D. Borba 1 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Excitation of Alfvén eigenmodes with sub-Alfvénic neutral beam ions in JET and DIII-D plasmas Duarte Borba on behalf of JET EFDA contributors in collaboration with Raffi Nazikian on behalf of the DIII-D program

D. Borba 2 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Outline Objectives –Determine experimentally the V b / V A threshold for the excitation of Alfvén cascades (RSAE) using Neutral Beam Injection (NBI) Requirements –Core fluctuation measurements required due to high toroidal mode number (n) of instabilities Setup –Low energy beams with high magnetic field to minimise V b / V A Experiments in JET and DIII-D –Scan of the magnetic field, beam energy and direction and major radius Implications and Conclusions –Further opportunity to diagnose the safety factor profile (q-profile) –Core plasma turbulence and transport studies

D. Borba 3 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Outline Objectives –Determine experimentally the V b / V A threshold for the excitation of Alfvén cascades (RSAE) using Neutral Beam Injection (NBI) Requirements –Core fluctuation measurements required due to high toroidal mode number (n) of instabilities Setup –Low energy beams with high magnetic field to minimise V b / V A Experiments in JET and DIII-D –Scan of the magnetic field, beam energy and direction and major radius Implications and Conclusions –Further opportunity to diagnose the safety factor profile (q-profile) –Core plasma turbulence and transport studies

D. Borba 4 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Objective of the experiment Alfvén continuum deep reversed shear  TAE ~  A /2  RSAE <<  A Condition for instabilities requires: –Efficient energy exchange between the particles and the wave (resonance condition)  + p  t or b + n  d = 0 for TAEs V A = V b also (V b = V A /3) for Cascades (RSAE) V A >> V b Objective of the experiment: Determine experimentally the V b /V A threshold for the excitation of Alfvén cascades (RSAE) S.E.Sharapov et al., Nuclear Fusion v.46 (2006) S868.

D. Borba 5 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Outline Objectives –Determine experimentally the V b / V A threshold for the excitation of Alfvén cascades (RSAE) using Neutral Beam Injection (NBI) Requirements –Core fluctuation measurements required due to high toroidal mode number (n) of instabilities Setup –Low energy beams with high magnetic field to minimise V b / V A Experiments in JET and DIII-D –Scan of the magnetic field, beam energy and direction and major radius Implications and Conclusions –Further opportunity to diagnose the safety factor profile (q-profile) –Core plasma turbulence and transport studies

D. Borba 6 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Diagnostic Requirements Condition for instabilities requires: Efficient energy exchange between the particles and the waves Wave Particle Radial Mode width  m > Radial Orbit width  p  m ∞ 1/ (n s) Toroidal mode number Magnetic Shear mm pp In reversed shear scenarios, around the zero shear (s=0) point, large toroidal mode number (n) waves can be excited Core localised Waves with large n are invisible for external diagnostics (magnetics), so internal measurements are required

D. Borba 7 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Diagnostic Setup (JET) Far infrared DCN interferometer 3 vertical chords, measured line- integrated  n e L fluctuations out of the 4 available channels Microwave reflectometer radially localized measurement of density fluctuations using O-mode and X- mode Magnetic probes New diagnostic capability allows core fluctuation measurements, essential for the detection of NBI driven cascades

D. Borba 8 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Diagnostic Setup (DIII-D) CO 2 interferometer, ~ 1.6 MHz bandwidth, 4 chords, line-integrated  n e L 65 and 42 GHz Quad-reflectometer, ~10 MHz bandwidth, radially localized measurement of density fluctuations Beam-Emission Spectroscopy (BES), 500 kHz bandwidth, providing spatially localized measurements of  n e with increased sensitivity Far-Infrared Scattering (FIR), ~10 MHz bandwidth, line-integrated measurement of low-k density fluctuations (k = cm -1 ) CO 2 Interf. 65 GHz 42 GHz

D. Borba 9 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Measurements at JET Far infrared DCN interferometer most versatile data gives information on the instabilities throughout the shot Microwave reflectometer (O-mode) working as an interferometer gives the best signal, but relies on the density to be such that the diagnostic frequency is above the cut off Magnetic probes are mostly sensitive to low toroidal mode number instabilities (n=2,3,4), due the radial extent of these modes Using these diagnostics allows an detailed analysis of the instability threshold in different configurations Microwave reflectometer Magnetic probe Far infrared DCN interferometer 3.0 Time (s) 4.4 Frequency (kHz) # S. Sharapov et al Phys. Rev. Lett. 93, (2004); Berk et al, Phys. Rev. Lett. 87, (2001)

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Measurements at JET time (s) Alfvén Cascades near r/a=-0.2 using X-mode Reflectometer # 67732

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Outline Objectives –Determine experimentally the V b / V A threshold for the excitation of Alfvén cascades (RSAE) using Neutral Beam Injection (NBI) Requirements –Core fluctuation measurements required due to high toroidal mode number (n) of instabilities Setup –Low energy beams with high magnetic field to minimise V b / V A Experiments in JET and DIII-D –Scan of the magnetic field, beam energy and direction and major radius Implications and Conclusions –Further opportunity to diagnose the safety factor profile (q-profile) –Core plasma turbulence and transport studies

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Experimental Scenario JET Maximum Magnetic field B t =3.45 Tesla Lowest beam energy possible Lower Hybrid Current drive to obtain reversed shear (P LHCD =2MW) Low density to maximize the fraction of fast ions Objective: minimise V b /V A ~ E beam 0.5 n e 0.5 /B T in order to find the threshold for the excitation of Alfvén cascades (RSAE)

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Difficult to reduce velocity ratio V b /V A below 0.4 in DIII-D Experimental Scenario DIII- D Objective: minimise V b /V A ~ E beam 0.5 n e 0.5 /B T in order to find the threshold for the excitation of Alfvén cascades (RSAE) Time (s) V b /V A P NBI (MW) nene # #125975

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Outline Objectives –Determine experimentally the V b / V A threshold for the excitation of Alfvén cascades (RSAE) using Neutral Beam Injection (NBI) Requirements –Core fluctuation measurements required due to high toroidal mode number (n) of instabilities Setup –Low energy beams with high magnetic field to minimise V b / V A Experiments in JET and DIII-D –Scan of the magnetic field, beam energy and direction and major radius Implications and Conclusions –Further opportunity to diagnose the safety factor profile (q-profile) –Core plasma turbulence and transport studies

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Neutral Beam energy scan (JET) Cascades clearly visible with 50 keV beams at JET Beam energy scan performed at JET using low (80 keV) and high (140 keV) voltage NBI The energy of the 80 keV NBI was also decreased to 50 keV Microwave reflectometer (working as a interferometer) # 66957# # 66963# 66964

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October keV V b /V A ≈ keV V b /V A ≈0.60 Cascade Modes Observed with 50 keV Beam injection in DIII-D though at reduced level and number compared to 80 keV Neutral Beam energy scan (DIII-D) # # R. Nazikian et al Phys. Rev. Lett. 96, (2006), M. Van Zeeland et al Phys. Rev. Lett. 97, (2006).

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Alfvén Eigenmode Excitation is Sensitive to Beam Ion Direction in DIII-D Reverse Shear Alfvén modes excited mostly by co-injected ions in DIII-D Difference in drive is either due to finite orbit width effects, the radial distribution of co/counter ions and/or on an intrinsic sensitivity of the mode to ion direction. Results suggest that modes can drive current in fusion plasma from the preferential redistribution of co going alpha particles Neutral Beam direction (DIII-D) # # B.N.Breizman et al., Physics of Plasmas v.10 (2003) 3649

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Outline Objectives –Determine experimentally the V b / V A threshold for the excitation of Alfvén cascades (RSAE) using Neutral Beam Injection (NBI) Requirements –Core fluctuation measurements required due to high toroidal mode number (n) of instabilities Setup –Low energy beams with high magnetic field to minimise V b / V A Experiments in JET and DIII-D –Scan of the magnetic field, beam energy and direction and major radius Implications and Conclusions –Further opportunity to diagnose the safety factor profile (q-profile) –Core plasma turbulence and transport studies

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 q-profile 4 5 Time (s) 6 7 Frequency (kHz) q min = integer

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 t=4.8 s Frequency (kHz) t=7.2 s Frequency (kHz) 20 q-profile evolution (JET) q=4 Cascade q=3 Cascade q=2 Cascade t=3.44 s Frequency (kHz) Information from the Alfvén waves is used to reconstruct the q-profile evolution more accurately and validate the Motional Stark Effect (MSE) measurements

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October V b /V A ≈0.45 q min =4 q min =3 q min =2 Cascade Modes with 50 keV Injection also clearly indicate rational q-min crossings in DIII-D q-profile evolution (DIII-D) #125975

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Conclusions I Alfvén waves (RSAE/ Alfvén Cascades), can be excited by sub-Alfvén Neutral Beam Injection as low as V b /V A ~0.17 (JET). Observations suggest that the interaction of these modes with the energetic particles may impact the current drive in vicinity of q min in advanced tokamak regimes using NBI Alfvén eigenmode excitation is sensitive to NBI direction (DIII-D) Results suggest that modes may drive current in vicinity of q min in burning plasmas from the preferential redistribution of co-passing alpha particles

D. Borba st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Conclusions II Alfvén cascades (RSAE) will be destabilised in ITER reversed shear auxiliary heated discharges Possible redistribution of the Helium ash from the plasma core Possible current drive in the vicinity of q min surface Powerful tool to diagnose the evolution of the q min surface Diagnostic setup in ITER must be able to measure Alfvén instabilities in the plasma core