Loads due to disruptions and prospects for mitigation

Michael Lehnen – 6th IAEA DEMO Programme Workshop – 1-4 October 2019

© 2019 ITER Organization

IDM UID: 29EHK3

Loads due to disruptions and

prospects for mitigation

M. Lehnen



IDM UID: 29EHK3

‘Damage’ limits are very low compared to what the target plasma

parameters are (already the case in ITER)

Disruption Mitigation can prevent damage during low

performance commissioning

Disruption Mitigation cannot prevent damage above certain

energies and especially not at those of the target scenario

The DEMO design has to take into account the need for

virtually 0 disruptions at target plasma

Main messages



IDM UID: 29EHK3

Melt limit: 𝑡𝑇𝑄~𝑅; 𝐴~𝑅𝜆; Δ𝑇 ~𝐸𝑡ℎ𝑒𝑟𝑚𝑎𝑙/𝐴 𝑡𝑇𝑄 𝐸𝑚𝑒𝑙𝑡~𝑅3/2

Beryllium

Tungsten

up to 350 MJ

Disruption Loads and Load Limits Thermal Energy

𝐸𝑡ℎ𝑒𝑟𝑚𝑎𝑙~𝑅5



IDM UID: 29EHK3

Disruption Loads and Load Limits Magnetic Energy

𝑃𝑟𝑎𝑑 ≪ 𝑃𝑐𝑜𝑛𝑑

Melt limit: 𝑡𝑣𝑒𝑟𝑡𝑖𝑐𝑎𝑙~𝑅2; 𝐴~𝑅𝜆; Δ𝑇 ~𝐸𝑡ℎ𝑒𝑟𝑚𝑎𝑙/𝐴 𝑡𝑇𝑄 𝐸𝑚𝑒𝑙𝑡~𝑅

2

𝐸𝑚𝑎𝑔𝑝𝑜𝑙

~𝐵𝑡2𝑅3

Assumption for ITER & DEMO: tVV > tCQ



IDM UID: 29EHK3

Disruption Loads and Load Limits Magnetic EnergyITER 5 MA , 1.8 T scenario*

Halo current density: 20 kA/m2

*scaled from 15 MA

t [ms] 275 325 375 425

DE [MJ] 5 14 19 18

lq [mm] 32 41 53 82

Total: 56 MJ

Coburn+ PhysScr 2019



IDM UID: 29EHK3

Disruption Loads and Load Limits EM loads

𝐼ℎ𝑎𝑙𝑜

FzBt

Impact of EM loads is very much

design dependent



IDM UID: 29EHK3

Bt

Mz

dB/dt𝐼𝑒𝑑𝑑𝑦

dB/dt dIP/dt

Disruption Loads and Load Limits EM loads

Impact of EM loads is very much

design dependent

Moment on blanket modules limits allowable current decay rates in ITER



IDM UID: 29EHK3

Disruption Loads and Load Limits asymmetric VDEs

Rotation frequency of

asymmetric VDEs can be in

the resonance range in ITER

Amplification factors of 10 after

2-3 rotations can be expected

Physics understanding and

model validation is still ongoing3D MHD models, sink & source model,

eddy current driven model

Myers+ NF 2018

Critical question:

How does the force amplitude scale with rotation?

Dedicated experiments with special divertor tiles in

COMPASS were performed last week for model validation



IDM UID: 29EHK3

Mitigation of heat loads and electro-magnetic forces ITER

Injection of high-Z impurities

Dissipate thermal and magnetic energy through photonic radiation

Shattered Pellet Injection



IDM UID: 29EHK3

Parameter Space for loads and mitigation ITER

ITER requires mitigation from

the first campaign onward

Limits for Eth are based on more

detailed calculations; Limits for

Emag is a first estimate

High radiation fractions for

magnetic energy on timescales

of 100 ms are possible (exp’s,

modelling)


thermal energy are not confirmed

yet (complexity of the thermal

quench)

EM load mitigation requirement

from worst case halo current

(aVDEs not accounted for)

Lehnen+ IAEA 2014



IDM UID: 29EHK3

Limits on thermal load mitigation

JOREK

Ne/D2 SPI

single injection

ITER L-mode

D. Hu, ITPA MHD April 2019

Radiation flash can cause surface melting / recrystallization

Energy limits assuming a peaking of 4 (TPF = PPF = 2)

ITER: 70 MJ (SS) and 150 MJ (Be)

DEMO: 200-400 / 450-900 MJ (W recryst./melt) [R = 7.5-9.5 m]

*

* Injection plume extends toroidally over 120 deg



IDM UID: 29EHK3

Parameter Space for loads and mitigation DEMO

Limits are based on simple

scalings shown earlier


magnetic energy on timescales

of 100 ms are possible (exp’s,

modelling)


thermal energy are not confirmed

yet (complexity of the thermal

quench)

Peaking of radiation is uncertain

EM load limits depend on VV and

in-vessel component design

(aVDEs not accounted for)

Recrystallisation threshold for DT = 1400 and PF = 4

Energy at L-H transition for EU DEMO 1 (R=9.1m, IP = 19.6 MA) [Wenninger+, NF 2017] and

using L-mode scalings [Kaye+, NF 97 and Martin J.Physics 2008]

R = 7.5 – 9.5 m



IDM UID: 29EHK3

Disruption Loads and Load Limits Runaway electrons

RE free disruptionswith Be wall in JET

9.5 MA

Maximum initial runaway population (IRE < 0.1 MA)

105

109

Runaway seed sources

• Dreicer

• Hot tail

• T decay

• Compton scattering

Avalanche amplification

increases exponentially

with plasma current

Highly conductive VV

can mitigate to some

extent

Main source during non-active phase in ITER

Possible sources during active phase in ITER

Main sources in DEMO?

No strong contribution in ITER



IDM UID: 29EHK3

Disruption Loads and Load Limits Runaway electrons

MEMOS: Temperature distribution

ENDEP: Energy distribution in FW panelLpoloidal = 100 mm, Dr = 2 mm, <ERE> = 15 MeV, Etot = 1.75 MJ, Dt = 100ms (Emag conversion)

Melt threshold estimate (ITER)

~0.3 MJ / FW panel (Be)

~0.3 MJ / divertor cassette (W)

Corresponding to IP @ TQ of

~0.5 MA (deposition on single PFC)

~3-8 MA (uniformly distributed)

Variation due to different impact time

Energy distribution depends on

• Misalignment of PFCs

• Kink mode that causes RE loss

not yet quantified

Water leaks are likely for significant RE currentLehnen+ IAEA 2016



IDM UID: 29EHK3

Runaway electron avoidance ITER

mo

re d

eute

riu

m

Quantities from simple models

MHD and assimilation will matter

RE avoidance presently based

on massive D2 (H2) injection

JOREK D2 SPI simulation for JETHu+ NF 2018Martín-Solís+ NF2017



IDM UID: 29EHK3

Runaway electron energy dissipation ITER

Presently foreseen as a second layer of defense

in ITER in case of accidental RE generation

Runaway current decays as expected with impurity

quantity for modest injections in experiments

But there are substantial issues:

• Significant current carried by RE at the final loss

[e.g. Hollmann+, NF 2019; Reux+, NF 2015]

in ITER (twall > tCQ): Z = f(IRE)

• Scraping-off effect could lead to early RE energy

deposition [Konovalov+, IAEA 2014]

• Limited fuelling efficiency

o Pellets are not better than MGI (in this

respect) [Shiraki+, NF2018]

o Slow particle transport times prevent fast

penetration [Hollmann+, NF2019]Hollmann+ NF 2019

DIII-D



IDM UID: 29EHK3

Runaway electron mitigation

Alternative schemes for mitigating RE other than material injection

Boozer NF2018

Can passive structures

in the first wall produce

long-wavelength

non-axisymmetric

perturbations?

Stochastisation

External coils?

ITER in-vessel perturbation coils

with n=3 cannot remove all REs

Papp+ IAEA2012

Passive structure?



IDM UID: 29EHK3



Carnevale+ PPCF2019

Possible on present

tokamaks, example here: FTU

RE beam position control

Enables applying other techniques that require longer timescale

Position control only possible under

special conditions in ITER

(plasma to be moved to the neutral point)

Lukash+ EPS2013

0,4

0,6

0,8

1dI

p/dt,MA/s

9,5 10 10,5 11 11,5 12

Ire max

,MAcontrolled

not controlled

(a)

250

300

350

400

9,5 10 10,5 11 11,5 12

tloss min

,mscontrolled

not controlled

(b)

IRE,max [MA]



IDM UID: 29EHK3



Lvovskiy+ PPCF2018

Kinetic instabilities and

RE formation in DIII-D

RE-wave interaction

Kinetic instabilities may…

…help dissipating RE energyPlasma parameters in RE plateau result in

low growth rates D2 injection may help

Aleynikov+ NF2015 / Paz-Soldan+ IAEA 2018

…prevent RE formation



IDM UID: 29EHK3

Shattered Pellet Injection

24 flight tubes in 3 equatorial ports + 3 barrels in 3 upper ports

Each pellet D = 28.5 mm and L = 57 mm

Each flight tube of D 60 mm with a shatter bend at the end

Present design of the ITER Disruption Mitigation System

SPI cryostat

(prismatic)

Cold head

Propellant

gas recovery

Optical pellet

diagnostic

Double

bellows

Pellet

shatter exit

segment

Vacuum

valve

Flyerplate

propellant

valve

x1 SPI Injector

Pellet

guide tube

Torus

vacuum

boundary

Diagnostic

first wall

(Plasma

facing

Component)



IDM UID: 29EHK3

ITER Shatter Pellet Injection Quantities

13 mm20 mm

17 mm

28 mm

Required assimilated quantities based on present knowledge

Quantity Species Purpose Minimum # of pellets

6x1024 D RE avoidance 3

~1025 Ar RE energy dissipation 11

5x1021 Ne EM loads

Mixed into D pellets5x1021 Ne CQ heat loads

4x1022 Ne TQ heat loads

Significant uncertainties on RE avoidance and energy dissipation!

Minimum required number of pellets for 100% assimilation and without redundancy

Multiple injection effective? experiments at DIII-D, KSTAR + Modelling



IDM UID: 29EHK3

ITER Shatter Pellet Injection Reaction time

13 mm20 mm

17 mm

28 mm

Overall reaction time includes:

• Delay from trigger decision to trigger arrival at the PS

• Valve opening and gas release

• Pellet acceleration + flight time

Length of flight tube in the equatorial ports: 6.3 m

He propellant

m [g] v [m/s] Dtflight [ms]

Argon 60 ~200 32

Deuterium 7 ~600 11

Hydrogen 3.5 ~800 8

Estimate of flight time



IDM UID: 29EHK3

Injection techniques

Pros

MGI Fast response if located close to

plasma

Inefficient for long delivery tubes

No direct fueling of plasma core

SPI Efficient delivery of material for

long flight tubes

Potential to fuel plasma core more

efficiently

Restricted to cryogenic pellets

Statistical fragment size distribution

could possibly not fulfil requirements

Massive multiple injection could be

ineffective

EPI* Possibility of injecting non-

cryogenic material

High velocities

Not yet explored on a tokamak

Requires sabot recovery system

Shell

Pellet

Can deliver material to plasma

core

Possibility of injecting non-

cryogenic material

Shell lifetime needs to be adjusted to

plasma parameters to guarantee core

deposition

Cons

*Electromagnetic particle injector



IDM UID: 29EHK3

Triggering the mitigation system

Trigger system to run independently of any other exception handling tools

Physics based approaches focused so far on mode lock and needs to be

further developed

AI approaches require learning and so far lack sufficient portability, but

can inform how to improve physics based schemes

Post-TQ detection has higher

success rate than pre-TQ

prediction (important for high

current operation)

DEMO: Which measurements

can contribute to the disruption

prediction scheme and what

prediction times are achievable?

Esquembri+, IEEE 2018

SPAD - Mode lock anomaly detector

Achievable success rates and

warning times in JET (ILW)



IDM UID: 29EHK3

Disruption Management

Disruption Mitigation is required from early operation onward.

The number of unmitigated disruptions is controlled by containing disruptivity (disruption avoidance) and maximising the mitigation success rate (disruption prediction and system reliability).

How many unmitigated and mitigated disruptions are acceptable at what energy and current?

The Disruption Budget Consumption is introduced as a lifetime consumption indicator.



IDM UID: 29EHK3

Disruption Budget

Based on pre-2017 ITER research plan

Lehnen+ IAEA 2016



IDM UID: 29EHK3


Disruption Budget



IDM UID: 29EHK3

Conservative assumption: unmitigated disruptions at high current always generate high halo currents


Disruption Budget



IDM UID: 29EHK3


Mitigation is load reduction, but not load avoidance!

Here: photonic radiation flash

Disruption Budget



IDM UID: 29EHK3


Disruption Budget



IDM UID: 29EHK3

Disruption rate and mitigation targets




IDM UID: 29EHK3





IDM UID: 29EHK3


Lifetime consumption




IDM UID: 29EHK3

Mitigation will be required at still low energies and currents

Mitigation at energies still far away from targets exceeds critical temperatures

RE avoidance at high current is not yet confirmed (even stronger avalanche

multiplication than in ITER)

Alternative mitigation methods for RE can still be taken into account

Prospects for Mitigating Disruptions in DEMO

Disruption Mitigation in ITER

Mitigation will be required at still low energies and currents

High confidence in mitigation of thermal and EM loads through particle injection

Mitigation at high energies is likely to exceed critical temperatures

RE avoidance at high current is not yet confirmed

Effectiveness of particle injection for RE energy dissipation questionable

Conclusions



IDM UID: 29EHK3

ITER is the Nuclear Facility INB no. 174.

The views and opinions expressed herein do not necessarily reflect those of the

ITER Organization.

This publication is provided for scientific purposes only. Its contents should not be

considered as commitments from the ITER Organization as a nuclear operator in

the frame of the licensing process.

Disclaimer

Loads due to disruptions and prospects for mitigation

Documents

Transcript of Loads due to disruptions and prospects for mitigation