Beam Driven Alfvén Eigenmodes and Fast Ion Transport in the DIII-D and ASDEX Upgrade (AUG) Tokamaks

Transcription

1 Beam Driven Alfvén Eigenmodes and Fast Ion Transport in the DIII-D and ASDEX Upgrade (AUG) Tokamaks by M.A. Van Zeeland 1 M. García-Muñoz 2, W.W. Heidbrink 3, I. Classen 4, R.K. Fisher 1, B. Geiger 2, R. Nazikian 5, D.C. Pace 3, B.J. Tobias 6, R.B. White 7, S. Aekaeslompolo 8, M.E. Austin 9, J. Boom 4, S. da Graca 10, M. Gorelenkova 5, N.N. Gorelenkov 5, A.W. Hyatt 1, N. Hicks 2, G.J. Kramer 5, N. Luhmann 6, M. Maraschek 2, G.R. McKee 11, H. Park 12, S. Sharapov 13, W. Suttrop 2, G. Tardini 2, Y. Zhu 3, and the ASDEX Upgrade and DIII-D Teams 1 General Atomics 8 Aalto University 2 Max-Planck-Institut fur Plasmaphysik 9 University of Texas-Austin 3 University of California-Irvine 10 Associacao EUATOM/IST 4 FOM-Institute for Plasma Physics Rijnhuizen 11 University of Wisconsin-Madison 5 Princeton Plasma Physics Laboratory 12 POSTECH 6 University of California-Davis 13 Euratom/UKAEA 7 Princeton Plasma Physics Laboratory Presented at the 52 nd Annual Meeting of the APS Division of Plasma Physics Chicago, Illinois November 8-12, 2010

2 Why Look at Alfvén Eigenmodes (AEs)? Alfvén Eigenmodes are routinely observed in present tokamaks and are predicted to be unstable in ITER. Measurements show AE activity is correlated with transport of fast ions This increased transport can cause: Reductions in fusion performance* Damage to first-wall components** Understanding the properties of AEs through modeling and experimental observation is essential in order to have confidence in predictions for their impact on ITER and future devices. NN Gorelenkov, et al., Nucl. Fusion 43, 749 (2003) *GY Fu and JW Van Dam, Phys. Fluids B, 1, 1949 (1989) **HH Duong et al., Nucl. Fusion 33, 749 (1993)

3 AUG/DIII-D Collaboration Focused on Enabling Both Devices to Better Address AE Physics Problems Pre ~ 2009 DIII-D Beam Driven AEs YES 1,2 Impact of AEs on core fast ion profile (FIDA) AE induced fast ion loss (FILD) YES 3 ASDEX Upgrade NO NO NO YES 4 1 M.A. Van Zeeland et al., Phys. Rev. Lett. 97, (2006) 2 R. Nazikian et al., Phys. Plasmas 15, (2008) 3 W. W. Heidbrink et al., Phys. Rev. Lett. 99, (2007) 4 M.Garcia-Munoz et al., Phys. Rev. Lett. 104, (2010) Historically, beam driven AEs in DIII-D are common and observed to cause large core fast ion transport No loss measurement capability ASDEX AE studies typically focused on AEs driven by RF accelerated ions Excellent loss measurement capability Beam driven AEs not common and impact on confined profile unknown

4 AUG/DIII-D Collaboration Focused on Enabling Both Devices to Better Address AE Physics Problems DIII-D ASDEX Upgrade Beam driven AEs YES YES 1 Impact of AEs on core fast ion profile (FIDA) AE induced fast ion loss (FILD) YES YES 1,2 YES 3,4 YES 1 M. Garcia-Munoz, et al., IAEA FEC, Daejeon, Korea (2010) 2 B. Geiger, et al., PPCF (In Preparation) Thesis Work 3 D.C. Pace, R.K. Fisher, et al., RSI 81, 10D305 (2010) 4 D.C. Pace, et al., IAEA FEC, Daejeon, Korea (2010) Historically, beam driven AEs in DIII-D are common and observed to cause large core fast ion transport No loss measurement capability ASDEX AE studies typically focused on AEs driven by RF accelerated ions Excellent loss measurement capability Beam driven AEs not common and impact on confined profile unknown

5 AUG/DIII-D Collaboration Focused on Enabling Both Devices to Better Address AE Physics Problems DIII-D ASDEX Upgrade Beam driven AEs YES YES 1 Impact of AEs on core fast ion profile (FIDA) AE induced fast ion loss (FILD) YES YES 1,2 YES 3,4 YES 1 M. Garcia-Munoz, et al., IAEA FEC, Daejeon, Korea (2010) 2 B. Geiger, et al., PPCF (In Preparation) Thesis Work 3 D.C. Pace, R.K. Fisher, et al., RSI 81, 10D305 (2010) 4 D.C. Pace, et al. IAEA FEC, Daejeon, Korea (2010) Questions Addressed With New Capability: Is DIII-D observed fast ion transport reproducible in similar ASDEX plasmas? Do these modes cause fast ion loss? Important question for future devices and can also help understand details of physics

6 Outline Primary Diagnostics Background on Alfvén Eigenmodes and associated fast ion transport measured in DIII-D ASDEX Upgrade beam driven Alfvén Eigenmode results DIII-D fast ion loss measurements and modeling Summary, Conclusions and Future

7 Fast Ion Loss Detector (FILD) Provides Key Information on Alfvén Eigenmode Induced Losses Collimator and Magnetic Field Provide Energy and Pitch Discrimination Scintillator Image with Energy and Pitch FILD measures the pitchangle and energy of lost fast ions 1,2 Large bandwidth allows measurements at Alfvén Eigenmode frequencies (~100 khz) key for identifying coherent losses and discriminating between individual modes Local phase-space measurements like these help to isolate the physics 1 M. Garcia-Munoz et al., RSI 80, (2009). 2 D.C. Pace, R.K. Fisher, et al., RSI 81, 10D305 (2010) D.C. Pace, UP POSTER This Afternoon

8 Fast Ion D-alpha (FIDA) Measures the Impact of AEs on the Confined Fast Ion Profile FIDA is a charge exchange measurement exploiting the large Doppler shift of fast neutrals Fast neutrals created when fast ions charge exchange on injected beams W.W. Heidbrink, RSI 81, 10D727 (2010) FIDA signal n FI n neutral Several channels radial profiles of quantity related to fast ion density FIDA SIM code constructed to simulate FIDA emission for a given fast ion distribution

9 Outline Primary Diagnostics Background on Alfvén Eigenmodes and associated fast ion transport measured in DIII-D ASDEX Upgrade beam driven Alfvén Eigenmode results DIII-D fast ion loss measurements and modeling Summary, Conclusions and Future

10 Alfvén Eigenmodes are Common in DIII-D Plasmas They are driven by 80 kev deuterium beams* (V B /V A ~ 0.4) qmin Typically unstable during the current ramp phase when Central magnetic shear is reversed P NB Ptot *R. Nazikian, et al., Phys. Plasmas 15, (2008) Minimum q (q min ) is decreasing Neutral beam pressure (P NB ) is a significant fraction of the total pressure (P tot ) Beam driven AEs are not typically observed in ASDEX Upgrade plasmas

11 TAEs and RSAEs are Most Common AEs Observed and are Easily Identified Through Their Spectral Behavior Crosspower Beam Emission Spectroscopy (BES) and CO 2 Interferometer TAEs Toroidicity-induced Alfvén Eigenmodes (TAEs) Global modes Frequency changes gradually q min decreasing M.A. Van Zeeland, et. al., Phys. Plasmas 14, (2007)

12 TAEs and RSAEs are Most Common AEs Observed and are Easily Identified Through Their Spectral Behavior Crosspower Beam Emission Spectroscopy (BES) and CO 2 Interferometer TAEs Toroidicity-induced Alfvén Eigenmodes (TAEs) Global modes Frequency changes gradually RSAEs q min decreasing Reversed Shear Alfvén Eigenmodes (RSAE*) Localized near q min Frequency sweeps upward as q min decreases *A. Fukuyama, et al., IAEA 2002 TH/P3-14 M.A. Van Zeeland, et al., Phys. Plasmas 14, (2007)

13 Delaying Beam Injection Alters AE Stability and Provides Example of Mode Impact on Fast Ions NBI NBI Injection at 300 ms shows clear TAEs and RSAEs Up to 50% neutron deficit relative to classical TRANSP predictions due to fast ion transport Delaying beam injection until 500 ms alters current profile NO AEs With no AEs neutron emission is classical ASDEX Upgrade typically injects beams later during the current ramp Likely reason no beam driven AEs observed previously

14 Delaying Beam Injection Alters AE Stability and Provides Example of Mode Impact on Fast Ions NBI NBI Injection at 300 ms shows clear TAEs and RSAEs Up to 50% neutron deficit relative to classical TRANSP predictions due to fast ion transport Delaying beam injection until 500 ms alters current profile NO AEs With no AEs neutron emission is classical ASDEX Upgrade typically injects beams later during the current ramp Likely reason no beam driven AEs observed previously

15 Outline Primary Diagnostics Background on Alfvén Eigenmodes and associated fast ion transport measured in DIII-D ASDEX Upgrade beam driven Alfvén Eigenmode results DIII-D fast ion loss measurements and modeling Summary, Conclusions and Future

16 Alfvén Eigenmodes Obtained With Early 60 kv Neutral Beam Injection In ASDEX Upgrade RSAEs Early 60 kv neutral beam injection, similar to DIII-D reference case, created spectrum of RSAEs Modes are accompanied by large neutron deficit (relative to TRANSP predictions) - indicative of fast ion transport As RSAEs disappear, neutron emission returns to classical levels M. Garcia-Munoz, et al., IAEA FEC, Daejeon, Korea (2010)

17 ASDEX Upgrade FIDA Data Indicate Large Reduction in Core Fast Ion Density During RSAE Activity n fi (arb) FIDA FIDA SIM FIDA FIDA SIM FIDA SIMulation code predicts FIDA emission assuming classical fast ion profile Large deficit in FIDA emission relative to FIDA SIM indicates central depletion of fast ion density As with neutron emission, FIDA profile returns to classical levels after modes disappear M. Garcia-Munoz, et al., IAEA FEC, Daejeon, Korea (2010) B. Geiger, et al., PPCF (In Preparation) Thesis Work

18 ASDEX Upgrade FIDA Data Indicate Large Reduction in Core Fast Ion Density During RSAE Activity n fi (arb) FIDA FIDA SIM FIDA FIDA SIM FIDA SIMulation code predicts FIDA emission assuming classical fast ion profile Large deficit in FIDA emission relative to FIDA SIM indicates central depletion of fast ion density As with neutron emission, FIDA profile returns to classical levels after modes disappear M. Garcia-Munoz, et al., IAEA FEC, Daejeon, Korea (2010) B. Geiger, et al., PPCF (In Preparation) Thesis Work

19 ASDEX Upgrade FIDA System Measures a Drop in Central Fast Ion Population as q min Passes Through an Integer qmin=2 At q min =2 crossing, several RSAEs are excited by 60 kv beams (Grand Cascade) ECEI #25528 Rapid drop in central fast ion density corresponding to peak in RSAE amplitude FIDA( ~0) No fast ion losses observed during this event May be geometrical blocking due to small outer gap

20 Higher Energy Beams Observed to Drive Spectrum of TAEs and RSAEs Repeating discharge with 90 kv beams drove spectrum of RSAEs and TAEs unstable Edge magnetics detect combination of RSAEs, TAEs and additional mode ECEI at mid-radius detects primarily RSAEs

21 TAEs Observed to Cause Fast Ion Loss FILD spectrogram shows clear coherent losses from beam driven TAEs FILD FILD Scintillator indicates TAE induces losses appear near gyroradius corresponding to injection energy

22 TAEs Observed to Cause Fast Ion Loss FILD SCINTILLATOR t~0.675 FILD spectrogram shows clear coherent losses from beam driven TAEs t~0.775 FILD Scintillator indicates TAE induced losses appear near gyroradius corresponding to injection energy FILD

23 Outline Primary Diagnostics Background on Alfvén Eigenmodes and associated fast ion transport measured in DIII-D ASDEX Upgrade beam driven Alfvén Eigenmode results DIII-D fast ion loss measurements and modeling Summary, Conclusions and Future

24 FILD Was Commissioned on DIII-D and it Sees Coherent Losses at Alfvén Eigenmode Frequencies FILD Using what we learned from ASDEX Upgrade, a series of experiments were carried out at DIII-D to utilize the new FILD diagnostic # FILD sees coherent losses from TAEs, RSAEs, and other modes D.C. Pace, R.K. Fisher, et al., RSI 81, 10D305 (2010) D.C. Pace, et al. IAEA FEC, Daejeon, Korea (2010)

25 Modeling Focused on DIII-D Discharge and Two Time Ranges with Very Different Levels of Fast Ion loss Strong AE Loss Weaker AE Loss FILD # Time windows have large difference in levels of fast ion loss yet similar mode amplitudes Goals are to see if modeling can explain/reproduce: Origin of the losses Change in loss levels Energy and pitch of losses observed with scintillator

26 Methodology For Simulating Fast Ion Loss

27 ECE and ECEI Data Combined With FILD Are Used to Identify Primary Modes Many RSAEs and TAEs observed by ECE and other diagnostics

28 ECE and ECEI Data Combined With FILD Are Used to Identify Primary Modes Many RSAEs and TAEs observed by ECE and other diagnostics White points represent modes with significant coherence between several adjacent ECE channels and FILD (these are the modes that cause fast ion loss)

29 ECE and ECEI Data Combined With FILD Are Used to Identify Primary Modes n=2 RSAE n=3 RSAE Many RSAEs and TAEs observed by ECE and other diagnostics White points represent modes with significant coherence between several adjacent ECE channels and FILD (these are the modes that cause fast ion loss) n=1 TAE n=5 TAE n=3 TAE n=4 TAE n=3 RSAE n is determined from magnetics and ECEI/BES give m

30 NOVA* Calculated Eigenmodes are Selected Based on Mode Type and Match to ECE and ECEI Data NOVA* solves for linear ideal MHD eigenmodes using experimentally measured profiles T e is used to determine experimental amplitude Process is repeated for t~725 ms case *C.Z. Cheng, Phys. Rep. 211, 1 (1992) B.J. Tobias, et al., PRL (Submitted)

31 ORBIT* Code Is Used to Calculate Transport and Additional COM Based Calculation Follows Particles To Wall ORBIT* is a Hamiltonian guiding center code that calculates particle trajectories in a tokamak in the presence of wavefields (taken from NOVA) as well as collisions Follows particles to last closed flux surface only Recently used to successfully model impact of AEs on confined fast ions in DIII-D plasmas 1,2 *R.B. White & M.S. Chance, Phys. Fluids 27 (1984) 1 R.B. White, et al., PPCF 52, (2010) 2 R.B. White, et al., PoP 17, (2010)

32 ORBIT* Code Is Used to Calculate Transport and Additional COM Based Calculation Follows Particles To Wall ORBIT* is a Hamiltonian guiding center code that calculates particle trajectories in a tokamak in the presence of wavefields (taken from NOVA) as well as collisions Follows particles to last closed flux surface only For a given equilibrium, energy, position, and pitch, Constants of Motion (COM) define unique poloidal trajectory easily obtained with contour routine Used to follow particles outside LCFS to wall Approach verified with full orbit code

33 Simulations Used 1M Particles Sampled From TRANSP Calculated Distribution Function Distribution contains Co- and Countercurrent ions Only considered energies above ~20 kv Initial distribution is peaked on axis and distributed uniformly toroidally

34 ORBIT Calculations Find Losses Due to Modes SAMPLE OF LAUNCHED LOST (Cross Last Closed Flux Surface) All particles which fall onto orbits intersecting wall were initiated on counter current orbits HIT WALL Of the lost orbits only a small subset intersect FILD, with the majority being closer to the injection energy HIT FILD Other particles could also possibly hit FILD Model does not allow particles to re-enter plasma and continue interaction with modes

35 When a Particle s Trajectory Passes Near FILD, Its Gyoradius and Pitch are Recorded Particles striking FILD are localized in pitch (~43-49 degrees) Losses peak at FILD near injection energy (~80 kev)

36 When a Particle s Trajectory Passes Near FILD, Its Gyoradius and Pitch are Recorded Particles striking FILD are localized in pitch (~43-49 degrees) Losses peak at FILD near injection energy (~80 kev) Typical loss situation - initial green orbit (confined) is pushed to red (unconfined orbit). Ions strike FILD on cocurrent leg of orbit

37 Majority of Ions Hitting Near Peak on FILD Scintillator Were Initialized on Counter Passing Orbits Counter passing ions are pushed to larger minor radius losing energy and toroidal canonical angular momentum Eventually counter passing orbit crosses loss boundary and is on unconfined trapped orbit intersecting FILD Once on unconfined orbit, trajectory is very close to unperturbed trajectory

38 Energy and Pitch of Losses Correspond to That Measured Experimentally Simple synthetic scintillator is constructed by binning particles in gyroradius and pitch Prompt Losses FILD Scintillator Simulated loss spot is in same range of Gyroradius and Pitch as non-prompt loss spot on FILD

39 Simulations Reproduce Observation of Significantly Less Loss at Higher Current (Later Time) ORBIT - ALL LOST PARTICLES t~525 ms ORBIT - ALL LOST PARTICLES t~725 ms Same simulation procedure results in fewer total losses for later time case Fraction of particles hitting FILD is even smaller Even though mode amplitudes are similar, result may have been expected: Fewer modes extending to large radii (where loss region is) Higher current at later time moves loss region out further away from modes

40 Simulations Reproduce Observation of Significantly Less Loss at Higher Current (Later Time) B Modes Used in ORBIT t~525 ms B (arb) Same simulation procedure results in fewer total losses for later time case Fraction of particles hitting FILD is even smaller B Modes Used in ORBIT t~725 ms B (arb) Even though peak mode amplitudes are similar, result may have been expected: Fewer modes extending to large radii (where loss region is) Higher current at later time moves loss region out further away from modes

41 Summary / Conclusions Beam driven Alfvén eigenmodes have been observed in ASDEX Upgrade using early beam injection as in DIII-D experiments During periods of AE activity, ASDEX Upgrade FIDA measurements show depletion of the central fast ion density and reduced neutron emission AUG, ECEI #25528 DIII-D, CO2 # Coherent losses of fast ions induced by AEs have been observed in both DIII-D and ASDEX Upgrade experiments Modeling of DIII-D fast ion losses is consistent with many features of the measurements including: energy, pitch, and evolution with plasma current

42 Future: Carry Out Similar Alfvén Eigenmode Induced Transport Calculations for ITER Unstable AEs calculated for projected ITER scenarios Fast ion loss simulations analogous to those shown here will be run for a range of B/B ITER and DIII-D simulations will also be extended to include particles leaving and re-entering plasma (G. Kramer s SPIRAL code)