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Pellet Fueling Technology Leading to Efficient Fueling of ITER Burning Plasmas

Pellet Fueling Technology Leading to Efficient Fueling of ITER Burning Plasmas. presented by L.R. Baylor in collaboration with P.B. Parks*, S.K. Combs, W.A. Houlberg, T.C. Jernigan, S. Maruyama # , L.W. Owen, G.L. Schmidt, D.A. Rasmussen Oak Ridge National Laboratory, *General Atomics,

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Pellet Fueling Technology Leading to Efficient Fueling of ITER Burning Plasmas

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  1. Pellet Fueling Technology Leading to Efficient Fueling of ITER Burning Plasmas presented byL.R. Baylor in collaboration with P.B. Parks*, S.K. Combs, W.A. Houlberg, T.C. Jernigan, S. Maruyama#, L.W. Owen, G.L. Schmidt, D.A. Rasmussen Oak Ridge National Laboratory, *General Atomics, #ITER International Team at the Burning Plasma Workshop 5-July-2005 Tarragona, Spain

  2. Overview • ITER requires significant fueling capability to operate at high density for long durations • Gas fueling will not be able to sustain high density in ITER due to limited neutral penetration in the thick dense scrape off layer • Pellet fueling from the inner wall looks promising for core fueling with high efficiency despite limited pellet speeds • The ITER pellet injection system requires capabilities well beyond the current state-of-the-art • Throughput enhancement of nearly an order of magnitude • Reliability at high repetition rate is required for BP control • The use of pellets for ELM triggering and amelioration remains a possibility for ITER • Understanding pellet interaction with NTMs, ELMs, RWMs etc needed

  3. DIII-D Cross Section ITER Fueling Needs are Significant 4 m • ITER plasma volume is 840 m3 and scrape-off layer is ~20 cm thick. This compares to 20 m3 and ~ 5 cm for DIII-D. ITER Cross Section

  4. ITER Fueling Needs are Significant 4 m • ITER plasma volume is 840 m3 and scrape-off layer is ~30 cm thick. This compares to 20 m3 and ~ 5 cm for DIII-D. • ITER is designed to operate at high density (> 1x 1020 m-3) in order to optimize Q. ITER Cross Section

  5. ITER Fueling Needs are Significant 4 m • ITER plasma volume is 840 m3 and scrape-off layer is ~30 cm thick. This compares to 20 m3 and ~ 5 cm for DIII-D. • ITER is designed to operate at high density (> 1x 1020 m-3) in order to optimize Q. • Gas to be introduce from 4 ports on outside and 3 in the divertor region Gas Injectors ITER Cross Section

  6. ITER Fueling Needs are Significant 4 m • ITER plasma volume is 840 m3 and scrape-off layer is ~30 cm thick. This compares to 20 m3 and ~ 5 cm for DIII-D. • ITER is designed to operate at high density (> 1x 1020 m-3) in order to optimize Q. • Gas to be introduce from 4 ports on outside and 3 in the divertor region • NBI fueling to be negligible (< 2 x 1020 atoms/s or < 3 torr-L/s ) Gas Injectors Note that DIII-D at 10 MW is ~ 10 torr-L/s ITER Cross Section

  7. ITER Fueling Needs are Significant 4 m • ITER plasma volume is 840 m3 and scrape-off layer is ~30 cm thick. This compares to 20 m3 and ~ 5 cm for DIII-D. • ITER is designed to operate at high density (> 1x 1020 m-3) in order to optimize Q. • Gas to be introduce from 4 ports on outside and 3 in the divertor region • NBI fueling to be negligible (< 2 x 1020 atoms/s or < 3 torr-L/s ) • Inside wall pellet injection planned for deep fueling and high efficiency. Reliability must be very high. Gas Injectors Pellet Injection ITER Cross Section

  8. ITER Fueling Needs are Significant 4 m • ITER plasma volume is 840 m3 and scrape-off layer is ~30 cm thick. This compares to 20 m3 and ~ 5 cm for DIII-D. • ITER is designed to operate at high density (> 1x 1020 m-3) in order to optimize Q. • Gas to be introduce from 4 ports on outside and 3 in the divertor region • NBI fueling to be negligible (< 2 x 1020 atoms/s or < 3 torr-L/s ) • Inside wall pellet injection planned for deep fueling and high efficiency. Reliability must be very high. • Pellet injector must operate for up to 1 hour continuously and produce up to 4500 cm3 of DT ice per discharge. Gas Injectors Pellet Injection ITER Cross Section

  9. Gas Fueling in ITER is Much Less Efficient than in Current Machines Gas Fueling Source Profile 1000 Fueling efficiency is DNplasma / Nsource 100 Gas Fueling Efficiency < 1% 10 D+ 1019 m-3 s-1 1 0.1 DIII-D Gas 0.01 ITER Gas 0.001 0.0 0.2 0.4 0.6 0.8 1.0 r (normalized minor radius) • This B2-Eirene slab calculation shows that gas puff core fueling in ITER will be much less effective than in current experiments such as DIII-D. • Gas fueling rate of 100 torr-L/s for DIII-D • Gas fueling rate of ~1000 torr-L/s for ITER case (L. Owen and A. Kukushkin) (see also Kukushkin & Pacher, Plasma Phys. Control. Fusion 44, 931, 2002 )

  10. 2.7 mm pellets -HFS 45°vsLFS 10 DIII-D 98796 - measured Dne HFS 45° vp = 118 m/s Dt = 5 ms LFS vp = 586 m/s Dt = 1 ms ne (1019 m-3) 5 Calculated Penetration 0 0.8 0.0 0.2 0.4 0.6 1.0 1.2  Pellet Injection from Inner Wall Looks Very Promising for Tokamak Plasma Fueling Fueling Efficiency: HFS - 95% LFS - 55% • Net deposition is much deeperfor HFS pellet in spite of the lower pellet velocity used to survive curved guide tube • Pellets injected into the same discharge and conditions (ELMing H-mode, 4.5 MW NBI, Te(0) = 3 keV)

  11. Theoretical Model for Pellet Radial Mass Drift • Polarization of the ablatant occurs from B and curvature drift in the non-uniform tokamak field: • The resulting E yields an ExB drift in the major radius direction, V^ = (ExB)/B2 ExB Polarization Drift Model of Pellet Mass Deposition(Rozhansky, Parks) B µ1/R • JÑB= - 2p/RB and this balances the polarization return current Jp = (r/B2) dE/dt. (p is cloud pressure and r is cloud density) • Therefore the pellet cloud motion equation is dV^/dt = 2p/rR • DR drift distance is stronger at higher plasma b due to higher cloud pressure • Detailed model by P.B. Parks, [Phys. Rev. Lett. 94, 125002 (2005)]. Pellet Ablatant (Cloud) - E LFS HFS - + + ExB R

  12. Pressure Relaxation Lagrangian (PRL) Code Solves Coupled Drift and Parallel Dynamics for a Series of Cloudlets Cloudlet • The PRL code uses the pellet size and plasma parameters at each point along the ablation track determined by PELLET code [Houlberg, Nucl. Fusion 1988] to initialize the cloudlet parameters using model of Parks, et al. 10-20 cloudlets are assumed per pellet. [see Phys. Rev. Lett. 94, 125002 (2005)]. • The cloudlets form a tube of high density plasma along the field lines. The ends of the cloudlet are sheared off as it drifts inward (mass shedding). • The experimental plasma profiles are used by PRL to calculate the cloudlet pressure relaxation, drift velocity, and shedding location. • The deposition profiles from each cloudlet are summed, yielding a net n deposition profile. Deposition profile PELLET Ablation (No drift) Dne Cloudlet drift r Pellet + Cloud Cloudlet Q Lagrangian cells of constant mass Heat flux

  13. PRL Model PRL Model Experiment and PRL Model Compare Well Inside launch (45 deg above mid-plane) Outside midplane launch 10 10 DIII-D 98796 2.7mm pellet, vp = 586 m/s DIII-D 99477 2.7mm pellet, vp = 153 m/s NGS Ablation Model x0.3 Data ne (1019 m-3) ne (1019 m-3) NGS Ablation Model x0.5 5 5 Data 0 0 0.0 0.2 0.4 1.0 0.6 0.8 0.8 0.0 0.2 0.4 0.6 1.0   • Vertical arrows indicatepellet burnout location • Fueling efficiency for inside launch is much higher (even with slower pellets) • outside launch theory = 66% , exp = 46% (discrepancy due to strong ELM) • inside launch theory = 100% , exp = 92% (discrepancy due to weak ELM) • PRL model is a major breakthrough in understanding the physics of pellet mass drift

  14. Pellet Injection is Crucial for Effective Core Fueling in ITER as Shown in H-mode Fueling Source Profile Comparison HFS pellet Gas puff DIII-D ITER 1000 1000 HFS Pellet 100 LFS Pellet 10 100 Gas D+ 1019 m-3 s-1 1 0.1 10 HFS Pellet Gas Fueling Efficiency < 1% LFS Pellet 0.01 Gas 0.001 1 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 r r • Gas puff core fueling in ITER will be much less effective than in DIII-D • ITER pellet profiles are from PRL (P. Parks) ( 5-mm @ 16 Hz ) • gas fueling rate of ~1000 torr-L/s for ITER case B2-Eirene slab calculation (L. Owen and A. Kukushkin)

  15. ITER Deposition Profiles 5.0 Outside Midplane Pellets x 0.3 5mm 4.0 3mm 3.0 D ne (1019 m-3) 2.0 Inner Wall Pellets 1.0 0.0 0 0.2 0.4 0.6 0.8 1 r Density Change in ITER as a Function of Inner Wall Pellet Size • Pellet fueling deposition calculations from PRL for ITER with different size pellets. Larger pellet size yields marginally deeper mass penetration. Mass drifts well beyond the pedestal for all pellet sizes. Outside midplane injection deposition profiles (dashed) with no drift are shown for comparison. • Pellets injected into the same discharge conditions from the inner wall guide tube port. (H-mode, Te(0) = 20 keV, Tped = 4 keV, Dped=0.04)

  16. Weaker Shear Leads to Deeper Mass Deposition for ITER Inner Wall Pellet Injection 8 4 k=2 6 k=5 3 k=20 D ne (1019m-3) 4 Safety factor, q k=2 2 2 k=5 k=20 0 1 0 0.2 0.4 0.6 0.8 1 1 0 0.2 0.4 0.6 0.8 r r • Pellet fueling deposition calculations from PRL for ITER with different plasma q profiles. Stronger shear at the edge leads to more rapid mass shedding of the cloudlets and hence shallower mass penetration. • Pellets (6mm from inner wall) injected into the same discharge conditions (H-mode, Te(0) = 20 keV, Tped = 4 keV, Dped=0.04)

  17. Pellet Path in ITER ITER Pellet Fueling Requirements • ITER will initially have 2 pellet injectors that each provide D2, DT, T2 pellets (5mm @ 16Hz, 3mm @ 32Hz). • Inside wall pellet injection for deep fueling beyond the pedestal and high efficiency. Reliability must be very high. • Guide tubes bring the pellets in from divertor ports and routes them to the inner wall. • Pellet injector must operate for up to 1 hour continuously and produce ~ 1.5 cm3/s of ice.

  18. ITER Fueling Systems Requirements & Present Design Requirements refined at ITER Pellet Injector Workshop in Garching, May 2004 • Gas injection system • Supplies H2, D2, T2, DT, Ar, Ne, and He via a gas manifold • Primary use for initial gas fill, control of SOL, and flushing impurities to divertor • Makes use of conventional gas handling hardware and requires minimal R&D • Pellet injection system • Supplies H2, D2, and DT pellets: 3 to 5 mm diam. (32 to 16 Hz, respectively) • Only at pre-conceptual design level and some R&D still needed

  19. ITER Inner Wall Guide Tube Tests Indicate 300 m/s Speed Limit • Initial tests with 5.3 mm pellets • Pellet speeds limited to ≈300 m/s for intact pellets • Guide tube mass loss ≈10% at speed limit Pellet Path in ITER S. Combs, et al. SOFT 2004

  20. Cryocooler Cryocooler D2, T2, DT D2, T2, DT Supply Supply Cutter Cutter Extruder Extruder Rotor Arm Rotor Arm Guide Guide Tube Tube Centrifuge Centrifuge Vacuum Vacuum Guard Vacuum Guard Vacuum Tritium Tritium Pump Pump Reprocessing Reprocessing Plant Plant Pellet Injection Cask Pellet Injection Cask ITER Pellet Injection System Conceptual Design • ITER initially will have 2 pellet injectors for deep core fueling as the primary fuel delivery system. • Up to 6 injectors planned for future • Requires continuous, highly reliable, high throughput, tritium rich pellets • significant throughput extension of present-day designs • Centrifuge accelerator witha continuous screw extruder • Inner wall pellet injection with curved guide tubes • Maximum T concentration is ~80% due to tritium processing plant limitation • PIS to be enclosed in cask that rolls up to a divertor port 6.5 m 2.5m 3.6 m

  21. Tritium Pellet Formation has been Proven and Can Be Used to Control Plasma Burn Tritium Pellet Injector at TSTA Tritium Extrusion (≈8 mm) • Tritium pellet formation and acceleration were found to be readily achievable with present technology. • Pellets with high T2 concentration are envisioned for fueling ITER using the isotopic tailoring scheme • T2 rich pellets combined with D2 gas puffing (Gouge, et al., Fusion Tech. 1995) • Multiple pellet injectors with different T fractions can be used to control fusion power

  22. Screw D2 Heat Exchanger Solid D2 He LHe Vacuum Pump Batch Piston and Continuous Screw Extruders are Possible to Meet the Needs for the ITER Pellet Injection System Batch Piston Extruder Continuous Screw Extruder • Both batch piston and continuous screw extruders have been developed as possible ITER ice sources • Multiple batch extruders have produced 1.3 cm3/s (S. Combs, et al, RSI (1998)while a continuous screw extruder by PELIN has produced steady-state H2 ice up to 0.3 cm3/s. (~1/5 of rate needed for ITER) (I. Viniar, SOFT 2004). • Throughput enhancement may be possible or multiple such extruders could be used on the ITER pellet injection system. • Simpler operation makes the screw extruder preferable over a batch extruder. LHe OFHC Copper 8mm H2 ice extrusion

  23. Pellet ELM Triggering May Provide Tool for ELM Amelioration • Pellet injection has been found to trigger ELMs in ELMing H-mode plasmas ( AUG, DIII-D, JET). • LFS pellets trigger larger ELMs than the same pellets from the inner wall, leading to a possible sensitive LFS pellet ELM trigger. • AUG has succeeded in increasing the ELM frequency and lowering the ELM size using small pellet triggers. (P. Lang et al., Nuc. Fusion 2004) • ITER 3mm size pellet is for ELM triggering using a LFS guide tube. • Further research is needed to investigate the pellet induced ELM mechanism and its scaling to ITER. • Interaction of pellets with NTMs, RWMs, ELMs, etc. needs better understanding. Time (s) LFS Pellets for ELM triggering

  24. Summary • ITER will require significant fueling beyond that provided by gas • Gas fueling and recycling expected to be very inefficient • Inner wall injection port will allow up to 300 m/s pellet injection • Modeling of the proposed ITER pellet injection scenario looks promising for core fueling well beyond the H-mode pedestal • Further validation of the ExB polarization drift model is needed with diagnostics and scaling studies • The pellet fueling system for ITER presents challenges for the technology developers in throughput and reliability, concepts look promising • Development is underway and expected to take ~ 5 yrs • Centrifuge and extruder prototypes will be produced which can be available to test on existing tokamak devices • ELM triggering by small LFS pellets also a promising technique for ITER • Further research to optimize and understand physics of pellet induced ELMs and ELM amelioration is required as well as other MHD interactions.

  25. Pellet Droplet Concept for ELM Triggering Pellet Droplet Device • A batch extruder with curved nozzle and pellet cutter can supply sub 1mm D2 pellets at up to 100 Hz for triggering ELMs on DIII-D • The extruder is to be cooled with a GM cryocooler for simplified installation and operation • Gravity acceleration limits speed to < 50 m/s • Low speed and small pellet size minimize fueling, but make strong enough density perturbations to trigger ELMs V+3 Solenoid Cutter D2 Ice 0.7-1.3 mm @ 50-100 Hz

  26. ITER Pellet Injector Pumping System LHe • The mass loss rate is on the order of ~12 Pa-m3/s (90 torr-L/s) per pellet injector when operated at the full fueling rate. (~10% of fueling rate) • Assuming an operating pressure of 0.1 Pa (1 mtorr) implies a required pumping speed of 120,000 L/s. • A continuously regenerating cryopump, i.e. a snail pump, is an ideal pumping scheme as the output can be fed back in the fuel input stream. The snail pump by CAF, Inc. (C. Foster) is the most promising technology for this task. • Prototypes of this pump have been developed with recent tests at LANL achieving > 120,000 L/s pumping speed for D2(S. Willms, C. Foster, et al.) Inlet Exhaust to blower Snail in operation

  27. ITER Inner Wall 6mm Pellet Deposition as a Function of Pedestal Te 6.0 Tped = 1 keV 5.0 Tped = 2 keV Tped = 4 keV 4.0 3.0 D ne (1019m-3) 2.0 1.0 0.0 0 0.2 0.4 0.6 0.8 1 r • Pellet fueling deposition calculations from PRL for ITER with different size pellets. Larger pellet size yields marginally deeper mass penetration. • Pellets injected into the same discharge conditions (H-mode, Te(0) = 20 keV, Tped = 4 keV, Dped=0.04)

  28. 2.0 DIII-D 120775 H-mode 5 MW NBI neL (1014 cm-2) 1.5 1.8mm V+3 pellets 1.0 6 4 Divertor Da (a.u) 2 0 4.1 4.0 3.9 3.7 3.8 3.6 Vertically Injected Pellets on DIII-D from V+3 Port Trigger ELMs with Minimal Density Perturbation • 1.8 mm pellets injected from the DIII-D V+3 port trigger ELMs with minimal fueling of the core plasma. Pellets injected at ~200 m/s through a curved guide tube.

  29. H2 Extrusions Achieved with Cryocooler Technology Coldhead Extruder • Extruders for long-pulse pellet injectors are usually cooled with LHe (dewars or cryogenic plant) • Pellet injectors with cryocooler ice formation offers some advantages • low operating cost after initial investment (up to several $k per day for laboratory operation with LHe) • independent from LHe dewars or facility cryogenic systems • Both piston and screw extruders have been successfully mated to cryocoolers. Extrapolation to higher throughput extruders for ITER is under development. (Pfotenhauer, Combs, Viniar )

  30. Multiple Extruders Operated in Sequence Provided a Steady-State Supply of H2 Ice for Feeding a Pellet Injector • ITER EDA prototype testing at ORNL showed that batch piston extruders could be employed for the needed pellet source. • With the three-extruder prototype, a rate of 1.3 cm3/s was produced and ≈0.33 cm3/s was maintained for a duration of 1 hour (~1/4 of rate needed for 5mm@16Hz). • Operation of batch piston extruders at > 1 cm3/s is possible with 4-6 such extruders operating in tandem. LHe OFHC Copper 8mm H2 ice extrusions (S. Combs, et al, RSI (1998).

  31. Inner Bore Pellet Injector for ITER? • Pellet injection through the central solenoid as shown in this ITER cross-section has been suggested. (F. Perkins, PPPL, see Poster EP1.114 Tuesday morning) • The impetus for this idea is to maximize penetration with faster pellets. • Penetration distance l of pellet will scale linearly in velocity in ITER pedestal instead of the customary lµ v0.3.

  32. 5.0 Vp = 0.3 km/s Vp = 1.0 km/s midplane 4.0 3.0 D ne (1019m-3) 2.0 1.0 0.0 0 0.2 0.4 0.6 0.8 1 r ITER Inner Wall 6mm Pellet Comparison Inner Bore vs CurvedGuide Tube • Higher speed inner wall pellet injection made possible by placing the injector inside the central solenoid (F. Perkins, Poster EP1.114). • PRL calculations show a deeper fuel penetration for the same size pellets assuming > 3x the speed.

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