reliability challenges for circular ads drivers mike seidel, psi reliability workshop cern, june 221

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Reliability challenges for circular ADS drivers

Mike Seidel, PSI

Reliability Workshop CERN, June 22

Outline• Suited Circular Accelerator Concepts

– requirements for ADS accelerators– cyclotrons, rapid cycling synchrotrons, FFAG

• Generics on failure probability / trip rates– redundancy in accelerators; AND vs OR fault logics; failure and survival

probability – measured trip statistics at PSI

• Comments on FFAG, J-PARC RCS, PSI cyclotron• New cyclotron ideas

– H2+ Daedalus, stacked/flux coupled cyclotrons, reverse bend cyc.

• Discussion– Pro’s and Con’s of Circular concepts

Reliability Workshop CERN, June 22 M.Seidel, PSI

Requirements for ADS Accelerators• energy: flat optimum 1.2GeV; however 0.8 ..

2.5GeV under discussion • power: 2...10MW; Ptherm = Pbeam G/(1-k)• low losses: 1W/m; PSI: 100W at critical location• reliability & stability: 0.01…0.1 trips per day(!)• efficiency: as best as possible, =Pbeam/Pgrid= 20…

30%• cost: as low as possible; optimize for series

production; modern nuclear power plant: (5B€) Reliability Workshop CERN, June 22 M.Seidel, PSI

High Intensity Accelerator Landscape

2.4mA = 1.416MW

PSI

SNS, Sep 2013 (today target limits intensity)Reliability Workshop CERN, June 22

M.Seidel, PSI

J-PARC RCS ramping up!

1MW routine operation expected 2016

reliability, trip performance• todays trip performance of accelerators is orders of magnitude worse than desired

for ADS, e.g 10…100d-1 achieved vs. 0.01…0.1d-1 desired by reactor experts• in recent years discussions took place and requirements were somewhat relaxed:

– fatigue failure of fuel elements is not seen extremely critical anymore– short trips (few seconds) can be accepted; shorter than thermal time constants

• accelerator and reactor developers must find compromises, my impression: reactor community in general not very flexible due to strict safety rules; e.g. studying fast reactor startup, bridging trips

IBR-2 pulsed reactordemonstrates fast cycling

even without technical reasons, for industrial power production (and consumption) reliability is very important !


Suited Accelerator Concepts?

Linear Accelerators Cyclic Accelerators

normalconducting linac

superconducting linac

electrostatic(E limited!)

• pulsed• CW: very low

gradient

• CW possible• cooling power!• cost!

Synchrotron[rapid cycling]Cyclotron

Synchro-Cyclotron(cycling)

Other: Betatron, Microtron

FFAG

• CW possible• beam dynamics!• extraction!

• pulsed• power limited

(1MW?)

• compact• cycling / CW

questionable• no demonstrator

exotic(laser, plasma, diel.

efficiency!)


classification of circular acceleratorsbending radius

bending field vs. time

bending field vs. radius

RF frequency vs. time

operation mode (pulsed/CW)

comment

betatron induction

microtron varying h

classical cyclotron

simple, but limited Ek

isochronous cyclotron

suited for high power!

synchro- cyclotron

higher Ek, but low P

FFAG strong focusing!

a.g. synchrotron

high Ek


reliability calculation in a nutshell


example: lightbulbs (MTBF=1000h)

N(t) number of surviving lightbulbsN0 original number of funct. bulbs(t) fractional failures per time

• constant , i.e. no aging/history(Markov process)

• simple, but for some applications too simple!

mathematical treatment:

S(>t) survival probabilityF(>t) failure prob. (1-S(t))f(t) failure density function =F‘(t)(t) hazard function

beyond =const: Weibull distribution

duration statistics for un-interrupted run periods


PSI run data analyzed:• integrated distribution of un-interrupted beam times• double log scale

How many runs per day with duration longer than t.

total trips per day

modelling with exponential and Weibull distributions


fault topology of an accelerator


&1

&

2

system A

system A´

system B

system B‘

system C

system C‘

system C‘‘

system C‘‘‘

classical

simple redundancy

multiple redundancy

beam on

clearly redundancy is desirablewhether redundancy is possible with major subsystems depends on the choice of accelerator concept

reliability, concepts

numerical example:tube: MTBF=5000h; MTTR=8h• Linac with 80 tubes, accepting 0 fault:

MTBFeff = 62h• Linac with 80 tubes, accepting 1(k=2) fault:

MTBFeff = 1.074h• Linac with 80 tubes, accepting 2 faults:

MTBFeff = 26.067h• cyclotron with 4 tubes, accepting 0 faults:

MTBFeff = 1.250h

binomial distribution, Bp = incomplete Beta Function


circular accelerator linear accelerator

only few resonators / magnetsredundancy difficult

redundancy wrt. resonators possible

distribution of trip durationD. Vandeplassche, Proc. IPAC 012

PSI analysis of trip-periods


double logarithmic:power law is straight line.

PSI

PSI

total trips per day

next : circular concepts compared…


complexity = less

availability (?

)

isochronous cyclotron: continuous, nothing cycles

FFAG: pulsed, RF cycling

synchrotron: pulsed, magnets + RF cycling

[investigation towards CW operation: S.Machida, FFAG, EMMA, serpentine acceleration]

Fixed Field Alternating Gadient Accelerator ?


• strong focusing, large E acceptance, magnets fixed in time• RF must be cycled faster than synchrotron, but less intensity than CW cyclotron• not ramping the magnets should be an advantage for reliability• otherwise same arguments as for RCS hold• cyclic injection/extraction potentially difficult• high intensity: not demonstrated

[EMMA]


M.Seidel, PSI

[M.Shirakata, J-PARC]

fRF: 0.94MHz – 1.67MHz

Outline of the J-PARC RCSCircumference 348.333 m

Superperiodicity 3

Harmonic number 2

Number of bunches

2

Injection Multi-turn,Charge-exchange

Injection energy 181 MeV

Injection period 0.5 ms (307 turns)

Injection peak current

30 mA

Extraction energy 3 GeV

Repetition rate 25 Hz

Particles per pulse

5 x 1013

Output beam power

600 kW

Transition gamma 9.14 GeV

Number of dipoles

24

quadrupoles 60 (7 families)

sextupoles 18 (3 families)

steerings 52

RF cavities 12

Now the RCS is in the final beam commissioning phase aiming for the design output beam power of 1 MW.

Recently the hardware improvement of the injector linac has been completed.

⇒ 400 MeV in 2013

⇒ 8.3 x 1013

⇒ 1 MW 400 MeV H-

3GeVproton

MLF : Material and Life Science Experimental FacilityMR : 50-GeV Main Ring Synchrotron

⇒ 50 mA in 2014

[H.Hotchi, IPAC2015]

Earthquake

300 kW

Hg-target replacement

Incident at Hadron Facility

532 kW

300 kW

• as of 3rd of June 2015

〜 560 kW

〜 10 months interruption due to the earthquake

593 kW

〜 1 month interruption due to the fire in MLF

Beam Power History at MLF

Interruption due a trouble of Hg-target

500 kW

400 kW

[T.Koseki]

J-PARC Rapid Cycling Synchrotron [RCS]• 1.01MW avg beampower achieved (Hotchi, IPAC15); still high losses, not

routine• high average availability: 90% (!)• drives neutron source (mercury target); i.e. ADS like application thus J-PARC RCS has demonstrated a respectable high intensity performance!


® one would think the rapid cycling of the magnets would cause specific problems, more than in an CW accelerator; but nothing outstanding was reported and 90% was achieved, on par with SNS and PSI.

typical failures from the past (taken from M.Shirakata presentation)• LINAC HV DC supply failure (2012: 59h, 2013:103h)• bending magnet supply (resonant circuit, 2014: 115h, oil pump)• radiation accident caused 23 months interruption (Au target got full

charge in 5ms instead 2s) not specific to RCS• Japan’s great earthquake: recovered in 286 days!! not specific to RCS

possibly related

to cycling

http://neutrons-old.ornl.gov/conf/arw2015/presentations/A%20-%20Shirakata%20-%20Operation%20Reliability%20of%20J-PARC%20Main%20Ring.pdf

PSI Ring Cyclotron

8 Sector Magnets: 1 T

Magnet weight: ~280 tons

4 Accelerator Cavities: 860 kV (1.2 MV)

1 Flat-Top Resonator 150 MHz

Accelerator frequency: 50.63 MHz

harmonic number: 6

kinetic beam energy: 72 590 MeV

beam current max.: 2.4 mA

extraction orbit radius: 4.5 m

outer diameter: 15 m

RF efficiency Grid/Beam

0.900.640.55 = 32%

rel. losses @ 2.2mA: -~1..210-4

transmitted power: 0.32 MW/Res.


Typical Trip Causes for Cyclotrons


• electrostatic elements: high voltage breakdowns due to plasma discharges; stray electrons collected on insulators; presence of intense proton beam in vicinity of electrodes and insulators; comparably poor vacuum of 10-6mbar

• loss tuning: depends critically on complete accelerator chain incl. source; low tail density very sensitive to all parameters

• cavities & RF systems: breakdown, multipacting, MW level amplifier chains (eg. 50MHz)

see presentation by J.Grillenberger on concrete examples

critical: injection/extraction schemes• deflecting element should affect just one turn, not neighboured turn

critical, cause of losses• often used: electrostatic deflectors with thin electrodes• alternative: charge exchange, stripping foil; accelerate H- or H2

+ to extract protons (problem: significant probability for unwanted loss of electron; Lorentz dissociation: B-field low, scattering: vacuum 10-8mbar)

0-

HV foil

extraction electrodeplaced between turns

extraction by charge exchange in foileg.: H- H+

H2+ 2H+

binding energies

H- H2+

0.75eV 15eV


extraction profile measured at PSI Ring Cyclotron

dynamic range:factor 2.000 in particle density

red: tracking simulation [OPAL] black: measurement

position of extraction septum

d=50µm

turn numbers

from simulation

[Y.Bi et al]


proposed cyclotrons I: H2+ Daedalus cyclotron

[neutrino source]

[L.Calabretta, A.Calanna et al]


purpose: pulsed high power beam for neutrino production• 800MeV kin. energy• 5MW avg. beam power

note: complex extraction path

binding energies

H- H2+

0.75eV 15eV

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proposed Cycl. II: H2+ AIMA Cyclotron w reverse

bend and multiple 60keV injection [P.Mandrillon]

The reverse valley B-field concept avoids the internal loop (cf. DAEdALUS extraction) for the stripped proton beam from H2+.

Vacc=150kV

Vacc=165kV


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Texas A&M University

• Two Stages Cyclotron: 100 MeV SF injector + 800 MeV SF booster.

• Stack of 3 Cyclotrons in //• Booster: 12 Flux coupled

stack of dipole magnet sectors

• 10 Superconducting 100 MHz RF cavities providing a 20 MeV Energy Gain/turn

• multiple power couplers per cavity

• Large turn separation allowing to insert SF beam transport channels made of Panofsky Qpoles (G=6T/m)

proposed cycl. III: TEXAS A&M: 800 MeV SUPERCONDUCTING STRONG-FOCUSING CYCLOTRON

[P.McIntyre, Texas A&M]Reliability Workshop CERN, June 22

recently: DOE awards stewardship funding for idea of strong focusing channels in cyclotrons

Summary – Reliability of Circular Accelerators


• reliability of high intensity circular accelerators today is around 90% (PSI, J-PARC), on par with the s.c. linac and accumulator of SNS; trip rates are 10…100d-1 for PSI, at least three orders of magnitudes worse than desired for ADS

• implementation of redundancy difficult in circular accelerators; on the upside circular acceleration is an economic concept; injection/extraction elements in cyclotrons are critical devices; their reliability could be improved by certain measures when considering ADS

• cyclotrons with CW operation should have best stability; next is FFAG with ramping of RF; rapid cycling synchrotron needs magnet ramping; nevertheless high reliability demonstrated by J-PARC

personal remark on choice of technology: today a s.c. linac is a straightforward solution for a multi-MW facility; however, despite of it‘s greater beam dynamics complexity and intrinsically higher susceptibility for trips, an optimized cyclotron, built in series can be very cost effective and could reach high availability as well.

thank you for the attention!


reliability challenges for circular ads drivers mike seidel, psi reliability workshop cern, june 221

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