# Presentation: 106325

## LHC BPM Talk to LARP

**Bob Webber   August 2, 2005**

## OUTLINE

**I will show slides from:**
**Rhodri Jones**
**Tune Feedback Workshop                                                  BNL - 9th-11th March 2005 **
_**[http://www.agsrhichome.bnl.gov/LARP/050309_TF_workshop/index.html](http://www.agsrhichome.bnl.gov/LARP/050309_TF_workshop/index.html)**_
**Eva Barbara Holzer and Hermann Schmickler**
**Workshop Chamonix XIV, Second LHC Project Workshop, January 2005 at CERN**
_**[http://indico.cern.ch/conferenceDisplay.py?confId=044](http://indico.cern.ch/conferenceDisplay.py?confId=044)**_
**See also D. Coqc et al. on BPM electronics**
**D. Cocq, “The wide band normaliser – a new circuit to measure transverse bunch position in accelerators and colliders”, Nuclear Instruments and Methods in Physics Research A 416 (1998) p.1-8.**
_**[http://www.esrf.fr/conferences/DIPAC/Proceedings/stampedpdfs/PM-17.pdf](http://www.esrf.fr/conferences/DIPAC/Proceedings/stampedpdfs/PM-17.pdf)**_
**DIPAC01**
_**[http://bel.gsi.de/dipac2003/papers/PT08.pdf](http://bel.gsi.de/dipac2003/papers/PT08.pdf)**_
**DIPAC03**

## LHC Beam Instrumentation&Commissioning

- Tune Feedback Workshop
- BNL - 9th-11th March 2005
- Rhodri Jones (CERN – AB/BDI)

## Outline

- LHC Beam Instrumentation
-  The BPM system
-  The BLM system
-  Luminosity monitors
-  Emittance measurement
- Tune, chromaticity & coupling measurement – this workshop
- LHC Commissioning
-  General commissioning strategy
-  Commissioning the beam instrumentation

## LHC BPM System - General Layout 

- LHC BPM System - General Layout

- 922 Button Electrode BPMs (24mm)
- for the
- Main Arcs & Dispersion Suppressors

- Total of **1158** BPMs for the LHC
- and its Transfer Lines

## The Arc BPM - SSS Layout

## Arc BPM - Detailed Layout

- Beam
- Pipes

- Cryogenic
- Coaxial Cables

- BPMs

## Arc BPM - Button Feedthrough

- Beam Screen

- Button Feedthrough

- Liquid Helium
- Cooling Capillary

## Insertion Region Directional Couplers

## The Front-End Electronics

## The Wide Band Time Normaliser

**A**

**B**

## The Wide Band Time Normaliser

**A**

**B**

**A + (B + 1.5ns)**

## The Wide Band Time Normaliser

**A**

**B**

**A+(B+1.5ns)**

**B+(A+1.5ns)+10ns**

**System output**

- Interval = 10  1.5ns

## Sensitivity Observation	

**WideBand Time Normalizer**
**Full range of 1.5nsec for 24.5mm radius arc pickup**
**~61ps/mm**
**Tevatron AM/PM system**
**6 degrees at 53MHz (300ps) per dB difference **
**Pickup sensitivity of 1.5 mm/dB **
**~200ps/mm**

## The Wide Band Time Normaliser

## The LHC BPM Acquisition System

- Tunnel

- Surface

## WBTN - Linearity v Intensity

- For LHC Arc BPMs 1% ~ 130m

## WBTN - Linearity v Position

- For LHC Arc BPMs 1% ~ 130m

## Bunch-by-bunch Results in the CERN-SPS

- System extensively used in SPS for electron cloud & instability studies.

## Orbit feedback results from the CERN-SPS

**ramp**

**injection at 26 GeV**

**450 GeV**

- Time
- (ms)

**~ measurement noise !!**

- BPM
- Reading
- (m)

**feedback off**

**feedback on**

- Position distribution
- @ 100 Hz
-  = 8.5 m

## The LHC Beam Loss System

- Calculation of quench level equivalent chamber signal
- Electric currents from 600 pA to 300 A
- A dump should be requested at 50% of the quench level
- i.e. from 300 pA to 150 A
- Extend dynamic range for sufficient sensitivity at low losses
- Measuring current from 60 pA to 300 A
- Arc BLM acquisition rate not faster than one turn (89 s)
- Fastest total loss is ~ 6 turns & will be detected by special BLMs.

- Role of the BLM system:
- Protect the LHC from damage
- Dump the beam to avoid magnet quenches
- Diagnostic tool to improve the performance of the LHC

## Structure of the BLM Readout Chain

-  transforms particle losses into an electric current
-  6 per quadrupole (3 for each LHC ring)  ~3000 monitors
- Analogue Front-End
-  measures current and transmits data from Tunnel  Surface
- Dump Controller
-  processes data and interfaces to the beam interlock system

- Ionisation Chamber

## Quench Level EquivalentChamber Current

**One turn**

**300 ********A**

**600 pA**

**60 pA**

## Charge-Balanced Converter

*i**in*(*t*)

*i**in*(*t*) + I*ref*

## Current-Frequency Characteristics

## Pre-commissioning of Critical Beam Instrumentation Systems

- _E.B. Holzer_, O.R. Jones
- CERN AB/BDI
- Second LHC Project Workshop - Chamonix XIV
- January 18, 2005
- CERN

## Outline

- Beam Position Monitor (BPM) System
- Polarity errors
- Testing of electronics
- BPM Database issues
- Timing issues
- Beam Loss Monitor (BLM) System
- Hardware set-up and testing
- Calibration
- Threshold determination
- Emittance and Current Measurement Systems
- Sector Test
- Summary – Critical Issues for Commissioning

## Beam Position Monitors

- Polarity errors
- Testing of electronics
- BPM database issues
- Timing issues

## Polarity – Cryostat Cabling Errors

- Each SSS contains 2 BPMs (beam 1 and beam 2).
- Each BPM measures both planes. The 4 pick-up electrodes are connected to 4 semi-rigid coaxial cables.
- Mounting of the cables is performed in SMI2.
- Since the cables are preformed, no mix-up is possible on the BPM side.
- Exit flanges for beam1 and beam2 pick-ups are separated. Crossing beam1 and beam2 cables should not be possible.
- The connections to the outer cryostat flange, however, allow the possibility of an error. In order to minimise this risk the following procedure is adhered to:
- Installation of each cable in a predefined sequence.
- Test of the complete installation.

## Polarity – Cryostat Cabling, SSS BPM

**Polarity – Cryostat Cabling, SSS BPM**

## Polarity – Test Procedure of complete Installation

- Connect 600 MHz generator to one electrode via flange outside cryostat (one horizontal and one vertical tested)
- Verify amplitude and phase response on 2 neighbouring electrodes
- If amplitude is out of range this will signal one of the following
- Unconnected or broken cable
- Broken button
- Incorrectly cabled pick-up (H and V mixed up)
- Beam 1 / Beam 2 cables mixed up
- Phase out of range will indicate in addition
- Bad cable connection
- Incorrectly mounted button
- Expert is called in either case

- Cabling errors which will not be detected:
- Swap H1 with H2
- Swap V1 with V2
- Rotation of all contacts by arbitrary number of positions

## Polarity – Cabling Errors before Front-end Electronics

- Would result in incorrect polarity or in measuring adjacent electrodes
- Possible sources:
- Arc Case
- 2 cable connections before electronics
- Cryostat cables (verified during installation)
- Short coax cables
- DSS and Warm BPMs
- 3 cable connections before electronics
- Cryostat cables (verified during installation)
- Long coax cables of up to 200m
- Small coax cables
- Errors before electronics impossible to verify remotely after installation – will be seen with beam and can be visually inspected.

## Polarity – Cabling Errors after Front-end Electronics

- Would result in mixed up BPMs
- Possible Sources:
- 2 fibre patch links per plane after front-end
- Errors after electronics are easier to track down and should be spotted during hardware commissioning as each station is turned on individually.

- WBTN Mezzanine Card
- (10bit digitisation at 40MHz)

- VME based
- Digital Acquisition Board
- TRIUMF (Canada)

- Single-Mode Fibre-Optic Link

- Very Front-End WBTN Card

## Tests of Electronics without beam

**All** front-end cards:
- Adjusted and calibrated individually in the lab (data stored in MTF).
- Individual linearization will reduce errors from 6% to 1%.
- Calibrator sits at the very input (only one resistor before) of the electronic circuitry and enables the testing of the complete acquisition chain.
- Front-end cards will be tested in calibration mode locally during installation.
**All** digital conversion cards (on the surface):
- Adjusted and calibrated individually in the lab (data stored in MTF).
- Once installed their correct functioning can be verified by setting the front-end to calibration mode.
- Same electronics and procedure had been used in TI8:
- 3 planes of 51 gave problems (5%)
- 2 wrongly cabled special BPMs (measure CNGS and LHC beam). Only detected with beam!
- 1 malfunctioning plane (electronics was replaced)
- 5% for LHC would imply 50 incorrect or broken planes per beam.

## BPM Database Management

- Important during and after installation
**During **installation have to take into account
- Beam 1 and beam 2 position (internal or external) in each sector.
- Rotated cryostats where beam 1 and beam 2 BPM output ports change places within the same sector.
- Directional coupler BPMs where upstream and downstream ports on the same BPM provide the 2 beam signals (one of them rotated by 45°).
**After **installation
- Complete database of components for the whole acquisition chain will be required to calculated the beam positions:
- BPM Type - Linearization for BPM geometry will depend on type of BPM.
- Electronics - Calibration will require knowledge of which card is installed where. Currently trying to implement automatic identification of all cards.

## Timing Issues

- All setting-up and calibration is performed in asynchronous mode
- Data throughput is driven by the auto-triggered front-end. No external timing is used or required.
- In calibration mode the signals are generated by a 40 MHz crystal oscillator.
- Setting–up **with beam**
- Single Pilot over few turns (RF synchronization ?)
- FIFO stores all valid auto-triggers.
- Single Pilot over many turns
- Can use asynchronous mode as for calibration.
- Single or multiple pilots over many turns with RF synchronized
- Use BST to give 40 MHz bunch synchronous clock.
- Requires individual timing adjustments for all BPMs to compensate for different cable lengths.
- Phase margin quite large (auto-triggered input is stable during 20 ns out of 25 ns).
- Currently looking into ways of automatically adjusting phase if errors are detected.
- Allows bunch tagging and turn counting.
- Once BST is in use real-time data is available for orbit feedback.

## Sector Test

- BPM
- Commissioning of BPMs in the sector (polarity checks, timing, database issues) and a part of the functionality of the BPM system.
- Possibility to find problems and fix them before LHC start-up.
- BLM
- Commissioning of a part of the functionality of the BLM system (dump signal, setting of thresholds and beam flags, database issues, logging, post mortem, offline analysis).
- Quench level calibration: Controlled beam loss in cold magnet equipped with several BLMs.
- Longitudinal loss patterns (only way for measurements before LHC start-up).
- Possibility to find problems and fix them before LHC start-up.
- Could prove very useful considering the complexity of the system and the time needed to implement changes or fix problems.

## Summary – Critical Issues for Commissioning

- BPM system:
- Cabling errors (< 5% in TI8) – access time during beam commissioning
- Calibration / linearization database errors
- wrong BPM type  wrong position readings
- wrong linearization / calibration constants  reduced accuracy
- BLM system:
- Accuracy of quench level determination (factor 10 should be acceptable for initial commissioning)
- Accuracy of the prediction of loss locations (accuracy of the aperture model)
- Availability of application software (already for sector test)

## Beam Commissioning of LHC instrumentation

- Chamonix XIV, January 2005
- H.Schmickler on behalf of the AB-BDI group

## Prerequisites (same slide as last year)

- Working and exploitable instruments need:- sensors and electronics : AB-BDIdetails see AB-LHC review; **recurrent problem: Design support**- controls infracstructure : AB-CO, in progress- RT software and expert tools: AB-BDI-SW   will be based on FESAII framework
- Operational application programs		AB-CO + AB-OPwork needs to be organized soon (that is the text of last year)work needs to be organized now (see following slides)

## Design effort still needed

- There are still major BDI components, for which the mechanical design is not done / finished.
- The situation was similar at Chamonix 2004.
- After the workshop a significant improvement in design office support was notable.
- With the QRL problem higher priorities defined, present support insufficient
- Without the requested support - some special BPMs (= special BPMs of the normal orbit system and special BPMs for tune measurements)- the synchrotron light telescope- the wire scannerswill not be ready

## Prerequisites: Software effort

- Example: Time resolved chromaticity measurementsat day 0: We should not count on PLL tune tracking and momentum modulation  use head-tail method (despite emittance growth)
- SPS application program:- needs visual inspection of result- gives chromaticty as number onthe screen- needs:automation, integration into a measurement system...

- Solution:
- Define for the key system “System commissioners”and start definition, design and coding of the application programs

## Lessons learnt from LEP (1/2)

- Provide reduced functionality of instruments without beam synchronous timing (BST)- BPM-system: works without BST, 		but no bunch/turn identification, reduced noise immunity- BLM system: 		most of the system acquires data in an asynchronous 1ms 		rhythm- BCT DC: 	no timing- BCT BB: 	needs beam synchronous timing,		was a problem for the downstream BCT in TI8, people are 		working on a input filter to stretch the signal for single bunch 		operation- tune measurement systems:		tune kicker needs rough timing, tune meter will have one 		front-end not depending on timing basic functionality 		without BST- all TV based monitors: 		will work without BST, obviously no bunch gating		(sync-light telescope, ionisation profile monitor, screens)- wire scanners and luminosity monitors:		 will have an acquisition mode without BST- AGM and LDM:		 need the BST in order to work.

## Lessons learnt from LEP (2/2)

- Hardware can not be tested because software not ready,Software can not be developed because hardware not available...- The FESA framework provides the possibility to have device emulation of missing hardware, such that the software can be developed and tested at a time when the hardware is still partially missing:
- This feature will help having software/hardware ready in time

## Commissioning with Beam: BPM system

| Day 0 performance without adjustments | No BST adjusted,
1 pilot bunch; can measure orbit, turn by turn data possible with BST turn clock | Immediate! |
| --- | --- | --- |
| Systematic check for inverted cables/planes/rings | Per ring and per plane non closed bumps with two correctors at 90^0 phase advance. | If well automated, one shift for detection of problems.Requirement for scripting tool.
If 5% error rate, one week to fix problems.  |
| Adjustment of BST | One circulating bunch;
Scan of timing setting for all front ends.  | Two shifts interleaved by a few days; check for stability of settings |
| Systematic confirmation of system performance | Resolution as function of beam intensity, linearity over aperture, dependence on long. beam structure, crosstalk between beams | Long process. Several months. 
Demands LDM to be operational. |

## BPM system:

- Application software:This should be in good shape:System commissioner at work since years. Application programs very similar to those of LEP and SPS.Attention: The turn by turn analysis program as provided for the SPS needs more work.

## Commissioning with Beam: BLM system

| Day 0 performance:
All “slow (= 1ms)” monitors will deliver loss rates | Calibration of loss rates from simulation.Tolerated error: factor 5 | Initial working condition.
Channel assignment tested with radioactive source. |
| --- | --- | --- |
| Adjustment of BST for fast loss monitors (collimation region) | One circulating bunch.
Automated scan. | A few hours. 
Repeated after some time to check stability.
Parasitic. |
| Verification of calibration. | Delicate machine experiment. Calibrated loss rates have to be produced into the cryo-magnets | Dedicated experiment.
A couple of shifts.
Necessary step in order to establish dump thresholds. |
| Crosstalk between channels, in particular between the two beams, longitudinal coverage | Dedicated machine experiments | Crosstalk between beams simulated to be below 10%. Only a problem if beam currents very different. |
| Determination of dump thresholds | Many dedicated machine experiments and operational experience | See next slide |

## Beam Commissioning: BLM dump thresholds

- There will be more than 3000 beam loss monitors. Each of them can be enabled to dump the beam.
- The dependence of the dump level as function of the duration of the loss (3 orders of magnitude) is realized with 11 different integrators with exponential staggering of integration times (about factor 2)
- The dependence on beam energy represents another 2 orders of magnitude (multiplication factor)

- Clear need for
- a system-
- commissioner

## Conclusions

- The BDI group makes an attempt to have integrated into all instruments basic modes of operation, which will give acceptable measurements in the conditions of “bootstrap”.
- The essential systems for commissioning have an acquisition mode independent of beam synchronous timing.
- The BLM system for machine protection will need a considerable commissioning effort.
- All systems need a large investment into the corresponding application programs.
- The recently proposed concept of system commissioners should be implemented soon with the right people selected for the key beam instruments. This is for me the most urgent line of action.

## My Impressions

**What must be done is known**
**Critical aspects that will lead to success have been identified (many in common with those faced here,     e.g. in implementing new Recycler, Tevatron and MI BPM systems)**
**Resource limitations that also feel familiar are being experienced**
**It’s a long road to commissioning and then on to operations**