Bernert, M.

Gal, K.

Hölzl, M.

Kappatou, A.

Maraschek, M.

Mlynek, A.

Pautasso, G.

Suttrop, W.

English

MPG.PuRe

MGI in plasmas with locked modesG. Pautasso, M. Bernert, A. Mlynek, M. Maraschek, W. Suttrop,A. Kappatou (∗), K. Gal, M. Hoelzl and the ASDEX Upgrade Team.Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-85748 Garching,Germany; (*) FOM Institute DIFFER, Association EURATOM-FOM, Nieuwegein,The Netherlands.E-mail contact of main author: gabriella.pautasso@ipp.mpg.deIntroduction. Massive gas injection (MGI) is foreseen as a mitigation technique forITER disruptions. Typically, MGI experiments have been carried out by injecting impu-rity gas in a H-mode plasma, in order to test the capability of the contaminated plasma inradiating a large amount of thermal energy and to have a target plasma with predefinedparameters. Nevertheless MGI will be used in ITER in discharges with a high probabilityof disrupting and therefore likely with large non-rotating modes.A series of plasmas with locked modes (LMs) were terminated by MGI on ASDEX Up-grade (AUG) in order to study the influence of these modes on the fuelling efficiencyand radiation asymmetry. This paper reports on the experimental findings and discussestheir implications for ITER.Experimental conditions. Two types of plasma scenarios with LMs were first createdand then terminated by MGI. A series of ohmic-heated plasmas with a toroidal current(Ip) of 1 MA, low plasma thermal energy at the time of valve triggering (Eth(ttrig) < 0.1MJ ) and a safety factor q95 ∼ 4.1 were driven to the density limit, by means of strongfeed-forward gas puffing. These discharges developed a MARFE and a slowly rotating(m,n) = (2,1) or (3,1) mode. The second scenario was a repeat of a high-beta discharge,previously developed on AUG for experiments on disruption avoidance and known togenerate a (2,1) NTM, which grows and disrupts the plasma. In this case Eth(ttrig) ∼ 0.32MJ, q95 ∼ 3.8 and the plasma core is still rotating at some tens of km/s at the time ofMGI.In both scenarios, the tearing modes were artificially slowed down and locked by applyingan n=1 radial magnetic field with the RMP coils turned on well before mode develop-ment. In this way, the position of the LM could be controlled by the choice of the coilconfiguration. The amplitude of the n=1 radial magnetic field, measured by a pair ofsaddle coils, located on the high field side (HFS) of the torus (routinely employed to de-tect a LM), was then used to trigger the MGI valves. In addition, two recently-installedpairs of saddle coils - with each coil covering a 90 degree toroidal angle - permit thelocalization of the n=1 and n=2 modes.AUG is equipped with two fast valves, located circa 10 cm from the plasma edge, oneon the HFS and the other on the low field side (LFS) of the plasma. In this experiment,only one valve at a time was triggered to inject 4× 1022 atoms of neon (corresponding toa pressure of 20 bar in the gas reservoir volume of 80 cm3). This quantity, divided by theAUG plasma volume (14 m3), corresponds to the number of neon atoms per m3, plannedto be injected by the ITER thermal quench (TQ) mitigation system (plasma volume of830 m3, gas quantity of 10 kPa×m3 [1]).Pre- thermal quench (pre-TQ) time and fuelling efficiency. The pre-TQ time140th EPS Conference on Plasma Physics O5.104(∆tpreTQ) is defined as the time interval between the appearance of the injected impuritygas at the plasma edge and the start of the TQ. This time is an important design pa-rameter for a mitigation system since it is the amount of gas, which is assimilated by theplasma in this time interval, that radiates a fraction of Eth before and during the TQ,and therefore prevents this fraction from being conducted into the divertor. Therefore,the longer is the pre-TQ time, the larger is the amount of gas delivered to and assimilatedby the plasma.0 4 8 12 16x 10221234Ninj∆ tpre−TQ   (ms)Eth > 0.5 MJ         0.3 MJ < Eth < 0.5 MJEth < 0.3 MJ         LM           TM         HFS   (ms)0 4 8 12 16x 10221234Ninj∆ t pre-TQLFS0 4 8 12 16x 102200.20.40.60.8NinjF effEth > 0.5 MJ         0.3 MJ < Eth < 0.5 MJEth < 0.3 MJ         LM       TM      CQ       HFS0 4 8 12 16x 102200.20.40.60.8NinjF effLFS(a) (b)(c) (d)Figure 1 .Duration of the pre-TQphase versus the number ofinjected neon atoms for (a)HFS and (b) LFS MGI.Fuelling efficiency versusthe number of injected neonatoms for (c) HFS and (d)LFS MGI.In AUG, ∆tpreTQ was found to be 50 % longer during the HFS MGI into H-mode plasmas,with respect to the LFS MGI [2]. This is probably due to the larger distance between theplasma edge and the low q (2 and 3) surfaces, on which the MHD modes rapidly grow andinitiate the TQ. In addition, an increasing amount of injected gas (Ninj) accelerates thedestabilization of the plasma and decreases ∆tpreTQ. The plasma Eth does not seem toinfluence this time interval; this observation is counter-intuitive and implies that ∆tpreTQis not simply proportional to the amount of energy which has to be radiated outside ofa critical low q flux surface. The presence of LMs and also large tearing modes is nowfound to reduce the pre-TQ time by a factor up to two when the gas is injected fromthe HFS, as one would expect for a plasma which is more unstable (see fig.s 1(a) and1(b)). In the case of LFS injection, the pre-TQ time is already relatively short, even atsmall Ninj , and the presence of rotating or locked modes decreases this time by a smallerfactor.The fuelling efficiency, Feff , is defined in this and previous MGI-related AUG publica-tions as the increase of the total number of electrons per number of injected impurityatoms, averaged over the plasma volume and the current quench duration. Feff is afundamental figure of merit and is a complex function of plasma and gas parameters.The detailed physics, determining its magnitude, has not yet been formulated but someof the mechanisms and parameters influencing it are known. For example, Feff decreaseswith Ninj - in both LFS and HFS MGI - because ∆tpreTQ and therefore the amount ofgas assimilated up to the TQ decrease. The presence of tearing modes is seen to have asignificant influence on the HFS MGI Feff (up to a factor of 1.8). No influence of tearingmodes is observed on the LFS MGI Feff , probably because of their small influence on240th EPS Conference on Plasma Physics O5.104the already small ∆tpreTQ and Feff .Fig. 1(c) also shows four discharges with MGI starting during the TQ (therefore with∆tpreTQ=0) and exhibiting a Feff ∼ 0.1. These data points prove that a density increaseby MGI into TQ and CQ is possible and, therefore, that MGI can be used for runawayelectron suppression.Radiation asymmetry. During the pre-TQ, both the impurity density and the radiatedpower are strongly poloidally and toroidally asymmetric, and larger in the vicinity of thevalve. It was calculated in [3] that an increase of a factor of two in the energy deposited onthe wall with respect to the energy assumed uniformly radiated during an ITER pre-TQ(Eth = 350 kJ within 1 ms) would melt the Be wall.AUG is equipped with several AXUV diode arrays in sector 5 and 13, and foil bolome-ters in sector 5, which allow the reconstruction of the power radiated at these toroidallocations. In order to quantify the degree of asymmetry of the energy radiated in thesector of (or close to) the valve, φ = 0, and in the sector ∆φ = π away from the injectionlocation, the ratioAF (∆t) ≡ Erad(φ = 0, ∆t)/Erad(φ = π, ∆t), Erad(φ, ∆t) =∫∆tPrad(φ)dt (1)is defined and calculated during the pre-TQ and TQ phases. In this definition, Erad(φ, ∆t)is the energy radiated during the time interval ∆t, measured in a given sector and ex-trapolated to the whole torus. As shown in fig.s 2 (a) and (b), the AF(∆tpreTQ) spansa wide range of values (up to 5) and the presence of LMs at low Eth clearly tends toincrease this ratio. The inspection of the Prad time traces reveals that the degree oftoroidal asymmetry of the radiated power can be of the same order of magnitude in bothLM and no-LM cases.no-LM          LM          Eth (ttrig)    [MJ]E    (φ=0) / E    (φ=π)radradTQ time intervalE    (φ=0) / E    (φ=π)radradpre-TQ time interval0 0.2 0.4 0.6 0.802460 0.2 0.4 0.6 0.8012300.20.40.60.80 0.2 0.4 0.6 0.8E     [MJ]radpre-TQ time intervalEth (ttrig)    [MJ] Eth (ttrig)    [MJ](a) (b) (c)Figure 2. (a) Values of the toroidal asymmetry factor, AF, versus thermal energy of thetarget plasma during the (a) pre-TQ and (b) TQ phase. (c) Energy radiated during thepre-TQ as measured by the foil bolometers versus thermal energy of the target plasma.Nevertheless, in the rotating plasmas the asymmetry can reverse sign in the presenceof a rotating short lived n=1 mode structure seen in SXRs and magnetic measure-ments. Therefore the presence of rotating modes - or just plasma rotation - can decreaseAF(∆tpreTQ). The foil bolometers indicate that the overall energy radiated during thepre-TQ phase is of the order of Eth(ttrig)(fig. 2(c)).340th EPS Conference on Plasma Physics O5.104The TQ phase is defined here as the interval of +/- 0.3 ms about the maximum of thetoroidally averaged (between two sectors) radiated power, as measured by the AXUVvertical arrays. AF(∆tTQ) is typically of order unity and reaches the value of 2 in fig.2(b). The LM cases exhibit a larger radiation asymmetry also in this phase, with theexception of the higher beta discharges which have AF(∆tTQ) ∼ 1. How the radiatedpower can become toroidally symmetric in these last discharges is not clear.The tendency of AF(∆tTQ) to be smaller than AF(∆tpreTQ) is understood in terms ofan enhanced mixing of impurities and target plasma during the TQ, due to the heatingof the impurities, caused by the out-flowing thermal energy, and to the MHD activity,causing large convective flows. Nevertheless, recent calculations carried out with theNIMROD code, suggest that the radiation asymmetry can be very large, with factors ofAF(∆tTQ) down to 1/6 and toroidal peaking factors down to 1/3.5 reported in [4] for LFSMGI. However, the current AUG experimental results do not confirm such large radiationasymmetry during the TQ. These simulations have also pointed out a possible dependenceof the gas assimilation efficiency and radiation asymmetries on the relative location of thegas injection footprint and tearing mode phase. The values of AF presented here seem tovary independently of this relative phase, which has been artificially varied by choosingdifferent RMP n=1 configurations. Additional work is necessary to clarify the origin ofthe discrepancy between modelling and experimental findings. Moreover, a consistentnumerical model of the whole gas-plasma interaction dynamics need to be developed inorder to extrapolate these experimental observations to ITER.The ITER disruption mitigation system (DMS) foresees several gas injectors at differ-ent toroidal locations, which potentially can distribute the radiated power more evenlyaround the torus. Only few attempts have been made up to now on AUG to synchro-nize the different valves and to study the effect of multiple injection on the radiationasymmetry.Summary. Experiments of MGI in plasmas with LM have been performed in AUGwith neon quantities per plasma volume comparable to ITER. A reduction of the pre-TQ phase is observed when large rotating or locked modes are present. These effectsdecrease the amount of gas assimilated by the plasma up to the TQ (up to a factor of 2)and up to the middle of the CQ and should be taken into account when dimensioning theITER DMS. Large radiation asymmetries are observed during the pre-TQ phase whenthe plasma is locked; nevertheless, in these cases the plasma thermal energy is small(order of 10 % of the maximum Eth) and would not cause the melting of the ITER wall.Years of disruption mitigation studies have contributed to a conceptual picture of thephysics processes determining MGI dynamics but a quantifiable physical model, whichallows to extrapolate present experimental observations to ITER, is still missing.Acknowledgements. The first author thanks Val Izzo and Michael Lehnen for usefuldiscussion and input.References[1] S. Putvinski et al., Proc. 23rd Int. Conf. on Fusion Energy 2010 (Daejeon, Korean,2010) (Vienna: IAEA) CD-ROM file ITR/1-6[2] G. Pautasso et al., Proc. 24th Int. Conf. on Fusion Energy 2012 (San Diego, USA,2012) (Vienna: IAEA) CD-ROM file EX/P8-12[3] M. Sugihara et al., Nuclear Fusion, 47, 337 (2007)[4] V.A. Izzo, Physics of Plasmas, 20, 056107 (2013)440th EPS Conference on Plasma Physics O5.104

MGI in plasmas with locked modes

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MGI in plasmas with locked modes

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