beam losses and machine protection (real life) by kay wittenburg, deutsches elektronen synchrotron...

Beam Losses and Machine Protection (real life)

By Kay Wittenburg,Deutsches Elektronen Synchrotron DESY, Hamburg, Germany

Experiences from HERA (accidental losses)

Beam Dump: why?

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

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

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.

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

No quench

Dump due to losses

HERA BLM Alarm System

Alarmtest:Set threshold 1 or 2.Will be overwrittenafter some seconds!

Should show 30 or 5

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

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

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

5 ms event, PS failure, HF failure

92 BLM Alarms

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

Clean Dump due to HF alarm

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

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

What is a critical PS?

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!

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, …

Improvements Beam-Dump:before: 570 safter: 10 s

Alarm at 0

Turn by turn current of bunch #1

DCCT beam current

BPM SL345had wrongreadings. =>local Bump atone Quad. => < 4 BLM- alarms

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

Alarm Zentrale failure:Threshold went from 5 to 30

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

Injection:10 bunches injected into first Dipoles

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

Operating (1):Wrong rampfile was chosenby operator.

Operating (2):Fast switch-onof magnets.Alarm loop (A1) was still disabled

Diverse: Hitting a cable during drilling(no quench)

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

Dump of 19% coasting beam isnot a problem in HERA.

Some loss induced quenches were not documented in the Logbook?!?!?

< 4 BLMsStill 4 loss induced quenches in 2004:

The Endnow open for discussion

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.“