Fast Beam Loss Monitor
H.IkedaIntroduction
KEKB Beam Abort System
Loss Monitor
Installation of Data Logger
Data Analysis
SummaryIntroduction••••

KEKB is high current machine in order to get the high luminosity andBelle (SVD) is placed as close to the beam line as possible to obtain
the best position resolution.
A controlled beam abort system is necessary for protection of thehardware components of accelerator and detector from the high beamcurrent. At the beginning of experiment, abort system was installed, but
movable masks were damaged and Belle detector received a hugeradiation dose.
PIN PD was installed at the movable masks to get a fast response
from the beam loss and abort the beam before the mask is damaged.
Data logger was installed to monitor the beam loss directly and find themethod of hardware protection. By using this system, we identified thereason of the beam abort and decided the best condition and method ofthe beam abort.

KEKB Beam Abort System••••

The kicker magnet is activatedwith a rise time of less than 1 µs
and an entire beam is aborted
in one turn (The kicker timing issynchronized to an abort gap).
HER/LER have independentinterlock module in each local
control room.
Trigger signal = Loss Monitor,RF, Belle B.G. etc.
Time necessary from the inputof the interlock module to thetermination of the beam abort =80 -160 µs.

Loss Monitor••

Ionization Chamber =>installed in the tunnel when the KEKB
accelerator was constructed.
PIN photo-diode => replaced with ionization chambers to protect themovable masks after two year operation.
– Fast trigger signal = 20 µs (fast enough compare with the delay inabort system.)– All of the movable mask = 16/ring,– PIN PD 2x4/mask,– acceptance surface= 2.65mm x 2.65mm

Loss Monitor•••

PIN PD is used without bias.
Pulse signal is shaped bytrack and hold (3.3 ms decaytime) circuit for ADC readout.
Compatible with ion chamber
signal after gain adjustment.

Installation of Data LoggerýýIn order to monitor the beam loss
directly and diagnose the reason of
the beam abort, input signals areexpected to have a strong relation
with the beam condition.
– Input signal = beam current, PIN
Loss monitor signal, RF statussignal.
– Set up in local control rooms.
– Logged time = 600ms.
– Sampling interval = 5 µs.
Using this information, we could
identify how much beam loss wascaused and which mask responded.

Data AnalysisăManual Abort••••

An example of manual abort isshown to identify the correct abort
signal.
Monitor beam current = 40 µs +90µs delay (inside the cable and theelectronics circuit).
Real beam current = aborted in 10µs.
If the decay time and slope isdifferent from this example, weconsider the abort is abnormal
and analyze the data logger
information to determine thereason of the beam abort.

Data Analysisăan Example of Movable Mask
Damage••••PIN showed the large signal
with beam loss when themovable mask was damaged.
RF down -> Beam Phase signal
start to rise -> 1.4ms -> Beam
Loss -> Loss Monitor Abort &Beam Phase Abort request.
Beam abort request was toolate and the beam hit themovable mask directly.
Each abort trigger level weretuned carefully after this trouble.

Data Analysisăan Example of Movable Mask
Damage•••Injected beam hit the mask andproduced a large signal when theinjection tuning was not fine. Since
this dose not damage the mask,
the generation of the abort isvetoed for 3 ms in synchronouswith the injection timing.
After applying this veto, the trigger
level of loss monitor was tighten
and the trigger level became to afew mA beam loss.
We expect the loss monitor abort isgenerated in time to protect theKEKB and Belle hardware.

Data AnalysisăAbort Reason••

In order to decrease the numberof aborts, the reason of abort
was checked one by one. (Thedata logger data can be checkin control room within 3 minutesafter a beam abort.) -> Main
reasons are beam loss and RFtrouble.
It is important to know the beamloss.

Data AnalysisăRelation between Beam Loss and Beam Oscillation••

Relation between beam loss and beam
oscillation was checked using beam
oscillation recorder (BOR).
– Most of the beam loss were caused by
the beam oscillation and several typeof beam oscillation is observed.
– When a LER beam is aborted, an
instability of the HER beam isgenerated and the horizontal
oscillation is observed.
– When we find no oscillation despite ofa large beam loss, we often found thetune was shifted off.
We figured out the reason of the beam loss
and it is used to improve the control
parameters of the accelerator.

Summary•••

A controlled beam abort system was installed at the KEKB accelerator toprotect the hardware component from the loss of the high current beams,
but the movable masks were damaged and Belle detector received ahuge radiation dose due to the delay of trigger signal in some abort.
We installed PIN PD loss monitor at the movable masks to get a faster
response (20 µs) from a beam loss.

And the signal of PIN PD and the RF status signal are supplied toa data logger to make a diagnosis of the beam abort. By analyzing
the data, the reason of beam loss and beam abort have been
identified and the best condition of the beam abort has been
determined. As the result of the optimized beam abort, we
succeeded to eliminate catastrophic beam losses which has
caused a serious damage to movable mask. A radiation dose to
the Belle detector also decreased.