Accelerated MHD-induced Ion Loss in NSTX H-mode Discharges

1. S. S. Medley, R. Andre and A. L. Roquemore 45th American Physical Society Division of Plasma Physics Meeting Albuquerque, NM, October 27 – 31, 2003

2. Abstract Accelerated MHD-induced Ion Loss in NSTX H-mode Discharges* S. S. Medley, R. Andre and A. L. Roquemore Princeton Plasma Physics Laboratory, Princeton, New Jersey, 08543, USA A rich variety of energetic ion behavior resulting from magnetohydrodynamic (MHD) activity is observed in the National Spherical Torus Experiment(NSTX) using a horizontally scanning Neutral Particle Analyzer(NPA) whose sightline views across the three co-injected neutral beams. For example, onset of an n = 2 mode leads to relatively slow decay of the energetic ion population (E ~ 5 - 100 keV) and consequently the neutron yield. The effect of reconnection events, sawteeth and bounce fishbones differs from that observed for MHD modes. In this case, prompt loss of the energetic ion population occurs on a time scale of ≤ 1 msec and a precipitous drop in the neutron yield occurs. This paper focuses on MHD-induced ion loss during H-mode operation in NSTX. After H-mode onset, the spectrum typically exhibits a significant loss of energetic ions only for E>Eb/2. The magnitude of the energetic ion loss was observed to decrease with increasing tangency radius, Rtan, of the NPA sightline, increasing toroidal field and increasing NB injection energy, Eb. Increasing values of these parameters reduces the fraction of trapped particles that is either generated or viewed by the NPA. TRANSP modeling suggests that MHD-induced ion loss is accelerated during H-mode operation due to an evolution of the q and beam deposition profiles which feeds trapped ions into the region of low-n MHD activity. * This work was supported by the United States Department of Energy under contract number DE-AC02-76CH03073.

3. The Neutral Particle Analyzer (NPA) on NSTX Scans Horizontally Over a Wide Range of Tangency Angles on a Shot-to-Shot Basis • Covers Thermal (0.1 - 20 keV) and Energetic Ion (≤ 150 keV) Ranges

4. Slowing Down and Pitch Angle Scattering of NB Ions in Quiescent NSTX Plasmas is Consistent with Classical Behaviour Start of NBI 40 msec later • Perpendicular distribution for E ≤ Ecrit (~15 keV) fills in over ~40 msec (classical time: 50 msec)

5. TRANSP Simulations are in Reasonable Agreement with the NPA Horizontal Scans of NB Energetic Ion Spectra Start of NBI 40 msec later Absolute magnitude of measured and simulated NB flux agree to within ~ 5x!

6. Slowing Down of Short NB Ion Pulses is Classical • Short ~ 3 ms pulses of Eb = 80 keV deuterium neutrals were injected into NSTX deuterium plasmas • NPA measurements of the evolution of the NB energy components are shown • The full energy component slows down according to simple theory as E = Eb exp( - nEt) (see arrows) where nE is the classical deceleration rate calculated assuming the beam ions are localized in the plasma core W. W. Heidbrink and M. Miah, UC Irvine

7. Various Mechanisms Produce Energetic Ion LossesObserved by the NPA Diagnostic √ MHD Effects - Strong n=1 or n=2 mode activity and reconnection events [1] - Fishbones [2] √ Plasma Opacity Effects - Outer gap width (i.e. plasma radius) - High density, broad ne(r) profiles √H-Mode Effects - MHD-induced ion loss is accelerated during H-mode operation due to high, broad density profile effects. [1] “Neutral Particle Analyzer Measurements of Ion Behavior in NSTX,” S. S. Medley, et al. PPPL-3668 (February, 2002) [2] “Wave Driven Fast Ion Loss in the National Spherical Torus Experiment,” E.D. Fredrickson , et al. Phys. Plasmas 10, 2852 (2003).

8. Reconnections and Sawteeth Cause Rapid Loss of Energetic Ions (ED ~ 5 - 85 keV) • Shown are two large reconnection events (solid vertical lines) preceded by two sawteeth (dashed lines). IREs and sawteeth occur when q(0)≤ 1 - panel (b) • Neutron yield crashes due to IREs - panel (c), then recovers as NBI continues • NPA fast ion spectrum is promptly depleted during IREs (panel d) without the redistribution signature seen in the thermal energy range • After IREs, NBI continues and the fast ion spectrum rebuilds. The NPA signal becomes larger during Ip rampdown due to increasing CX neutral target density

9. Fishbones Redistribute Energetic Ions Which Causes a Reduction in Neutron Rate • Fishbone frequency chirps down rapidly with mainly n = 1, 2, 3 modes and is thought to be bounce resonant • Large NPA flux bursts coincide with fishbones, but only when the frequency chirps to near zero. This ion loss causes ≤ 25 % drop in the neutron rate • Fishbone induced bursts in the NPA flux have been observed at all energies in the NB ion spectra (E ~ 2 - 80 keV) and over a wide range of NPA tangency radii (Rtan = 15 - 92 cm) • NPA flux bursts at lower energies are delayed by ~ 1 msec relative to higher energies E. Fredrickson, et al. Phys.Plasmas 10, 2852 (2003)

10. Evolution of NB Energetic Ion Spectra during a Fishbone Event • Fast ion distributions with full, half and third neutral beam injection energies marked. Four spectra are shown with 1 msec time resolution through a single fishbone event.

11. Illustration of “Ion Loss” due to Plasma Opacity EffectsBT = 4.8 kG, IP = 0.8 MA, Source B @ 100 keV, Low MCP Bias • Following H-Mode onset at 375 msec, the NPA spectra show significant loss of ions at all energies. H-mode Onset

12. Discharge Parameters for SN109067BT = 4.8 kG, IP = 0.8 MA, NPA RTAN ~ 75 cm, Low MCP Bias •For SN109067 (loss at all E), source B @ 100 keV, PB = 2.25 MW. •MHD n = 2 activity rolls over SNPA and SN prior to onset of the H-mode. H-mode Onset

13. TRANSP Simulation Agrees Well with the Measured NPA Spectra and Neutron Rate • “TRANSP and NPA spectra are normalized at one point only. • “Ion loss” in this case is due solely to plasma opacity effects: i.e. reionization of neutral efflux during plasma exit.

14. Ion Loss is not an Instrumental EffectFull Energy Ion Loss is Approximately One e-folding (~ 60%) •Ion loss is independent of the spectra location on the MCP detector. Loss also observed with D => H mass column. •Ion loss is independent of the MCP bias (higher bias increases SNPA) •Fishbone spikes and an IRE-induced burst on SNPA show that the MCP detector is not in saturation.

15. Illustration of MHD-induced Ion Loss during H-mode BT = 4.8 kG, IP = 0.8 MA, Source A & B @ 90 keV, Low MCP Bias • Following H-Mode onset at 230 ms, the NPA spectra show significant loss of energetic ions only for E>Eb/2. H-mode Onset

16. MPTS ne and Te data for SN108730BT = 4.8 kG, IP = 0.8 MA, NPA RTAN ~ 70 cm, Low MCP Bias •Electron density and temperature differ significantly compared with the “opacity” case just shown. •Here neL is 20% lower and the density profile is not centrally peaked. •The electron temperature flattens rather than collapsing. •Effect of opacity on the evolution of the NPA signal is negligible.

17. TRANSP Simulation Disagrees with NPA Ion Spectra & Neutron Yield during MHD Active H-mode •Based on a comparison of the measured and calculated neutron yields, the total ion loss is not large: i.e. ~ 10%.

18. H-mode Ion Loss Decreases with Increasing NPA RtanBT = 4.8 kG, IP = 0.8 MA, Sources A & B @ 90 keV, Low MCP Bias •Increasing Rtan corresponds to the NPA viewing more passing ion orbits.

19. H-mode Ion Loss Decreases with Increasing BT IP = 0.8 MA, Sources A & B @ 90 keV, Rtan = 92 cm, Low MCP Bias •Increasing BT corresponds to a reduction in the trapped ion fraction.

20. MHD-induced Ion Loss Decreases with Increasing NPA Tangency Radius, NB Energy and Toroidal Field •TRANSP calculations show that the trapped particle fraction viewed by the NPA decreases with increasing Rtan. •Reduced ion loss with increasing Rtan, ENB and BT is due to reduction of either the viewed or generated trapped ion fraction associated with MHD-induced ion loss.

21. The L-H TransitionGenerally Occurs At or Before Onset of MHD-induced Ion Loss •The solid line is a visual guide to simultaneous events and not a fit to the data.

22. MHD-induced Energetic Ion Loss is Accelerated during H-mode Operation •At 2nd NB turn-on, onset of low-n MHD activity causes a slow rolloff of the NPA signal, (Snpa), and the neutron yield, (Sn). •Subsequently, an H-mode at ~ 360 msec accelerates the MHD-induced ion loss observed on Snpa and Sndecays . •HYPOTHESIS: MHD-induced ion loss is accelerated during the H-mode due to an evolution of the q and beam deposition profiles which feeds trapped ions into the region of low-n MHD activity.

23. Evolution of Te, ne (and hence Pressure) Profiles during H-mode Drives other Profile Changes •Pressure profile evolution modifies Ip distribution, mainly by bootstrap driven current. •Density profile evolution increases full energy NB deposition in outboard region.

24. q-profile Evolution Introduces Low-n MHD activity: Elevated Trapped Ion Fraction Feeds Ion Loss. • Ip profileevolution modifies q profile, introducing q = 2.5 region around r/a ~ 0.63. •Out-shifted NB deposition increases full E trapped ions in m/n = 5/2 MHD active region.

25. Initial ORBIT Modeling Shows Resonant Ion Loss occurs for E>E/2 but Not Lower Energies as Observed •ORBIT modeling of SN108730 uses TRANSP input. •Model invokes n = 2, m = 4,5 MHD activity with f = 20 kHz, consistent with data. •MHD modes have a broad radial structure in the low shear region around q min at r/a ~ 0.5. •Particle loss increases with increasing mode amplitude. Courtesy N. N. Gorelenkov

26. Summary • During H-modes, the NPA always observes significant MHD-induced ion loss at E ≥ Eb/2 but seldom at lower energies. This loss generally occurs after the L-H transition. • Ion depletion only at higher energies is not consistent with attenuation due to simple plasma opacity effects with increasing ne(r) . • The magnitude of the ion loss decreases with increasing NB injection energy, toroidal magnetic field and NPA tangency radius. Increasing values of theseparameters reduces the fraction of trapped particles that is either generated or viewed by the NPA. • TRANSP modeling indicates that the effect is driven by the high, broad density profiles endemic to H-modes: i.e. a pressure-driven evolution of the q profile introduces low-n MHD activity whilst the beam deposition profile broadens to feed trapped ions into the MHD active region. This effect can also occur in high density L-mode discharges.