R. Wenninger 1 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Status of ELM trigger investigations on JET and AUG R. Wenninger IPP Garching, EFDA

R. Wenninger 1 (35) Sat. Workshop to EPS on Fuelling 16th June 2008

Status of ELM trigger investigations on JET and AUG

R. WenningerIPP Garching, EFDA JET


Outline

• Pellet technology at AUG and JET• Pellets – a candidate ELM mitigation method• Observation of pellet triggered ELMs in H-

mode regimes• Direct pellet driven MHD• Careful considerations towards trigger

mechanism• Pellet impact on tokamak (ITER) operation• Summary


Outline





AUG Pellet Centrifuge

AUG fuelling systemVolume to freeze and compress ice

Extrusion nozzel Extrusion arm (s=0.25mm)

Cutter

VHFS


AUG Blower Gun

• • High frequency: Up to 143Hz

• 2nd injection line for tangential transfer through edge plasma close in or outside separatrix

AUG pacing system


AUG Injection Capabilities

Centrifuge Blower Gun

Species H2, D2, doped D2

Frequency Up to 83Hz Up to 143Hz

Pellet speed240 - 1200 m/s

(limited by looping to 1000m/s)

100 – 350m/s

Pellet size/mass (D2)

~ 2.8 – 7.2 mm3

(1.7 - 4.3 1020 atoms)

~ 1.7 mm3

(1020 atoms)

Reliability 95% 95%

Injection line VHFS LFS (Invest. line)


JET – High Frequency Pellet Injector

High Frequency Pellet Injector (HFPI): • Worldwide 1st system fully optimised for ELM

pacing• Deep fuelling also possible

PELIN pellet injector


JET – Overall Pellet system

Pellet Injection Locations

• Injection from 3 poloidal locations

• Centrifuge – Future option for parallel fuelling and pacing

V

LH


Fuelling ELM pacing

Species H2, D2 H2, D2

Frequency ≤15 Hz ≤60 Hz

Pellet speed 150-600 m/s 50-200 m/s

Pellet size/mass (D2)

~ 64 mm3 (4 1021 atoms )

~ 1.5 mm3 (9 1019 atoms )

Reliability 98% 98%

JET – HFPI Injection capabilities

Values from HFPI specification


Outline





ELM control – a necessary requirementAcceptable ELM size for ITER:

• Target plate erosion negligible at 0.5MJ/m2 for CFC and W (Zhitlukhin 2007)

• Assume strong asymmetry of deposition on inner/outer targets: Pout/Pin = 1 : 2

• Tolerable energy deposition on target plates per ELM: WELM,target=0.5MJ/m2

1.3m2 (1+1/2) 1MJ (Polevoi EPS 2008)

F. Federici et al PPCF 45 (2003)

Reliable ELM control technology mandatory for ITER

• WELM,target fELM = Psep, = 0.2 – 0.4

(Herrmann 2002) fELM = 20 – 40Hz (spont. ~ 2 - 4Hz)


ELM control technologies

• Edge ergodisation by resonant magnetic perturbation (Y. Liang PRL 98 (2007)) Plasma edge rarefaction without cooling (needs pellet fuelling)

• Magnetic ELM pacing by vertical kicks – accelerating plasma in vertical direction (Sartori et al.)

• Impurity seeding Type III (higher frequency) + higher radiation fraction

• Pellet ELM Pacing (P. Lang NF 2004)

• …

DIIID

None of these technologies is yet proven to work at ITER


Pellet ELM Pacing – Proof of Principle at AUG

AUG

JE

T

• fELM ≥ fPEL fELM more than doubled at AUG

• WELMfELM=const at const. Pheat confirmed for pellet triggered ELMs (Lang NF 2002)


Scaling aspects: size, magnitude, location of required perturbation?

• Local perturbation imposed by pellet particle deposition is strong enough for triggering ELMs at AUG and JET (Until now every pellet injected into ELMy regime triggered an ELM)

• But does this still hold at ITER size? • More physics understanding necessary!• Threshold might be defined by

– local (e.g =T, n, p, j,...) and– relative extension (e.g. x/R)

Readjustment might be possible but at the expense of again stronger fuelling (and pumping) and hence convective losses.

x


Outline





Pellet can trigger an ELM at any time between Type-I ELMs

AUG

• ELMs triggered by with feff up to 350Hz (temporal resolution problem in higher frequency)

•Triggered ELM: 50s delay between pellet causes perturbation and ELM

G. Kocsis


Quiescent H-Mode: Pellets don’t trigger ELMs in any H-mode regime

• QH: Obtained by counter injection

• Good confinement (H98y~1)

• High pedestal and core ion temp.

• ELMs replaced by ‘edge harmonic oscillation’ (EHO, ~10kHz) + ‘high frequency oscillation’ (300 – 400kHz)

• Even large pellets do not trigger ELMs

AUG


Pellet can terminate phase free of spontaneous ELMS

• Pellets trigger first ELM ~0.5s earlier than first spont. ELM occurs in reference shot

• Avoid spontaneous Giant ELM (e.g. 1st after ELM free phase)

• Each pellet triggered an ELM of smaller size, with <20% the loss in energy, than the spontaneous Giant ELM

B. Alper 2002

JET

14MW NBI


Pellets lead to fastest ELM-growth (I)

Comparison of growth time of MHD signal up to its max. value (ELM rise time):spontaneous Type I pellet driven between Type I

pellet driven between Type III < spontaneous Type III

AUG


Pellets lead to fastest ELM-growth (II)

Type I

Rad. cooled Type I

Type III

• ELM rise time of pellet driven ELMs const. ( spont. Type I: fastest growth)

• ELM rise time increases from Type I, via rad. cooled Type I to Type III

Correlated e.g. with parallel resistivity at pedestal top

AUG


Outline





Simplistic causal model

Pellet

Perturbation of plasma parameters (n, T, j,...)

Direct pellet driven MHD

ELM

Option 1:

Option 2:

Pellet

Perturbation of plasma parameters (n, T, j,...)

Direct pellet driven MHD

ELM


Magnitude of required ELM trigger perturbation: Tiny in pellet terms?

JET

Magnetic signal for pellet driven ELMs can be separated in• ELM related MHD• Directly pellet driven part

observed even in L-mode stops abrupt with burn out @ trigger time below resolution Direct pellet driven MHD sufficient, if threshold would be > x100, if option 1


OH: Direct pellet driven MHD only

≈ 50 µs

AUG

• Ohmic plasma (OH) Only direct pellet driven component

• Faintest pellet provokes stronger MHD than at typical ELM onset


OH: No sign. Variation of directly pellet driven MHD with pellet parameters

•Repeated use of same stationary scenario

• Averaging of amplitude of dB/dt (MHD) over entire shot for all pellets (about 10)

• MHD clearly correlated to radial position of pellet and thus on local plasma parameters (e.g. p, j, Te, …)

• No significant dependence on pellet parameters (mass, velocity) and thus on ablation / deposition saturation effect?

• Pellet driven MHD depends mainly on plasma parameters

AUG


Outline





Spont. type I and triggered ELMs – No significant difference on level of density fluctuation

• Reflectometry at fixed frequency mode density fluctuation

• Compare frequency power spectra (Integration: 2.5ms) for

• 3 different densities

• LFS and HFS

• Spontaneous and triggered ELMS

No significant difference between spontaneous type I and triggered ELMs AUG


Spont. type I and triggered ELMs – No significant difference in target power load pattern

• Infrared Thermography: Observation of type I ELMs reveals non-axissymmetric stripes on divertor targets mapped to filaments (Eich PRL 2003)• For later ELM phase mode structure can be identified

No significant difference in patterns between spontaneous type I and triggered ELM

AUG


Pellets in the Peeling-Ballooning-Picture

• PB-Theory quantifies ELM dynamics via growth Rate of driving mode

• Type I ELM cycle (Connor 1998)

• Type I ELMs:

Typical av. inter ELM time > ms

• Pellet triggered ELMs:

t(pellet at location – ELM) < 0.1ms

If there is a similarity in the mechanism, pellet must shortcut the type I ELM cycle significantly

J

Stable <C

Instable >C

Transport time scale

Res

istiv

e tim

e sc

ale


Outline





Pellet impact on tokamak (ITER) operation – Penetration depth and required pellet mass

Kocsis 2007

• Cumulative distribution function of trigger locations ~100% at ped. top

Assumption: Pellet penetration to pedestal top is required

• Pellet mass required to reach pedestal top for different scenarios:Reference: width 20cm, Te at top 4keV

Wide pedestal: width 30cm, Te at top 4keV

High pedestal: width 20cm, Te at top 5keV

• Lower speed More mass required but stronger perturbation

• Assumption: Ref. scenario

ITER

K.Gál (Hybrid-LLL-Code)


Pellet impact on tokamak (ITER) operation – 3 injection scenarios

Option v (m/s) Feasibility PerturbationMin. pellet

massAdditional

fuelling rate

Pessimistic (P)

100 Demonstrated Max 351020D 1401021D/s

Optimistic Standard

(OS)500

JET – HFPI at lower mass

Mid 101020D 401021D/s

Optimistic Advanced

(OA)1000

To be demonstrated

– straight injection line!

Min 11020D 41021D/s

Results from Hybrid-LLL code

Assuming 40Hz pellet frequency

X

?ITER total pumping rate: 40 – 50 1021 D/s


Pellet impact on tokamak (ITER) operation – Convective power losses due to pellets

• Pellet particles are heated by the plasma up to 75% Tped,ref

• Steady state cond.: Pellet = add. loss

Padd. loss= 3 Pellet kB <T>

• Assuming <T>=3keV (75% Tped,ref)

Padd. loss 200MW (P)

60MW (OS)

6MW (AO)

ITER total heating power: 40MW

Conclusion:1. Self-consistent modeling of the localized particle deposition and

enhanced transport required for more robust figures2. At least OS is needed – better AO


A new idea – Beryllium pellet injection

• Advanced Optimistic: 1000m/s of 1020D (1.6mm3) extremely challenging

• High speed should be easier for Be pellet (melting point 1278°C, crystal structure)

• Simulation with a C pellet (K.Gál) indicate 1 * 1019 Be (a Ø 500μm sphere) would be sufficient to reach pedestal top

• Fuel dilution: Taking a plasma particle content of 1023e, P=1s, finjection=40Hz :

ΓP ≈ 4 * 1020/s Be, but ≈ 4 * 1021/s expected from wall

Need to demonstrate:

- ELM triggering by a Be pellet

- Pellet transfer through a tube

ITPA Activity


Summary

• JET-HFPI: Milestone in development of pellet injector technology

• Pellet ELM Pacing: Candidate technique for ITER ELM control

• So far every pellet injected into ELMy regime triggered an ELM – not clear, if this holds for ITER

• Pellet can trigger an ELM at any time between Type-I ELMs

• Pellet triggered ELMs grow as fast as the fastest spontaneous ELMs (Type I)

• Pellet driven MHD depends mainly on plasma parameters

• If there is a similarity in the mechanism, pellet must shortcut the type I ELM cycle significantly

• 500 or 1000m/s injection mandatory for tolerable pump load and additional power losses in ITER


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Documents

R. Wenninger 1 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Status of ELM trigger investigations on JET and AUG R. Wenninger IPP Garching, EFDA