ENEA for EUROfusion, via E. Fermi 45, Frascati (Rome), Italy 2. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, , China 3

Quasi-Snowflake divertor studies on EAST G. Calabrò 1, B.J. Xiao 2,3, S. L. Chen 2, Y.M. Duan 2, Y. Guo 2, J.G. Li 2, L. Liu 2, Z.P. Luo 2, L. Wang 2, J. Xu 2, B. Zhang 2, R. Albanese 4, R. Ambrosino 4, F. Crisanti 1, V. Pericoli Ridolfini 4, F. Villone 4, B. Viola 1, L. Barbato 4, M. De Magistris 4, G. De Tommasi 4, E. Giovannozzi 1, S. Mastrostefano 4, S. Minucci 4, A. Pironti 4, G. Ramogida 1, A.A. Tuccillo 1, R. Zagórski 5 and EAST team 1 ENEA for EUROfusion, via E. Fermi 45, 00044 Frascati (Rome), Italy 2 Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China 3 School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, 230026, China 4 CREATE, Università di Napoli Federico II, Università di Cassino and Università di Napoli Parthenope, Via Claudio 19, 80125 Napoli, Italy 5 Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland Thanks to D. D. Ryutov and H. Reimerdes 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. The views and opinions expressed herein do not necessarily reflect those of the European Commission. In addition, this work was partly supported by Italian MIUR under PRIN Grant No. 2010SPS9B3, by the National Magnetic Confinement Fusion Research Program of China under Grant No 2014GB103000, the National Natural Science Foundation of China under Grant No. 11305216. ASIPP G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 1

Outline Introduction EAST alternative magnetic configurations: modelling and optimization First experiments in 2014 and next steps Conclusions towards future facility as Divertor Tokamak Test (DTT) G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 2

Discussion on a Dream Divertor One approach to handling the heat exhaust power is to use alternative magnetic configurations, such as Snowflake Divertor (SF) Original Theory [Ryutov PoP 07] TCV [Vijvers NF 14] NSTX [Soukhanovskii NF 11] DIII-D [Allen IAEA 12] SF divertor: second-order null ( B P 0). Longer connection length Larger divertor volume Larger region of low poloidal field G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 3

Motivations for SF experiments on EAST Large superconducting tokamak producing Steady State H-mode discharges (see I-3 talk J.Qian) W divertor In the coming years will operate with DEMO relevant P Sep /R (~15MW/m) EAST main parameters: R=1.9m, a=0.5m B T up to 3.5-4T, I P up to 1MA =1.5 2.0 and =0.3-0.6 Unlike DIII-D, TCV and NSTX the divertor coils are outside the toroidal field coils DEMO relevant G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 4

EAST two nearby nulls divertor configuration I P =200kA An exact SF configuration is topologically unstable [Ryutov PoP 2007] for instance: a PF currents splits the 2 nd order null in two 1 st order nulls Difficulties in exactly controlling it on a power plant Exact SF in EAST limited to ~200kA [Guo APPC 13] We are interested in divertor magnetic topologies (two nearby nulls divertor configuration called quasi-snowflakes ) where the secondary null acts in concert and communicates with the primary one in a significant way [Calabrò submitted to NF 2015] G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 5

QSF optimization by CREATE-NL tools Exploiting the linearized relation between plasma-wall gaps and PF coils [Ambrosino NF 14]: I P = 400-500kA; low (~0.1) and high (~0.5) p I PF <10% max I PF (14.5kA); vertical instability growth rate same order SN configuration; SPs on vertical target; 40mm clearance from FW QSF far nulls QSF close nulls Increase of flux expansion f m (up to factor ~12) and connection length L (~40%) Secondary x-point may be brought inside the vessel at price of slightly lower I P,orhigher and/or further optimization of PF currents SN: f m =2.1, L=95 G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 7

Flaring or contracting geometry near the target? Optimized far nulls QSF far nulls: R B P decreases field lines converging from the null to strike point contracting geometry Optimized close nulls QSF close nulls: R B P first increases then decreases flaring geometry (feature of single-legged X-divertor [Kotschenreuther PoP 13]) G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 8

Is conversation between the nulls active? A necessary condition for including a particular configuration to the SF family, and its physics, isthe proportionality of the field gradient to the distance between the two nulls, described in [Ryutov Plasma Phys. 2008 and PPCF 2010] QSF far nulls QSF close nulls Direct manifestation of the conversation between the nulls flux flaring in the main null (characterized by grad B P ) is proporational to the distance between the nulls G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 9

Edge predictive simulations by TECXY TECXY takes into account all the main physics processes into the SOL, but the neutral dynamics treated with an analytical model instead of the more rigorous Monte Carlo method EDGE2D used to validate TECXY results Power load reduction for QSF: I P =300kA, B T =1.8T, no impurity According to previous studies[pericoli FED 13] combination of flux exp. and enhanced,far nulls dissipation process (likely due the longer conn. Length) Significant drop at 2.5x10 19 m -3 start of detachment? G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 10

First QSF experiments in 2014 Experimental set-up EAST restared after 20 month-long upgrading break I P =250kA and only far nulls case for safety reasons RZIP control used, leaving the shape and x-points distance in QSF conf. to freely evolve Ohmic and with 500kW of NBI heating #47660 #47660 #47660 G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 12

L-mode discharges at low n e : QSF vs. SN @t=4.5s P LH P NBI LPs PF6 ~ 8kA as max PF current I P could be further increased up to ~500kA Increase of connection length by ~30% and flux expansion by a factor ~4 G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 13

Edge results and interpretative simulations Langmuir Probes Infrared Camera and TECXY heat flux at 4.5s,TECXY,TECXY QSF stable P NBI ~500kW [Viola to be presented at EPS 2015] [Calabrò FEC-IAEA 2014] Once QSF shape becomes stable peak of j SAT on LP,OT observed to drastically drop IR data point out peak heat load reduction of factor ~2 (as espected at low density) Reasonable agreement between IR and TECXY QSF simulated heat flux G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 14

Isoflux-SF implemented on PCS and tested Control algorithm verified only in few ohmic shots due to limited experiment time and conditions [Xiao IAEA TM Control, India, 2015] G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 15

Next steps: P SOL, n e and D nulls scan TECXY predictive simulations by using experimental QSF far nulls and SN equilibria QSF load mitigating properties expected to be exalted at higher density Sligthly improvement by increasing the P SOL Simulated Power density at OT by TECXY at low n e,lcms =6x10 18 m -3 SN far A further reduction (of a factor ~3) is expected by decreasing D nulls (i.e. moving from QSF,far nulls to QSF,close nulls) mainly due to flux exp. variation: f m-ot,sn ~2 f m-ot,qsf-far ~8 f m-ot,qsf-close ~24 [Viola to be presented at EPS 2015] G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 16

Conclusions First ever demonstration of the possibility of creating and controlling two-null divertor configuration (so-called QSF) on a large superconducting tokamak, as EAST, has been realized First QSF experiments at low I P and n e : Secondary x-point is placed far from the primary one forming a contracting geometry near the target plates; I P could be increased up to ~500kA Increase of the connection length by 30% and the flux expansion of a factor 4 w.r.t. SN IR and LPs edge data highlighted a peak heat load reduction for QSF of the same order of flux expansion increase; further reduction expected by going to higher n e and/or reducing the distance of two nulls Reasonable agreement between 2D edge TECXY code and QSF experimental results Isoflux-SF control implemented and initially tested G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 18

Discussion on a future facility in Europe Power Exaust study in view of DEMO foreseen in European Fusion Road Map: on a dedicated test on specifically upgrading existing facilities or a on a dedicated Divertor Tokamak Test (DTT) facility, within 2020 s: DTT should test several alternatives to the conventional SN divertor with solid W targets Dedicated WPDTT1-2 groups present in EUROfusion Programme Within the European Framework very preliminary work to arrive to a self consistent DTT design started [Albanese IAEA TM-DEMO, Hefei 2015] This will allow to be ready for the EU Juncker Funds selection around June/July 2015 G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 19

DTT draft proposal [Albanese IAEA TM-DEMO, Hefei 2015] -QSF, - SN Superconducting machine with major Radius R 0 =2.15m, I P =6MA, B T =6T, in order to satisfy the main constraints: the machine must operate in a Physics regime Reactor relevant Z(m) The overall cost of the machine must be within a given budget (500M in our case; i.e. the budget Italy is officially proposing within the EU Juncker development plan) The additional heating (P ADD ~45MW) cost cannot exceed 1/3 of the total cost Test different divertor structure and different local R(m) magnetic topologies (need of internal coils) Lessons on large superconducting EAST (i.e. QSF on steady state H-mode, relevant DEMO P/R, W divertor, with PFCs outside TFCs) may be crucial G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 20

Quasi-Snowflake divertor studies on EAST Back-up slides ASIPP G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 22

QSF terminology to describe two-null divertor conf. The term QSF does seem to fit the physicists tradition of using the word quasi to designate effects or structures that are similar but not identical to their prototypes ( quasiclassics, quasisymmetry, quasiparticle, etc.) Naming the magnetic field structures with two nearby nulls as quasi-snowflakes does seem to match this approach When applying a term quasi-snowflake, one has to be reasonable in the definition of what the nearby means: In the most general sense, nearby may mean that the nulls have significant effect on each other. If, say, a field flatness (dbp/dr) in one null is strongly affected by the other, this already may mean that the configuration can be called a quasi-snowflake. G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 23

EAST QSF configuration with even closer nulls 2 I P =300kA _ g _ 1.5 1 0.5 0-0.5-1 Not only far and close nulls QSF configuration even closer nulls (study ongoing) configuration as clear link to SF divertor: design of QSF configuration with the second null in the vicinity of the divertor plates maximizing the plasma current ~300kA (work on-going) -1.5-2 0 0.5 1 1.5 2 2.5 3 G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 24

Contracting or flaring for EAST QSF exps? #47660 #47660 #47660 Obtained QSF configuration with a significant distance between the two nulls and with a contracting geometry near the target plates G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 25

Ryutov s criteria for experimental EAST QSF Ryutov s criteria on conversation between the two nulls for QSF experimental discharge #47660: Weak conversation G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 26

On the differences between exp.s and TECXY Infrared Camera and TECXY heat flux at 4.5s Qualitatively: TECXY well reproduces the profile shapes and great mitigation of the load with QSF.,TECXY,TECXY Large quantitative discrepancy refers to the peak values of SN: a strong candidate for this is the diffusion into the private region that should have a nonnegligible magnitude in front of the strong [Viola to be presented at EPS 2015] gradients found in the main SOL Indeed the differences of the integrals over this region are consistent with the integral of the experimental curve on the left side of the graph Conversely the QSF discrepancies could be attributable to experimental inaccuracies. This matter is presently under further investigation by taking into account the effect of the impurities. Recent upgrade of TECXY would also allow in a short time to consider the private region too. G. Calabrò, 8 th IAEA TM-SSO, Nara (Japan), 26 May 2015 27