciting theinstability. The detail of the oscillation source will be
studied at the various temperatures by using the new
cooling water system. The cure method of the oscillation
will be found in these measurements.

ACKNOWLEDGEMENTS
Fig. 5 A) Vc=300kV, f=10.4kHz

We would like to thank facility staff of KEK for theirassistance. We also would like to express our gratitude toProfessors Y. Kamiya and S. Enomoto for theircontinuous encouragement.

REFERENCES

Fig. 5 B) Vc=150kV, f=7.14kHz[1] T. Naito, et al., “Installation and measurement of the
ATF injection kicker”, KEK preprint 97-135[2] http://atfweb.kek.jp/atf/2002/report/021115.pdf[3] Kubo, “Simulation of longitudinal multi-bunch
instabilities in ATFDR”, ATF internal ATF-03-02A[4] T. Naito, et al., “SR monitor for the ATF Damping
Ring”, KEK Internal 97-43[5] F. Hinode, et al., “ATF Design and Study report”,
KEK Internal 95-4

Fig. 6 A) 0.04ms after the injection
Fig. 7 A) 180kV, 24.3°C

Fig. 6 B) 2ms after the injection
Fig. 7 B) 300kV, 29.1°C