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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?
                     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, …




                                                First            Total
                 Failure                        losses           Beam
                                                                  loss
X or Y


Aper- 0
tur                                                                                t


                             Dt, Dl    long              short
    HERA experience with Beam loss induced Quenches 1994 - 2004
                                                                    Injection
                                                                    Magnet PS
                                                                    5 ms events
                            unknown                                 RF
                                                                    ALZ
                  Diverse     5%                                    BLMs <4
                 6%                                                 Operating
        Kollimator                                  Injection       Kollimator
           2%                                         27%           Diverse
      Operating
                                                                    unknown
         3%


    BLMs <4
       8%                                                          S = 189 Quenches
                                                                    More statistics

         ALZ
         12%                                           Magnet PS
                                                            13%



                      RF
                      8%              5 ms events
                                         16%

Note: A quench in HERA is not a disaster! It takes typ. 1-2 h to recover from cryogenic
                                                                                   Statistic of BLM events 1993 - 1995


              21                                                                                                                                                             100

                                           1993                                1994                                             1995
                                                                                                                                                                             90
              18

                                                                                                                                                                             80


              15                                                                                                                                                             70




                                                                                                                                                                                   beam current [mA]
                                                                                                                                                                             60                         Errors
              12
events/week




                                                                                                                                                                                                        Quenches
                                                1/3 BLMs        all                                                                                                          50                         5 ms events
                        no BLMs
                                                                                                                                                                                                        BLM-Alarms
               9
                                                                                                                                                                             40                        I beam current


                                                                                                                                                                             30
               6

                                                                                                                                                                             20

               3
                                                                                                                                                                             10


               0                                                                                                                                                             0
                   9




                                                                                                                       0
                       12
                            15
                                 18
                                      21
                                           24
                                                27
                                                     30
                                                          33
                                                               36
                                                                    39
                                                                         21
                                                                              24
                                                                                    27
                                                                                         30
                                                                                              33
                                                                                                   36
                                                                                                        39
                                                                                                             42
                                                                                                                  45


                                                                                                                           20
                                                                                                                                23
                                                                                                                                     26
                                                                                                                                          29
                                                                                                                                               32
                                                                                                                                                    35
                                                                                                                                                         38
                                                                                                                                                              41
                                                                                                                                                                   44
                                                                                                                                                                        47
                                                                                              week
                                                                                           Statistic BLM events 1995 - 1997


              21                                                                                                                                                                        100


                                  1995                                                      1996                                      1997                                              90
              18

                                                                                                                                                                                        80


              15
                                                                                                                                                                                        70




                                                                                                                                                                                              beam current [mA]
                                                                                                                                                                                        60                         Errors
              12
events/week




                                                                                                                                                                                                                   Quenches
                                                                                                                                                                                                                   5 ms events
                                                                                                                                                                                        50
                                                                                                                                                                                                                   BLM-Alarms
              9                                                                                                                                                                                                   I beam current
                                                                                                                                                                                        40


                                                                                                                                                                                        30
              6


                                                                                                                                                                                        20

              3
                                                                                                                                                                                        10


              0                                                                                                                                                                         0
                                                                                                                         6
                                                                                                                             9
                   19
                        22
                             25
                                  28
                                         31
                                              34
                                                   37
                                                        40
                                                             43
                                                                  46
                                                                       49
                                                                            22
                                                                                 25
                                                                                      28
                                                                                           31
                                                                                                34
                                                                                                     37
                                                                                                          40
                                                                                                               43
                                                                                                                    46




                                                                                                                                 12
                                                                                                                                      15
                                                                                                                                           18
                                                                                                                                                21
                                                                                                                                                     24
                                                                                                                                                          27
                                                                                                                                                               30
                                                                                                                                                                    33
                                                                                                                                                                         36
                                                                                                                                                                              39
                                                                                                                                                                                   42
                                                                                                     week
            HERA BLM Alarm System




Dump due
to losses
No quench
5 ms event, PS failure,
HF failure


        92 BLM Alarms
           Old HERA Beam-Loss-Alarm-
                   Topology
                          ALIs

Internal                                              DUMP
Power-      Galv.
            Trenn.
Supply-
Alarm                                      Alarm loop-Zentrale
           ALIs       Alarmloop            ALIs

                                                       HF failure
                                                       input


     BLMs + BPMs          ALIs
     +Alarm-modules              “Alarm-Loop-
                                   Interface”
Clean Dump due to HF alarm
                                                                                 Statistic BLMp events 1998 - 2000
  start after 5 month shutdown
                20

                                              1998                                     1999                                 2000
                18                                                                                                                                                    100


                16

                                                                                                                                                                      80
                14

                                                                                                                                                                                                    Errors




                                                                                                                                                                            beam current [mA]
                12                                                                                                                                                                                  Quenches
events / week




                                                                                                                                                                      60                            5 ms events
                                                         Since Week 39 (1998)
                10                                       RF Interlock active                                                                                                                        BLM-Alarms
                                                         Less BLM alarms,                                                                                                                           beam current
                                                         less 5 ms events,
                                                                                                                                                                                                I
                8                                        less quenches
                                                                                                                                                                      40

                6


                4
                                                                                                                                                                      20

                2


                0                                                                                                                                                     0
                     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
                                                                                          Statistic BLMp events 2001 - 2003
                start after 5 month shutdown (Lumi upgrade)
                20

                               2001                2002                                       2003                                    2004
                18                                                                                                                                                                     100



                16


                                                                                                                                                                                       80
                14                                                                                                                                                                                        Errors


                                                                                                                                                                                                          Quenches
                12




                                                                                                                                                                                             beam current [mA]
                                                                                                                                                                                                          5 ms events
events / week




                                                                                                                                                                                       60

                10                                                                                                                                                                                        BLM-Alarms
                                                                                                             I

                                                                                                                                                                                                I beam current
                 8
                                                                                                                                                                                       40

                 6



                 4
                                                                                                                                                                                       20


                 2



                 0                                                                                                                                                                     0
                31

                     36

                          41

                               46

                                    51

                                         15

                                              20

                                                   25

                                                        30

                                                             35

                                                                  40

                                                                       45

                                                                            50




                                                                                         13

                                                                                              18

                                                                                                   23

                                                                                                        28

                                                                                                             33

                                                                                                                  38

                                                                                                                       43

                                                                                                                            48

                                                                                                                                 53



                                                                                                                                          10

                                                                                                                                               15

                                                                                                                                                    20

                                                                                                                                                         25

                                                                                                                                                              30

                                                                                                                                                                   35

                                                                                                                                                                        40

                                                                                                                                                                             45

                                                                                                                                                                                  50
                                                                                 3

                                                                                     8




                                                                                                                                      5


                                                                                                   w eek



                                         All by 5 ms PS failure events
What is a critical PS?
              Alarm timing during
   failure of a critical magnet power supply
           Power supply                      Total-
              failure                         loss

X oder Y


Aper- 0
tur                                                                t


                                                        Too late

 Improved and             Magnet-    ACCT-      BLM-
 faster internal          current-   Alarm      Alarm
 Power-Supply-            Alarm
 Alarm                                         No faster BLM Alarms
                                               due to spiky background!
           Beam-Loss-Alarm-Topology
           Magnet
           current-
                          ALIs       ACCT-Alarm
           Alarm
                                       DCCT-Alarm
Internal                                               DUMP
Power-       Galv.
             Trenn.
Supply-
Alarm                                       Alarm loop-Zentrale
           ALIs       Alarmloop             ALIs
                                                      More Failure
                                                      inputs:
                                                      PS, HF, …

                                                        faster
     BLMs + BPMs          ALIs
                                                       Active
     +Alarm-modules              “Alarm-Loop-
                                  Faster clock
                                   Interface”
                                       rate            New
             Improvements Beam-Dump:
             before: 570 ms
             after: 10 ms




                  Turn by turn current
                  of bunch #1

                  DCCT beam current




Alarm at 0
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
                     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)
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)
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
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)
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


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

				
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