1. Fault Tolerance in the MYRRHA superconducting linac
Frédéric Bouly (LPSC, Université Grenoble-Alpes, CNRS/IN2P3 )
Thursday 20 March 2014
CERN - Meyrin, Suisse
Workshop on Accelerators for ADS

2. High power proton beam (up to 2.4 MW)
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Introduction: A high reliability linac for MYRRHA
 Demonstrate the ADS Concept & Transmutation
Coupling : Accelerator + spallation source + subcritical reactor
High power proton beam (up to 2.4 MW)
 Avoid beam trips longer than 3 seconds to minimise thermal stresses and fatigue on target, reactor & fuel assemblies and to ensure 80 % availability.
 Actual Specification : Less than 10 trips per 3 months operation cycle.
Extreme reliability level
 Reliability guidelines are needed for the ADS accelerator design:
 Strong design i.e. robust optics, simplicity, low thermal/mechanical stress, operation margins…
 Redundancy (serial where possible, or parallel) to be able to tolerate failures
 Repairability (on-line where possible) and efficient maintenance schemes
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Fault tolerance in the MYRRHA SC linac

3. Global strategy for faults compensation
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Global strategy for faults compensation +
Operational injector 1: RF + PS + beam ON
Warm stand-by injector 2: RF+ PS ON, beam OFF (on FC)
 Strategy for a fault in the injector :
 Parallel redundancy
Switching magnet
Change polarity in ~1s
R. Salemme : The MYRRHA LEBT test stand
D. Mäder : The MAX injector R&D
 Strategy for a fault in the main linac :
 Serial redundancy & independently powered cavities
 A failure is detected anywhere
 Beam is stopped by the MPS in injector at t0
 The fault is localized in a SC cavity RF loop
 Need for an efficient fault diagnostic system
 New V/φ set-points are updated in cavities adjacent to the failed one
 Set-points determined via virtual accelerator application and/or at the commissioning phase
 The failed cavity is detuned (to avoid the beam loading effect)
 Using the Cold Tuning System
 Once steady state is reached, beam is resumed at t1 < t0 + 3sec
 Failed RF cavity system to be repaired on-line if possible
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Fault tolerance in the MYRRHA SC linac

4. Consequences on the linac architecture
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Consequences on the linac architecture
 The Local Fault-Recovery scheme
 A minimal number of cavity settings need to be modified
 Significant cavity Voltage increase (up to ~30%) needed for compensation  Efficient SC cavities initially used at half of their capabilities in nominal conditions – Ability for fast field increase in CW (no multipacting, low field emission…)
 Fast tuning system for failed cavity detuning & to minimise RF power consumption
 Large acceptance for phase retuning and margins on the available RF power
 Followed rules for the longitudinal beam dynamics design
 1. Keep phase advance at zero-current σL0 < 90° / lattice
→ GOAL = avoid SC-driven parametric resonances & instabilities in mismatched conditions
→ Implies limitations on Eacc
 2. Provide high longitudinal acceptance
→ GOAL = avoid longitudinal beam losses & easily accept fault conditions
→ Implies low enough synchronous phases (φs= -40° at input, keep φs< -15°) & to keep constant phase acceptance through linac, especially at the frequency jump
 3. Continuity of the phase advance per meter (< 2°/m)
→ GOAL = minimize the potential for mismatch and assure a current independent lattice
→ Implies especially limitations on Eacc at the frequency jump
J-L. Biarrotte: Talk at SLHiPP2, Catana
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Fault tolerance in the MYRRHA SC linac

5. 1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Linac architecture
5-ELLIPT47 2 cav/module 2-SPOKE50 (ESS) is also a viable back-up candidate
5-ELLIPT65 4 cav/module
1-SPOKE35 2 cav/module
Section # #1 #2 #3
Einput (MeV)
17.0
80.8
183.9
Eoutput (MeV)
184.2
600.0
Cav. technology
Spoke
Elliptical
Cav. freq. (MHz)
352.2
704.4
Cavity optim. β
0.35
0.47
0.65
Cavity geom. β
0.375
0.510
0.705
Nb of cells / cav. 2 5
Focusing type
NC quadrupole doublets
Nb cav / cryom. 4
Total nb of cav. 48 34 60
Nominal Eacc (MV/m) *
6.4
8.2
11.0
Synch. phase (deg)
-40 to -18
-36 to -15
Beam load / cav (kW)
1.5 to 8
2 to 17
14 to 32
Nom. Qpole grad. (T/m)
5.1 to 7.7
4.8 to 7.0
5.1 to 6.6
Section length (m)
73.0
63.9
100.8
*Eacc is given at βopt normalised to Lacc = Ngap.β.λ/2
 Overall linac: 233 metres & 142 cavities
J-L. Biarrotte et al., Proc. SRF 2013
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Fault tolerance in the MYRRHA SC linac

6. Beam dynamics simulations
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Beam dynamics simulations
 Rules for transverse beam dynamics
 Keep phase advance at zero-current σT0 < 90° / lattice to avoid structure and Space charge driven resonances
 Keep σT > 70% σL to stay away from the dangerous parametric resonance σT = σL/2
 Avoid emittance exchange between T & L planes via SC-driven resonances
 Keep smooth transverse phase advance & provide clean matching between sections in all planes to minimise emittance growth
J-L. Biarrotte: Talk at SLHiPP2, Catana
J-L. Biarrotte et al., Proc. SRF 2013
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Fault tolerance in the MYRRHA SC linac

7. Beam dynamics and acceptance
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Beam dynamics and acceptance
 Transverse and longitudinal lattice phase advance & cavities set points in the Hofmann diagram
J-L. Biarrotte et al., Proc. SRF 2013
 Transverse acceptance:
 Ø tube / RMS envelope > 15
Longitudinal acceptance:
 Up to 50 times nominal RMS emittance
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Fault tolerance in the MYRRHA SC linac

8. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Retuning feasibility and beam dynamics aspects
Preliminary studies on the Retuning procedures with the “perfect” linac design :
Evaluate the retuning feasibility and critical scenario (transitions between two cavity sections, full cryomodule loss…)
Quantify requirement for the RF technologies
keep the acceptance (smooth phase advance, low synchronous phase…)
One example : A complete spoke cryomodule is lost (2 cavities)
Retuning strategy used in the TraceWin code for compensation optimisation
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Fault tolerance in the MYRRHA SC linac

9. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Retuning example: 1 spoke module lost
Max. voltage increase : 27 %
Max. synchronous phase : 16°
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Fault tolerance in the MYRRHA SC linac

10. Emittances growth along the linac (RMS) Longitudinal acceptance
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Retuning example : 1 spoke module lost
Nominal Tuning
Fault-recovery
Emittances growth along the linac (RMS)
~+1%
Longitudinal acceptance
(SC linac + MEBT + HEBT)
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Fault tolerance in the MYRRHA SC linac

11. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Conclusions on these fault-recovery scenario analyses
 Several scenarios studied even with multiple cryomodules failures in different section
 The main conclusion is : the fault recovery scheme is a priori feasible everywhere in the MYRRHA main linac to compensate for the loss of a single cavity or even of a full cryomodule.
 Scenarios with several failed modules already tested. Further studies are required to evaluate the limits of multiple failed cavities/cryomodules.
 Impact on the R&D of superconducting cavities and associated systems
1. RF power amplifiers and margins : To take into account errors of control systems & fault recovery
2. Performant Superconducting cavities must accept fast gradient changes in CW operation
3. Control systems must enable to :
_ Retune the compensation cavities in less than 3 seconds
_ frequency detune the failed cavities which could perturb the beam
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Fault tolerance in the MYRRHA SC linac

12. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
RF Power Needs
 Power delivered to the beam :
 RF power required for a cavity which has an optimum frequency tuning :
 Optimal incident coupling :
 Ideally, each cavity has an optimal Qi (function of (r/Q), ϕs, Vcav & Ib0)
Qi =
5-cell (βg = 0.47)
 Coupling for the 3 cavity families of the MYRRHA accelerator:
Spoke (βg 0.35) : Qi =  BandWidth = Hz
 5-cell (βg 0.47) : Qi =  BandWidth = Hz
 5-cell (βg 0.65) : Qi =  BandWidth = Hz
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Fault tolerance in the MYRRHA SC linac

13. Estimation of needs due to errors
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Estimation of needs due to errors
 RF generator power - general formula :
Ex : Cavity n° 76 (βg 0.47) which is compensating a failure
 Errors taken into account for statistical errors study : Uniform Distribution
22.35 kW
2.106 draws
 Vcav : ± 2%
 ϕs : ± 2°
Ib0 : ± 2%
Δf : ± 20 Hz
 Qi : ± 2 mm (± 20%)
 (r/Q) : ± 10 %
~ 7%
 + 10 % margins added to errors study to take into account attenuation and calibration errors.
Maxi.
24.9 kW
~ 18.5%
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Fault tolerance in the MYRRHA SC linac

14. Minimum RF spare +70% RF Power Requirements See presentation
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
RF Power Requirements
See presentation
Minimum RF spare
+70%
 75 % Margin forseen
 Needs for reliable and flexible power supplies : with possibility for online reparability  In MAX R&D on 700 MHz solid-state amplifiers
5-cell β 0.65
Spoke β 0.35
S. Sierra : Development of 700MHz SS amplifiers
5-cell β 0.47
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Fault tolerance in the MYRRHA SC linac

15. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Control Systems requirements
 Requirement on Energy accuracy : 600 MeV ± 1 MeV at the linac ouput .
 Control systems must ensure the stability of the accelerating field and the synchronous phase
Δ 𝑉 𝑐𝑎𝑣 𝑉 𝑐𝑎𝑣 <0.5 %
Δ ϕ 𝑆 <0.5°
 With a limited amount of CW power the mechanical frequency tuning systems must enable to retune the compensation cavities and quickly detune the failed ones.
 Study the feasibility of retuning procedures (< 3 sec.) for the individually controlled cavities with a limited margin of CW RF power.
 Worked based on the β 0.47 linac section
 Model : cavity + tuning system + feedback/control loops
 Use of Matlab SimulinkTM for time simulation
 Define the best control strategy for the tuning system - R&D on an adaptive & predictive controller (ADEX).
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Fault tolerance in the MYRRHA SC linac

16. Control scheme for a superconducting cavity
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Control scheme for a superconducting cavity
 Model of digital system in I/Q formalism - Transfer function in Laplace domain:
Numerical system effects : Delay + ZOH + modulator.
PI correctors adjusted to minimise beam loading effect.
Complete board with
analogue mezzanine
C. Joly : DLLRF for reliability-oriented linacs
 Transfer function of the cold tuning system modelled from measurements
 Simualtions for feasibility studies achieved with a PI controller
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Fault tolerance in the MYRRHA SC linac

17. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
A fault recovery scenario
-45
 Compensation cavity can be easily retuned in less than100 ms
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Fault tolerance in the MYRRHA SC linac

18. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Failed cavity and requirement on the cold tuning system
 If the cavity is still superconducting, the important criterion is the induced decelerating voltage (to be lowered below 0.5% of nominal voltage); otherwise, it is the dissipated power, especially for a quenched but still cold cavity.
Requirement :
 Cavity frequency must be detune by more than 100*BandWidth in less than 3 seconds
 Minimum detuning speed capabilities for the CTS: 5 kHz/sec
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Fault tolerance in the MYRRHA SC linac

19. Controller of the tuning system
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Controller of the tuning system
 An adaptive and predictive system (from ADEX) was chosen to improve the cavity frequency control (michrophonics damping – fast detuning)
I. Martin Hoyo : C&C activities for cold tuning systems
Example: Simple frequency control
Example: strong microphonics perturbations
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Fault tolerance in the MYRRHA SC linac

20. A real scale experiment
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
A real scale experiment
 Dispose of a “real scale” experimental facility to Carry out Reliability-oriented experiments with a fully-equipped 700 MHz prototypical cryomodule controlled by feedback systems (RF and Fast Cold Tuning System).
 Component robustness
 retuning procedures
M. El Yakoubi : the MAX 700MHz test stand
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Fault tolerance in the MYRRHA SC linac

21. Fault tolerance in the MYRRHA SC linac
1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions
Conclusions
 Fault tolerance studies involved many instrumentation developments for experimental demonstration (real-scale cryomodule, Digital LLRF, Control systems for cavity frequency tuning, solid state amplifier)
 Based on existing systems a model of the cavity and its feedback loops have been developed
 Results from simulations showed that it is feasible to retune the cavities in less than 3 seconds.
 Need fast tuning system to unable the fault recovery procedure to minimise beam loading in the failed cavity. In worst case, the minimum required detuning (>100*Bandwidth) has to be achieved in less than 3 seconds.
 These Procedures were defined with beam dynamics simulations. The retuning strategy has been successfully assessed in several test scenarios: RF cavities but also for quads failure.
 It rests upon a consolidated superconducting linear accelerator design with large transverse and longitudinal beam acceptances.
 Several steps of this procedure appear to be non straightforward and will require further studies (consolidate errors studies, retuning with non-homogenous cavity gradients distrib.,….)
 FUTUR WORK : Development of a dedicated retuning tool. Fast calculation of retuning setpoints from actual cavity working points.
Idea : Adjust the voltage, the phase and smooth the phase advance cavity /cavity
THANK YOU !
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Fault tolerance in the MYRRHA SC linac