evolution of the pedestal on mast and the implications for elm power loadings

A. Kirk, 21st IAEA Fusion Energy Conference, Chengdu, China, October 2006

Evolution of the pedestal on MAST and the implications for

ELM power loadings

Presented by Andrew KirkEURATOM / UKAEA Fusion Association

UKAEA authors were funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by the European Communities under the contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission.


ITER

Size of type I ELM Energy Loss

Understanding the size of ELM energy loss on future devices such as ITER is essential

Most predictions are based on scaling data from present devices

There is a large uncertainty due to our lack of understanding of the processes involved.


• Extension of MAST operational space to lower pedestal collisionality

• The filament structures observed during type I ELMs and their relevance to the ELM energy loss process

• Working towards a model for ELM energy losses

Outline


Contribution from MAST

Most MAST discharges have a high pedestal collisionality due to a low temperature pedestal and the energy losses due to ELMs

are up to 5 % of the pedestal energy

MAST data


Contribution from MAST

New data

A dedicated campaign using optimised fuelling and vessel conditioning has been used to increase the temperature

pedestal and extend the MAST data set to lower collisionalities

Agrees well with the scaling observed on

other devices


Pedestal profiles

The lowest pedestal collisionality shot (*ped ~ 0.08) produced to date has:

31910)05.062.2( mn pede eVT ped

e 20435 kJW ped 3.13


Stability analysis

Edge stability calculations, using the 2D ELITE code, have been performed on these profiles, which were obtained just before an ELM

A parameter scan shows that the point lies close to the ballooning boundary- Characteristics associated with type I ELMs


Comparison of the pedestal at high and low *

Low * High *

(kPa/N) 29.0 30.3 (N) 0.035 0.017

Nd

dP

Similar pressure gradient but low * case is > 2xbroader


Comparison of ELM losses at high and low *

Low * High *

No TeLarge Te

%12

pedW

W%3

pedW

W

But why are ELM losses greater at lower collisionality ?


What happens during the ELM energy loss process


Spatial structure during type I ELMs

MAST

DIII-DASDEX Upgrade

but what role do they play in the ELM loss mechanism ?

NSTX

At the last IAEA it was known that filament structures exist during ELMs


Studies of the evolution of the filaments during type I ELMs on MAST

MAST has installed new imaging systems and a new edge TS system to study the evolution of

the filaments

10 kHz frame rate


Analysis of the images

3-D field lines are mapped onto the 2-D

images taking into account the distortion

of the system.


Comparison of type I ELMs at different *

Very little difference in the size, number or the behaviour of the filaments

5 s exposure 10 s exposure


Evolution of the filament during an ELM


Reduced views allows the evolution of the filaments to be studied

100 kHz frame rate


Evolution of the filaments

The filaments are observed to rotate in the co-current

direction, decelerate toroidally and accelerate out

radially The width of the filament is ~

constant in time

The toroidal and radial location of the filaments are tracked in consecutive frames


Evolution of the filaments

The filaments exist during the time that particles are lost from the pedestal


Summary of the evolution of ELM filaments

• Filaments exist for the entire “ELM loss” time

• Individual filaments detach from the LCFS at different times

• Follow the local field line, decelerate toroidally and accelerate radially.

• The width perpendicular to the field line is ~ constant in time

But what is the energy/particle content of the filaments?


The energy/particle content of the filaments

0. Pre-ELM pedestal



0. Pre-ELM pedestal

1. Formation of tail



0. Pre-ELM pedestal


2. Radial expansion - no local

loss in pedestal density

Energy in largest filament < 2.5 % WELM



0. Pre-ELM pedestal




3. Filament appears to detach

at mid-plane (leaving a

depression)

Energy in largest filament < 2.5 % WELM



0. Pre-ELM pedestal




3. Filament appears to detach

at mid-plane (leaving a

depression)

4. Accelerates radially –

particle content decreases

exponentially with distance


A possible mechanism for type I ELM energy losses


A possible mechanism for type I ELM energy losses

• Filaments remain close to the LCFS for 50 - 200 s - during this time they enhance transport into the SOL.

• Individual filaments detach during this period - slow toroidally and accelerate radially.

• At the time of detachment each filament contains up to 2.5 % WELM

• As the filaments travel out radially this content is reduced by parallel transport to the targets


Energy lost through the filaments

ELMfilamentfilfilamentN #

Total particles down the filaments

Assume that during ELM the filament acts as a conduit for

losses from the pedestal region

ELM(MHD) ~(A2E)1/3 ~100 s (MAST)

21

841

41

dm

kTncn

Number of filaments Area of filament


Energy lost through the filaments

ELMfilamentfilfilamentN #

Total particles down the filaments

Assume that during ELM the filament acts as a conduit for

losses from the pedestal region

ELM(MHD) ~(A2E)1/3 ~100 s (MAST)

ELM energy loss filped

eped

iconv

ELM NTTkqW )(25

//

filament, #fil and pedestal T and n measured


Results from the simple model

MAST

Given the simplifications and approximations used there is

moderate agreement with the data

But what about other devices?


Results from the simple model

Assume #fil=10

and filament a

Size ~ 20 cm

The next step should be to use our improved knowledge of the ELM loss process to build a “real” model

Extrapolation of model to JET


Summary

• High temperature pedestal plasmas have been achieved on MAST with collisionalities one order of magnitude lower than in previous discharges.

• The structure and evolution of the filaments observed during type I ELMs is similar at both high and low collisionalities

• The evolution of the filaments have been tracked through individual ELMs showing how they remain attached to the LCFS, decelerate toroidally and accelerate radially and at the time of detachment each filament carries up to 2.5 % of WELM

• Based on this evolution a mechanism for ELM energy loss has been proposed and a simple ELM energy loss model derived – the next step is to build a “real” predictive model.

evolution of the pedestal on mast and the implications for elm power loadings

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