Baciero, A.

Bernert, M.

Boom, J.

Brezinsek, S.

Carraro, L.

Casson, Francis

Coffey, I

De la Luna, E.

Drewelow, P.

Flanagan, J.

Giroud, C.

Guillemaut, C.

Henderson, S. S.

Huber, A.

JET Contributors

Lawson, K. D.

Lomanowski, B.

Maslov, M.

Meneses, L.

Menmuir, S.

O'Mullane, M.

Puiatti, M. E.

Putterich, T.

Reinke, M.

Schmuck, S.

Sergienko, G.

Sertoli, M.

Solano, Emilia R.

Stamp, M.

Summers, H.

Valisa, M.

Solano, E.

Coffey, I.

Henderson, S.

Casson, F.

Pütterich, T.

Puiatti, M.

Lawson, K.

de la Luna, E.

MPG.PuRe

Characterising W radiation in JET-ILW plasmas Emilia R. Solano1, I. Coffey2, A. Huber3, S. S. Henderson4, M. O'Mullane4 , F. Casson2, H. Summers4, T. Pütterich5, M.E. Puiatti6, L. Carraro6, M. Valisa6, S. Menmuir2, K. Lawson2, M. Sertoli5, B. Lomanowski7, M. Reinke8, M. Bernert5,  P. Drewelow9, M. Maslov2, J. Flanagan2, G. Sergienko3, M. Stamp2, S. Brezinsek3,  L. Meneses10, S Schmuck2, J. Boom2, C. Giroud2, C. Guillemaut10, E. de la Luna1, A. Baciero1 and JET Contributors* EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK 1Laboratorio Nacional de Fusión, CIEMAT, Avda Complutense 40, 280040 Madrid, Spain; 2CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK; 3Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung - Plasmaphysik, 52425 Jülich, Germany; 4Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK; 5Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany; 6Consorzio RFX, corso Stati Uniti 4, 35127 Padova, Italy; 7Aalto University, P.O.Box 14100, FIN-00076 Aalto, Finland,  8Oak Ridge National Laboratory, Oak Ridge, TN 37831-6169, USA; 9Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, D-17491 Greifswald, Germany; 10Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Portuga. *See Appendix of F Romanelli et al. Proc. of the 25th IAEA Fusion Energy Conf. 2014, Saint Petersburg, Russia A fundamental change of plasma behaviour in JET since the installation of the ITER-like-Wall (ILW) is that the pedestals in ELMy H-modes are considerably colder than in the Carbon phase of JET, even for comparable fuelling levels [1,2]. This is interesting in itself, and also has implications for scenario development, since reduced core confinement is correlated with the lower pedestal electron temperature, Te,ped, in ELMy H-mode plasmas. H-mode pedestals exhibit strong gradients in electron density, ne, which drive a neoclassical inward pinch of W from the SOL towards the pedestal top. From there ne is typically flat, so further penetration of W would be much slower. Thus, in between ELMs, W could peak in the Te,ped region, where now typically Te,ped< 1.5 keV. W is known to be a good radiator in the  0.5-2 keV Te range, where calculation of the W radiation from first principles is challenging.  In figure 1 we illustrate the dependence of radiated power density on Te for various plasma species, based on ADAS atomic models: ,   for each species, where LZ(Te) is the cooling function. Be "burns through" below 100 eV: this means that from then on, as Te rises Prad,Be drops, so Te can continue to rise. In contrast we see that for both W curves, there are regions with positive slope: as Te rises, Prad,W rises, so it is harder for Te to continue to rise. Calculations from two rather different cooling functions for W are shown: W 89 is based on the JET-ADAS working version (commonly referred to as ADAS 89, unpublished). The ADAS 89 data included only low level configurations for estimation of the line radiated power. It was created as a  Fig. 1: Computed radiated power density (MW/m-3) for various species, with two different curves shown for W. It was assumed that ne=3×1019m-3, nZ/ne=2×10-4, except for Be, nBe/ne=2×10-4. We do not advocate that either W curve is correct: they just illustrate types of behaviour in the interesting Te range. 43rd EPS Conference on Plasma Physics P2.005  starting point. W TP is based on more recent work [3,4]: a different choice of configurations was made, and recombination coefficients were adjusted to match ASDEX measurements in the W24+-W46+ range (>1 keV). W TP also has more sophisticated ionization rates. Further studies, using configuration average estimates of omitted power from further configurations, indicate that both ADAS 89 and TP had important omissions, likely to introduce/modify the structure of the Te dependency of W radiation below 1 keV. Additionally the ionization balance must be re-examined. Recent experimental studies of dielectronic recombination (DR) [5, and references therein] indicate a major flaw in the DR coefficients for mid-range ions with open 4f shell in the ground state. This effect, known as the low-temperature DR effect, is greatly enhanced in tungsten, such that it can increase the rate, at the temperatures considered here, by a factor of 10. This would significantly move the ionisation balance towards temperatures of interest in the JET pedestal region (0.5-1keV), with the expectation that structure may appear in the cooling function below 1 keV.  Both W curves would be affected by the modified ionisation balance and improved configuration modelling. Work is ongoing to incorporate these effects into new W radiation calculations. Here we use the two existing models as test cases, to investigate the evidence for or against structure below 1 keV, as in W 89, or in W TP, respectively. To gather experimental data in relevant plasma conditions we used laser ablation to inject W into cold L-mode plasmas. We injected ~1018 atoms (estimated from the size of the hole left in the target) of W at t=11.005 s into 2.4 T, 1.7 MA JET plasmas, heated by PNBI=1.2 MW and Pohm~0.9 MW. Soon after injection we observed a fast rise in radiation (Fig. 2a) and  Fig. 2: a: temporal evolution of Te, ; b) total radiated power and Dα,  showing W injection at 11.005 s and its effects. c) and d)  profiles of Te and ne at the times indicated a,b. e) evolution of profiles of Te, f)  ne and g) Prad/ne, normalised radius in vertical axis , 0 on axis, contour level values on left. h) Te on axis.  Note that Prad/ne  ~ nW LW(Te): we can see W penetrating into core after 3rd sawtooth. 43rd EPS Conference on Plasma Physics P2.005  drastic cooling of the plasma edge, as shown in Figs. 2c, 2e. The outer 15 cm of the plasma becomes power detached from 11.05 till 11.55 s (ρ>0.9, R>3.65 m), with Te below 100 eV. This low Te allows deep penetration of neutrals leading to a rise in edge ne, and hollow ne profiles, Figs. 2d, 2f. Near the edge Te is so low that radiation has contributions from Deuterium and Be, not only W. But inside of ρ<0.8, and after t=11.1s, the low Z species have burnt through and only W remains as a radiator. This is the area of interest for our study. Tomographic reconstruction of bolometry from vertical and poloidal arrays shows poloidally and radially localised structures. Because W is very collisional, it can take a long time for nW to become poloidally symmetric. Assuming classical collisional diffusion, a random walk estimate of the time required for W to propagate along a full poloidal arc would be given by  =  || , with  = + 	.	 We show some typical values in Table 1, assuming q=3, ne=3×1019 m-3. Here we must note that the two bolometer cameras at JET are separated toroidally by 135°, about 9 m, resulting in W propagation times of order 1-10 ms: thus toroidal propagation time is not a concern for diagnostic interpretation.  From the tomographic reconstruction of bolometry we computed the flux-surface averaged radiation density as a function of normalised minor radius and time, which we divided by ne to estimate . Looking at Prad/ne in Fig. 2 it is evident that sawteeth play an important role carrying W inward until the 3rd sawtooth, and outwards from then on. From t=11.3 s we see clear evidence of structure in Prad/ne near 0.5<ρ<0.8, where 150<Te (eV)<800. Is this structure due to the cooling function?  To investigate this we turn to transport modelling. We can’t model the interaction of W with sawteeth, so we enhance the inward particle flux during the time period 11.-11.2 s, to mimic the effect of the sawtooth carrying W inwards. This allows a centrally peaked nW profile to form. Starting with that initially peaked nW profile at t=11.2 s the subsequent temporal decay from t>11.2 s is modelled with SANCO [6] using two different sets of atomic data taken from the ADAS baseline data (W 89) and from Pütterich (W TP), including recombination, ionisation and line power rate coefficients. In each case, the radial diffusion profile is the same, shown in Fig. 3a, while the radial convective velocity profile is varied, for both W 89 and W TP, to match both the 2D reconstructed bolometry profiles and the mid-plane, line-integrated bolometry trace. As shown in Fig. 3c, the convective velocity profile associated with the W 89 model gives rise to a strong inward pinch around ρ=0.8 and a weak outward velocity at ρ=0.7, where ∇ne is outward. When using the W TP atomic data the inward pinch is smaller and changes sign twice inside of ρ=0.8. ZW  Te=Ti (keV) D|| 104m2/s time (ms) W8+ 0.160 1.1 85 W10+ 0.210 1.4 67 W13+ 0.290 1.9 51 W17+ 0.420 2.8 34 W19+ 0.500 3.4 28 W23+ 0.685 5.1 18 W27+ 0.881 7.0 14 W30+ 1.230 13.0 7 Table 1: Collisional random-walk estimate of time required for W to spread poloidally. 43rd EPS Conference on Plasma Physics P2.005  Calculations of the particle flux at each radial location have been carried out in the transport code JETTO [7,8] using the mixed Bohm/gyro-Bohm model [9]  to compute the anomalous diffusive transport and the local drift kinetic neoclassical code NEO [10,11] to calculate the convective velocity. The inward pinch needed to match the radiated power calculated using the W TP atomic data with the reconstructed bolometry profile is hard to justify between ρ=0.6-0.7 in comparison to NEO, because only a strongly peaked density profile can facilitate an inward pinch. Due to the flatness of the electron density profile around ρ=0.6-0.7, the convective velocity profile associated with the W 89 model provides the best match to the theoretical calculations. These results could imply that there is structure in the shape of the tungsten cooling function between Te=100 – 1000 eV, but, alas, they are not conclusive. In summary: Different ADAS-based atomic models can lead to very different distribution of W radiation. Work is ongoing to improve ADAS predictions of W radiation in the 0.1-2 keV range of Te, of interest for pedestal studies in JET-ILW. Transport models required to match observed radiation data require less manipulation of the convection term when there is structure in LW(Te) in the 0.3-1 keV range, but the results are still inconclusive. We also note how slow diffusion of W along a field line can be. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. [1] M. Beurskens et al,  PPCF 55, 124043. (2013) [2] E. Stefanikova, 42nd EPS Conf. on Control. Fus. and Plasma Physics, Lisbon 2015, P2.131. [3] T. Pütterich et al, PPCF 50 085016 (2008). [4] T. Pütterich et al, Nucl. Fusion 50 (2010). [5] C. Krantz et al, J. Phys. Conf. Ser. 488, 012051(2014). [6] L. Lauro-Taroni et al, 21st EPS Conf. on Controlled Fusion and Plasma Phys. (Montpelier, France) Vol 21,102 (1994). [7] G. Cenacchi, A.Taroni, Report JET-IR 88 03 (1988). [8] G. Cenacchi,  A. Taroni, Rapporto ENEA RT/TIB 88 5 (1988).  [9] M. Erba et al. Nucl. Fusion 38 1013 (1998). [10] E.A. Belli,  J Candy PPCF 50 095010 (2008). [11] E.A. Belli, J. Candy, PPCF 54 015015 (2012)  Fig. 3: a) diffusion coefficient used in SANCO during the 11.3-11.6 time window, compared with theoretical calculations; b) convection used to fit radiation data for the two atomic models considered; c) Prad/ne measurement, normalised to its maximum value. d) modelling result for W 89, from 11.2 s. The shaded areas in 1,b indicate regions were radiation from low Z species may also be present, but is ignored in models. 43rd EPS Conference on Plasma Physics P2.005

English

Characterising W radiation in JET-ILW plasmas

University of Strathclyde Institutional Repository

Solano, Emilia R. and Coffey, I and Huber, A. and Henderson, S. S. and O'Mullane, M. and Casson, Francis and Summers, H. and Putterich, T. and Puiatti, M. E. and Carraro, L. and Valisa, M. and Menmuir, S. and Lawson, K. D. and Sertoli, M. and Lomanowski, B. and Reinke, M. and Bernert, M. and Drewelow, P. and Maslov, M. and Flanagan, J. and Sergienko, G. and Stamp, M. and Brezinsek, S. and Meneses, L. and Schmuck, S. and Boom, J. and Giroud, C. and Guillemaut, C. and De la Luna, E. and Baciero, A. (2016) Characterising W radiation in JET-ILW plasmas. In: 43rd European Physical Society Conference on Plasma Physics. Europhysics Conference Abstracts, 40A . European Physical Society (EPS), Norwich. ISBN 2914771991 , This version is available at http://strathprints.strath.ac.uk/60343/Strathprints is  designed  to  allow  users  to  access  the  research  output  of  the  University  of Strathclyde. 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It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output.			Emilia R. Solano1, I. Coffey2, A. Huber3, S. S. Henderson4, M. O'Mullane4 , F. Casson2, H. Summers4, T. Pütterich5, M.E. Puiatti6, L. Carraro6, M. Valisa6, S. Menmuir2, K. Lawson2, M. Sertoli5, B. Lomanowski7, M. Reinke8, M. Bernert5,  P. Drewelow9, M. Maslov2, J. Flanagan2, G. Sergienko3, M. Stamp2, S. Brezinsek3,  L. Meneses10, S Schmuck2, J. Boom2, C. Giroud2, C. Guillemaut10, E. de la Luna1, A. Baciero1 and JET Contributors* 				 !"#$			%	&'()*	+,-.,,,) +/-&    	 0	   !" #/ &	12 34 ( 2 5  #	 5 6+78 9--9 237/!+	67*7	73	:3,%3#/9)05685(26+78!5.9;.337/<		1	&		 9-;6	*(7/;	*76=="	0,,&(%5,,,;<	&.8%	$		78% ;. 5<<>/>)05685(26+783:!5;>3:37/,(	6&?	%(	+	@	*$	6	=*See Appendix of F Romanelli et al. Proc. of the 25th IAEA Fusion Energy Conf. 2014, Saint Petersburg, Russia A fundamental change of plasma behaviour in JET since the installation of the ITER<like<Wall (ILW) is that the pedestals in ELMy H<modes are considerably colder than in the Carbon phase of JET, even for comparable fuelling levels [1,2]. This is interesting in itself, and also has implications for scenario development, since reduced core confinement is correlated with the lower pedestal electron temperature, Te,ped, in ELMy H<mode plasmas. H<mode pedestals exhibit strong gradients in electron density, ne, which drive a neoclassical inward pinch of W from the SOL towards the pedestal top. From there ne is typically flat, so further penetration of W would be much slower. Thus, in between ELMs, W could peak in the Te,ped region, where now typically Te,ped< 1.5 keV. W is known to be a good radiator in the  0.5<2 keV Te range, where calculation of the W radiation from first principles is challenging.  In figure 1 we illustrate the dependence of radiated power density on Te for various plasma species, based on ADAS atomic models: ,  for each species, where $ABC is the cooling function. Be "burns through" below 100 eV: this means that from then on, as Te rises Prad,Be drops, so Te can continue to rise. In contrast we see that for both W curves, there are regions with positive slope: as Te rises, Prad,W rises, so it is harder for Te to continue to rise. Calculations from two rather different cooling functions for W are shown: W 89 is based on the JET<ADAS working version (commonly referred to as ADAS 89, unpublished). The ADAS 89 data included only low level configurations for estimation of the line radiated power. It was created as a  &=D	++	:7B)EF5 C	*	+::	*	:	E=(:G ×,>5 AFG-×,50+	""FG-×,5=E		*	E*	D7H7+	*	= 43rd EPS Conference on Plasma Physics P2.005  starting point. W TP is based on more recent work [3,4]: a different choice of configurations was made, and recombination coefficients were adjusted to match ASDEX measurements in the W24+<W46+ range (>1 keV). W TP also has more sophisticated ionization rates. Further studies, using configuration average estimates of omitted power from further configurations, indicate that both ADAS 89 and TP had important omissions, likely to introduce/modify the structure of the Te dependency of W radiation below 1 keV. Additionally the ionization balance must be re<examined. Recent experimental studies of dielectronic recombination (DR) [5, and references therein] indicate a major flaw in the DR coefficients for mid<range ions with open 4f shell in the ground state. This effect, known as the low<temperature DR effect, is greatly enhanced in tungsten, such that it can increase the rate, at the temperatures considered here, by a factor of 10. This would significantly move the ionisation balance towards temperatures of interest in the JET pedestal region (0.5<1keV), with the expectation that structure may appear in the cooling function below 1 keV.  Both W curves would be affected by the modified ionisation balance and improved configuration modelling. Work is ongoing to incorporate these effects into new W radiation calculations. Here we use the two existing models as test cases, to investigate the evidence for or against structure below 1 keV, as in W 89, or in W TP, respectively. To gather experimental data in relevant plasma conditions we used laser ablation to inject W into cold L<mode plasmas. We injected ~1018 atoms (estimated from the size of the hole left in the target) of W at t=11.005 s into 2.4 T, 1.7 MA JET plasmas, heated by PNBI=1.2 MW and Pohm~0.9 MW. Soon after injection we observed a fast rise in radiation (Fig. 2a) and  &=-DD+	*			/C	+	:!α, 	:EH	=,,9=CC+		=C*			+		CC6F	*0,	0		**	=C	0=%	6FIE$EBCD:E+		 :		=43rd EPS Conference on Plasma Physics P2.005  drastic cooling of the plasma edge, as shown in Figs. 2c, 2e. The outer 15 cm of the plasma becomes power detached from 11.05 till 11.55 s (ρ>0.9, R>3.65 m), with Te below 100 eV. This low Te allows deep penetration of neutrals leading to a rise in edge ne, and hollow ne profiles, Figs. 2d, 2f. Near the edge Te is so low that radiation has contributions from Deuterium and Be, not only W. But inside of ρ<0.8, and after t=11.1s, the low Z species have burnt through and only W remains as a radiator. This is the area of interest for our study. Tomographic reconstruction of bolometry from vertical and poloidal arrays shows poloidally and radially localised structures. Because W is very collisional, it can take a long time for nW to become poloidally symmetric. Assuming classical collisional diffusion, a random walk estimate of the time required for W to propagate along a full poloidal arc would be given by  =  || , with  = + 	.	 We show some typical values in Table 1, assuming q=3, ne=3×1019 m<3. Here we must note that the two bolometer cameras at JET are separated toroidally by 135°, about 9 m, resulting in W propagation times of order 1<10 ms: thus toroidal propagation time is not a concern for diagnostic interpretation.  From the tomographic reconstruction of bolometry we computed the flux<surface averaged radiation density as a function of normalised minor radius and time, which we divided by ne to estimate . Looking at Prad/ne in Fig. 2 it is evident that sawteeth play an important role carrying W inward until the 3rd sawtooth, and outwards from then on. From t=11.3 s we see clear evidence of structure in Prad/ne near 0.5<ρ<0.8, where 150<Te (eV)<800. Is this structure due to the cooling function?  To investigate this we turn to transport modelling. We can’t model the interaction of W with sawteeth, so we enhance the inward particle flux during the time period 11.<11.2 s, to mimic the effect of the sawtooth carrying W inwards. This allows a centrally peaked nW profile to form. Starting with that initially peaked nW profile at t=11.2 s the subsequent temporal decay from t>11.2 s is modelled with SANCO [6] using two different sets of atomic data taken from the ADAS baseline data (W 89) and from Pütterich (W TP), including recombination, ionisation and line power rate coefficients. In each case, the radial diffusion profile is the same, shown in Fig. 3a, while the radial convective velocity profile is varied, for both W 89 and W TP, to match both the 2D reconstructed bolometry profiles and the mid<plane, line<integrated bolometry trace. As shown in Fig. 3c, the convective velocity profile associated with the W 89 model gives rise to a strong inward pinch around ρ=0.8 and a weak outward velocity at ρ=0.7, where ∇ne is outward. When using the W TP atomic data the inward pinch is smaller and changes sign twice inside of ρ=0.8. ZW  Te=Ti (keV) D|| 104m2/s time (ms) W8+ 0.160 1.1 85 W10+ 0.210 1.4 67 W13+ 0.290 1.9 51 W17+ 0.420 2.8 34 W19+ 0.500 3.4 28 W23+ 0.685 5.1 18 W27+ 0.881 7.0 14 W30+ 1.230 13.0 7 D			5:8	J	E	++		7=43rd EPS Conference on Plasma Physics P2.005  Calculations of the particle flux at each radial location have been carried out in the transport code JETTO [7,8] using the mixed Bohm/gyro<Bohm model [9]  to compute the anomalous diffusive transport and the local drift kinetic neoclassical code NEO [10,11] to calculate the convective velocity. The inward pinch needed to match the radiated power calculated using the W TP atomic data with the reconstructed bolometry profile is hard to justify between ρ=0.6<0.7 in comparison to NEO, because only a strongly peaked density profile can facilitate an inward pinch. Due to the flatness of the electron density profile around ρ=0.6<0.7, the convective velocity profile associated with the W 89 model provides the best match to the theoretical calculations. These results could imply that there is structure in the shape of the tungsten cooling function between Te=100 – 1000 eV, but, alas, they are not conclusive. 	 Different ADAS<based atomic models can lead to very different distribution of W radiation. Work is ongoing to improve ADAS predictions of W radiation in the 0.1<2 keV range of Te, of interest for pedestal studies in JET<ILW. Transport models required to match observed radiation data require less manipulation of the convection term when there is structure in LW(Te) in the 0.3<1 keV range, but the results are still inconclusive. We also note how slow diffusion of W along a field line can be. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014<2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. [1] M. Beurskens et al,  PPCF 55, 124043. (2013) [2] E. Stefanikova, 42nd EPS Conf. on Control. Fus. and Plasma Physics, Lisbon 2015, P2.131. [3] T. Pütterich et al, PPCF 50 085016 (2008). [4] T. Pütterich et al, Nucl. Fusion 50 (2010). [5] C. Krantz et al, J. Phys. Conf. Ser. 488, 012051(2014). [6] L. Lauro<Taroni et al, 21st EPS Conf. on Controlled Fusion and Plasma Phys. (Montpelier, France) Vol 21,102 (1994). [7] G. Cenacchi, A.Taroni, Report JET<IR 88 03 (1988). [8] G. Cenacchi,  A. Taroni, Rapporto ENEA RT/TIB 88 5 (1988).  [9] M. Erba et al. Nucl. Fusion 38 1013 (1998). [10] E.A. Belli,  J Candy PPCF 50 095010 (2008). [11] E.A. Belli, J. Candy, PPCF 54 015015 (2012)  &= DC		%= 5=<:	:	+:		/C	*				:				/C6F		0*=C		E.>	=-=	:			:A+7	+		=43rd EPS Conference on Plasma Physics P2.005

Characterising W radiation in JET-ILW plasmas

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