# Presentation: 012330

## Decay and Snap-back in LHC Main Magnets

- Presented by L. Bottura
- Prepared for the Mini-Workshop on Decay and Snapback in Superconducting Magnets
-  Fermilab, November 8th, 2002

**Notes:**
Acknowledgements are due to:
the MTA measurement team
Z. Ang, O. Bruening, A. Burns, M. Haverkamp, Q. King, L. Larsson, P. Proudlock, M. Schneider, L. Walckiers, R. Wolf

## Overview

- Definition of decay & SB
- Decay & SB statistics on LHC dipoles
- Operation, history and memory effects
- Physics understanding
- Fun ideas
- degaussing
- injection on-the-fly
- The *Multipoles Factory* and other ideas for the LHC

## Definition of decay & SB
Decay & SB statistics on LHC dipoles
Operation, history and memory effects
Physics understanding
Fun ideas 
degaussing
injection on-the-fly
The Multipoles Factory and other ideas for the LHC

- Decay & SB statistics on LHC dipoles
- Operation, history and memory effects
- Physics understanding
- Fun ideas
- degaussing
- injection on-the-fly
- The *Multipoles Factory* and other ideas for the LHC

## LHC dipole

## Static Field Quality

- geometric (linear) contribution
- T = 0.713 T/kA

- persistent currents (and other effects ?)
- T = -0.6 mT (0.1 %)
- overshoot forbidden !

*partial compensation* of persistent currents at injection

- iron saturation

- systematic b2 from two-in-one geometry

## Decay and Snap-back – 1

- snap-back

- decay

- accelerator operation cycle

## Decay and Snap-back – 2

- decay during simulated 10,000 s injection

- exponential fit
- i = 900 s

**Notes:**
The decay is a slow process, with characteristic times in the range of several thousands of s. 

## Decay and Snap-back – 3

- Snap-back at the start of the acceleration ramp

- decay during injection

**Notes:**
The second class of effects usually classified under dynamic effects is a decay of the field during periods of constant excitation (e.g. injection) followed by a rapid snap-back to the original value as soon as the field is ramped. This effect was first observed at the Tevatron, caused a wide concern and was carefully addressed at HERA. Both machines have developed successful strategies for dealing with decay and snap-back.

## Decay and Snap-back – 4

- snap-back fit:
- b3 [1-(I-Iinj)/I]3
- b3= 3.7units
- I = 27A  B = 19 mT

- snap-back

- decay

**Notes:**
On the other hand the snap-back is a very fast recovery of the original multipole values, taking place in a small field sweep (typically some 10 to 20 mT for the MB dipoles). The agreement with a cubic fit suggests that snap-back may be associated with penetration of de-magnetized filaments.

## Measurement conditions

- current regulation at injection
- typically 10 ppm, 0.1 A
- timing of current cycle and measurements
- automatic control of current cycle, measurement devices and acquisition
- logging (... *which cycle did we use in that measurement 3 months ago* ?)
- temperature stability
- typically better than 5 mK
- always, always, always quench the magnet before a measurement

## Measurement sample

- 10-m and 15-m long LHC prototypes
- short dipole models (1-m long) of the LHC R&D program (two X-sections):
- 11 single aperture models
- 7 twin aperture model
- 15-m long pre-series dipoles
- 13 magnets (26 apertures) integrally tested

## Definition of decay & SB
Decay & SB statistics on LHC dipoles
Operation, history and memory effects
Physics understanding
Fun ideas 
degaussing
injection on-the-fly
The Multipoles Factory and other ideas for the LHC

- Decay & SB statistics on LHC dipoles
- Operation, history and memory effects
- Physics understanding
- Fun ideas
- degaussing
- injection on-the-fly
- The *Multipoles Factory* and other ideas for the LHC

## Measurement of b3 decay

- large spread both in:
-   dynamics, and
-   magnitude

- decay is systematic in all magnets measured

**Notes:**
Expected decay and snap-back values have been estimated using an upper bound of 1/3 of the persistent current magnetization. Standard deviations for the MB dipoles has been estimated from measurements on a small series (approximately 10) of short model dipoles. As shown above the values measured on the latest 15 m long prototype dipoles are well within the expected values for series production.

## Measurement of b5 decay

- behavior similar to the one observed for b3 (this holds for all allowed harmonics)

**Notes:**
Expected decay and snap-back values have been estimated using an upper bound of 1/3 of the persistent current magnetization. Standard deviations for the MB dipoles has been estimated from measurements on a small series (approximately 10) of short model dipoles. As shown above the values measured on the latest 15 m long prototype dipoles are well within the expected values for series production.

## Dipole body and end behavior

- visible end effects in the magnitude of the decay (see later...)

**Notes:**
Expected decay and snap-back values have been estimated using an upper bound of 1/3 of the persistent current magnetization. Standard deviations for the MB dipoles has been estimated from measurements on a small series (approximately 10) of short model dipoles. As shown above the values measured on the latest 15 m long prototype dipoles are well within the expected values for series production.

## Decay and Snap-back statistics

- expected values from Field Quality WG, MB-99-02,
- based on decay = 1/3 persistent (see later why...)

- first 13, 15-m long,
- pre-series dipoles

**Notes:**
Expected decay and snap-back values have been estimated using an upper bound of 1/3 of the persistent current magnetization. Standard deviations for the MB dipoles has been estimated from measurements on a small series (approximately 10) of short model dipoles. As shown above the values measured on the latest 15 m long prototype dipoles are well within the expected values for series production.

## Decay and Snap-back statistics

- a systematic decay is observed on allowed harmonics only

- first 13, 15-m long,
- pre-series dipoles

**Notes:**
Expected decay and snap-back values have been estimated using an upper bound of 1/3 of the persistent current magnetization. Standard deviations for the MB dipoles has been estimated from measurements on a small series (approximately 10) of short model dipoles. As shown above the values measured on the latest 15 m long prototype dipoles are well within the expected values for series production.

## Is there a b2 decay in the LHC ?

- small decay observed on b2, changing in sign from aperture 1 to aperture 2
- no systematic effect on the beam

**Notes:**
Expected decay and snap-back values have been estimated using an upper bound of 1/3 of the persistent current magnetization. Standard deviations for the MB dipoles has been estimated from measurements on a small series (approximately 10) of short model dipoles. As shown above the values measured on the latest 15 m long prototype dipoles are well within the expected values for series production.

## Effect of Snap-back in LHC

- An uncorrected snap-back (of the expected magnitude) will cause in LHC:
- b1(MB)=2.6  Q = 0.026 vs. 0.003
- b2(MQ)=1.7  Q = 5.4 10–3 b2 =0.009 vs. 0.003
- b3(MB)=3.3   = 52 b3 = 172 vs. 1
- (source: O. Bruening, CERN)

## Definition of decay & SB
Decay & SB statistics on LHC dipoles
Operation, history and memory effects
Physics understanding
Fun ideas 
degaussing
injection on-the-fly
The Multipoles Factory and other ideas for the LHC

- Decay & SB statistics on LHC dipoles
- Operation, history and memory effects
- Physics understanding
- Fun ideas
- degaussing
- injection on-the-fly
- The *Multipoles Factory* and other ideas for the LHC

## Few important parameters

## Effect of flat-top current

- nearly linear growth of decay & SB as a function of the flat-top current

- short model dipoles

## Effect of flat-top current

- scaling for long magnets similar to short models

## Effect of flat-top time

- short model dipoles

- clear saturation of decay & SB for few magnets, peaks ?

## Effect of flat-top time

- saturation of decay & SB after < 1 hour flat-top

## Effect of pre-injection

## Multiple pre-cycles

- saturation observed after few cycles (> 3)

## Run sequences

- good repeatability already as of second cycle

## Injection current

- 1/Iinjection

- approximate 1/B dependence as if decay were constant in field

## Acceleration ramp

- no dependence of SB on the acceleration ramp-rate

## Injection ramp

- weak dependence on the ramp-rate to injection

## Temperature changes

## Measurements of Decay & SB

- Space of parameters:
- flat-top current
- flat-top time
- pre-injection duration
- injection duration
- magnet temperature
- ramp-rates
- ...
- too large for series
- measurements !

## Definition of decay & SB
Decay & SB statistics on LHC dipoles
Operation, history and memory effects
Physics understanding
Fun ideas 
degaussing
injection on-the-fly
The Multipoles Factory and other ideas for the LHC

- Decay & SB statistics on LHC dipoles
- Operation, history and memory effects
- Physics understanding
- Fun ideas
- degaussing
- injection on-the-fly
- The *Multipoles Factory* and other ideas for the LHC

## One... 

- Current distribution is not uniform in the cables...
- ...and changes as a function of time generating a time-variable, alternating field along the strands...

- (after the ideas of R. Stiening, SSC, and R. Wolf, CERN)

## ... two ...

- ...the field change affects the magnetization of the super-conducting filaments...

## ... three ...

- ... and the magnetization change averages to a net decrease (rectifying effect) – the decay !

- maximum decay is 1/ of M0

## ... et voilà !

- The magnetization state is re-established as soon as the background field is increased by the same order of the internal field change in the cable (5 to 30 mT) – the snap-back !

## Checks

- current distribution depends on powering history;
- larger decay & SB observed close to the ends of long magnets, where current imbalance is larger;
- current distribution saturates for times comparable (or longer) than the characteristic time;
- the current distribution change at injection does not depend on temperature changes;
- the SB does not depend on the acceleration ramp, quasi-DC process;

## A demonstration experiment

- measured

- computed

- Cu strands

- NbTi strand

- Courtesy of M. Haverkamp, CERN, experiment performed at U. Twente

## Summary on physics status

- basic understanding of physics principle available:
- interaction between cable transport current re-distribution and filaments magnetization
- L. Bottura, et al., Field Errors Decay and "Snap-Back" in LHC Model Dipoles, IEEE Trans. Appl Sup., 7(2), 602, 1997
- R. Wolf, The Decay of the Field Integral in SC Accelerator Magnets Wound with Rutherford Cables, Proc. of 15th Mag. Techn. Conf., Beijing, Oct. 20-24, 1997
- flux-creep not important (at most 10 % ... 30 % of effect)
- but cannot be controlled at production
- Rc, Ra, joints, n-value not in the control set

## Modelling approaches

- Empirical scalings:
- analytical model based on a charging and discharging R-L analogy
- neural network training based on measured magnets
- (M. Schneider, CERN)
- Direct simulation
- (M. Haverkamp, CERN and Twente University)

## R-L circuit analogy

- R-L circuit simulates the response of the induced currents in the cable
- The scaling assumes a linear relation:
- I(x,y,z,t)  B  M  b3

- idea *borrowed* from A. Gosh and W. Sampson, BNL

## Biological and digital neurons

- +

- +

- +

- =

- courtesy of M. Schneider, CERN

- biological

- digital

## Artificial neural network (ANN)

- courtesy of M. Schneider, CERN

- three layer *perceptron*

*training* achieved by matching expected response (e.g. b3 decay) to input (e.g. powering history)

## Modelling of Decay and SB

- Analytical model accurate to 30 %

- Neural network accurate to 5 %

## Modelling results

- R-L analogy:
- based on parameters with (some) physical analogy 
- lacks adaptivity 
- Artificial neural network:
- lacks physical insight 
- adaptive 
- Direct simulation:
- tantalizing task 

## Definition of decay & SB
Decay & SB statistics on LHC dipoles
Operation, history and memory effects
Physics understanding
Fun ideas 
degaussing
injection on-the-fly
The Multipoles Factory and other ideas for the LHC

- Decay & SB statistics on LHC dipoles
- Operation, history and memory effects
- Physics understanding
- Fun ideas
- degaussing
- injection on-the-fly
- The *Multipoles Factory* and other ideas for the LHC

## What is degaussing ?

*Degaussing removes permanent magnetization by introducing an alternating magnetic field that is stronger than the offending permanent magnetization... *
*... **if the amplitude of the alternating magnetic field is gradually reduced to zero, the material will be demagnetized...*
- ... degaussing restores focus, image sharpness (tune) and color purity (chromaticity)...
- ...on video screens 

## How is it achieved ?

- degaussing *cycle*

- a *suitable *AC current modulation is added before injection

## Degaussed state (1/3)...

- b3geom

- all multipoles tend to the *geometric* value after de-gaussing

## Degaussed state (2/3)...

- only allowed multipoles are largely affected by de-gaussing

- zero offset

- large offset

- scatter in persistent currents

## Degaussed state (3/3)...

- allowed multipoles are brought close to geometric value

## ...injection (1/2)...

- allowed multipoles have  negligible decay after de-gaussing

## ...injection (2/2)...

- non allowed multipoles also show no decay after de-gaussing

## ...and snap-back 

- multipole change equals the full persistent current effect (*giant* SB)

## A (yet) more efficient way !

- degaussing *blip*

- standard cycle

## Easy and cheap...

- http://www.periphman.com/degausser.html

## Injection on-the-fly

- Continuous injection ramp, 20 mT in 20 min

- standard decay and SB

- continuous ramp

## Injection on-the-fly

- standard decay and SB

- continuous ramp with negligible decay and SB...

- ... but is difficult for operation (injection energy tracking)

## Definition of decay & SB
Decay & SB statistics on LHC dipoles
Operation, history and memory effects
Physics understanding
Fun ideas 
degaussing
injection on-the-fly
The Multipoles Factory and other ideas for the LHC

- Decay & SB statistics on LHC dipoles
- Operation, history and memory effects
- Physics understanding
- Fun ideas
- degaussing
- injection on-the-fly
- The *Multipoles Factory* and other ideas for the LHC

## Correction magnets in the LHC

- LHC half-cell 53.5 m

- MQ

- MBA

- MBB

- MBA

## Control strategy for decay & SB

- Optimized ramp to minimize effects
- Cycling policy to guarantee reproducibility
- Feed-forward from the LHC magnetic reference
- Feed-forward from previous operating cycles
- Feed-back from on-line (BI) measurements

## Optimized ramp

- energy ramp

- preparation and access

- beam dump

- injection phase

- injection

- pre-injection

- I  t2

- I  et

- I  t

- A. Faus-Golfe, LHC Project Note 9, 1995.
- L. Bottura, _P. Burla_, R. Wolf, LHC Project Report 172, 1998.

- coast

- coast

## Additional cycling policy

- A pre-cycle before injection (1 hour) to condition magnets is foreseen at present
- Pre-injection stop to decrease magnitude of decay/snap-back (if mandatory)
- grace time limits for pre-injection stops and injection (re-cycle magnets if violated)
- TBD based on results of series measurements !

## The LHC magnetic reference

- Machine
- Operating
- Conditions:
- I, dI/dt, T

- Machine
- Operating
- History:
- I(-t), dI/dt(-t), T(-t)

- B1, B2,
- angle,
- multipoles

*Multipoles*
*Factory*

- Courtesy of Q. King

## Inside the Multipoles Factory

- dataBase tables from series measurements on 100 % of magnets

- dataBase tables from series measurements on 10 % of magnets

- machine operating conditions:
- , d/dt, T

- machine powering history:
- (-t), d/dt(-t), T(-t)

- multipoles from reference magnets

- multipoles from BI:
- tune (b2),
- chromaticity (b3)

- linear physical model of reproducible effects

- non linear model of decay and snap back

- non linear adjustment for actual powering conditions

- B1, B2, angle, multipoles

## Database generation

- Cold measurements on 100 % of MB and MQ
- ...
- ramp-rate harmonics
- decay and snap-back for standard measurement cycle
- Extended measurements on  10 % of MB and MQ
- decay and snap-back as a function of operating parameters for training of non-linear scaling

## Database growth

- I know, we are late...

- projected database size

- present growth rate (1 m/week)

- expected growth rate (10 m/week)

## Reference magnets layout

- use existing test benches (12) for the reference magnets

- wide spread in magnet properties:
- - 5 cable producers
- - 3 (2?) dipole producers

- open questions:
- - how many magnets ?
- - selection criterion ?

## Equipment: NMR for slow B1 calibration

- Equipment: NMR for *slow* B1 calibration

- magnet bore

- NMR signal =    NMR-1 + NMR-2

- SC cable

## Equipment: rotating coils for slow harmonics measurement 

- Equipment: rotating coils for *slow* harmonics measurement

- 16 m

- Rotating snakes: 0.1 T, 0.05 mrad resolution, 100 ppm accuracy, 3 Hz maximum bandwidth

## Equipment: Hall plates for fast b3/b5 mesurement

- 6 x b3 rings, 2 x b5 rings: 0.1 units resolution, 10 Hz maximum bandwidth

## Reference Magnets Control Interface

**C**

**Gateway**

**Multipoles**
**Factory**

**DB**

**I**

- SM18 Magnet
- Test Benches
- WorldFIP
- fieldbus

- Real-time LHC controls network

- FB

**Power**
**Converter**

**Real-Time**
**LHC Control**
**System**

**Instrumented Magnet**

**3-10Hz**

- Courtesy of Q. King

## Feed-back from BI

- b1
- 500 H and V beam position monitors (each ring)
- 10 Hz
- b2
- R&D on tune-loop running in the 1 Hz range (0.2 Hz possibly enough for snap-back correction)
- b3
- R&D on chromaticity measurement at  1 Hz
- (source: A. Burns, LHC-SL-BI)

## WG’s, Workshops, Research

- Working and study groups:
- Dynamic Effects Working Group (dormant)
- LHC Controls Project
- LHC Machine Commissioning Committee
- International Workshops and seminars:
- Dynamic Effects in Super-Conducting Magnets and their Impact on Machine Operation, CERN, October 6th, 1995.
- LHC Workshop on Dynamic Effects and their Control, CERN, February 5th to 7th, 1997.
- LHC Controls-Operation Forum, CERN, December 1st-2nd, 1999.
- Mini-workshop on Decay and Snapback in Superconducting Magnets, FNAL, November 8th, 2002
- Students and Ph.D.’s
- M. Schneider: Decay and Snapback Studies on the LHC Dipole Model Magnets. A Scaling Law, Ph.D. Thesis, Technical University of Vienna, 1998.
- L. Larsson, Sextupole Snapback Detector, Master Thesis, University of Luleå, 2000.
- E. Benedico-Mora: A Fast Sextupole and Decapole Probe for Chromaticity Corrections, Master Thesis, Universitat Politècnica de Catalunya, 2002.
- M. Haverkamp: Ph.D., in progress, Twente University, 2003.
- T. Pieloni: Master Thesis, in progress, Universita’ di Milano, 2003.

## Summary - 1

- We do not know everything...
- How reproducible will be the machine ?
- How well will we predict these variations ?
- assume 80 % for the moment, 20 % residual error
- What will be the spread among octants/magnets ?
- How long will be the learning curve between commissioning and high performance ?
- A deterministic model of decay and snap-back seems to be out of reach (for the moment) ...

## Summary - 2

- ... but, 5 years before the first p in LHC,
- we already know a lot !
- Treasured TeV and HERA experience
- Physics principle behind decay and snap-back assessed
- Phenomenology and working empirical scaling available
- Plans for 100 % cold measurements
- Involvement of machine control and operation teams for early integration
- Sector test could provide early vital information !

## Acknowledgements

- I am grateful to (at least) the following people for
- the ideas, results, analyses presented here:
- 	A. Akhmetov, E. Benedico-Mora, M. Breschi, A. Den Ouden,
- 	A. Devred, M. Haverkamp, A. Kuijpers, L. Larsson, S. Sanfilippo, M. Schneider, N. Smirnov, B. Ten Haken, H. Ten Kate,
- 	A. Tikhov, W. Venturini, L. Walckiers, R. Wolf, and the LHC-MTA measurement teams in the test stations of Block-4 and SM-18
- special thanks to P. Bauer and M. Lamm for the
- invitation and organization of this mini-workshop