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Fault - PPT Presentation

Tolerance in the MYRRHA S uperconducting Linac Workshop Reliability of Accelerators for ADS PrévessinMoëns CERN Monday 22 June 2015 F Bouly LPSCIN2P3CNRS 22 June 2015 FautTolerance in the MYRRHA SC linac Workshop Reliability of accelerators for ADS F Bouly ID: 500602

reliability linac ads myrrha linac reliability myrrha ads tolerance bouly workshop accelerators june 2015 faut amp beam cavity compensation section fault retuning

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Slide1

Fault-Tolerance in the MYRRHA Superconducting Linac

Workshop: Reliability of Accelerators for ADSPrévessin-Moëns (CERN), Monday 22 June 2015F. Bouly (LPSC/IN2P3/CNRS)Slide2

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly2 MYRRHA Reliability Requirements

 Demonstrate the ADS Concept & Transmutation

Coupling

: Accelerator + spallation source + subcritical reactor

High power proton

beam

(up to 2.4 MW)

Extreme reliability

Avoid

beam trips longer than 3 seconds

to

minimise thermal

stresses and fatigue on target, reactor & fuel assemblies and to ensure

80 %

availability (reactor re-start procedures).

Actual

Specification

:

Less than 10 trips per

3-month

operation cycle.Slide3

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly3Reliability Guideline & Linac Layout

 In any case, reliability guidelines are needed for the ADS accelerator design:  Robust design

i.e. robust optics, simplicity, low thermal stress, operation margins…

Redundancy

(serial where possible, or parallel) to be able

to tolerate/mitigate failures

Repairability

(on-line where possible) and efficient maintenance schemes Layout of the MYRRRHA linac

Serial redundancyParallel redundancySlide4

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly4Fault Compensation Strategy: SC Linac

 A failure is

detected anywhere

Beam is stopped by the MPS in injector at t

0

 The

fault

is

localised in a SC cavity RF

loop Need for an efficient fault diagnostic system

 New V/φ set-points are updated

in

cavities

(

cryomodule

)

adjacent to the

failed

one

Set-points

determined in advance: via virtual accelerator application and/or during

the

commissioning phase

 The

failed

cavity

is

detuned (to avoid the beam loading effect) Need an efficient Cold Tuning System

 Once the steady state is reached: the beam is resumed at t1 < t0 + 3sec Failed RF cavity system to be repaired on-line if possibleSlide5

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly5Strategy Choices & Linac Design Consequences

RF Power & Accelerating Gradient

overhead needed: 30 % Margin on

E

acc

chosen

(EUROTRANS FP6)

Considered

MTBF for RF

S

ystem

units : 10 000 hours.

Leads to a Global MTBF for RF units

of 70 hours.

At maximum 25 % the

total number of cavities can fail during the ADS operation cycle.

 Idea

: locally compensate the failures, local matching capabilities are

needed.

Overcost

on the

linac

compared

to a

conventional

design

Guidelines for the longitudinal beam dynamics design

 1. Keep phase advance at zero-current σL0 < 90° / lattice

→ GOAL = avoid space

c

harge

driven parametric resonances & instabilities in mismatched conditions

→ Implies limitations on

E

acc

2. Provide high longitudinal acceptance

→ GOAL = avoid longitudinal beam losses & easily accept fault conditions

→ Implies low enough synchronous phases (

φ

s

=

-40° at input, keep

φ

s

<

-15°) & to keep constant phase acceptance through the

linac

;

especially at the frequency jump

3. Continuity of the phase advance per meter (< 2°/m)

→ GOAL =

minimise

the potential for mismatch and ensure a current independent lattice

Implies especially limitations

on

E

acc

at the frequency jumpSlide6

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly6Linac Design and Acceptance

 Transverse

rules : smooth phase advance matching, avoid resonances and emittances exchange…

J-L. Biarrotte et al.,

Proc. SRF 2013

Transverse acceptance

:

Ø tube / RMS envelope > 15

Longitudinal acceptance:

Up to 50 times nominal RMS emittanceSlide7

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly7Main Parameters & Lattice

Section #

#1

#2

#3

E

input

(MeV)

17.0

80.8

183.9

E

output

(

MeV

)

80.8

184.2

600.0

Cav. technology

Spoke

Elliptical

Cav. freq. (MHz)

352.2

704.4

Cavity

geom. β

0.35

0.47

0.65

Cavity

optim

. β

0.375

0.510

0.705

Nb

of cells / cav.

2

5

5

Focusing type

NC

quadrupole

doublets

Nb

cav

/

cryom

.

2

2

4

Total nb of cav.

48

34

60

Nominal

Eacc (MV/m) *6.48.211.0Synch. phase (deg)-40 to -18-36 to -15Beam load / cav (kW)1.5 to 8 2 to 1714 to 32Nom. Qpole grad. (T/m)5.1 to 7.7 4.8 to 7.05.1 to 6.6Section length (m)73.063.9100.8

*Eacc is given at βopt normalised to Lacc = Ngap.β.λ/2

Section #1 : 2 * spoke

(

β

opt

=

0.375) /

cryomodule

Section #2 : 2 * 5-cell elliptical (

β

opt

=0.51) /

cryomodule

Section

#3

: 4 * 5-cell elliptical (

β

opt

=0.705) /

cryomodule

Replacement

with

spoke

(ESS

design ) to

be

studied

J-L. Biarrotte et al.,

Proc. SRF 2013Slide8

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly8Retuning Feasibility & Beam Dynamics First Goal : carry out preliminary retuning studies

 Evaluate the retuning feasibility and critical scenario (transitions between two cavity sections, full

cryomodule

loss…)

Quantify requirement for the RF technologies

Keep as best

as possible

the acceptance in the retune area (smooth phase advance, low synchronous phase…)

Simulations achieved with

TraceWin

code Several strategies with “DIAG” functions have been testedRem : More and different diagnostics than what we will have on the real accelerator Slide9

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly9How is it done in TraceWin?

 Errors function to set Eacc= 0 in the cavity

 ADJUST command is used to find the retuned cavity set points, in relative to the initial one:

Factor on the field map amplitude

Advance or

delay

on the RF phase

Diagnostics to recover the energy, the phase, the beam size and the

T

wiss parameters

 Calculations are long and need several adjustments before converging to an acceptable solutions Slide10

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly10Studied Scenarios

Several critical scenarios individually studied

The main conclusion was :

the

fault-recovery

scheme is a priori feasible everywhere in the MYRRHA SC

linac

to compensate the failure of a single cavity or even of a full

cryomodule

Most advanced scenario with multiple failures simulated at the end of the MAX project:

 Section #1 : 1 failed spoke cavity → 4 compensation cavities

Section #2

:

1

failed

cryomodule

8 compensation cavities

section

#3

: 1 failed

5-cell cavity

4 compensation

cavities Slide11

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly11Multiple Failures Scenario (1) : Beam envelope

Input beam from injector simulation

MEBT

Section #1:

Spoke

(

β

opt

=0.51

)

Section #2: 5-cell (βopt =0.51)

Section #3: 5-cell (βopt =0.51)Slide12

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly12

Max. Eacc increase : 30 %

Max.

ф

s

change : 56 %

Multiple

Failures

Scenario (2) : compensation settingsSlide13

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly13Multiple Failures Scenario (3) : Phase Advance & Acceptance

Nominal

Fault compensationSlide14

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly14Error Study with the fault-compensation scenario (1) Error study on the nominal MYRRHA

linac : No significant lossesAn error study have been carried out on the multiple failure scenario

Static and dynamic errors taken into account in

TraceWin

Simulation from the RFQ output to the SC

linac

output

 1000

linacs simulated with 10

6 macro-particles : significant longitudinal losses

D.

Uriot

et al., MAX

project

deliverable

1.4Slide15

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly15 In the last section the losses reach the acceptable limit of 1 W/m

 L

ongitudinal acceptance is the key point for beam loss

control

D.

Uriot

et al., MAX

project

deliverable

1.4

Error

Study with

the fault-compensation scenario (2) Worst

case

Average

over the 1000

linacs

simulated

Emittance

growth too important

in section

#2

: failed

cryomodule

Slide16

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly16Improving the Fault-Compensation method ? The retuning calculations were done with an “internal” matching algorithm of the TraceWin

code

 “Black-Box”

The optimisation criteria and used diagnostics may not be relevant

Can we find other settings which less affect the acceptance and the emittance?

Is it possible to find/define a “systematic methodology”, in view of the development of a retuning tool that can be applied on the real machine?

 Consideration over the retuning area: o

ne example with one failed cavity

Beginning of the retuning area

End of the retuning areaSlide17

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly17Cavity Transfer Matrix & Phase Advance

 Cavity

Longitudinal transfer matrix (1

st

order) – with the approximation

Δβ

<<

β

β

cste

out

=

cavity

.

in

 

:

synchrotron phase advance per length unit

 

 

The

transverse

focussing

is

also

l

inked

to k

1

st

criterion

:

recover the same transfer matrix of the retuned area than in nominal condition

In this case 4 non-linear equations, 4 unknowns (

k

i

)

Find the best compromise on

k

i

(

optimisation routine – matlab fuction ‘lsqnonlin’: solves nonlinear least-squares problems )Slide18

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly18Energy Gain & time of flight

2

nd

criterion

:

the total Energy gain should remain the same than in the nominal case

1

+

2

+

4

+

5

 

Problem can be solved by optimization of

β

i

(

Δ

W

i

,

k

i

)  (Eacc, фs )  (Amplitude increase, ϕRF )

 The solution with the simplified model is injected into the TraceWin

model for

fine/adjustments

tuning of the solutions

 

Assumption on the cavity time of flight:

 

 

 

 

 

 

 

 

 

3

rd

criterion

: the time of flight should remain the same than in the nominal caseSlide19

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly19Example result : first try!

First optimisation:

directly with

TraceWin

With pre-optimisation

WORK IN PROGRESSSlide20

22 June 2015Faut-Tolerance in the MYRRHA SC linac - Workshop: Reliability of accelerators for ADS - F. Bouly20Final Remarks The adopted Fault-compensation scheme enables a very local retuning of the linac

 In principle it can be applied on the whole SC accelerator Multiple failures can be compensated

 The retuning affects the longitudinal acceptance and can create important emittance growth

Non-acceptable beam losses, especially on a real (“non-perfect”) machine

A

retuning algorithm

has

to be

developed &

optimised

for an application to the real accelerator Work in progress – one of the goals within the MYRTE (Virtual Accelerator)

The Fault compensation scheme is effective, but only if the linac has an intrinsic optimised reliability

Diagnostics - High performant SC cavities (no

multipacting

)- Fast tuning system-

RF power margin of 70 % required

Other solutions to explore in the MYRTE project

Linac

design adjustment : more compact lattice? Lower emittance from inj.? How to increase the acceptance?

Global re-phasing of the

linac

after a failure: is it more simple ?

Relax the constraints for the compensation cavities: use more cavities ?

 According to

reliability

studies the number of trips should not be as high as initially foreseen

See

next presentation by A. Pitigoi