1. BEAM LOSS MONITORING SYSTEM
B. Dehning, E. Effinger, G. Ferioli, J.L. Gonzalez, G. Guaglio,
M. Hodgson, E.B. Holzer, L. Ponce, V. Prieto, C. Zamantzas
CERN AB/BDI
External Review of LHC Collimation Project
July 1, 2004

2. Outline BLM System Hardware Dynamic Range Positioning of Monitors
Signals from the BLM system
Simulations of Cleaning Insertions
Momentum Cleaning (Igor A. Kurochkin, IHEP)
Betatron Cleaning (M. Brugger, S. Roesler, CERN SC/RP)
Summary

3. THE BLM SYSTEM Purpose: Challenges:
Machine protection against damage of equipment and magnet quench
Setup of the collimators
Localization of beam losses and identification of loss mechanism
Machine setup and studies
Challenges:
Reliable (tolerable failure rate 10-7 per hour per channel)
High dynamic range (108, 1013)
Fast (1 turn trigger generation for dump signal)

4. Families of BLM’s
Type
Area of use
Time resolution
Number of monitors
BLMC
Collimation sections
1 turn
~ 100
BLMS
BLMS*
Critical aperture limits or critical positions
1 turn (89 us)
~ 500
BLMA
All along the rings (ARC, …)
2.5 ms
~ 3000
BLMB
Primary collimators
bunch-by-bunch
~ 10
BLMC & BLMS: In case of a non working monitor this monitor has to be repaired before the next injection

5. Loss Detectors: Ionization Chamber
SPS Chamber
Gas: N2, Volume: ~ 1 Liter, 30 Al disks of 0.5 mm,
Typical bias voltage: 1500 V.
New LHC chamber design
Diameter = 8.9 cm,
Length 60 cm, 1.5 litre,
Filled with Ar or N2

6. Secondary Emission Monitor
Diameter = 8.9 cm
Length 15 cm

7. Dynamic Range (I) Secondary Emission Monitor P < 10-7 bar
Ionization Chamber
P > 1bar
Efficiency SE ~ 0.05 charges/particle
Efficiency ioniz. Chamber ~ 50 charges / (particle cm)
Efficiency Ionization Chamber / Efficiency SE ~ 3 104

8. Dynamic Range (II) Beam Loss Current BLMC and BLMS*
Pilot bunch: 5e9 p, lower limit of measurement in the collimation region 400 – 4000 protons -> level of 10e-6 !

9. System Layout
Threshold Comparator: Losses integrated in 12 time intervals to approximate quench level curve.

10. Quench and Damage Levels
Detection of shower particles outside the cryostat or near the collimators to determine the coil temperature increase due to particle losses
BLMS* & BLMC
Quench level and observation range
450 GeV
7 TeV
Damage levels
Dynamic
Arc: 108
Collimator: 1013
BLMS*: 3.2E13 protonen (nominal SPS injection) lost in about 10E-5 seconds.
Special & Collimator1 turn
Arc
2.5 ms

11. Loss Levels and Required Accuracy
Relative loss levels
450 GeV
7 TeV
Damage to components
320/5
short/long
1000/25 short/long
Quench level 1
Beam dump threshold for quench prevention
0.3
0.3/0.4 short/long
Warning
0.1
0.1/0.25
Specification:
Absolute precision (calibration)
< factor 2
initially:
< factor 5
Relative precision for quench prevention
< 25%

12. Monitor Positions in Arc
Installation of BLMAs on a SSS quadrupole

13. Monitor Positions in Collimation
Collimator interconnect with ion pump and BLM (possible positions)

14. Signals from the BLM system
Dump signal to beam interlock controller (BIC), 2 types:
Not mask able: BLMC and BLMS, ~ 600 monitors.
Can be masked when “safe beam” flag is set: BLMA, ~ 3000 monitors
Post mortem:
2000 turns plus integral of 10 ms.
Logging:
Once a second
Stored in database
Used for graphical representation in the control room:
Values measured for each detector and time interval are normalized by their corresponding threshold values.

15. “Artist View” of the Logging Display

16. Momentum Cleaning Igor A. Kurochkin
IR3 (6.2) – length and positions of collimators have changed
BLM position
TCS 6,5,4
TCS3,2
TCP1
TCS1
Collimator length: 20cm primary and 50 cm secondary, reduced shielding present. Codes: K2 and MARS.
Activation will reduce the sensitivity of the monitors in the low signal range. Expected activation: 10-2 to 10-4 of mean loss rate (SPS 10-3)
Monitors close to vacuum chamber to reduce cross talk and background.
Monitors 30 cm downstream of collimator

17. Relative contribution to BLM signal from primary inelastic interactions in the collimators
Igor A. Kurochkin
“Good” signal (from upstream collimator)
BLM1 – 100%
BLM2 – 4%
BLM3 – 57.4%
BLM4 – 9%
BLM5 – 5%
BLM6 – 4%
BLM7 – 1%
TCP1 - major contributor to background
BLM2 – 96%
BLM7 – 20%
Igor A. Kurochkin
7 TeV
TCP 1
TCS 1
BLM signal:
Good measure for heat load in the corresponding collimator
Does not represent the number of proton inelastic interactions of the corresponding collimator

18. Transversal Variation of Monitor Location
Igor A. Kurochkin
TCS1
Igor A. Kurochkin
Total energy deposition. The contribution from beam 2 (crosstalk) is small (<1%) due to longitudinal distance.
Best signal to background and signal to cross talk at position near to the beam

19. Betatron Cleaning M. Brugger, S. Roesler Primary interactions
Inelastic interaction rate (star), threshold 20MeV
Comparison of energy deposition [GeV] to inelastic interaction rate in IP3:
They scale for the downstream secondary collimators (2 – 3 GeV per inelastic interaction)
Primary collimator: 0.4 GeV
First secondary collimator: 3.3 GeV
“Primary” refers to 7TeV proton interactions, whereas the “Loss Distribution” includes all secondary particles (threshold of 20MeV), however the latter are dominated (~98%) by high-energy particles close to 7TeV (single- or multiple-scattering).
Secondary particles are dominated (~98%) by high-energy particles close to 7TeV

20. Contribution to the number of inelastic interactions from beam particle losses in upstream collimators
M. Brugger, S. Roesler
Values similar to momentum cleaning.
Higher inter beam crosstalk can be expected due to reduced longitudinal distance between collimators of beam 1 and 2.

21. Summary BLM system: machine protection First priority (downtime)
Detectors next to possible loss locations protect local equipment
Monitors in collimation region
Measure energy deposition in the collimators
Can not measure primary inelastic interactions (response matrix)
High activation (reduce sensitivity)
Possible noise problems to be investigated (analogue signal cables of BLMs are up to 300 m long and close to numerous stepping motor control cables)