High radiation scenarios in pronounced detached divertor conditions - PowerPoint PPT Presentation

High radiation scenarios in pronounced detached divertor conditions at ASDEX Upgrade M. Bernert 1 , F. Reimold 2 , R. Dux 1 , T. Eich 1 , A. Huber 2 , A. Kallenbach 1 , B. Lipschultz 3 , M. Wischmeier 1 , the EUROfusion MST1 team 4 and the ASDEX Upgrade Team 1 Max-Planck-Institut für Plasmaphysik, Garching, Germany 2 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung – Plasmaphysik, Jülich 3 University of York, York Plasma Institute, Heslington, York, United Kingdom 4 See http://www.euro-fusionscipub.org/mst1. 1 st IAEA TM on Divertor Concepts, Vienna, 29 th Sep – 2 nd Oct 2015

Motivation DEMO requirements: – Detached divertor necessary to reduce power and particle flux With deuterium: No full / pronounced detachment in H-mode ⇒ Seeding impurities necessary for power dissipation – 95% of exhaust power needs to be dissipated Only achievable with radiation inside and outside confined region ⇒ Core and edge radiators necessary Possible with a conventional divertor using strong impurity seeding M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 2

Motivation DEMO requirements: – Detached divertor necessary to reduce power and particle flux With deuterium: No full / pronounced detachment in H-mode ⇒ Seeding impurities necessary for power dissipation – 95% of exhaust power needs to be dissipated Only achievable with radiation inside and outside confined region ⇒ Core and edge radiators necessary Goal: Test scenarios in present day experiments Match SOL plasma • What is the maximum f rad ? ( P sep /R ≈ 15MW/m, • How stable are these regimes? f GW ≈ 1, H 98 ≈ 1) M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 3

The ASDEX Upgrade Divertor • P heat = 27 MW ( available 20 MW NBI, P heat /R ≈ 16 MW/m 6 MW ECRH, 6 MW ICRH) • R = 1.65 m • Fueling - Main fueling and seeding from divertor - High-Z seeding from outer midplane • Divertor - Closed divertor - Vertical targets - Tungsten coated CFC - Solid tungsten tiles (outer target) ( q max > 10 MW/m 2 [Hermann, NF 2015] ) M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 4

Detachment (in H- mode) • Inner divertor typically detached • Progress of outer divertor detachment: – Partially – Pronounced – Fully [A.Kallenbach, NF 2015] • Outer divertor detachment correlated with increase of plasma density – Changed fueling? • Detachment in H-mode only achieved with intense seeding, e.g. N – Full detachment only inter-ELM – ELMs: Complex sequence of detachment states M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 5

The different radiators [T.Pütterich, EPS 2015] Various seeding impurites possible • Nitrogen: Divertor • Neon: SOL • Argon: SOL & pedestal • Krypton: Pedestal & core What is the optimal impurity mix? N Ne Ar Kr M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 6

High radiation scenarios Nitrogen: P heat = 21 MW • Pronounced P heat /R = 12.7 MW/m detachment f GW ≈ 0.95 H 98 ≈ 0.9 • Small reduction of confinement c N,core ≈ 2-3% f rad ≈ 90% • Dominant radiation from inside confined region M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 7

High radiation scenarios: Nitrogen • Strong radiator at X-point ⇒ MARFE-like radiation condensation • With ongoing detachment: Radiator moves at X-point from outside to inside of confined region • Time evolution of several seconds ⇒ RT control possible Constant N puff -> increasing N density M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 8

High radiation scenarios: Nitrogen [F.Reimold, PSI 2014] • Radiator can be reproduced by SOLPS • Temperature reduction within confined region - T e < 5eV - D line radiation observed ⇒ Parallel temperature gradients inside confined region! M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 9

High radiation scenarios: Nitrogen [F.Reimold, PSI 2014] • Radiator can be reproduced by SOLPS • Temperature reduction within confined region - T e < 5eV - D line radiation observed ⇒ Parallel temperature gradients inside confined region! • Pedestal top pressure reduced at similar core performance M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 10

High radiation scenarios Krypton: P heat = 19 MW • Pronounced P heat /R = 11.5 MW/m detachment f GW ≈ 0.8 H 98 ≈ 0.9 • Small reduction of confinement c Kr < 0.1% • Low impact on dilution f rad ≈ 75% • Radiating ring around pedestal top M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 11

High radiation scenarios: Krypton • Krypton radiates in ring at pedestal top • Nonlinear response to Kr seeding level ⇒ poloidally symmetric radiation condensation? Kr replaces (RT controlled) N • Discharges only quasi-stable (stable for less than 10 τ E ) • Kr modulated by ELMs • ELM frequency reduces M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 12

Krypton: High radiation scenarios: Confinement Stable (AUG #30503) Reducing (AUG #31648) P heat = 19 MW P heat = 10.5 MW - Radiation within pedestal - Radiation moves inside - Temperature reduction - Density increase too small compensated by density increase ⇒ Radiation outside pedestal top M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 13

High radiation scenarios: Comparison Nitrogen: Krypton: • Pronounced detachment of outer divertor at highest heat fluxes • Most radiation inside confined region - Poloidally localized radiation - Radiating ring (above X- point) ρ pol ≥ 0.985 0.8 ≤ ρ pol ≤ 1 - Small reduction of confinement - Impact on confinement (<10%) varying - f ELM increases - f ELM decreases - Quasi-stable for more than 2s - Stable vs ELMs, full stability not shown yet - Lower impact on fuel dilution Z eff (3%N) ≈ 1.91, Z eff (1‰Kr) ≈ 1.91, d fuel (3%N) ≈ 6% d fuel (1‰Kr) << 1% M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 14

JET M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 15

Summary Detached divertor possible at high heating powers using N and/or Kr – Conventional divertor – P heat /R ≈ 12 MW/m (Demo: P sep /R=15MW/m) – f rad ≤ 90% – Dominant radiation inside the confined region – Scenarios quasi-stable (N ) – Real time control most likely possible – Impact on confinement differs with radiation location (Kr ) – A possible solution for DEMO and ITER? Outlook • Stability (Kr ) and controllability ( N ) to be tested • Where do Ne and Ar radiate in AUG? M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 16

Open questions, points of discussion • What is the stabilizing mechanism for the X-point radiator? • Why didn’t it work for carbon walls? • How does the radiation influence the H-mode threshold? • Does maybe P ped matter instead of P sep ? • How to estimate P sep ? • What is the best impurity mix? • Do future machines need other impurities (e.g. xenon)? • Impurity behaviour with pellet fueling? • Increased impurity divertor compression? M. Bernert 1st IAEA TM on Divertor Concepts, 29th Sep 2015 17