1. Chamonix Workshop XIV CERN - 17th-21st January 2005
Machine Protection Issues affecting Beam Commissioning Beam Instrumentation other than BLMs linked to the Machine Protection System
Chamonix Workshop XIV
CERN - 17th-21st January 2005
Rhodri Jones (AB/BDI)
from discussions with
David Belohrad, Brennan Goddard, Jose Gonzalez, Patrick Odier,
Hermann Schmickler, Rüdiger Schmidt, Jorg Wenninger and others

2. Outline Beam Position Interlock for the LHC Beam Dump System in IR6
Interlock for Fast Beam Position Changes and Oscillations
Capabilities of Fast Beam Current Transformers for machine protection
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

3. Beam Position Interlock for the LHC Beam Dump
Why do we need an interlock?
Dump channel aperture is very tight
particularly tight at injection
Need to avoid damage of beam dump equipment
for all failure scenarios
Limit induced Q4 quenches to real failure modes
no quench of Q4 for normal dump operation
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

4. A Clean On-Axis Beam Dump at 450GeV4σ 8σ
7.2σ
aperture
To Dump
(TDE) M S D Q4 Q4
MKD
TCDQ
7.2σ
aperture M S D
TCDS
7.8σ
aperture
8.2σ
aperture M S D
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

5. +4mm Off-Axis Beam Dump at 450GeV4σ 8σ
To Dump
(TDE)
4.6σ
aperture M S D Q4 Q4
MKD
TCDQ M S D
TCDS
4.6σ
aperture
+4mm M S D
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

6. -4mm Off-Axis Beam Dump at 450GeV4σ 8σ M S D
To Dump
(TDE) Q4 Q4
MKD
TCDQ
4.2σ
aperture M S D
TCDS
5.0σ
aperture
-4mm M S D
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

7. -4mm & 14/15 MKD Beam Dump 450GeV To Dump (TDE) 8σ 4σ D S M Quench Q4
TCDQ
1.2σ
aperture M S D
Particle Shower
TCDS
-4mm M S D
No Damage to Any Equipment
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

8. Summary of Acceptable Orbit Variations for the Beam Dump System (see talk by Brennan Goddard at Chamonix 2003)
 4mm Limit at Injection (450 GeV)
Safe for ALL beams
Q4 quench may occur for large emittances
Q4 quench likely to occur in 14/15 MKD case
 4mm Limit at 7TeV
Safe for MOST beams
quench of Q4 will only occur in 14/15 MKD case
damage may occur for large emittance nominal beams
Can be avoided by reducing the limit to  3.5mm
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

9. Beam Position Interlock Layout
New IR6 layout sees the addition of 4 interlock BPMs per beam
2 redundant BPMs added near TCDQ and 2 near preceding Q4
90° phase advance to minimise chance of an unfortunate orbit bump
BPM diameters have to be different due to integration constraints
large diameters (80mm & 130mm) require the use of stripline pick-ups
will use standard LHC orbit BPM analogue electronics
2 x 130mm diameter
stripline BPMs
2 x 80mm diameter
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

10. Acquisition – Response and Threshold
To take into account all scenarios need to look at worst case:
D1 failure gives ~ 60 mm/turn at the TCDQ
Response time required:
1 turn (detection) + 2 turns (BIC delay & abort gap synchronisation)
 200 mm movement between detection and dump.
For a single pilot bunch of 5109 protons the BPM system has a single shot resolution of ~1.5% of half radius:
~ 300mm for 80mm diameter BPM (at Q4)
~ 500mm for 130mm diameter BPM (at TCDQ)
Interlock threshold:
set to = 3.3mm to give effective threshold of 4mm
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

11. Acquisition - Detection
Modified LHC orbit digital acquisition card
Direct comparison of position & threshold performed in FPGA
 no dependence on external software
Autotriggered system
 no dependence on external timing
Hard wired output to BIC
Open issues
Limiting spurious triggers
Do we require a certain number of bunches out of limit before dumping?
For single or few bunches this will imply an increased latency
Alignment and position offsets
Do we measure these with the beam or reduce the threshold to include them?
Sensitivity switching (at around 3  1010 cpb)
Direct link from intensity card?
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

12. Interlock for Fast Beam Position Changes and Oscillations
What needs to be protected?
Arc, triplets & collimators from asynchronous beam dump
beam position relative to TCDQ needs to be maintained
Collimators from fast orbit changes
Collimators from oscillating bunches
generated by instabilities such as electron cloud
not seen by orbit feedback system
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

13. Off-Axis Asynchronous Beam Dump at 450GeV
To Dump
(TDE)
Within dump channel aperture Q4 Q4
MKD
TCDQ
Protected by TCDQ M S D
Beam
Sweep
Protected by TCDS
TCDS
Will hit arc aperture – OK for < 20 bunches
-4mm
Oscillate & ejected at next turn M S D
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

14. Protection against an Asynchronous Beam Dump
Injection (450 GeV) – arc protection
Arc aperture at 7.5s
TCDQ will sit at around 7.5s
Can accept up to around 20 bunches hitting the arc
Fully protected by 4mm beam position interlock
7 TeV – TCDQ, triplet and collimator protection
TCDQ will sit ~0.5-1s outside of secondary collimators
avoid turning TCDQ into secondary or primary collimator
beam position to be stabilised via orbit feedback to 0.5s (300mm at TCDQ)
Require software interlock on orbit feedback system (slow ~1s)
will provide protection for most asynchronous dumps
will not provide protection if there is a fast orbit change at time of the asynchronous dump (i.e. 2 simultaneous failures)
e.g. D1 failure + asynchronous dump
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

15. Protection against oscillations & fast orbit changes
Injection (450 GeV) – collimator protection
Arc aperture at 7.5s
TCDQ will sit at around 7.5s & collimators at 6-7s
4mm beam position interlock at < 2s
this already allows for orbit drift due to D1 failure during dump request
Fully protected by 4mm beam position interlock
Allows Q kicks at max kicker strength of 1.75s for centred beam
7 TeV – collimator protection
Collimators will sit at 6s
D1 failure will result in collimator damage after ~3ms
loss of around 1012 protons
Orbit only moves by only ~1mm during this time at TCDQ
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

16. Protection against oscillations & fast orbit changes
Can we reduce the beam position interlock to 1mm at 7TeV?
only valid for nominal intensity beams
where BPM single bunch resolution < 100mm
will require a relative (not absolute) position threshold value
i.e. 1mm offset with respect to “stable orbit”
can be done by comparing current position to last orbit reading
Global orbit reference updated every 20ms
Can compare single bunch position (for oscillations) & /or 1 turn orbit
can all be implemented in hardware (FPGA) with no external input
should be separate channel from the beam dump position interlock
Can still have data treatment in the same FPGA
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

17. Fast Beam Current Decay Monitoring
Some orders of magnitude for fast losses
(see talks by V. Kain and A. Siemko for latest figures)
At 450 GeV:
Damage limit ~6  1011 protons lost per metre in 1ms
Quench limit ~2  109 protons lost per metre in 1ms
At 7Tev:
Damage limit ~ 1010 protons lost per metre in 1ms
Quench limit ~ 107 protons lost per metre in 1ms
Things to keep in mind
Losses will normally be distributed over more than 1m
extra margin (e.g. pilot bunch considered below 450GeV quench level)
At 7TeV collimators will be closed
if correctly positioned, fast losses will impact first on collimators
graphite can withstand > 1012 protons in 1ms
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

18. Fast Beam Current Decay Monitoring
From R. Schmidt’s MAC presentation (Dec 2004)
Monitoring a loss of 1011 protons within 1ms:
would fully protect the LHC at 450GeV
would fully protect the LHC at 7TeV
provided graphite collimators are hit first
possibly some damage of copper collimator surface
would for many types of failure protect the LHC at 7TeV, even if collimators are not correctly positioned
could reduce damage by more than 3 orders of magnitude
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

19. What can we Reasonably Expect to Detect with BCTs?
DC Beam Transformer
resolution of 10mA for integration over 20ms
equivalent to change of 1.2  1012 protons in 20ms
can just about detect loss of 6  1010 protons per ms
response time of 40ms
Would be pushed to detect loss of 1011 protons per ms
Response time too slow to deal with such a loss
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

20. What can we Reasonably Expect to Detect with BCTs?
Fast Beam Current Transformer (using figures based on SPS system)
Characteristics
40MHz integration
12-bit bipolar digitisation (to handle baseline droop)
2048 effective bits for 2  1011 protons
rms noise of system around 3 bits per 25ns integration window
3 bits corresponds to 3  108 protons
Total Intensity (integration over 5 turns ~0.5ms)
calculated by adding up all 3564 slots of 25ns per turn
noise limit √(5  3564)  3108 = 4  1010 charges
not much margin, but detection of 1  1011 loss in 1ms possible
hope of improving LHC system based on SPS experience
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

21. What can we Reasonably Expect to Detect with BCTs?
Fast Beam Current Transformer
Advantages
independent method to measure beam losses
provides redundancy for BLM system
can react within ms time scale
will detect bunch core losses resulting in unbunched beam
limited complexity (one instrument)
Open Issues (some can be studied in SPS during 2006)
should be independent of bunch to bunch timing
will require active filtering of signal  amplifiers & added noise
40MHz integrators only integrate ~21ns out of every 25ns
how much error does this introduce on filtered signal?
to be robust put in FPGA what is currently done in PowerPC – possible?
influence of 50Hz interference for integration times < 20ms ?
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

22. Summary 1 LHC Beam Dump Interlock
solution exists using modified LHC orbit system electronics
will protect the dump line from damage for all scenarios
will limit Q4 quenches to dump extraction failure modes
Oscillation and fast orbit change interlock
protection provided by 4mm interlock at injection
protection possible at 7TeV if 1mm interlock is implemented
Fast beam current decay monitoring
fast BCT seems capable of detecting a loss of 1011 protons in 1ms
provides redundancy to “complex” BLM system
will fully protect the LHC at 450GeV & for most cases at 7TeV
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)

23. Summary 2 Other instruments linked to the machine protection system
BCT for safe beam flag
probably the DCCT as this also measures unbunched beam
Longitudinal Density Monitor
to protect against beam in abort gap – currently under development
Reliability & availability issues * see BDI disclaimer!
The aim is to produce a redundancy in the diagnostics of machine protection without severely reducing the viability of the system as a whole
unlike BLM system, however, none of the systems mentioned was originally designed to be used for machine protection
currently no numbers for reliability & availability of these systems
further studies required to make these systems as simple and robust as possible
* The aim is to produce a redundancy in the diagnostics of machine protection without severely reducing the viability of the system as a whole.
Unlike BLM system, however, none of the systems mentioned was originally designed to be used for machine protection.
Currently no numbers for reliability & availability of these systems. Further studies required to make these systems as simple and robust as possible
Machine Protection Issues affecting Beam Commissioning - Rhodri Jones (AB/BDI)