Liquid metal vapour shielding in linear plasma devices T.W. Morgan 1 - PowerPoint PPT Presentation

Liquid metal vapour shielding in linear plasma devices T.W. Morgan 1 , G.G. van Eden 1 , P. Rindt 2 , V. Kvon 1 , D. U. B. Aussems 1 , M. A. van den Berg 1 , K. Bystrov 1 , N.J. Lopes Cardozo 2 and M. C. M. van de Sanden 1 1 Dutch Institute for Fundamental Energy Research- DIFFER, Eindhoven, The Netherlands 2 Eindhoven University of Technology, The Netherlands IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 1/28

Going from ITER to DEMO involves large jumps in several parameters Timescales/fluence much larger o Neutron loading much higher o Narrow path to avoid excessive o exhaust power Property ITER DEMO 1 Pulse length ~400 s ~7200 s Duty cycle <2% 60-70% Neutron load 0.05 dpa/yr 1-9 dpa/yr Exhaust power 150 MW 500 MW Divertor area ~4 m 2 ~6 m 2 Radiated power 80% 97% Courtesy G. Matthews IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 2/28

Limiting factors for W in DEMO Neutrons Big ELMs/VDEs/disruptions Thermal shock/fatigue High heat/particles Cracking (small Melting- irreversible Erosion ELM-like loading) 2 damage For 5 mm W Progressive Runaway failure? lifetime ~2 years 1 deterioration 3 Transmutation, H+He creation, defects Smaller operational temperature window 1 Maissonier NF 2007 2 Linke NF 2011 Increased brittleness 3 Loewenhoff FED 2012 IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 3/28

Capillary porous structures (CPSs) create conduction based stabilized PFCs Replace solid surface with liquid o MHD forces ( j x B ) destabilize liquids in o tokamaks (droplets) Use surface tension/capillary refilling o Replace top region with this combined o material Evtikhin 1999 Thin CPS layer W monoblock Capillary supply Coolant pipe to surface Coolant LM reservoir IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 4/28

Benefits of liquid metals for DEMO Neutrons Sputtering Big ELMs/VDEs/disruptions Thermal shock/fatigue No cracking Already molten Self replenishment Higher heat fluxes Lowered stresses Vapour protection substrate (see later) ELMs possible(?) Only influences substrate Separation of PSI from neutron issue IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 5/28

Material options of Li, Sn both have strengths and weaknesses Choices once cost, availability, activation etc. taken into account Lithium Tin/Ga Low Z Higher Z High vapour pressure Lower vapour pressure High T retention Lower T retention Allain and Taylor PoP (2012) Wesson, Tokamaks (2004) IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 6/28

Magnum-PSI/Pilot-PSI utility for LM study due to DEMO relevant heat/particle loading IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 7/28

Linear devices have good flexibility and diagnostic access for basic physics and test module studies Skimmer plates (differentially pumped chambers) Plasma beam Water cooled vacuum vessel Rotatable/tiltable target holder Plasma source Diagnostic ports through coil Superconducting magnetic field coil IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 8/28

Linear devices have good flexibility and diagnostic access for basic and test module studies IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 9/28

Vapour shielding: additional loss channels for heat flux (impurity stimulated “ detachment ” ) Solid metal: 𝑟 𝑞𝑚𝑏𝑡𝑛𝑏 = 𝑟 𝑑𝑝𝑜𝑒 𝑟 𝑞𝑚𝑏𝑡𝑛𝑏 = 𝑟 𝑑𝑝𝑜𝑒 + Liquid metal: 𝑟 𝑓𝑤𝑏𝑞 + 𝑟 𝑠𝑏𝑒 + 𝑟 𝑛𝑏𝑡𝑡 IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 10/28

Experiment compared performance of Sn CPS with solid Mo reference targets Ion species T n e Qref n.b. Deliberately poorly cooled to e (10 20 m -3 ) (eV) (MW m -2 ) reach VS temperature regime H or He 0.4-3.1 0.6-7.0 0.47-22 IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 11/28

Vapour interaction with plasma decouples input power from surface temperature Poorly cooled Sn samples exposed to power load series in pilot-PSI T surf at target centre Temperature locked through shot Temperature rise cut off van Eden PRL 2016 IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 12/28

T emperature locking when vapour pressure and plasma pressure ~ matches Plasma pressure vs vapour pressure Equilibrium T surf at target centre IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 13/28

Overall reduction in power to cooling water of ~one third Evaporation alone cannot explain energy loss: relatively high re-deposition rate means most energy returns to surface IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 14/28

Strong recombination occurs due to lowered T e T e Sn shots vs Mo shots Atomic/Molecular processes IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 15/28

Oscillatory self-regulatory behaviour is observed: dynamical equilibrium REPEAT IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 16/28

Shielding behaviour is oscillatory in nature T surf centre and edge Vapour cloud size and emission • correlated to surface temperature 1 𝜏𝑜 𝑓  d mfp ↑ ; T e and/or n e ↓ 𝑒 𝑛𝑔𝑞 ∝ • IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 17/28

Oscillations in floating target potential indication of T e variations IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 18/28

Oscillation in continuum emission – a sign of n e variation • Continuum emission increasing throughout cycle. n e increases by factor 4 • Increased mean free path of Sn neutrals thus explained by reduced collision rate due to lower T e • Plasma pressure still ~conserved. ( ∝ n e T e ) IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 19/28

Cyclical equilibrium leads to dynamic locking of temperature at pressure balance point Detachment timescale: τ ie = τ ei = τ e m He • /2m e ≈ 0.2 µ s (at 0.8 eV and n e =10 20 m -3 ) Vapour extinction timescale: τ v = • d ax / (2 𝑙𝐶 𝑈 𝑡𝑣𝑠𝑔 )/𝑛 ≈ 16 µs Cooling timescale: T surf = (T 0 - T cool ) • 𝑓 −𝑢/τ 𝑑 → τ c ≈ 250 µs Oscillation freq. (~10 Hz) set by • thermal equilibrium timescale IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 20/28

Increased (visual) emission near Li CPS target compared to solid reference Applied conditions: 150 A, 14 slm He, 0.8 T  11.4 MW m -2 Li CPS Mo reference: IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 21/28

T echnological challenge- performance on long timescales via surface replenishment Component similar to and designed to test design for NSTX-U Rindt FED 2016 IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 22/28

Similar vapour shielding effect observed for Li as for Sn High heat load can be sustained (8 MW m -2 o Temperature cut off peak heat load) Sn vapour limit is ~1700 ° C (P vap ~3P plasma ) o Prediction for Li is therefore ~700-900 ° C o oscillations Prediction well matched by observation o Similar oscillatory behaviour as for Sn o Heat flux (MW m -2 ) 8 Time (s) IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 23/28

Analytical description of VS mechanism Heat conducted through target. Power dissipated by lost Li. 𝑹 𝒒𝒎𝒃𝒕𝒏𝒃 = 𝐑 𝐝𝐩𝐨𝐞 (𝑈 𝑡𝑣𝑠𝑔 ) + 𝚫 𝐌𝐣𝐮𝐢𝐣𝐯𝐧 𝐌𝐩𝐭𝐭 𝑈 𝑡𝑣𝑠𝑔 , 𝑆 ∙ 𝛝 co𝐩𝐦 𝛝 𝐝𝐩𝐩𝐦 =? Net lithium loss rate. Limited by available supply IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 24/28

Lithium energy dissipation through ionization and radiation Dissipated energy per particle 𝝑 𝒅𝒑𝒑𝒎 [𝒇𝑾] 𝑼 𝒇 [𝒇𝑾] Calculated from collisional radiative modeling in: Goldston et al., Nuclear Materials and Energy, 2017. IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 25/28

A temperature plateau occurs when dissipation via Li becomes dominant. Tungsten substrate melting, 3422 o C Li dissipated regime ~800 o C surface conductive temperature regime ~10 MW/m 2 deposited power IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 26/28

FEM shows good agreement with experiment Temperature plateau when Li dissipation becomes bare molybdenum significant. lithium Temperature [ o C] Time [s] IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 27/28

Benefit for DEMO of vapour shielding Decouples incoming heat load from surface o temperature and reduces cooling requirements Maximum impurity influx ~fixed o For Sn adds protection for off-normal events: o adds robustness and is more forgiving For Li constant operation could be possible o IAEA VS Meeting | 19-20 th March 2018 | T.W. Morgan LM VS in linear plasma devices 28/28