Fault Tolerance in the MYRRHA superconducting linac

Frédéric Bouly (LPSC, Université Grenoble-Alpes, CNRS/IN2P3 )

Workshop on Accelerators for ADSThursday 20 March 2014

CERN - Meyrin, Suisse

Fault tolerance in the MYRRHA SC linac F. Bouly20/03/2014

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Demonstrate the ADS Concept & Transmutation Coupling : Accelerator + spallation source + subcritical reactor

Introduction: A high reliability linac for MYRRHA

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.

High power proton beam (up to 2.4 MW)

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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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Global strategy for faults compensation

Strategy for a fault in the injector : Parallel redundancy

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Operational injector 1: RF + PS + beam ON

Warm stand-by injector 2: RF+ PS ON, beam OFF (on FC)

Switching magnet

Change polarity in ~1s

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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

R. Salemme : The MYRRHA LEBT test stand

D. Mäder : The MAX injector R&D

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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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

J-L. Biarrotte: Talk at SLHiPP2, Catana

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Linac architecture

Section # #1 #2 #3Einput (MeV) 17.0 80.8 183.9Eoutput (MeV) 80.8 184.2 600.0Cav. technology Spoke Elliptical Cav. freq. (MHz) 352.2 704.4Cavity optim. β 0.35 0.47 0.65Cavity geom. β 0.375 0.510 0.705Nb of cells / cav. 2 5 5Focusing type NC quadrupole doubletsNb cav / cryom. 2 2 4Total nb of cav. 48 34 60Nominal Eacc (MV/m) * 6.4 8.2 11.0Synch. phase (deg) -40 to -18 -36 to -15Beam load / cav (kW) 1.5 to 8 2 to 17 14 to 32Nom. Qpole grad. (T/m) 5.1 to 7.7 4.8 to 7.0 5.1 to 6.6Section length (m) 73.0 63.9 100.8

1-SPOKE352 cav/module

5-ELLIPT47 2 cav/module2-SPOKE50 (ESS) is also a viable back-up candidate

Overall linac: 233 metres & 142 cavities

5-ELLIPT654 cav/module

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

J-L. Biarrotte et al., Proc. SRF 2013

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

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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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

J-L. Biarrotte et al., Proc. SRF 2013

J-L. Biarrotte: Talk at SLHiPP2, Catana

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Beam dynamics and acceptance

Transverse and longitudinal lattice phase advance & cavities set points in the Hofmann diagram

Transverse acceptance: Ø tube / RMS envelope > 15

Longitudinal acceptance: Up to 50 times nominal RMS emittance

J-L. Biarrotte et al., Proc. SRF 2013

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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Retuning example: 1 spoke module lost

Max. voltage increase : 27 %

Max. synchronous phase : 16°

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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Nominal Tuning Fault-recovery

Longitudinal acceptance

(SC linac + MEBT + HEBT)

Retuning example : 1 spoke module lost

Emittances growth along the linac

(RMS) ~+1%

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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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.

Impact on the R&D of superconducting cavities and associated systems1. RF power amplifiers and margins : To take into account errors of control systems & fault recovery2. Performant Superconducting cavities must accept fast gradient changes in CW operation3. Control systems must enable to :

_ Retune the compensation cavities in less than 3 seconds_ frequency detune the failed cavities which could perturb the beam

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

Scenarios with several failed modules already tested. Further studies are required to evaluate the limits of multiple failed cavities/cryomodules.

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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)

Coupling for the 3 cavity families of the MYRRHA accelerator:

Qi = 8.2 106

5-cell (βg = 0.47)

Spoke (βg 0.35) : Qi = 2.2 106 BandWidth = 160.2 Hz 5-cell (βg 0.47) : Qi = 8.2 106 BandWidth = 86.05 Hz 5-cell (βg 0.65) : Qi = 6.9 106 BandWidth = 102.2 Hz

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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Estimation of needs due to errors RF generator power - general formula :

Vcav : ± 2% ϕs : ± 2° Ib0 : ± 2% Δf : ± 20 Hz Qi : ± 2 mm (± 20%) (r/Q) : ± 10 %

Errors taken into account for statistical errors study : Uniform Distribution

Ex : Cavity n° 76 (βg 0.47) which is compensating a failure

22.35 kW

Maxi.24.9 kW

+ 10 % margins added to errors study to take into account attenuation and calibration errors.

2.106 draws

~ 7%

~ 18.5%

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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RF Power Requirements

Spoke β 0.35

5-cell β 0.47

5-cell β 0.65

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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

S. Sierra : Development of 700MHz SS amplifiers

See presentation

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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

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).

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.

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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Control scheme for a superconducting cavity

Complete board withanalogue

mezzanine

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.

Transfer function of the cold tuning system modelled from measurements Simualtions for feasibility studies achieved with a PI controller

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

C. Joly : DLLRF for reliability-oriented linacs

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A fault recovery scenario

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Compensation cavity can be easily retuned in less than100 ms

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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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.

Failed cavity and requirement on the cold tuning system

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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

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Controller of the tuning system

Example: Simple frequency control Example: strong microphonics perturbations

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

I. Martin Hoyo : C&C activities for cold tuning systems

An adaptive and predictive system (from ADEX) was chosen to improve the cavity frequency control (michrophonics damping – fast detuning)

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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

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. Conclusions

M. El Yakoubi : the MAX 700MHz test stand

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Conclusions

1. Introduction – 2. Linac Design – 3. RF Fault tolerance – 4. Recovery procedures and technological requirements – 5. 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.,….)

THANK YOU !

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