ELM mitigation using the recently installed active in-vessel coils
at ASDEX UpgradeT. Pütterich1, W. Suttrop1, L. Barrera1, R. Dux1, A. Herrmann1, T. Eich1, R. Fischer1,Ch. Fuchs1, B. Kurzan1, P.T. Lang1, R.M. McDermott1, A. Mlynek1, S.K. Rathgeber1,M. Rott1, T. Vierle1, E. Viezzer1, M. Willensdorfer2, E. Wolfrum1, I. Zammuto1 andthe ASDEX Upgrade Team1
2

Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-85740 Garching, Germany
Institut für Angewandte Physik, TU Wien, Assoc. EURATOM-ÖAW, A-1040 Vienna, Austria

For the 2011 campaign in-vessel coils have been installed in ASDEX Upgrade which allow theapplication of a magnetic perturbation with a mode number of up to n=2. If the peripheral plasma density
is above a critical value, the n=2 perturbation leads to a strong mitigation of type-I edge localized modes(ELMs). This is found independent of whether the resonant perturbation field is maximized orminimized. The particle confinement increases in ELM mitigation phases, i.e. no density ‘pump-out’ isobserved. Pellets are used to fuel these plasmas up to densities 1.5 x the Greenwald limit with highfuelling efficiency, while also pellet-triggered ELMs are suppressed by the magnetic perturbations.

1. Introduction
In ITER, the type-I ELMy H-mode is foreseen
to be the basic operational scenario. The activityof edge localized modes (ELMs) presents the riskof overloading the plasma facing components.
Thus, a way of reliably suppressing or mitigating
ELMs is required to guarantee the success ofITER and later reactors. One way to do this could
be additional coils close to the plasma that
introduce
non-axis-symmetric,
magneticperturbations.

Fig. 1 Cross-section of ASDEX Upgrade indicating thelocation of perturbation coils and diagnostics

In the present work, results on ELM mitigation
employing such perturbation coils in ASDEX
Upgrade are presented. More details about thedescribed experiments can be found in [1,2].
2. Experiment and Observations
In 2010, eight in-vessel coils, labelled
perturbation coils in the following, were installedin the ASDEX Upgrade tokamak. They aremounted close to the plasma, as can be seen in
Fig. 1. The perturbation coils form an upper and alower ring each of which consists of four coils. InFig. 1, the upper and lower coils are labelled Bucoils and Bl-coils, respectively. The wiring allows
for the application of magnetic perturbations of up
to n=2, while at each toroidal position the Bucoils either create a perturbation field that has thesame direction as that of the Bl-coils (even) or theopposite direction (odd).
First results demonstrate that ELM mitigationis possible in the n=2 configuration. Themitigation is found only above a critical electron
density ncrit. When the coils are switched on andthe plasma density is above ncrit the type-I ELMs
become less frequent and are replaced by smaller
ELM-like instabilities within a few energy
confinement times. The change of ELM type leads
to a small increase of plasma density, while thetemperatures are slightly decreased. No
considerable effect on the stored energy isobserved and neither the high-Z nor the low-Z
impurity levels are changed. All observations sofar suggest that ncrit shows a linear dependence on
the toroidal plasma current which can be
expressed as a minimum Greenwald fraction ofapprox. 0.65. Further parameter dependencies, onsafety factor and plasma shape, are underinvestigation. When perturbation coils are applied
and the pedestal density is above the threshold,type-I ELMs disappear and give way to a minor
ELM-like edge instability. The collapse ofpedestal parameters at each ELM is much reduced
and thus, the power load due to the ELM is rathersmall in both of the divertors. In Fig. 2, thecorresponding measurements are presented. The
application of the perturbation fields leads to anincrease of the baseline power-load at the outer
divertor, while at the inner divertor the baseline
level is slightly decreased. In any case, the large
excursions that are observed during type-I ELMyphases are eliminated by the perturbation fields.

Fig. 2 Comparisons of stored energy, edge density,edge temperature and the divertor power loads for
type-I ELMs and mitigated ELMs

ELM-mitigated phases are characterised by aslight increase in particle confinement and nodegradation of energy confinement as comparedwith the no coil reference phase. No large ELMs
are induced by injected pellets if the perturbation
fields are applied, while with perturbation coils
off ELMs are triggered as generally observed in
ELMy H-mode [3]. A train of pellets has been
employed to push the plasma density to 1.5 times
the Greenwald density, while type-I ELMs
remained mitigated by the application of theperturbation fields.
The access condition for the ELM-mitigatedregime is not found to be dependent on therelative alignment of magnetic field lines and theperturbation field structure applied by the coils(resonant/non-resonant). In the pedestal, theprofiles of electron temperature, electron density,
ion temperature, toroidal and poloidal rotation
exhibit no obvious difference to the cases without
perturbation coils. At the same time, strike line
splitting is observed in the divertor demonstrating
an influence of the perturbation coils at radii
outside of the separatrix.

Up to now the plasma rotation was not foundto influence the ELM mitigation effect. Thus, it isunclear what the interconnections between plasmarotation, the penetration of the magnetic
perturbations and the effect on the ELMs are.
3. Discussion
The observations reported in the present work
are remarkable, because they add additional
phenomenology to the observations at otherexperiments [4-7]. New aspects are the fact thatfor most of the cases neither density pump-out,
nor confinement degradation are observed in thephases during which ELMs are influenced by themagnetic perturbations. This is connected to thefact that the kinetic profiles are not degraded by
the perturbation fields. The latter suggests thatfield line stochastization is not playing a majorrole inside of the separatrix. However, it ispossible that the observations described in thepresent work relate to a density regime that was
not investigated in detail by other experiments upto now. At DIII-D, the existence of a highcollisionality regime was mentioned earlier [4], inwhich ELM-mitigation but not ELM-suppression
was possible. However, ELM suppression inplasmas corresponding to the DIII-D low
collisionality regime [5] has not been found as yetin ASDEX Upgrade. Current experimentation
aims to match dimensionless parameters andfuelling and pumping parameters in bothmachines. The ASDEX Upgrade in-vessel coil set
is now upgraded by installation of another eightcoils to allow n=4 resonant and non-resonant
perturbations. This upgrade allows theinvestigation of effects due to the details of thefield structure on ELM mitigation.
It should be noted that for the discovered
high-density regime, no satisfactory theory isavailable to describe the observations. Even if theregime is related to the high-collisionality regimeobserved at DIII-D, it is unclear how theperturbation coils can affect the ELM-stability inthe observed way.
References[1] W. Suttrop et. al. Phys. Rev. Lett. 206 (2011)
225004[2] W. Suttrop et. al. PPCF accepted[3] P. Lang et. al., Nucl. Fus. 44 (2004) 665[4] T. Evans et. al. Nucl. Fus. 45 (2005) 595[5] T. Evans et. al. Phys. Rev. Lett. 92 (2004) 235003[6] Y. Liang et. al. Nucl. Fus. 50 (2010) 025013[7] A. Kirk et. al. Plas. Phys. Contr. Fus. 53 (2011)065011