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

S. S. Medley and A.L. Roquemore

Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ, 08543USA

The Spherical Tokamak Workshop 2003

Culham Science Centre

Abingdon, UK September 15 - 17, 2003

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The Neutral Particle Analyzer (NPA) on NSTX ScansHorizontally Over a Wide Range of Tangency Angles

• Covers Thermal (0.1 - 20 keV) and Energetic Ion (≤ 150 keV) Ranges

Neutral Beam Injector

PlasmaAxis

Neutral Particle Analyzer

A: Rtan = 69.4 cmB: Rtan = 59.2 cmSource C: Rtan = 48.7 cmRtan = 135cmRtan = -65 cm

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Slowing Down and Pitch Angle Scattering of NB Ions in Quiescent NSTX Plasmas is Consistent with Classical Behaviour

40 msec laterStart of NBI

• Perpendicular distribution for E ≤ Ecrit (~15 keV) fills in over ~40 msec (classical time: 50 msec)

4

TRANSP Simulations are in Reasonable Agreement with the NPA Horizontal Scans of NB Energetic Ion Spectra

40 msec laterStart of NBI

Absolute magnitude of measured and simulated NB flux agree to within ~ 5x.

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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).

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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.

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Discharge Parameters for SN109067 BT = 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.

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TRANSP Simulation of Energetic Ion Spectraand Neutron Measurements

• TRANSP simulation agrees well with the NPA spectra and measured neutron rate.

• “Ion loss” in this case is due solely to plasma opacity effects.

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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.

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Discharge Parameters for SN108730 BT = 4.8 kG, IP = 0.8 MA, NPA RTAN ~ 70 cm, Low MCP Bias

n = 1 n = 2

• For SN108730 (loss at restricted E), sources A & B @ 90 keV, PB = 4.2 MW.

n = 3

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• In a quiescent plasma, good agreement is observed between TRANSP simulation and NPA spectra, but not during H-mode phase with MHD activity.

TRANSP Simulation of Energetic Ion Spectraand Neutron Measurements for SN108730

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H-mode Ion Loss Decreases with Increasing NPA Rtan

BT = 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.

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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.

NB Energy

NPA Rtan

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MHD-induced Energetic Ion Is Not an H-mode Trigger

• H-mode onset precedes or coincides with the decay of the NPA signal and neutron yield.

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• 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).

MHD-induced Energetic Ion Loss is Accelerated during H-mode Operation

• 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.

• Subsequently, an H-mode at ~ 360 msec accelerates the MHD-induced ion loss observed on Snpa and Sndecays .

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• Pressure profile evolution modifies Ip distribution, mainly by bootstrap driven current.

Evolution of Te, ne (and hence Pressure) Profiles during H-mode Drives other Profile Changes

• Density profile evolution increases full energy NB deposition in outboard region.

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• Ip profile evolution modifies q profile, introducing q = 2.5 region around r/a ~ 0.63.

q-profile Evolution Introduces Low-n MHD activity: Elevated Trapped Ion Fraction Feeds Ion Loss.

• Out-shifted NB deposition increases full E trapped ions in m/n = 5/2 MHD active region.

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Initial ORBIT Modeling Corroborates NPA Ion Loss Measurements

• ORBIT modeling of SN108730 uses TRANSP input and invokes n = 2, m = 4,5 MHD activity with f = 20 kHz and a broad radial structure in the low shear region around q min at r/a ~ 0.5.

• Initial modeling treats passing particles, but comparable loss expected for trapped particles.

• Particles launched from NB location along NPA sightline with v||/v = 0.8 - 0.9 are deemed lost if pitch angle perturbation exceeds +/- 0.05 beyond this window.

• Loss occurs for E>E/2 but not lower energies as observed in the NPA measurements.

Courtesy N. N. Gorelenkov

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Summary

• During H-modes, the NPA always observes significant ion loss at E ≥ Eb/2 but seldom

at lower energies.

• 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 neutral beam injection

energy, toroidal field and tangency radius of the NPA sightline. Increasing values of

these parameters 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.

• This MHD-induced energetic ion loss is not an H-mode trigger.