a. kirk, itpa pedestal meeting, ga, 30 april 2008 1 h-mode pedestal characteristics on mast a.kirk,...

1A. Kirk, ITPA Pedestal meeting, GA, 30 April 2008

H-mode pedestal characteristics on MAST

A.Kirk, T. O’Gorman, R. Scannell

• Addition of new data at low pedestal collisionality

• New data from LFS Yag system

• Comparison of SND with CDN shots


Recap: Inboard versus outboard ne widths

Inboard and outboard widths in mm similar far from ELM

ne pedestal width is not a function of N alone

Can be explained if density pedestal width determined by neutrals

If we mapped inboard data to

outboard we would get the wrong answer

But what about the temperature/pressure width ?


Recap: Comparison of HFS and LFS Teped width

The outboard temperature pedestals are approaching the limit of thespatial resolution of the system and care is needed


New results on CDN discharges

• Looking at more shots from the Ruby system

• Using the new edge Yag data


New Ruby data on Inboard versus outboard widths

Again confirm that the density pedestal width is NOT a flux

surface quantity

Temperature pedestal width:

error bars are too large to be certain

Some indication that different to density dependence


Edge Thomson Scattering on MAST

Edge TS System Optics

Core System View

Edge System View

Core System Optics

Laser Beam Path

• Edge TS system views the laser from 1.25-1.45m

• The outboard pedestal and SOL region

• Uses the same lasers as the core system but different viewing optics

• Scattered light from 16 laser centres is transmitted to 8 spectrometers via fibre optics


Comparison of Yag and Ruby at LFS

New Yag system allows measurement of lower temperature pedestals on the outboard side


Comparison of HFS(Ruby) with LFS(Yag)

• As reported previously the density pedestal is wider on the LFS

• Temperature looks in good agreement but need statistics to be sure



Good agreement in the pedestal heights measured at the HFS and LFS



Confirms that the density pedestal width is NOT a flux surface quantity



BUT the temperature pedestal width does seem to be a flux surface quantity


Ratio of the temperature to density pedestal width

This factor of two would be sufficient to allow Te to be a flux surface quantity


What about the pressure pedestal?

The HFS and LFS pressure pedestal heights are similar

The ratio of the widths is consistent with the pressure being a flux surface quantity


What about the pressure pedestal?

The HFS and LFS pressure pedestal widths are similar in flux space –in spite of the fact that the density pedestal widths aren’t!!

Possible because the density and temperature barrier locations are different


Pedestal heights and dependencies


Pedestal heights

New low collisionality branch- “type IV ELMs”

The operational space in CDN discharges has been greatly expanded – including a

T Osborne, EPS 1997

DIII-D data

Te

PE

D(k

eV)


Pedestal width dependencies

The “type IV” ELMs have a different pedestal width dependence

- Generally on MAST Te Teped

- The “type IV” have a relatively narrow pedestal width that does not change with Te

ped


SND Discharges

For CDN no mapping required as ZTS~Zmag

For SND need to take into account that Zmag ~ 0.85*Z=0


SND vs CDN discharges – Pedestal characteristics

The SND discharges are at a higher Teped

and predominantly in the type I regionTe increases linearly with Te


SND vs CDN discharges – Pedestal width

For the SND discharges the density pedestal width IS a flux surface quantity

Ruby data


SND vs CDN discharges

Widths on the LFS are similar the difference is at the HFS

Is the HFS being modified through the SOL?


Pedestal width scalings


Scalings – banana orbit

As previously observed the MAST pedestal widths fall below the banana orbit scaling


Scalings – pol or pol?

Originally JT-60U had claimed that the temperature pedestal width scaled with *

pol

More recent experiments on JT-60U comparing pedestal widths in H and D plasmas have shown that it is really a pol scaling

BUTpol and pol are correlated


Scalings on MAST – pol or pol

pol and pol are correlated but the low ne high Te branch and the SND shots allow

some separation


Scalings – pol or pol

Using CDN data only and choosing similar pol and different pol

Width depends on pol for the same *pol


Using CDN data only and choosing similar pol and different pol


No positive correlation with *pol


Adding SND data and choosing similar pol and different *pol


No positive correlation with *pol


Summary

1) In CDN shots

• the density pedestal width is not a flux surface quantity but the temperature pedestal width is!

2) In SND shots

• Both the density and temperature pedestal widths are flux surface quantities.

This suggest that in addition to neutrals playing a role the SOL helps to determine the density pedestal width

3) The temperature pedestal width scales with pol rather than *pol.

a. kirk, itpa pedestal meeting, ga, 30 april 2008 1 h-mode pedestal characteristics on mast a.kirk,...

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