14 th pedestal and edge physics itpa topical group meeting

14th Pedestal and Edge Physics ITPA April 30 – May 2, GA, San Diego P. T. Lang et al. 1/31

14th Pedestal and Edge Physics ITPA Topical Group Meeting

Status of the pellet ELM triggering investigations at JET and ASDEX Upgrade and possible conclusions

for ITER

● ELM control: AUG & JET status● Can pacing like in AUG be maintained in an ITER like parameter regime at JET?● From JET to ITER: how to scale up the required local perturbation?● ELM features and directly pellet driven MHD perturbation: is there ample of headroom?● Three different scenarios● A silly idea?● Summary & Outlook

AUG

P.T. Lang et al (V1.5.)


ELM control status: pacing demonstrated at AUG

Full control for fPel > 1.5 x f0, raising fELM reduces WELM

Triggered and spontaneous type-I ELMs are essentially identical if they have the same frequency

AUG


ELM control status: pacing demonstrated at AUG

Pellet pacing can break the intrinsic relation between edge parameters (n,T,ν*,…) and fELM

fELM and hence PELM become control parameter

AUG


ELM control status: triggering demonstrated at JET

Pellet triggers ELM also already by local perturbation

pacing will work at JET as well

JET


ELM control status: AUG – JET – ITER step ladder

Pellet pacing works at AUG up to about fPel/f0 = 2 and will also work at JET, but will it still hold on at

- ITER parameters (fPel/f0 = 10, ν*Ped,… )?- ITER size (“size dilution of perturbation”)?

The first task will be tackled in the “Pellet ELM pace making” and “Pellet ELM pacing scenario integration”

experiments this year in JET,the second is subject to several dedicated experiments and analyses but should be investigated by theory and

modeling (XGCO?) as well!


“ITER-scenario” at JET:the new HFPI & 3 launch sites

Pelin Injector(Tore Supra

design)

Selectors

Valves

Main support frame

Microwave cavities

Tracks

Collector

Fuelling: 64 mm3 pellets at up to 15 HzPacing: 1.5 mm3 (~ 1020 D) at up to 60 Hz

V

LH

JET


Pacing at JET:still some convective losses expected

When doing pacing at JET, due to the macroscopic pellet sizeit will still come with noticeable fuelling… ~ 5 x 1021 D/s could cause Δne/ne ~ 0.1

…and probably with some confinement loss as well

AUG

AUG JET


Pacing at ITER:convective losses to be expected?

AUG: mm pellet size and high rates required for pacing do cause strong fuelling and hence confinement reduction (20 - 25 %)

JET: same pellet parameters are expected to have only a very modestimpact on fuelling and confinement (2 – 10 %)but already at ITER like fP/f0!

ITER: same pellet size but lower required rates would have no significant fuelling and confinement impact ( < 1 %)

But the question is: can such pellets still trigger ELMs?


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

Local perturbation imposed by pellet particle deposition is strong enough for triggering at AUG and JET,

but does this still hold at ITER size?

Perturbation amplitude is determined by local parameters,

reduction with plasma size below a critical level?

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


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

There are some indications that perturbations created by the pellets used at AUG and JET are far stronger than required

for just “get off triggered” an ELM

- Trigger at any time within the natural ELM cycle-“Explosive” growths following pellet trigger

- “Saturated” MHD perturbation monitor

Our hypothesis: ELMy H-mode edge always non-linear unstable with mm size pellets creating abundant

perturbation magnitude to kick local conditions into the explosive growth regime


Trigger at any time within the natural ELM cycle

AUG

“Perturbative” ELM triggering: “scan” elapsed time to previous spontaneous ELM

AUG


Triggered ELM: “explosive” growth kept over different ELM regimes

Triggered ELMs remain fast even in regimes the spontaneous ones do not

Type-IIIType-I

AUG


Triggered ELM stay “explosive”: radiative edge cooled type-I

Hot edge type-I: triggered/spont. ELMs virtually identical Cooled edge: triggered keeps characteristics of its regime, but keeps fast hot-edge type-I MHD onset

AUG


“Saturated” perturbation?: pellet and ELM driven MHD response

Taking the MHD monitor as perturbation monitor: does it make sense?

Indeed it is found the pellet creates “direct” MHD activity (declining rapidly after the pellet has burn out) stronger that those at the onset of an ELM: the method seems to be at least plausible

≈ 50 µs

AUG


Pellets in OH:what drives the MHD ?

Variation of pellet mass and/or velocity – changes magnitude of MHD amplitude…

AUG


… but neither pellet ablation nor deposition correlate with the MHD amplitude.→ Local pressure perturbation seems to play (no more) role

Pellets in OH:what drives the MHD ?

AUG


Pellets in OH: impact on MHD already “saturated”

The only correlation is found between penetration depth and MHD amplitude.→ no more effect of stronger pellet perturbation, reached “saturation” level

AUG


Pellets in OH:correlated MHD activity

Coherent MHD activity (drift Alfvén turbulence driven TAE, p ≈ 0.8) in ohmic phase, enhanced by a factor > 100 reaching ELM onset level during pellet

AUG


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

The large, slow pellet “survives” the ELM it has triggered and the directly driven MHD becomes visible

JET


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

JET

Directly driven MHD? -vanishes with pellet burn out. -when the ELM is triggered drops below the resolution.

Remind: -pellet changes the magnitude of some local parameters -compare to e.g. Δx/x << 1 from fluctuations triggering spontaneous ELM


Required ELM trigger perturbation: possible conclusions for ITER

Assumption: the magnitude of pellet imposed perturbation required to provide a suitable ELM seed structure is quite massive

Consequence: ELM pacing at ITER would be feasible indeed almost without any adverse effects

Detailed investigations at JET can confirm or challenge this assumption further (the final answer has to wait anyway for ITER), but for the moment we can regard it as one (plausible?) option.

For operational safety reasons, we should not only rely on.


Considering 3 different scenarios: required pellet penetration

Gas jet: close to and slightly beyond LCFS not sufficient

Pellet: mid-pedestal to top-pedestal, arrives at 100% trigger fraction around pedestal top

→ Assume penetration to pedestal top is requiredfor any scenario


Considering 3 different scenarios: required pellet mass

Using Hybrid-LLL-Code (K.Gál), take ITER profiles and pellet velocity;

"Tailor" pellet mass to achieve pedestal top penetration

ITER


Considering 3 different scenarios: max. safety to max. performance

Pellet velocity (and path) to optimize100 m/s: this represents the slowest technical feasible injection speed, imposing the maximum possible perturbation and hence the safest option

for ELM triggering. This option is considered as pessimistic suitable. “P”

500 m/s: assuming smaller perturbations can trigger ELMs, this option stands for the best one available with the currently planned pellet system.

Therefore, consider it as the optimistic standard one. “OS”

1000 m/s: assuming smaller perturbations can trigger ELMs, this option stands for the best one available with an upgraded pellet system allowing for straight injection (no significant bend in the guiding tube) at the horizontal mid plane from the LFS. Therefore, this 1000 m/s case can be

considered as the optimistic advanced one. “OA”


Considering 3 different scenarios: pellet particle fluxes

Taking the (straight LFS injection: adapted!) values from the code runs:

mP → Pessimistic: 35 * 1020 D Optimistic Standard: 10 * 1020 D Optimistic Advanced: 1 * 1020 D

Taking a 40 Hz pellet rate (A. Loarte: WELM < 1 MJ)

ΓP →

Pessimistic: 140 * 1021 D/sOptimistic Standard: 40 * 1021 D/s Optimistic Advanced: 4 * 1021 D/s

This was the required input for a pumping system assessment,but what is of coarse also of great interest is …


Considering 3 different scenarios: loss power due to convective losses

… is the impact of this particle loss on the confinement.

AUG JET

In steady state conditions

ΓP = Γloss the pellet fuelling (and pacing!) operational area boundary can be explained by taking into account the convective heat flux carried by the extra pellet particle flux

Ti = Te → Ploss = 3 ΓP kB <T>


Considering 3 different scenarios: loss power due to convective losses

Assuming <T> = 3 keV one gets (of course not self consistently)

Ploss ≈ Pessimistic: 200 MW Optimistic Standard: 60 MW Optimistic Advanced: 6 MW

showing immediately how severe the impact on confinement could be.A more profound statement requires a decent self-consistent modeling of the localized particle deposition and enhanced transport (forced losses!).But it is already evident from such a (stupid) estimate that we need at least optimism, even better advanced optimism,or a silly idea…


Considering scenario 4: Be pellet injection

The “Optimistic Advanced” scenario has two shortfalls:- new pellet track required- high launch speed might be troublesome

Such a high speed should be easier achievable by a solid state pellet, and (higher evaporation energy) a much smaller pellet size is sufficient as well.

Simulation with a C pellet (K.Gál has code available) indicate 1 * 1019 Be (a Ø 500 μm sphere) would be sufficient.

Now of course the fuel dilution is the main concern…


Considering scenario 4: Be pellet injection

Fuel dilution as the main concern:

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

Taking a plasma particle content of 1 * 1023 e,1 s confinement time (edge & ELM), 40 Hz rate,this causes a ΔZeff ≈ 0.01

The approach would further need the demonstration of- ELM triggering by a Be (C) pellet- Pellet transfer through a tube

Thinking of supersonic gas jet or LBO instead?Does not trigger prompt ELMs…


Supersonic gas jet, LBO: no local/prompt trigger

Insufficient penetration and/or too dispersed perturbation:only trigger via change of global plasma parameters (no sense)

AUG

JET


Summary & Outlook

● ELM pacing demonstrated at AUG, local triggering at JET● Any pellet in ELMy H-mode triggers instantly with fast growth

● Pellets create a perturbation visible on MHD monitors stronger than according one present at ELM onset

● In pellet parameter regime applied the MHD perturbation is saturated, no correlation with local pressure change

● Indication for “massive oversized” pellet perturbation with respect to trigger requirements, optimization headroom left

→ Demonstrate ELM pacing in ITER parameter regime

→ Allow scaling up by physics insight

→ Investigate technical headroom left (“aggressive” pellets)


Pellet triggered ELMs:review of type-I results

Every pellet triggers an ELM (at any time within the natural ELM cycle!), but we could not find striking differences between triggered and spontaneous ELMs,neither with respect to temporal (fast onset) nor spatial (statistical onset position) structure, onset within ≈ 50 µs after imposed perturbation inside ETB, onset visible at any poloidal and toroidal location within ≈ 20 µs. [G. Kocsis et al., Nucl. Fusion 47 (2007) 1166]

PelletSpontaneous


Pellet triggered ELMs: edge radiative type-I

During control phase here fP = 44 Hz, fELM ≈ 80 Hz

mixture of spontaneous and triggered type-I ELMs

A.Kallenbach et al.,J. Nuc. Materials Vol. 337-339, 732-736


ELMs features: comparison hot/type-I to cold/type-III onset

Triggering from "quiet" phase hard to find, at high fELM impossible, but few clear phases show rapid onset like for type-I ELMs.

Triggering recovers the prompt / fast onset of type-I again!


Beyond baseline scenarios: pellets in quiescent H-mode plasmas

Spontaneous type-I ELM: clear burst like events in MHD and out flux of energy

Pellet in QH: burst like MHD of small size, no significant out flux of energy (small fuelling induced effects)

Pellets to increase density in QH mode [W. Suttrop et al., NF 45, 721]


Trigger potential in H-regime:results

Every pellet launched into a type-III, ELM-free and type-I plasma triggers a prompt ELM at any time in the natural ELM cycle -> edge is never stable against strong seed perturbations

The growth time of the triggered ELM is always as fast as that of a hot edge type-I ELM, the fastest onset time observed anyway-> triggered and spontaneous type-I ELMs evolve with the fastest observed onset times anyway, presumable in the "explosive non-linear" regime characterizing the natural evolution in a "deeply unstable" regime or the forced evolution with a strong seed perturbation kicking the edge locally in such a regime.

QH (and RMP) plasmas with H-mode confinement seem to be in an edge stable (or saturated instable) regime, even a hard external perturbation cannot cause further mode activity (beyond the transient directly driven one).

Pellets can probe edge stability (but careful, they can also change it!)Pacing requires fast small pellets, just reaching the pedestal top.


Features of MHD:trigger of n=3, m=2 NTM

H. Zohm et al.,Nucl. Fusion 41, 197

Pellet fuelling creates globaltrigger conditions (ρ*, ν*)and then triggers NTM=> Almost perfect method to map out the onset boundary


Features of MHD:trigger of NTM

Pellet with low frequency onset and "lethargic" phase.Pellet created seed island with low rotation in lab frame

Unlikely pellet or plasmoid hits q=1,5 surface

Mode grows several 10 ms on transport time scale

NTM triggered, but local and prompt triggering cannot be claimed


Features of MHD:pellet trigger of n=m=1 snake AUG


Features of MHD:trigger of n=m=1 snake JET

Again growth on time scale of several 10 ms,so prompt local seed perturbation cannot be claimed

J.A. Wesson.,PPCF 37, A337


Summary of pellet trigger features:ELM specific and general

Local pellet impact creates saturated seed perturbation, then growing at maximum speed even under conditions (type-III, radiation) spontaneous

ELMs show significantly slower growth,

provided edge can be destabilized

- not e.g. in QH and RMP phases.

Triggering of MHD instabilities seems to be common feature, but this cannot be claimed for the

initial local perturbation.

14 th pedestal and edge physics itpa topical group meeting

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