DSOL ITPA meetingW.Fundamenski Avila, Spain, 7-10/01/08 Integrated power exhaust strategies for...

DSOL ITPA meeting W.Fundamenski Avila, Spain, 7-10/01/08

Integrated power exhaust strategies for ITER:

a few remarks on the problem

W. Fundamenski (UKAEA, EFDA-JET)

with contributions from V.Philipps, G.Matthews, S.Brezinsek, A.Loarte, F.Sartori,

C.Lowry, P.Lang, Y.Liang, T.Eich


Ignition vs. exhaust criteria for a reactorIgnition vs. exhaust criteria for a reactor

In order for any exothermic reactor to operate in steady-state,

(i) fresh fuel must be added at the rate at which it is consumed,

(ii) this fuel must be heated, ideally by the reactions themselves,

(iii) fuel must be confined, by whatever means are available, for sufficiently long to allow the exothermic processes to continue,

(iv) the energy and ash must be removed from the system at the rate at which they are created,

(v) the impurities released from the reactor walls during this exhaust process must not inhibit the ignition (burn) of fuel, and

(vi) the reactor itself, primarily its walls, must not be damaged by all the exhaust processes.

Conditions (i)-(iii) represent ignition criteria, conditions (iv)-(vi) as exhaust criteria.

Taken together these constitute the criteria of mutual compatibility between the reaction processes and materials/components in an exothermic reactor

Since the latter provide the boundary conditions for the thermodynamic quantities, they effectively determine the maximum achievable energy gain for a given reactor design, i.e. a give combination of fuel and hardware.


Ignition vs. exhaust criteria for ITERIgnition vs. exhaust criteria for ITER

In order for ITER to operate in steady-state,

(i) D & T must be added (NBI & pellet fuelling) at the rate at which they are consumed

(ii) D & T must be heated by RF, NBI and dominant heating

(iii) D & T must be confined for sufficiently long to ensure Q = Pfus / Pheat = 10

(iv) The (neutron, photon and plasma) power and He ash must be removed from the system at the rate at which they are created

(v) the Be, C & W impurities released from the reactor walls must not inhibit the continued burn (Q=10) of D & T by either dilution and/or radiation,

(vi) The plasma facing components must not be damaged by the above processes.

e.g. joint requirement of partial detachment of both divertor legs (to reduce the inter-ELM heat loads below ~10 MW/m2), and ELM-mitigation (to reduce the ELM transient energy loads below ~1 MJ/m2).

(iv)-(vi) effectively impose the boundary conditions for the plasma density and temperature, and hence determine the maximum achievable Q in ITER, for a given set of internal and external hardware, i.e. TF, PF and control coils; PFCs; heating, fuelling, pumping and cooling systems, etc.


Ignition and MHD stability limitsIgnition and MHD stability limits


Power exhaust limitsPower exhaust limits


MHD stability, ignition & exhaust MHD stability, ignition & exhaust limits limits


energy density / MJm-2

0.5 1.0 1.5

neg

ligib

leer

osi

on

mel

tin

g o

f ti

le e

dg

es

mel

tin

g o

f t

he

fu

ll ti

le s

urf

ace

(no

dro

ple

t e

ject

ion

)

dro

ple

t ej

ecti

on

and

bri

dg

ing

of

tile

s a

fter

50

sho

ts

W

energy density / MJm-2

0.5 1.0 1.5n

eglig

ible

ero

sio

n

ero

sio

n s

tart

sat

PF

C c

orn

ers

PA

N f

ibre

ero

sio

n o

ffl

at s

urf

aces

afte

r 10

0 sh

ot

sig

nif

ican

tP

AN

fib

reer

osi

on

afte

r 50

sh

ots

PA

N f

ibre

ero

sio

naf

ter

10 s

ho

ts

CFC

Transient heat load limits for ITERTransient heat load limits for ITER

ITER adopted a value of 0.5 MJ/m2 for the maximum allowed ELM energy load


Using best estimates for divertor wetted area and in-out asymmetry, one finds

WELM = QELM x Sin x (1 + Pout/Pin) = 0.5 MJ/m2 x 1.3 m2 x 1.5 ~ 1 MJ

Assuming Wdia ~ 400 MJ and Wped / Wdia ~ 1/3, then WELM/ Wped < 1%

This requires a decrease in the ‘natural’ ELM size by a factor of ~ 20

Some caveats, e.g. the above assumes that the simulated ‘ELM’ pulse shape in the plasma gun is the same as the real ELM pulse in a tokamak

Maximum permitted ELM size for ITERMaximum permitted ELM size for ITER

1%2 %


Determination of divertor ELM power flux time dependence

2

,

2

,

2

,,, exp1)(

tttAtP oioioioioi

more than 60% of WELM,div arrives after qELM,divmax smaller Tsurf

ELM

This inherent skewness could allow for a moderately larger ELM load (~ 30% higher)

W. Fundamenski

AUG-Eich

T.Eich

JET-T. Eich

Divertor heat loads due to ELMsDivertor heat loads due to ELMs

TRINITI plasma gun

(from A.Loarte)


Steady and transient wall loads on ITERSteady and transient wall loads on ITER

Parallel (װ)

Perpendicular (┴)

~1°

Start-up & Ramp-down װ MW 15 15 8

Type of Interaction Units 2001 PID Latest Proposal

Outer Midplane

Radiation ┴ MWm-2 0.5 0.5 0.5

Power Conducted between ELMs

װ MWm-2 None None 3

Power Conducted by ELMs

װ MWm-2 None 0.93 2.5 (3.4)

Energy Conducted by ELMs

װ MJm-2 None 0.19 (0.93)

0.06 (1.7)

Near second X-point

Radiation ┴ MWm-2 0.5 0.5 0.5

Power Conducted between ELMs

װ MWm-2 None None 5

Power Conducted by ELMs

װ MWm-2 None 3.8 33

Energy Conducted by ELMs

װ MJm-2 None 0.77 (3.8) 0.8 (17)

Baffle Region

MARFE ┴ MWm-2 1.3 1.4 0.6-5

-4

-3

-2

-1

0

1

2

3

4

5

3 4 5 6 7 8 9

Z, m

R, m

ITER-FEAT

q=1.5

q=2.0

q=2.5

q=3.0

Scenario #2


– On ITER, ELM size can be reduced by a combination of• pellet pacing

– Necessary tool for deep (pedestal) plasma fuelling• external magnetic field perturbation (EFCC coils, RMP coils?, TF ripple)

– 6x3 EFCC coils envision for error field correction– TF ripple in the range of 0.2-0.5 %

• magnetic pacing (vertical kicks)– Not clear if suitable to ITER at present

• impurity seeding (Type-III ELMs)– In the absence of C as radiator with Be/W mix, impurity seedind necessary for partial

detachment

– For all the above techniques it is imperative to determine • The maximum reduction in ELM size (increase of ELM frequency)• Associated reduction in confinement (H98) and fusion gain (Q)• Associated increase in inter-ELM heat loads (q_div, Te_div, Ti_div, q_lim) and any

detrimental effects on divertor plasma detachment• Associated increase in core plasma pollution (Z_eff)• Synergistic effects

– all the above processes (pellets/EFCCs/TF ripple/seeding) concurrent on ITER– Quantitative prediction beyond our abilities in the near future

ELM control and effect on inter-ELM loadsELM control and effect on inter-ELM loads


fPel > 1.5 f0ELM

ELM size reduction by pellet injectionELM size reduction by pellet injection

Type-I ELM frequency can be increased by injection of small deuterium pellets, provided that pellet freq. > 1.5 natural ELM freq. (results from AUG)• Can the effects of plasma fuelling and ELM pacing be decoupled?

• Can ELM pacing be demonstrated at N_GW ~ 0.75?


fELM ↑ ~ 15 Hz ~ 40 Hz

Te ↓

~ 650 eV ~ 250 eV

n_e ↓(pump-out)

T_e ↑(not fully understood)

H98 ↓ (~ 0-20 %)

12 14 16 18 20Time (s)

PNBI

(x10 MW)IEFCC

(x16 kAt)

ne,l (x1020m-2)

D(a.

u)

JET #70472 Bt = 1.85 T / Ip = 1.6 MA/ q95=4.0

24 kAt

Te

(keV)

core

core

edge

edge

ELM size reduction by EFCC coils with n=2ELM size reduction by EFCC coils with n=2

Type-I ELM frequency can also be increased by introducing steady

state n=1 or n=2 fields

What is the change in confinement when the magnetic pump out is

compensated by external fuelling? What is the effect in impurity seeded,

highly radiative plasmas?


D

FRFA current

kick

Wdia

ne edge (LID4)

Te edge

ne core (LID3)

70426, 2MA 2.35TNatural ELM frequency (~ 5 Hz) increased to ~ 10-25 Hz

At freq. > ~35 Hz kicks do not always trigger an ELM

Small reduction of Wdia and pedestal quantities: ne , Te

Promising technique for ILW, in which case the ELM size need only be reduced by ~ 2-3 times

ELM size reduction with vertical kicksELM size reduction with vertical kicks

Type-I ELM frequency can likewise be increased by fast changes (vertical kicks) to magnetic equilibrium


Best pulse at 2.75MA/2.2T, high • frad=0.75, Zeff ~ 1.5 – 2, N-seeded

• H98(y,2) ~ 0.83 (~ 17% degradation)

• N ~ 1.9, * reduced by ~ 2.5

• Both divertors detached !

At present the only scenario compatible with ITER requirements of 1% ELM energy loss + partial detachment, although at the penalty of confinement degradation of 15-20%, which yields Q ~ 4-5 @ 15 MA.

Q=10domain

45.0295

89 q

H N

Zeff=1.7

Old results

Impurity seeded, highly radiative discharges Impurity seeded, highly radiative discharges

Finally, ELM frequency can increased substantially (factor of ~ 10) by affecting a I-III transition, that is, by replacing Type-I by Type-III ELMs.


ELM size, as well as H98, decrease with increasing TF ripple– For the same *ped, WELM/Wped decreases by factor of two when TF ripple increases from 0.1% to 1%

– Change related to smaller conductive loss (Te/Teped), rather than convective loss (n/nped)

– The reduction must less pronounced at higher density, i.e. close to N_GW ~ 1.

Effect of TF ripple on ELMsEffect of TF ripple on ELMs


THE END


Plasma compatibility issues

1. The risk of W contamination of (fuel dilution in) core plasma

2. Reduced edge and divertor radiation (in the absence of Carbon)

3. Hence, the need for impurity seeding to replace Carbon as the main radiating species

Energy and power limits

1. Main chamber PSI, mainly during transients

2. Divertor steady state & transient loads

3. NBI shine-through

4. Special effects associated with RF power (ICRH and LH)

Some lessons from Be/W wall on JETSome lessons from Be/W wall on JET

The presence of Be on main wall (limiters, dump plate) and W in the divertor (especially W-coated CFC tiles) imposes new limits on plasma scenarios


Maximum coating test temperature in cyclic loading (200 pulses): 1600C

• W-C carbide formation starts at about 1000C (exponential increase), “carbidisation” of the W layer should be avoided

• W-C carbides have lower melting point, less ductility, and release C

W-carbide

C

W

To have some margin for ELMs, Tmax should be below 1600C ( 1200C)

Surface temperature is limiting in most cases (presently set by energy limits given by metallic base structures)

Preliminary heat load tests in Judith simulator on 200m VPS (2000 Elms, 1 ms) found power limit of ~ 0.3 GW/m2 (T. Hirai): ongoing analysis about failure mode at higher loads

Recommend maximum transient heat load ~ 0.2 GW/m2

Transient heat load limits for W-coatings on JETTransient heat load limits for W-coatings on JET


Using best estimates for divertor wetted area and in-out asymmetry, one finds

WELM = QELM x Sin x (1 + Pout/Pin) ~ 0.2 GW/m2 x 500 s x 1 m2 x 1.5 ~ 150 kJ

For typical JET stored energies of Wdia ~ 5 MJ and Wped / Wdia ~ 1/3,

which translates into WELM/ Wped < 9 % (less for larger Wdia).

This requires a decrease in the ‘natural’ ELM size by a factor of ~ 2

Maximum ELM size for W-coatings on JETMaximum ELM size for W-coatings on JET

9%


Another possible methods of estimating maximum ELM size is based on maximum tile temperature rise, and is thus dependent on tile temperature.

This gives somewhat higher limit for cold tiles (~ 250 kJ with safety margin).

Maximum ELM size for W-coatings on JETMaximum ELM size for W-coatings on JET


W erosion proceeds by physical sputtering with an ion energy threshold (for deuterium ions) of ~ 150 eV, or T_e_div ~ 150 / 5 eV ~ 30 eV.

• Erosion of bulk W plate is not a critical issue for its life time

• Erosion of W coatings, might be an issue, especially for the thinner coatings, which pose a high risk of gradually revealing the C substrate

Need to find an optimum between cooling the divertor plasma (< 30 eV) to reduce the erosion of W and introduction of seeding impurities which can themselves increase erosion (lower energy threshold for higher Z)

Need to aim for partial detachment (T_e_div < 5 eV) at both divertor legs !

Maximum T_e_div for W-coatings on JETMaximum T_e_div for W-coatings on JET


Shinethrough Area

Restraint ring protection

Be limiter Surfaces moved forward 3cm (W- coated CFC recessed)

Be coating of inconel

10μm W - coated CFC recessed

Bulk Be ribbed Dump Plate

ITER-like wall (ILW): Be wall ITER-like wall (ILW): Be wall


Bulk W tile

W coatings

W coatings:

• 200 m VPS (Plansee) selected at first but more R&D show that thick VPS on CFC may not be reliable enough

• change to 14 m thin Re-W multilayer coating is very likely (to be decided January 2008)

Tungsten (>99.95%)ZrO2

Inconel 625

Inconel 706

TZM spacers (coated with Al2O3)

optionalcopper inserts

8000 W- lamellas

ITER-like wall (ILW): W/W-CFC divertorITER-like wall (ILW): W/W-CFC divertor


0 2 4 6 8 10 12300

600

900

1200

1500

1800

2100

Time (s )

Tem

p (

K)

Tungsten (>99.95%)ZrO2

Inconel 625

Inconel 706

TZM spacers (coated with Al2O3)

optionalcopper inserts

Surface Temperature limits:

• Steady state

• Transient (ELMs)

Energy limit for metallic for substructures

7.5 MW/m2 on JET CFC

Steady state

Elms

Power and energy load limits for ILWPower and energy load limits for ILW


0 2 4 6 8 10 120

5

10

15

20

25

30

35

40

Shot length (s)

Po

wer

(MW

/m2)

+INF

Tmax<1200C

forbidden

Start T = 220C

0 2 4 6 8 10 12300

600

900

1200

1500

1800

2100

Time (s )

Tem

p (

K)

Tungsten Graphite Beryllium

Similar thermal response for W, C and Be

7.5 MW/m2

Maximum coating temperature 1600C 1200C for ELM window

Steady state heat load limits for ILWSteady state heat load limits for ILW

The limit on the steady state heat load determined by the maximum allowed coating temperature, heating power, radiation, SOL width and shot duration


ELM affected area on JET ~ 1 m2. Hence, above increase occurs at ~ 0.7 MJ/m2, although the increase of radiation associated with ablation of surface layers, rather than bulk CFC

A. Huber/R. Pitts

JET experiments at high Ip with ITER-like values of ELM size up to 1 MJ

ELM induced material loss on JETELM induced material loss on JET


ELM energy deposition at main chamber given by competition of parallel and perpendicular transport and filament size + detachment dynamics

Do larger ELM filaments travel faster? What is their spatial structure?

T. Eich/W. Fundamenski/R. Pitts

Main chamber heat loads due to ELMsMain chamber heat loads due to ELMs

DSOL ITPA meetingW.Fundamenski Avila, Spain, 7-10/01/08 Integrated power exhaust strategies for...

Documents

Transcript of DSOL ITPA meetingW.Fundamenski Avila, Spain, 7-10/01/08 Integrated power exhaust strategies for...