High-density operation with pellets on ASDEX Upgrade - and its impact on the DEMO design

Transcription

1 78. DPG-Frühjahrstagung AMOP March 2014, Berlin High-density operation with pellets on ASDEX Upgrade - and its impact on the DEMO design M. Bernert, L. Casali, R. Fischer, O. Kardaun, G. Kocsis, P.T. Lang, M. Maraschek, A. Mlynek, B. Plöckl, F. Ryter, T. Szepesi, G. Tardini, H. Zohm, ASDEX Upgrade Team Introduction Prerequisites for efficient fuelling Set up for high-speed inboard launch Demonstration of efficient fuelling High-density operation in a reactor Fueling concept study for DEMO March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 1

2 Introduction: Need of efficient particle fuelling A fusion reactor, like any power plant, needs fuelling Efficient fuelling = reach the requested density with minimum particle flux Fuel cycle in DEMO reactor: particle flux puts burden on pumping and recovery system, increases T inventory (Licensing!) March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 2

3 Introduction: How to achieve efficient particle fuelling At present, particle fuelling is provided mostly by gas puffing Very simple and reliable method Disadvantage: efficiency drops with tokamak size (temperature) Density limit encountered: Empirical Greenwald limit observed for gas fuelling Edge limit restricts core density and finally fusion power as well Can be overcome by deep deposition of fuel creating peaked density profiles March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 3

4 Introduction: How to achieve efficient particle fuelling Deep efficient fuelling can be achieved by Pellets, size 1 6 mm produced from frozen (about 10 K) fuel (D, T) Injection with high speed into the plasma Injection location (poloidal) plays a crucial role for the efficiency In hot plasmas, high-β-plasmoid formed from ablation cloud Manifold advantages for inboard (HFS) launch: Drifting cloud shifts particles into plasma deeper deposition and cools plasma ahead deeper penetration Drag and asymmetric heat flux accelerates pellet March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 4

5 Prerequisites for efficient fuelling Injecting pellets with high speed from the torus outboard (LFS): technically simple but drastic loss of fuelling efficiency when increasing the plasma temperature and entering ELMy H-mode (ELM: edge localised mode) Outboard (LFS) AUG with C wall Ablation cloud form high pressure plasmoid Diamagnetic Drift to torus LFS Instant losses, shallow deposition Pellet causes strong local perturbation Unstable edge ELM triggered Shallow deposited particles removed March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 5

6 Prerequisites for efficient fuelling Pellet injection from the torus inboard (HFS) results in enhanced penetration and fuelling efficiency test with low injection speed (technical limitations) March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 6

7 Prerequisites for efficient fuelling ELMs expelling particles and energy can be triggered by pellets Resilience significantly improved when replacing C by W as first wall In AUG-W pellets have almost lost their capability for ELM control ( ELM mitigation by pellet pacing ) But significantly improved their capability for efficient fuelling! March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 7

8 Efficient fuelling: optimized pellet parameters Efficient fuelling = reach requested density with minimum particle flux Maximum speed Technology yield AUG-C JET-C Maximize particle sustainment time Deep deposition; Reduced ELMs Maximum acceptable mass & March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 8

9 Efficient fuelling: Set up for high-speed inboard launch System has proven its: Reliability Sustainability Flexibility Launch speed > 1000 m/s System was developed with AUG already in full operation System designed together with torus could do be even better March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 9

10 Demonstration of efficient fuelling High-density operation by pellet fuelling Efficient pellet fuelling enables access to high density regime Scenario established reliably, found reproducible and reversible Benign ELMs in high-density regime also without (R)MP coils March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 10

11 High density operational regime Greenwald limit = edge limit Core densities far beyond n Gw achieved ( m -3 = 4.4 n Gw ) Density gradient zone seems to correlate with penetration depth Enhanced pellet particle sustainment time in high density regime, only minor contribution from deeper pellet penetration March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 11

12 High density operational regime Confinement kept Careful TRANSP analysis confirms: initial confinement kept in steady state high density phase L. Casali et al., P16.1, Wednesday 14:00, SPA HS202 Kinetic profiles confirm W MHD evolution, W fast reduced Short transient W MHD drop when pellet train set in: Temperature drop is faster than density increase χ > D Moderate initial confinement due to strong gas puff required Can be overcome increasing P heat and applying N 2 seeding March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 12

13 High density operational regime Benign ELM behaviour In previous experiments high fuelling efficiency was realized once type-i ELMs were suppressed by (R)MP coils Successful ELM mitigation only in high density regime (n/n Gw > 0.6) Pellet assisted access to high density regime P.T et al., Nucl. Fusion 52 (2012) Without (R)MP coil actuation: when approaching n/n Gw > 1 type-i ELMs almost disappear Mainly type-iii ELMs with isolated weak type-i events Enhanced drift Alfvén turbulence driven TAEs March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 13

14 High density operation in reactor For DEMO, modelling predicts peaking In todays tokamaks, sufficient pellet penetration is needed to create peaked density profiles required for n > n Gw In DEMO (higher temperature) this will be hardly possible But: strong reduction of collisionality as well Anomalous inward pinch expected (TLGF modelling) to create peaked density profile H. Zohm, P7.1, today 17:30 Kinosaal H. Zohm et al., Nucl. Fusion 53 (2013) High β somewhat reduces peaking, but not significantly March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 14

15 High density operation in reactor System design aiming for efficient fuelling Modeling (ASTRA) for a steady state localized particle source: Required flux to establish target pedestal top density (0.85 n Gw, n Sep =4x10 19 m -3 ) in DEMO-1 Sufficiently deep deposition results in high fuelling efficiency Seems doable with available technology + some additional R&D Study led by IPP & KIT to design DEMO fuelling system March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 15

16 Summary Over the last two decades, AUG gathered all the elements required for efficient particle fuelling: High-speed inboard launch & benign ELMs Application resulted in reliable, reproducible, reversible operation in the high-density regime Edge density limit can be overcome by profile peaking (transiently up to 4.4 x n Gw ) No loss of confinement in steady-state phase Deep pellet penetration not attainable in DEMO reactor, but due to low collisionality intrinsic peaking Sufficient penetration required to reduced T flux Study under way: design of DEMO (pellet) launcher March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 16

17 Back up slides for discussion March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 17

18 Prerequisites for efficient fuelling Inboard launch brings strong enhancements Direct observation of the drifting plasmoid by fast framing camera Launch from torus inboard (HFS) into AUG-W, large size 570 m/s Pre-cooling deeper penetration Tracking shows: pellet accelerated Manifold advances achieved by inboard pellet launch March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 18

19 High density operational regime Greenwald limit = edge limit H-Mode density limit encountered applying strong gas puffing 1: Normal H-mode (density ) 2: degrading H-mode (only SOL ) 3: H-L back transition (pedestal erodes) 4: L-mode (density, MARFE, disruption) V. Mertens et al., PPCF 1994 M. Bernert et al., EPS 2013 March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 19

20 High density operational regime Potential for performance improvement Scenario suitable for higher confinement H98 = 1.12 without gas puffing, but with fast impurity accumulation Impurity accumulation (2,1) (3,2) NTM H98 = 0.76 High confinement achieved with C wall can be recovered at high P heat Pellet fuelling (at still higher v P ) in high power domain (N 2 seeding) March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 20

21 High density operational regime Confinement scaling no longer appropriate Scaling H98 in common use to characterize energy confinement Also applied in most investigations of ITER/DEMO scenarios H98P(y,2) derived using data up to 0.8 n Gw only H98 ~ n 0.41 not kept beyond Need better scaling validated also in high-n e regime H06-IP(y,dd) better suited [O. Kardaun, IAEA 2006, J. Johner, Fusion Sci. Tech. 59 (2010) 308.] New scaling in preparation, different approaches considered: With additional high n e data; Modelling based in reactor regime March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 21

22 High density operational regime A better suited scaling H06-IP(y,dd) best suited scaling available at present, it reads Work in progress: ITERH14-IP(y,dd)-type scaling (O. Kardaun) Adapted for medium to high densities ( n Gw ) March 17., 2014 DPG Frühjahrstagung AMOP, Berlin P2.2 P.T. Lang 22