Projected Fueling Efficiency and Implications for a DEMO Fusion Reactor

Projected Fueling Efficiency and Implications for a DEMO Fusion Reactor L.R. Baylor Oak Ridge National Laboratory IAEA DEMO Workshop 2016 Karlsruhe, Germany Nov. 15-18, 2016 ORNL is managed by UT-Battelle for the US Department of Energy

Outline Magnetic Fusion Fuel Cycle Background Fueling Efficiency Data from Tokamaks Fueling of DEMO Burning Plasmas Anticipated Fueling Efficiency and Implications Summary 2

Fuel Cycle What is it and Why is it Important? Fueling of fusion reactors is much more complicated than conventional power producing sources - Fuel is radioactive gas (Tritium) - Fuel burn fraction is low ( < few% is likely) --> significant fueling and exhaust flow rate - Exhaust contains additional species (He, hydrocarbons, H 2 0, etc.) - Exhaust impurities and fuel isotopes need to be separated - Fuel injection is non-trivial - Pumping technology is challenged by steady-state tritium - Compatibility with mitigation of transients is necessary - Fuel (Tritium) is bred in the reactor and has to be processed Without the fuel cycle the burning plasma will not persist 3

Fusion Fuel Cycle Challenges Steady-state Fuel Injection Pellet Fuel Formation Tritium Breeding Steady-state Exhaust Pumping Tritium Reprocessing Fuel Injection Fuel Injection Tritium Breeding Exhaust Pumping Pellet Fueling Pellet Formation Tritium Reprocessing & Separation Holistic Approach is Needed 4 4

How do you Fuel a Fusion Reactor Plasma? Gas Injection Neutral particle penetration is poor in a dense SOL plasma (opaque to neutrals) Pellet Injection Solid cryogenic pellet injection technology has matured to a level for implementation on ITER Physics is reasonably well understood and can be projected to DEMO CT Injection Attempted in small tokamaks, not mature enough to speculate on effectiveness in a reactor Other Concepts? Pure Tritium Extrusion Hydrogen, Deuterium and Tritium Pellets 5

Fuel Throughput (Pa-m 3 ) Fuel Throughput in Fusion Plasmas Progress and Needs DEMO ITER FNSF ITER Tore Supra JT60 JET ATF TFTR LHD TRIAM-1M DT *Requires recirculation ST DIII-D 1 Min 1 Hr 1 Day Plasma Duration (s) DT fuel throughput for ITER and beyond is well beyond what has been achieved in previous devices. Recirculation makes the fuel cycle much more complicated. 6

Fueling Burning Plasma with DT Recycling of neutrals is expected to be R ~ 0, high density operation makes neutrals opaque to entering the plasma. Fueling efficiency of pellets h f can be nearly 1 in small tokamaks using inner wall injection high for burning plasma less certain for stellarator Tritium burn fraction is the ratio of He production to T fuel input f B = G a /G in From plasma physics analysis of burning plasma conditions, with R=0, h f = 1, f B is anticipated to be ~2%. Jackson, et al, FST 2013 DT D2 + Neon gas 7

Outline Magnetic Fusion Fuel Cycle Background Fueling Efficiency Data from Tokamaks Fueling of DEMO Burning Plasmas Anticipated Fueling Efficiency and Implications Summary 8

Pellet Fueling is a 2-Step Process, Ablation Followed by Homogenization Pellet ablation well understood with neutral gas shielding (NGS) model (Parks, Milora, Lengyel, Kuteev, Pegourie, et al.) dn p /dt n e 1/3 T e 5/3 r p 4/3 Fast ions and alphas can also impact dn/dt During homogenization along field lines, B and curvature drift occurs leading to polarization of the ablatant, resulting in ExB drift v W eb B 3 2W B B Pellet Ablatant (Cloud) DR p (P CL, R, q) (Rhozansky, Parks, Pegourie, et al.) HFS R E - + - + LFS ExB

B in Stellarator is Much Weaker than a Tokamak 4 3 W7-X j=0 B (T) 2 DIII-D 10 1-1.0-0.8-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 r B and curvature drifts are weaker for stellarator geometry, thus ExB drift of pellet ablatant is expected to be weaker nonetheless it has been observed. (R. Sakamoto, Plasma Fus. Res. 2013)

Fueling Efficiency from Pellet Injection is Defined as Percent of the Pellet Mass that is Measured in the Plasma (Nucl. Fusion, 47 (2007) 1598) Data have been collected from the 5 different injection locations on DIII-D to examine the mass deposition process. The inner wall in particular the V+1 location to some extent indicate mass penetration deeper than expected from pellet ablation modeling without drifts taken into account. Fueling efficiency is higher with HFS injection 11

Pellet Injection is Crucial for Effective Core Fueling in ITER as Shown in H-mode Fueling Source Profile Comparison D + 10 19 m -3 s -1 1000 100 10 DIII-D (Experiment) HFS Pellet LFS Pellet Gas 100 torr-l/s 1 0.0 0.2 0.4 0.6 0.8 1.0 r 1000 100 0.1 0.01 HFS pellet 10 1 HFS Pellet LFS Pellet Gas ITER 0.001 0.0 0.2 0.4 0.6 0.8 1.0 r Gas puff Gas Fueling Efficiency < 1% Gas puff core fueling in ITER will be much less effective than in DIII-D - ITER pellet profiles are from PRL ( 5-mm @ 16 Hz ) (Parks, Phys. Rev. Lett. 2005) - Gas fueling rate of ~1000 torr-l/s (130 Pa-m 3 /s) for ITER case B2-Eirene slab (solid), SOLPS (dashed) calculations (L. Owen and A. Kukushkin) Nucl. Fusion 47 (2007) 443 448 12

Pellet Density Change Prediction in ITER after Homogenization Varies Depending on the Drift Model Used Parks, et al, PRL 2005 Pegourie, et al, NF 2007 Polevoi, et al, PPCF 2001 Pegourie, et al., Plasma Phys. Control. Fusion 51 (2009) 12402 Pellet fueling deposition calculations for ITER 5mm HFS pellets. Mass drifts beyond the pedestal for all models. Pellets injected into the same discharge conditions from the inner wall guide tube port. (H-mode, T e (0) = 20 kev, T ped = 4 kev, D ped =0.04) 13

Pellet Fueling Efficiency for LFS Pellets Scales with Increased Penetration Depth Fueling Efficiency 1.0 0.8 0.6 0.4 0.2 AUG L-mode AUG H-mode DIII-D H-mode Tore Supra L-mode 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Edge Gas Penetration Depth l /a Axis ASDEX-U and Tore Supra fueling efficiency data yield similar results to the DIII-D data. Only DIII-D H-mode (ELMing and ELMfree) data is shown here. Data is from IPADBASE (Nucl. Fusion, 37 (1997) 445.) 14

Pellet Fueling Efficiency for LFS Pellets Scales with Increased Penetration Depth Fueling Efficiency 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Edge Gas Penetration Depth l /a AUG L-mode AUG H-mode DIII-D H-mode Tore Supra L-mode Axis LHD ASDEX-U and Tore Supra fueling efficiency data yield similar results to the DIII-D data. Only DIII-D H-mode (ELMing and ELMfree) data is shown here. Data is from IPADBASE (Nucl. Fus., 37 (1997) 445.) LFS pellets injected in LHD follow this trend (R.Sakamoto, Nucl. Fus. (2001)) 15

Pellet Fueling Efficiency in DIII-D M-mode Plasmas Shows Improvement with Penetration Depth Nucl. Fusion 47 (2007) 1598 Fueling Efficiency on DIII-D is higher with deeper penetration, but a clear difference with injection location is apparent. Little difference in penetration depth is present in the HFS pellet data. 16

HFS Pellet Fueling Efficiency on DIII-D is High for Shallow Penetration in ELMing H-mode DIII-D Pedestal Nucl. Fusion 47 (2007) 1598 Edge Pellet Depth r Axis Fueling Efficiency on DIII-D is higher with deeper penetration, but a clear difference with injection location is apparent. Vertical HFS injection has lower fueling efficiency than inner wall HFS injection. 17 17

HFS Pellet Fueling Efficiency on DIII-D is High for Shallow Penetration in ELMing H-mode ~15% N e DIII-D ~30-45% N e Difficult to make smaller pellets to better mimic the ITER/DEMO fueling scenario Pedestal Nucl. Fusion 47 (2007) 1598 Edge Pellet Depth r Axis Fueling Efficiency on DIII-D is higher with deeper penetration, but a clear difference with injection location is apparent. Vertical HFS injection has lower fueling efficiency than inner wall HFS injection. 18

Outline Magnetic Fusion Fuel Cycle Background Fueling Efficiency Data from Tokamaks Fueling of DEMO Burning Plasmas Anticipated Fueling Efficiency and Implications Summary 19

Fueling System for a Burning Plasma Requirements: Provide D-T fuel to maintain the plasma density and isotope mix for a specified fusion power, P DT = E DT g (1 g )n i2 σv DT where g n T /(n D + n T ) P DT ~ g (1-g) n i 2 T i 2/3 Replace the D-T ions consumed in the fusion reactions and that escape Establish a density gradient for plasma particle (especially helium ash) flow to the edge. 0.25 FpP ( t DT ) 0 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 g 0 t 35-65% T is needed for high fusion power 1 20

ITER Inner Wall Guide Tubes Provided for HFS Pellet Fueling ITER HFS [S. Maruyama, IAEA2012] LFS Pellet Guide Tube at 3 Toroidal Locations for ELM Pacing Guide tubes to be installed behind blanket modules for HFS pellet injection Difficult to design inner wall fueling guide tubes without sharp bends that limits pellet speed 21

ITER HFS Guide Tubes not Optimal for Fueling Efficiency Complicated curved guide tubes will limit the intact pellet speed to <300 m/s, erosion and shallow penetration will limit the fueling efficiency. ~10% erosion is measured in lab pellet transport tests. Difficult to design inner wall guide tubes around existing infrastructure 22

Vertical HFS Injection with Straight Guide Tubes may Lead to More Efficient Fueling in a Burning Plasma EU DEMO 23

Vertical HFS Injection with Straight Guide Tubes may Lead to More Efficient Fueling in a Burning Plasma 24 The DEMO machine may need to be designed to provide straight (or nearly so) injection for optimal fueling. Injection needs to be as straight as possible for high speed and with suitable access through coils and blanket. Single stage gas guns can inject pellets at 1500 m/s, repeating 2-stage guns up to 2500 m/s. Curved guide tubes like in ITER will limit the intact pellet speed to < 300 m/s. Research needed on high speed pellets in slightly curved tubes. EU DEMO Frattolillo, Tokunaga (SOFT2016)

Dn e (10 20 m -3 ) Calculations of Pellet Penetration for DEMO Conditions Shows Penetration to the Pedestal Top EU DEMO (Te PED = 6 kev) Calculation of different pellet ablation perturbations for DEMO from VHFS at 1500 m/s. Increasing the speed to 2500 m/s (2-stage gun) is ~20% deeper penetration. 300 m/s ITER speed pellets lead to very shallow penetration less than the pedestal top. Calculation of pellet mass drift shows deposition extends inward to r ~ 0.8 for 1000 m/s inner wall pellets (Pegourie EPS2016) 1.0 0.8 0.6 0.4 0.2 0.7 10% 2500 m/s 0.8 0.9 1.0 r 15% 10% 300 m/s 10% 5% 27

Outline Magnetic Fusion Fuel Cycle Background Fueling Efficiency Data from Tokamaks Fueling of DEMO Burning Plasmas Anticipated Fueling Efficiency and Implications Summary 28

ELM Interaction on HFS Pellet Fueling Efficiency Remains an Open Question ELMs are triggered on DIII-D with HFS pellet injection, even with RMP ELM suppression. ELMs are triggered with HFS pellets during ELM pacing, but not larger ELMs than the triggered ELMs. HFS pellets do not trigger ELMs on AUG with RMP ELM suppression and density can be strongly increased (Valovic, et al., Nucl. Fus. 2016). What happens when pellet penetration is shallow, outside r=0.9 (top of pedestal) with or without ELM suppression? Difficult to mimic this scenario on present day tokamaks Pellets less than 1.5mm are difficult to transport intact to HFS Pedestal T e much less in smaller machines 29

h Fueling Efficiency with High Speed Pellets with Optimal Injection Geometry can Posssibly Exceed 50% DIII-D ITER DEMO Pedestal Edge Pellet Depth r Axis 30 30 Extrapolation from present small tokamaks to ITER and DEMO is highly uncertain, but h is likely less in DEMO than ITER from more shallow penetration.

31 Fusion DEMO Pellet Fueling Needs are Significant f B = 2% or less implies significant fuel recirculation Each D,T fuel atom on average must recirculate ~50 times or more before fusing This implies 50 or more phase changes from plasma to gas to solid How much refrigeration power? D,T enthalpy change from 300K to solid is: 6000 J/mol = 0.03 ev per atom. 17.6 MeV per reaction is significantly more Nonetheless, at reactor fueling rates over 100 kw of refrigeration power needed

D-T Mixture Pellet Formation is Possible and has Been Demonstrated Solidifier Liquifier D2 T2 32

Fusion Exhaust Pumping Is Equally as Important as Fueling for a DEMO Function: Remove the exhaust gases from the plasma losses and burn byproducts. The torus vacuum pumping must maintain low divertor neutral pressure (~10 Pa) while removing helium ash that will be generated by the fusion burn. - Conductance from divertor to pumps must be high. Reactor requirements: - A Fusion Reactor will require reliable steady state pumping that can be controlled and maintained in a nuclear fusion environment. - It is likely that the pumping speeds and throughputs will be 2-5x higher than ITER, and with a much higher duty factor. Gaps: Tritium compatible pumping at fusion pressures and throughputs is a challenge 33

Fusion Exhaust Pumping Schemes Batch Cryopumps ITER Method T2 inventory, Deflagration limit, Thermal cycling, Valve cycling, He pumping Continuous Cryopump Snail pump prototype developed and tested by Foster - Mechanical scraper - Cryo separation - Helium compression for conventional pumping - Pellet formation concept tested Liquid Metal Pumps (KIT): Diffusion pump and liquid metal ring roughing pump - Needs separation of impurities and helium cryogenic? - Super permeable membrane separation? ITER 34

Continuous Cryopump System Coupled to Pellet Injector DT Duct 30K Extruder 15K Fuel Recirculation Loop Impurity Trap 20K Snail Pump Snail Exhaust 30K Impurity Pump 35 He Pump Tritium Plant Extruder Exhaust 30K

Summary Significant additional R&D is needed in the fusion fuel cycle to make fusion viable for a DEMO A reactor must be designed from the beginning for optimal fueling and pumping efficiency High fueling efficiency > 50% can be achieved with suitable high speed HFS injection in a tokamak DEMO A stellarator DEMO would also need high speed pellets Its very useful to be planning ahead for the fueling and pumping needs beyond ITER 36

The End Tritium Pellet 37

Snail Continuous Cryopump Designed to Meet the Fusion Requirements Snail pump uses brass scraper to remove solid DT deposited on cold inner cylinder. Scraped solid heated to 30K for transport to downstream pumps He passes through pump after compression Invented by C. Foster at ORNL and Developed by SBIR project in 2005 38 38

D-T Continuous Pellet Formation and Acceleration is Being Developed Liquifier Solidifier Fast Pneumatic Valve Solenoid Cutter Nozzle Gas Gun Barrel 39