Proceedings of IBIC2012, Tsukuba, JapanTUPB53

ABORT DIAGNOSTICS AND ANALYSIS DURING KEKB OPERATION

Abstract
KEKB has stopped since June 2010 for upgrading theluminosity 40 times, i.e. SuperKEKB. During theoperation of 11 years, a pair of controlled beam abortsystems worked more than 10000 times to protect thehardware components of KEKB accelerator and thedetector against the high intensity beams of high and low
energy rings (HER and LER, respectively). Optimizationof the abort trigger was necessary to balance efficient
operation with the safety of the hardware. Therefore, we
analyzed one-by-one all of the aborts, and continuallyadjusted the abort system. The diagnostic system wasbased on a high-sampling-rate data logger that recorded
beam currents, RF signals and beam loss monitor signals.
The beam oscillation signals, vacuum pressure anddetector dose rate were also examined. This paper
describes the typical abort causes, optimizations of abortlevels, and abort statistics over approximately eight years
after having arrived at high current operation.

INTRODUCTIONKEKB was an energy asymmetric electron positron
collider dedicated to B meson physics. An electron ring of
8 GeV (HER) and a positron ring of 3.5 GeV (LER) were
installed in a tunnel. The maximum achieved currents ofthe electron and positron rings were 1.35 A and 2.0 A
respectively so far. When a large beam loss is expected,
the beam should be quickly dumped in order to avoid thedamage to the accelerator and the Belle detector
components due to high current beams. A controlled
beam abort system was installed for this purpose [1]. Thesystem triggered more than 10000 aborts during theKEKB operation for eleven years. Optimization of thecondition to issue the abort trigger was necessary tocompromise between efficient operation and safety of thehardware. Therefore, we analyzed all of the aborts one byone, and continually adjusted the abort system.

The signal flow of the data loggers is shown in Fig. 1.
Logged data were beam current measured by a directcurrent-current transformer (DCCT) [2], a part of loss
monitor signals from PIN photo-diodes (PD) and ion
chambers (ICs), signals from the RF cavities, i.e. cavity
voltages and output of klystrons, the beam phase signal
showing the deviation of the synchronous phase, theinjection trigger timing and the Belle PD signal. MostPDs were fixed on the movable masks of each ring, anddetermined the ring in which the beam loss occurred. On
the other hand, ICs were installed in the whole tunnel andcovered the wide range in space, but could not distinguishthe ring. These signals were useful to diagnose the causeof the beam abort since they had a strong correlation withthe beam condition. The recorded data were sent to theKEKB central control room (CCR) via the KEK internal
network then monitored by operators. The information
was ready for inspection within a few minutes after theabort. The beam oscillation signals were obtained fromthe beam oscillation recorder (BOR) [3]. The signals werealso ready within a few minutes after the abort. The BOR
recorded the bunch-by-bunch beam position over 4000
turns immediately before the beam abort so as to detectvertical and horizontal beam oscillations.

SETUP OF ABORT MONITOR
The diagnostic system was based on a high-samplingrate data logger that recorded beam currents, RF signals,signals from beam loss monitors and the Belle detector at
the moment of the beam abort. In addition to the datastored in the data loggers, beam oscillation, vacuum
pressure, the earthquake sensor and the dose rate of theBelle detector were also examined to analyze the beamabort.
Loss monitor signals of whole rings were collected atfour local control rooms (LCRs), and were sent to the dataloggers distributed in five LCRs where both loss monitor
signals and RF cavity signals were obtained.

Figure 1: A signal flow of data loggers.

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1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○H. Ikeda*, J. W. Flanagan, T. Furuya, M. Tobiyama, KEK, Tsukuba, Japan
M. Tanaka, MELCO SC,Tsukuba, JapanTUPB53

Proceedings of IBIC2012, Tsukuba, JapanTYPICAL ABORT EXAMPLESManual Abort
A manual abort is a beam abort triggered manually byswitches by operators. An example of the signals at themanual abort in HER is shown in Fig. 2 (a). The DCCTsignal shows delay of 40 μs and decay in 90 μs in spite of(a)the beam being aborted in 10 μs, i.e. one turn. This is anormal behaviour of the DCCT signal when the beam isaborted in one turn. If the decay time and the decay slope
differed from this example, the abort was judged
abnormal and the data logger information was analyzed todetermine the cause of the beam abort.

(b)c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○(c)Figure 2: Examples of logged signals at a moment of (a) a manual beam abort, (b) a beam phase abort caused by RFvoltage down and (c) a beam loss abort caused by vacuum problem. Signals in (c) are the LER beam current, the beamphase, the loss monitor PD and the Belle PD from top to bottom.

Beam Loss Abort
The beam aborts were categorized as the beam loss
aborts, when the loss of the beam current was observed
before RF cavity trips. About a half of the beam aborts
were the beam loss aborts. In most cases the signal of thePD at a movable mask was also detected. The PD signal
was useful to identify the location of the beam loss when
the beam loss happened at a movable mask. The PD
information was analyzed together with the data of BOR
to identify the cause of the beam loss. The analyzed result
was used to improve the operation parameters of KEKB.
Some beam loss aborts were caused by the beam____________________________________________*hitomi.ikeda@kek.jp

oscillation. For example, when the LER beam wasaborted, the HER beam sometimes remained alone in thering. Then an instability of the HER beam accompaniedby the horizontal oscillation was occurred, probably
because the damping effect by the beam collision was lost.
As a result the HER beam was also aborted. It depended
on beam condition and parameters of the bunch feedback
system. Figure 3 (a) and (b) show the statistics of theaborts caused by the beam loss. The ratio of the beam loss
aborts without the oscillations to the whole of beam loss
aborts increased in the latter period of the KEKB
operation. When no oscillation was found despite of alarge beam loss, we often found the tune was shifted.

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(a)

(b)

(c)

(d)

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(e)

(f)

(g)

Figure 3: Cause of beam loss in HER (a) and LER (b), status of beam loss and the abort when HER (c) or LER (d) was
aborted, signals which requested the abort of HER (e) and aborts triggered by the RF of HER (f) and LER (g).
Sometimes the LER beam loss triggered the HER abortand verse versa. We call this event a wrong abort. ThePIN PDs could identify the ring where the beam lossoccurred, thus did not generate the wrong abort. RFISBN 978-3-95450-119-9

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Copyright ○Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○TUPB53

Proceedings of IBIC2012, Tsukuba, Japansignals did not generate the wrong abort, too. On the other
hand the ICs and the Belle PDs could not identify the ringwhere the beam loss occurred because the sensors reacted
in both HER and LER beam loss. They could generate thewrong abort. To improve this situation Belle group
introduced a logic that checked which ring was being
injected when the beam loss at Belle happened, thenjudged which ring should be aborted. Anotherimprovement came from RF arc sensors which were PIN
PDs located at RF cavities. Figure 3 (d) shows the statusof the beam loss when the beam loss abort was requested
to LER. "HER abort" means the LER beam was lost after
the HER abort, probably because the LER beam become
unstable. "HER Loss", "LER Loss" and "Both Ring Loss"mean that the beam loss was observed by DCCT at HER,LER and both rings, respectively. The wrong abort
corresponds to "HER Loss" because the LER beam wasaborted although the beam loss was observed in HER.
Figure 3 (e) shows which sensors requested the abort ofHER. The “other” in (e) was issued almost by the RF arc
sensors. The number of the wrong aborts in LER
decreased as the “other” abort requests in (e) increased.
This correlation appeared after the installation of the crab
cavities. We suppose that the wrong aborts in LER
decreased because the RF arc sensors near the crab
cavities could identify the ring where the beam loss
occurred. Probably the extensive beam loss which caused
the IC abort was avoided, since the loss was detected firstby the arc sensors and then the right ring was aborted.
Figure 2 (c) shows another example of the beam loss
abort in which no beam oscillation was observed. Thetunes were stable. The beam phase (BP) started to swing
before the beam loss happened and also before the loss
monitor PD signals and the Belle PD signal appeared.
This type of the abort was found in Feb. 2005. It wasfound that some bellows were troubled. The pressure andtemperature near the bellows were higher than usual. By
fixing the vacuum problem, the abort ceased. This type ofabort happened again in 2006 as shown in Fig. 4. The
vacuum troubles were also found again in this case.
The number of the beam loss aborts decreased in 2004
after the continuous injection started to keep the beam
current at fixed level.

avoid this situation, the BP was used as an abort trigger[4]. The trigger level of the BP abort was set to one
degree in HER and five degrees in LER. We also installeda fast cavity voltage monitor in each RF station. Figure 3(f) and (g) show the rate of RF aborts. While the HER
was equipped with both superconducting cavities (SC)and normal conducting cavities (NC) [5], the LER wasequipped with only NCs. The rate of the abort was similar
in both types of cavities.

RF Abort

The statistics of the beam abort, which are categorized
by analyzing the information of the data loggers and othermonitors, is shown in Fig. 5. It shows the number ofaborts per day averaged over one month during eight
years operation of KEKB with high beam current. The
manual aborts issued by operators are not included. From
the figure, we see that there are two major origins of thebeam abort; the RF trouble and the beam loss. Since thecoupling between the beam and RF cavities are strong, it
is important to clearly classify the cause of the abort inorder to improve machine performance. The fractions of
the RF and beam loss aborts in all number of aborts were30% and 60%, respectively, before the crab cavity
installation. The ratio of the crab cavity abort was higher
than the other RF cavity aborts, but the sum of all aborts

Because of the strong interaction between an
accelerated beam and RF cavities, the cavities tripped
easily whenever the beam lost. On the other hand, when
one of the cavities tripped, the coherent synchrotron
motion of the beam occurred and gave a strong radiation
to the Belle detector. Figure 2 (b) shows an example oflogged signals at the beam phase abort caused by an RFtrip. The BP signal starts to rise in response to the RF trip.
On the other hand, when the beam loss happens earlier
than the RF trip, the BP starts to swing downward
because the RF can't compensate the loss of the beam
induced field immediately. In both cases, the induced
synchrotron oscillation causes a large beam loss which
could lead damages of hardware components. In order to

Crab Abort
Two crab cavities were installed in 2007 to improveluminosity [6]. Since the beam current was limited in first
several months after starting the crab operation in order totune the crab cavities, the breakdown of the crab cavitiesdid not affect the beam. However the beam was manuallyaborted to recover the cavity voltage after the breakdown.
In the early period of the operation the breakdown of theLER crab cavity occurred frequently. The breakdown
caused the beam loss at a few hundred mA of the beam
current. The luminosity was improved after the operationof the crab cavities became stable and the number of theaborts caused by the crab cavities decreased.

Figure 4: The number of beam loss aborts caused by
vacuum troubles.

STATISTICS

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were not changed after the installation of the crab cavities
since the beam current at the crab operation was lower
than that before the crab operation. There are many otheraborts due to earthquakes, troubles of vacuum system andmagnet power supplies and so on, though the fraction ofthese aborts was small. The number of the aborts did notstrongly depend on the beam current as shown in Fig. 5(c) and was reduced by optimization of the abortcondition. The number of HER and LER aborts was3.7/day and 2.3/day respectively. The total number of theaborts was about 6000 and 4000 in HER and LER,
respectively.

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CONCLUSION(c)

REFERENCES[1] T.Naitoh, KEKB MEMO No.187 (2001).
[2] M.Arinaga et al., “KEKB Beam Instrumentation
System", KEKB Accelerator Papers, Nucl. Instrum.
Meth. A499(2003)100.
[3] M.Tobiyama and E. Kikutani, Phys. Rev. ST Accl.
Beams 3,012801(2000).
[4] K.Akai et al. KEK preprint 2002-36(2002).
[5] K.Akai et al., Nucl. Instrum. Meth. A499, 45(2003).
[6] To be published in Prog. Theor. Exp. Phys.

Figure 5: Statistics of the beam abort for last eight years
of KEKB operation in HER (a) and LER (b). (c) Historyof the beam current and the luminosity of KEKB.

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5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○A controlled beam abort system was installed in
KEKB to protect the hardware components from the lossof high current beams. In order to make clear the realreason of each abort, various signals such as the PIN PD
beam loss monitors and the status signals of the RF
system were collected by the data logger system andanalyzed to identify the cause of the beam loss and thebeam abort. The results showed the real situation of theaccelerator hardware and gave us a lot of hints not only toprotect the hardware but also to optimize the operation
parameters. As a result, we could suppress theunnecessary beam aborts and improve the luminosity.