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-Mechanisms 1)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, … t 0 X or Y Failure First losses Total Beam loss Aper- tur long short  t,  l

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

No quench Dump due to losses HERA BLM Alarm System

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

Old HERA Beam-Loss-Alarm- Topology 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

start after 5 month shutdown

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 t 0 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 Alarms due to spiky background!

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  s after: 10  s Alarm at 0 Turn by turn current of bunch #1 DCCT beam current

BPM SL345 had wrong readings. => local Bump at one 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. 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

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 chosen by operator.

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

Diverse: Hitting a cable during drilling (no quench)

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

Dump of 19% coasting beam is not a problem in HERA.

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

The End now open for discussion

Dear colleagues, In Session V we are planning a discussion on failure scenarios leading to accidental beam loss. We would kindly ask you for your help, since the number of possible failures scenarios is (nearly) infinite. There are some obvious type of failures: power converter fault, magnet quench, unsynchronised beam dump,..... More subtle failures were also reported. One example: due to a wrong delay set in the beam dump kicker electronics, a number in a register became negative, and the dump kicker did not fire. We would greatly appreciate your help for a discussion on all kind of failures and accidental beam losses: a) What are the most common failures leading to accidental beam loss that you know of? b) What kind of (atypical) failures did you experience? c) What diagnostics allowed to identify such failures? d) Other comments are welcome... In the session we would hope to compile a list of failures - as an input to machine protection systems for various accelerators. It would be very nice if you would stimulate the discussion by presenting one or two slides on your experience. Thanks a lot, and see you soon, Rüdiger Schmidt and Kay Wittenburg Events from BNL (RHIC) and SNS (R. Keller) Slides from B. Macek LHC requirements (R. Schmidt)

Dear Conveners, I will prepare a slide or two on our experience and will be guided by your 4 questions. It is an important topic for operation of high intensity accelerators that I am pleased to see is being addressed. Regards, Bob Macek I probably won't have an occasion to participate in your session, but here are my two cents worth of input on loss-relevant failure scenarios caused by the injection system (front end) of the SNS accelerator chain. 1) Our biggest worry was and is failure of one of the two chopper systems not providing chopper pulses (due to failure of the timing system or the chopper electronics). This would result in spraying beam on the Ring extraction septum. Possible protection: interlock based on wave pattern recognition, monitoring the actual chopper excitation currents. 2) Interference between the global control system (EPICS) and the dedicated timing system could lead to extending the ion source discharge and Linac pulse length or macroscopic duty factor. This is primarily a dump load concern, but I could imagine it resulting in higher beam loss as well if the rf system cannot cope with the additional average load. A cure for this effect might be very hard to find because the root cause is incompatibility between two systems. Best regards, Rod Keller

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. “

< 4 BLMs Still 4 loss induced quenches in 2004: