Neutral recirculation the key to control of divertor operation A.S. - PowerPoint PPT Presentation

Neutral recirculation  the key to control of divertor operation A.S. Kukushkin 1,2 , H.D. Pacher 3 1 Kurchatov Institute, Moscow, Russia 2 NRNU MEPhI, Moscow, Russia 3 INRS ‐ EMT, Varennes, Québec, Canada Presented at the 1 st IAEA Technical Meeting on Divertor Concepts, Vienna, 29 Sept – 2 October 2015

Introduction Reactor plasma controls: Force balance ( pressure, magnetic field, currents ) Power balance ( heating, radiation, transport ) Particle balance ( fueling, pumping ) Fueling affects all the controls Pressure: p = n ∙ T P fus  n D ∙ n T ∙ F(T) Fusion reactions: P rad  n e ∙ n I ∙ L(T) Radiation: n <  ∙ n G limitation Transport: For steady state,  fuel =  pump (neutrals!)  Neutral recirculation affects everything ( unless Li walls are employed! ) A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 2

Introduction Divertor functions: Impurity screening ( plasma ‐ surface interaction further from the core ) Pumping ( neutral compression ) Improving core confinement ( experimental observation, unexpected ) Side effect: Power flux concentration on targets  severe operational constraint  Need to control  neutrals! ITER design studies 1995 ‐ 2014: neutral transport in the edge is the most important issue This presentation: review of those studies A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 3

SOLPS4.3 – computational model FED ‐ 2012 SOLPS4.3 development: primary attention to neutral transport puff Reasonably simple plasma model ‐ no drifts, no currents in the SOL flux ‐ limited   and   , constant   , D  ‐ Monte ‐ Carlo model for neutrals ‐ m ‐ i collisions, including c ‐ x (“MAR”) and elastic ‐ n ‐ n collisions elastic He 0 – D + collisions ‐  important for ITER (also for C ‐ mod [S. Lisgo, JNM 2003] ) to pump Scope: SOL & divertor plasma, target loading, particle fluxes, pumping, … Interaction with the core: feedforward P SOL ,  core , P rad_core (for surface loading) Methodology: density scans, matching detachment state for comparisons A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 4

NF ‐ 2003 Core ‐ SOL model integration PSI ‐ 2010 Direct coupling impractical – time scales too different “Mediated approach”: parameterization of SOLPS results  BC for the core model  Core solutions compatible with the edge Edge ‐ imposed restrictions for the core  operational window for whole plasma Realisation: ICPS model based on 1D ASTRA code for the core [G.W. Pacher et al.] Scope: operational flexibility, effect of divertor variations and operation mode Methodology: Vary core inputs and adjust puffing rate to keep q pk  q max , find the area in parameter space satisfying all known physics and design constraints A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 5

PSI ‐ 2014 Edge and core fueling: practically independent High ‐ power, big size machine (e.g. ITER): Increase of the puffing rate  saturation of n sep at rather low level depending on P SOL  strong neutral screening in the SOL  low neutral influx to the core  poor core fueling  Additional means of core fueling are needed (pellets) Gas puff efficient for q pk control  Nearly independent controls for • Core: pellets Divertor: gas puff • To be tested in experiment (ITER?) Indications from C ‐ mod A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 6

PSI ‐ 2012, PSI ‐ 2006 Power dissipation in divertor Divertor detachment: the path to reach acceptable q pk Energy dissipation disproportionately high in near SOL Radiation loss  n 2  effect of neutral fueling from PFR • Stronger with high p n Longer dome (i ‐ n contact area  ): higher q pk for same p n  higher p n needed Detachment ( I sat rollover ) at the same p n  Narrower window in p n (or Dome To Pump q pk ) if limited by detachment To Pump ( core confinement ) A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 7

PSI ‐ 2014 Overfuelling: discharge collapse normal baffled Typical in detachment modelling: I sat rollover as puffing rate increases (p n goes up) Then at some point the edge plasma collapses • no flux to targets • recycling inside separatrix Power loss and recombination zones • also move inside the core Stronger baffling delays the collapse  Collapse is caused by neutrals escaping from the divertor to the core  Neutral re ‐ circulation determines the limits of stable divertor detachment A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 8

PSI ‐ 2014 Gas puffing vs. impurity seeding Impurity seeding: Increase of divertor radiation  reduction of n sep and q pk Prerequisite for q pk : high density (fueling!) OK for divertor However, detrimental for the core plasma performance at high Q Integrated modelling: Operational window decreases in Q as c Ne grows (more P aux is needed)   puff with minimum seeding (to control n sep ) is preferable for q pk control A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 9

EPS ‐ 2005 In ‐ out asymmetry Neutral exchange between the inner and outer divertors: Outer divertor: more power (geometry effect) Inner divertor: cooler plasma, higher density  more neutrals  neutrals flow from inner to outer divertors, reducing the asymmetry When this neutral flow is impeded (reducing gas conductivity beneath the dome)  Stronger in ‐ out asymmetry, higher q pk (outer)  Transparency of dome Transparency  supporting structures important A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 10

PSI ‐ 2006, PSI ‐ 2010 Role of divertor dome From previous slides: Reduction of the dome furthers detachment Free inner ‐ outer neutral exchange beneficial  weaker asymmetry, lower q pk Why do we need the dome at all? – For pumping! If dome were removed, need a factor 10 higher pumping speed to keep the same He upstream or the same fuel throughput E pump_He [eV] Becomes critical in ITER 353_iDomGrid_A_s_z10_2 or reactor:  DT >  min ,  He  P fus 1 Major effect: 0.1 cooling down the neutrals thus condensing the flow 0.01 0.1 1  A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 11

q pk [MW/m ] FEC ‐ 2006 Effect of neutral leaks in structures 366_iLk4C_z2_5 Previous slides: divertor structures assumed vacuum tight. 10 What if they leak? Simplified model with leaks: p v >> p c >> p b ; p d > p c  q pk the same q pk [MW/m 2 ] 1  He removal better : leak (  v ) enhances pumping, 2 4 6 8 10 30 m=-1.17 p DT [Pa] He from bypass (  b ) recycles further from the separatrix  inter ‐ cassette gaps acceptable  n  c  n  i  i  b p b  b p b return flo w  b p c p c Cassette body p d throughput p v p v p c  v  d to pump p c p c  v  v  p A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 12

Conclusions Conditions of neutral re ‐ circulation in tokamak affect virtually all plasma controls SOLPS4.3 integrated with ASTRA  edge ( neutrals ) effect on total performance Lessons learnt from modelling and implemented in ITER design Core and edge fueling schemes are ~ independent  separate control possible • Neutral penetration from PFR provides detachment control even if  q ~ 1 mm • • Neutral confinement in divertor determines stability of detachment Gas puffing for divertor control is better for overall performance • than impurity seeding Free neutral exchange between divertors reduces in ‐ out asymmetry (q pk  ) • • Divertor dome is essential for efficient pumping (neutral compression) • Neutral leakage through divertor structures does not degrade performance  does not affect divertor control via neutral re ‐ circulation A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 13

Disclaimer In this paper, we use results of calculations done previously for ITER conditions and published elsewhere (see, e.g., the references in the paper) to discuss more general considerations resulting from those studies. We make neither recommendations for, nor assessments of, ITER here and our conclusions imply no liability for the authors, nor for their affiliated organizations. A.S. Kukushkin, H.D. Pacher. 1 st IAEA Tech. Meeting on Divertor Concepts, Vienna, 29 Sept – 2 Oct. 2015 14