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Beam Losses and Machine Protection (real life)
By Kay Wittenburg,Deutsches Elektronen Synchrotron DESY, Hamburg, Germany
Experiences from HERA (accidental losses)
Beam Dump: why?
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Loss-Mechanisms1) After an equipment failure (e.g. Power supply(ies) trip) the beam starts to
oscillate (position or size) with an exponential growing amplitude. First losses occur after a time (length) which depends on the failure typ– then the beam “explodes” within a very short time.
2) Mislead beam. Might be very fast (< 1 turn). Reasons: Kicker, Operation, …
t0
X or Y
FailureFirst losses
TotalBeamloss
Aper-tur
long shortt, l
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The Detector
• Two PIN-PDs in coincidence to count charged particles• Signal (in Si):
•dE/dx = 3.7 MeV/cm•3.7 eV/e-hole pair•=> 10-15 C/MIP•=> 10 000 e-/MIP
• Small dimensions:•Area: 2.75 · 2.75 mm2
•or 20 · 7.5 mm2
• Costs:•1 $ for small PD•100 $ for big PD
BPW34
PIN Photodiodes to satisfy the special conditions in HERA
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The AmplifierSensitive and fast amplifier with low noise and with a fast coincidence following
DESY BLM with lead hat (removed) on top of a sc quadrupole
Diodes Pre-ampl. Video ampl. Comperator +5V
+24 VBias
TTL driver
-5V
Threshold
-5V
•Efficiency to charged particles: 30%
•TTL output for counting
•Very low noise:•Dark count rate < 0.01 Hz•max. count rate > 10.4 MHz
•Very high dynamic range: >109
•Insensitive to synchrotron radiation:•Efficiency to : 3.5 · 10-5
•Coincidence + lead: 1 Hz at 1.5 Gy/h (e- ring at max.)
Pulse shape of the BLM output Response to MIPs: Blue line: Single diode; Green line: Coincidence; TTL compatible (90 )
500
mV
/Div
.
100 ns/Div.
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The counting module- Integration time: 5.2 ms (to be shorter than the cryogenic time constant of about 20 ms)- Short mode buffer: 128 · 5.2 ms = 666 ms- Long mode buffer: 128 · mean short = 85 s- Stop data taking in case of alarm- Archiving- Function check
Counter im „Grab“;immer neben Controller
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No quench
Dump due to losses
HERA BLM Alarm System
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Alarmtest:Set threshold 1 or 2.Will be overwrittenafter some seconds!
Should show 30 or 5
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Beam loss induced Quenches 1994 - 2004
Injection
27%
Magnet PS
13%
5 ms events
16%
RF
8%
ALZ
12%
BLMs <4
8%
Operating
3%
K ollimator
2%
Diverse
6%
unknown
5%
Injection
Magnet PS
5 ms events
RF
ALZ
BLMs <4
Operating
Kollimator
Diverse
unknown
Note: A quench in HERA is not a disaster! It takes typ. 1-2 h to recover from cryogenic
= 189 Quenches
More statistics
HERA experience with
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Statistic of BLM events 1993 - 1995
0
3
6
9
12
15
18
21
9
12
15
18
21
24
27
30
33
36
39
21
24
27
30
33
36
39
42
45 0
20
23
26
29
32
35
38
41
44
47
week
ev
en
ts/w
ee
k
0
10
20
30
40
50
60
70
80
90
100
be
am
cu
rre
nt
[mA
]
Errors
Quenches
5 ms events
BLM-Alarms
beam current
no BLMs 1/3 BLMs all
1993 1994 1995
I
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Statistic BLM events 1995 - 1997
0
3
6
9
12
15
18
21
19 22 25 28 31 34 37 40 43 46 49 22 25 28 31 34 37 40 43 46 6 9 12 15 18 21 24 27 30 33 36 39 42
week
even
ts/w
eek
0
10
20
30
40
50
60
70
80
90
100
bea
m c
urr
ent
[mA
]
Errors
Quenches
5 ms events
BLM-Alarms
beam current
1995 1996 1997
I
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5 ms event, PS failure, HF failure
92 BLM Alarms
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Old HERA Beam-Loss-Alarm-Topology
ALIs
ALIs
ALIs
ALIs
Alarm loop-Zentrale
Alarmloop
DUMP
BLMs + BPMs +Alarm-modules “Alarm-Loop-
Interface”
Internal Power-Supply-Alarm
Galv. Trenn.
HF failure input
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Clean Dump due to HF alarm
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Statistic BLMp events 1998 - 2000
0
2
4
6
8
10
12
14
16
18
20
28 32 36 40 44 48 4 8 12 16 20 24 28 32 36 40 44 48 52 3 7 11 15 19 23 27 31 35 33
week
even
ts /
wee
k
0
20
40
60
80
100
bea
m c
urr
ent
[mA
] Errors
Quenches
5 ms events
BLM-Alarms
beam current
1998 1999
I
2000
Since Week 39 (1998) RF Interlock activeLess BLM alarms,less 5 ms events, less quenches
start after 5 month shutdown
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Statistic BLMp events 2001 - 2003
0
2
4
6
8
10
12
14
16
18
20
31 36 41 46 51 15 20 25 30 35 40 45 50 3 8 13 18 23 28 33 38 43 48 53 5 10 15 20 25 30 35 40 45 50
week
even
ts /
wee
k
0
20
40
60
80
100
bea
m c
urr
ent
[mA
]
Errors
Quenches
5 ms events
BLM-Alarms
beam currentI
2003 2004
I
2001 2002
start after 5 month shutdown (Lumi upgrade)
All by 5 ms PS failure events
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What is a critical PS?
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Alarm timing during failure of a critical magnet power supply
t0
X oder Y
Power supply failure
Aper-tur
BLM-Alarm
ACCT-Alarm
Improved and faster internal Power-Supply-Alarm
Magnet-current-Alarm
Total-loss
Too late
No faster BLM Alarmsdue to spiky background!
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ALIs
ALIs
ALIs
ALIs
Alarm loop-Zentrale
Alarmloop
DUMP
BLMs + BPMs +Alarm-modules “Alarm-Loop-
Interface”
Beam-Loss-Alarm-Topology
Internal Power-Supply-Alarm
Galv. Trenn.
faster
Active
New
ACCT-Alarm
DCCT-Alarm
Faster clock rate
Magnet current-Alarm
More Failure inputs:PS, HF, …
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Improvements Beam-Dump:before: 570 safter: 10 s
Alarm at 0
Turn by turn current of bunch #1
DCCT beam current
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BPM SL345had wrongreadings. =>local Bump atone Quad. => < 4 BLM- alarms
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Story (1):Statement: In HERA each cold Quad has a BPM.Instruction: Install a BLM close to each BPM to cover all cold Quads.DONEEvents: Quenches of one Magnet in the middle of the arc during ramp.Observation: No Orbit distortion, no beam losses.?????
After a few days, some tries, some quenches:Observation 2: The correction coils in this area showed higher valuesCalculations: The correction coils drive a local closed bump.WHY THE BPM and BLM DIDN’T SHOW ANYTHING????Observation 3: There is no BPM (because there is a cold-box. No BPM foreseen)Observation 4: Therefore there is no BLM (see above)Analysis: The automatic Orbit correction makes the local bump by accident.
Consequence: Now we installed a BLM!=> flexible system
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Alarm Zentrale failure:Threshold went from 5 to 30
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Story (2):Due to a wrong cabling, the alarms of 20 BLMs were subtracted and not added
Story (3): Fieldbus-commands for other modules on the bus were interpreted by the ALZ
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Injection:10 bunches injected into first Dipoles
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Collimators went too far into the beam.=>Losses in the magnets behind. (no quench but happened in earlier years.Very high Collimator BLM thresholds)
protons
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Operating (1):Wrong rampfile was chosenby operator.
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Operating (2):Fast switch-onof magnets.Alarm loop (A1) was still disabled
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Diverse: Hitting a cable during drilling(no quench)
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Experiments PS-manipulation coasting beamOperating diverse diverses Operating
dump?
Remarks:What was first? Transient recorders most helpful. Here: p-beam was lost 8 ms before e-beam.
Quench Quench
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Dump of 19% coasting beam isnot a problem in HERA.
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Some loss induced quenches were not documented in the Logbook?!?!?
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< 4 BLMsStill 4 loss induced quenches in 2004:
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The Endnow open for discussion
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Solution 2: Proton and Positron ring in Lumi-Optic - BPM thresholds reduced to around 3 mm. Clean dump - no detectable current loss before dump triggered.Congratulations! Logbook comment to the test: “In dem Quencharchiv steht die Schwelle in der Alarmloopzentrale auf 30 und es werden 30 anstehende Alarme angezeigt. Wenn das Setzen von 40 Monitoren noetig war, dann deutet dies darauf hin, dass in den Alarmkassetten nicht alle BPMs Scharf geschaltet sind (leider sind dies Jumper im Tunnel unter Beton). Naja, wenn 3/4scharf sind, geht das ja noch.“