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
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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