HERA Beam Losses and Machine Protection: Real-Life Experiences

1. Beam Losses and Machine Protection (real life) By Kay Wittenburg, Deutsches Elektronen Synchrotron DESY, Hamburg, Germany Experiences from HERA (accidental losses) Beam Dump: why?

2. Loss-Mechanisms • 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, … Total Beam loss First losses Failure X or Y Aper-tur 0 t Dt, Dl long short

3. The Detector PIN Photodiodes to satisfy the special conditions in HERA • 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

4. The Amplifier Diodes Pre-ampl. Video ampl. Comperator +5V TTL driver -5V -5V +24 V Bias Threshold Sensitive and fast amplifier with low noise and with a fast coincidence following • 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 g: 3.5 · 10-5 • Coincidence + lead: 1 Hz at 1.5 Gy/h (e- ring at max.) DESY BLM with lead hat (removed) on top of a sc quadrupole Pulse shape of the BLM output Response to MIPs:Blue line: Single diode;Green line: Coincidence; TTL compatible (90 )

5. Counter im „Grab“; immer neben Controller 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

6. HERA BLM Alarm System Dump due to losses No quench

7. Alarmtest: Set threshold 1 or 2. Will be overwritten after some seconds! Should show 30 or 5

8. HERA experience with S = 189 Quenches More statistics Note: A quench in HERA is not a disaster! It takes typ. 1-2 h to recover from cryogenic

11. 5 ms event, PS failure, HF failure 92 BLM Alarms

12. ALIs DUMP Alarm loop-Zentrale ALIs Alarmloop ALIs HF failure input ALIs BLMs + BPMs +Alarm-modules “Alarm-Loop- Interface” Old HERA Beam-Loss-Alarm-Topology Internal Power-Supply-Alarm Galv. Trenn.

13. Clean Dump due to HF alarm

14. start after 5 month shutdown

15. start after 5 month shutdown (Lumi upgrade) All by 5 ms PS failure events

16. What is a critical PS?

17. Alarm timing during failure of a critical magnet power supply Power supply failure Total- loss X oder Y Aper-tur 0 t Too late Improved and faster internal Power-Supply-Alarm Magnet-current-Alarm ACCT-Alarm BLM-Alarm No faster BLM Alarms due to spiky background!

18. ALIs DUMP Alarm loop-Zentrale ALIs Alarmloop ALIs ALIs BLMs + BPMs +Alarm-modules “Alarm-Loop- Interface” Beam-Loss-Alarm-Topology Magnet current-Alarm ACCT-Alarm DCCT-Alarm Internal Power-Supply-Alarm Galv. Trenn. More Failure inputs: PS, HF, … faster Active New Faster clock rate

19. Improvements Beam-Dump: before: 570 ms after: 10 ms Turn by turn current of bunch #1 DCCT beam current Alarm at 0

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

21. Story (1): Statement: In HERA each cold Quad has a BPM. Instruction: Install a BLM close to each BPM to cover all cold Quads. DONE Events: 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 values Calculations: 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

22. Alarm Zentrale failure: Threshold went from 5 to 30

23. 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

24. Injection: 10 bunches injected into first Dipoles

25. protons Collimators went too far into the beam.=> Losses in the magnets behind. (no quench but happened in earlier years.Very high Collimator BLM thresholds)

26. Operating (1): Wrong rampfile was chosen by operator.

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

28. Diverse: Hitting a cable during drilling(no quench)

29. Remarks: What was first? Transient recorders most helpful. Here: p-beam was lost 8 ms before e-beam. Quench Quench dump? Experiments PS-manipulation coasting beam Operating diverse diverses Operating

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

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

32. < 4 BLMs Still 4 loss induced quenches in 2004:

33. The End now open for discussion

34. 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.“