Excitation of Alfv

1
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
2
Outline

• Objectives
• Determine experimentally the Vb / VA 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 Vb / VA
• 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

3
Outline

• Objectives
• Determine experimentally the Vb / VA 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 Vb / VA
• 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

4
Objective of the experiment

• Condition for instabilities requires
• Efficient energy exchange between the particles
  and the wave (resonance condition)
• p wt or b n wd 0
• for TAEs VA Vb also (Vb VA/3)
• for Cascades (RSAE) VA gtgt Vb

Alfvén continuum deep reversed shear
wTAEwA/2
wRSAEltltwA
S.E.Sharapov et al., Nuclear Fusion v.46 (2006)
S868.
Objective of the experiment Determine
experimentally the Vb/VA threshold for the
excitation of Alfvén cascades (RSAE)
5
Outline

• Objectives
• Determine experimentally the Vb / VA 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 Vb / VA
• 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

6
Diagnostic Requirements
Condition for instabilities requires Efficient
energy exchange between the particles and the
waves
Particle
Radial Mode width Dm gt Radial Orbit width
Dp Dm8 1/ (n s) Toroidal mode number
Magnetic Shear
Wave
Dm
Dp
In reversed shear scenarios, around the zero
shear (s0) 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
7
Diagnostic Setup (JET)

• Far infrared DCN interferometer
• 3 vertical chords, measured line-integrated DneL
  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
8
Diagnostic Setup (DIII-D)
CO2 interferometer, 1.6 MHz bandwidth, 4
chords, line-integrated DneL 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 Dne with increased
sensitivity Far-Infrared Scattering (FIR), 10
MHz bandwidth, line-integrated measurement of
low-k density fluctuations (k 0 - 2 cm-1 )
CO2 Interf.
65 GHz
42 GHz
9
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 (n2,3,4), due the radial extent of
  these modes

Far infrared DCN interferometer
Microwave reflectometer
Magnetic probe
Using these diagnostics allows an detailed
analysis of the instability threshold in
different configurations
S. Sharapov et al Phys. Rev. Lett. 93, 165001
(2004) Berk et al, Phys. Rev. Lett. 87, 185002
(2001)
10
Measurements at JET
Alfvén Cascades near r/a-0.2 using X-mode
Reflectometer
67732
11
Outline

• Objectives
• Determine experimentally the Vb / VA 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 Vb / VA
• 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

12
Experimental Scenario JET
Objective minimise Vb/VA Ebeam 0.5 ne0.5 /BT
in order to find the threshold for the excitation
of Alfvén cascades (RSAE)

• Maximum Magnetic field Bt3.45 Tesla
• Lowest beam energy possible
• Lower Hybrid Current drive to obtain reversed
  shear (PLHCD2MW)
• Low density to maximize the fraction of fast
  ions

13
Experimental Scenario DIII-D
Objective minimise Vb/VA Ebeam0.5 ne0.5 /BT in
order to find the threshold for the excitation of
Alfvén cascades (RSAE)
Difficult to reduce velocity ratio Vb/VA below
0.4 in DIII-D
14
Outline

• Objectives
• Determine experimentally the Vb / VA 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 Vb / VA
• 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

15
Neutral Beam energy scan (JET)
Microwave reflectometer (working as a
interferometer)

• 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

66957
66962
66963
66964
Cascades clearly visible with 50 keV beams at JET
16
Neutral Beam energy scan (DIII-D)
80 keV Vb/VA0.60
125961
50 keV Vb/VA0.45
125975
Cascade Modes Observed with 50 keV Beam injection
in DIII-D though at reduced level and number
compared to 80 keV
R. Nazikian et al Phys. Rev. Lett. 96, (2006), M.
Van Zeeland et al Phys. Rev. Lett. 97, (2006).
17
Neutral Beam direction (DIII-D)
Alfvén Eigenmode Excitation is Sensitive to Beam
Ion Direction in DIII-D
126581

•  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

126597
B.N.Breizman et al., Physics of Plasmas v.10
(2003) 3649
18
Outline

• Objectives
• Determine experimentally the Vb / VA 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 Vb / VA
• 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

19
q-profile
qmin integer
qmin integer
20
q-profile evolution (JET)
q4 Cascade
- Information from the Alfvén waves is used to
reconstruct the q-profile evolution more
accurately and validate the Motional Stark Effect
(MSE) measurements
q3 Cascade
q2 Cascade
120
21
q-profile evolution (DIII-D)
50 keV _at_ Vb/VA0.45
125975
qmin3
qmin4
qmin2
Cascade Modes with 50 keV Injection also clearly
indicate rational q-min crossings in DIII-D
22
Conclusions I

• Alfvén waves (RSAE/ Alfvén Cascades), can be
  excited by sub-Alfvén Neutral Beam Injection as
  low as Vb/VA0.17 (JET).
• Observations suggest that the interaction of
  these modes with the energetic particles may
  impact the current drive in vicinity of qmin 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 qmin in burning plasmas from the
  preferential redistribution of co-passing alpha
  particles

23
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 qmin
  surface
• Powerful tool to diagnose the evolution of the
  qmin surface
• Diagnostic setup in ITER must be able to measure
  Alfvén instabilities in the plasma core