Fast-ion energy loss during TAE avalanches in the National Spherical Torus Experiment

Fredrickson, E. D.; Crocker, N. A.; Darrow, D.S.; Kramer, G. J.; Kubota, S.; Podestà, M.; White, R. B.; Bortolon, A.; Gerhardt, S. P.; Bell, R. E.; Diallo, A.; LeBlanc, B.P.; Levinton, F. M.; Yuh, H.

doi:10.1088/0029-5515/53/1/013006

Cited by 41 publications

(37 citation statements)

References 63 publications

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“…This, and the redistribution of fast ions to regions of lower ion density with these peaked profiles can account for most of the measured decrease in neutron rate. The fast ion energy loss is comparable to the estimated energy lost by Alfvén wave damping during the burst [87].…”

Section: Fast Ion Phase Space Redistribution and Effects On Low And Hmentioning

confidence: 64%

“…TAE avalanches and associated neutron rate reduction studies, previously restricted to L-mode plasmas, have now been extended to H-mode plasmas with centrally peaked density profiles allowing reflectometer measurements of the mode structure [87]. Prompt, classical fast ion losses computed from the gyro-centre particle-following code ORBIT [88] are negligible and cannot fully account for the observed neutron rate reduction.…”

Section: Fast Ion Phase Space Redistribution and Effects On Low And Hmentioning

confidence: 99%

See 1 more Smart Citation

Overview of physics results from the conclusive operation of the National Spherical Torus Experiment

Sabbagh¹,

Ahn²,

Allain³

et al. 2013

Nucl. Fusion

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Research on the National Spherical Torus Experiment, NSTX, targets physics understanding needed for extrapolation to a steady-state ST Fusion Nuclear Science Facility, pilot plant, or DEMO. The unique ST operational space is leveraged to test physics theories for next-step tokamak operation, including ITER. Present research also examines implications for the coming device upgrade, NSTX-U. An energy confinement time, τ E , scaling unified for varied wall conditions exhibits a strong improvement of B T τ E with decreased electron collisionality, accentuated by lithium (Li) wall conditioning. This result is consistent with nonlinear microtearing simulations that match the experimental electron diffusivity quantitatively and predict reduced electron heat transport at lower collisionality. Beam-emission spectroscopy measurements in the steep gradient region of the pedestal indicate the poloidal correlation length of turbulence of about ten ion gyroradii increases at higher electron density gradient and lower T i gradient, consistent with turbulence caused by trapped electron instabilities. Density fluctuations in the pedestal top region indicate ion-scale microturbulence compatible with ion temperature gradient and/or kinetic ballooning mode instabilities. Plasma characteristics change nearly continuously with increasing Li evaporation and edge localized modes (ELMs) stabilize due to edge density gradient alteration. Global mode stability studies show stabilizing resonant kinetic effects are enhanced at lower collisionality, but in stark contrast have almost no dependence on collisionality when the plasma is off-resonance. Combined resistive wall mode radial and poloidal field sensor feedback was used to control n = 1 perturbations and improve stability. The disruption probability due to unstable resistive wall modes (RWMs) was surprisingly reduced at very high β N /l i > 10 consistent with low frequency magnetohydrodynamic spectroscopy measurements of mode stability. Greater instability seen at intermediate β N is consistent with decreased kinetic RWM stabilization. A model-based RWM state-space controller produced long-pulse discharges exceeding β N = 6.4 and β N /l i = 13. Precursor analysis shows 96.3% of disruptions can be predicted with 10 ms warning and a false positive rate of only 2.8%. Disruption halo currents rotate toroidally and can have significant toroidal asymmetry. of this phenomenon in designing future RF systems. The snowflake divertor configuration enhanced by radiative detachment showed large reductions in both steady-state and ELM heat fluxes (ELMing peak values down from 19 MW m −2 to less than 1.5 MW m −2 ). Toroidal asymmetry of heat deposition was observed during ELMs or by 3D fields. The heating power required for accessing H-mode decreased by 30% as the triangularity was decreased by moving the X-point to larger radius, consistent with calculations of the dependence of E × B shear in the edge region on ion heat flux and X-point radius. Co-axial helicity injection reduced the induct...

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Section: Fast Ion Phase Space Redistribution and Effects On Low And Hmentioning

confidence: 64%

Section: Fast Ion Phase Space Redistribution and Effects On Low And Hmentioning

confidence: 99%

Overview of physics results from the conclusive operation of the National Spherical Torus Experiment

Sabbagh¹,

Ahn²,

Allain³

et al. 2013

Nucl. Fusion

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“…Compared to the unperturbed gra dient both the gradients for the one and two fluid are reduced in the core and at the edge as is shown in figures 13(c) and (d), indicating that the MP fields can reduce the GAE fastion drive. Trapped particles can resonate with the GAE [26] when For a quantitative analysis of the GAE drive the spatial dis tribution of the eigenmode in the plasma is needed and can be obtained from reflectometer measurements as was shown in [44]. Unfortunately, these measurements were not available in this experiment because of the steep edge density gradients and the flat or hollow density profiles in the core as is shown in figure 14 and therefore, an accurate quantitative estimate of the change in GAE fastion drive is not possible.…”

Section: Effects Of Mp Fields On Aesmentioning

confidence: 99%

Mitigation of Alfvénic activity by 3D magnetic perturbations on NSTX

Kramer

Bortolon

Ferraro

et al. 2016

Plasma Phys. Control. Fusion

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Observations on the National Spherical Torus Experiment (NSTX) indicate that externally applied nonaxisymmetric magnetic perturbations (MP) can reduce the amplitude of toroidal Alfvén eigenmodes (TAE) and global Alfvén eigenmodes (GAE) in response to pulsed n = 3 nonresonant fields. From fullorbit following Monte Carlo simulations with the one and twofluid resistive MHD plasma response to the magnetic perturbation included, it was found that in response to MP pulses the fastion losses increased and the fastion drive for the GAEs was reduced. The MP did not affect the fastion drive for the TAEs significantly but the Alfvén continuum at the plasma edge was found to be altered due to the toroidal symmetry breaking which leads to coupling of different toroidal harmonics. The TAE gap was reduced at the edge creating enhanced continuum damping of the global TAEs, which is consistent with the observations. The results suggest that optimized nonaxisymmetric MP might be exploited to control and mitigate Alfvén instabilities by tailoring the fastion distribution function and/or continuum structure.

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“…In DIII-D and NSTX these corrections are small, 11% and 4%, respectively. The second effect is shown to lead to changes in the fusion reaction rate caused by the fast ion energy change associated with the energy transport induced by TAEs [18]. In this paper we use the following definition of the neutron deficit in the perturbative and nonperturbative CGMs as…”

Section: Critical Gradient Modelsmentioning

confidence: 99%

Validating predictive models for fast ion profile relaxation in burning plasmas

et al. 2016

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The redistribution and potential loss of energetic particles due to MHD modes can limit the performance of fusion plasmas by reducing the plasma heating rate. In this work, we present validation studies of the 1.5D critical gradient model (CGM) for Alfvén eigenmode (AE) induced EP transport in NSTX and DIII-D neutral beam heated plasmas. In previous comparisons with a single DIII-D L-mode case, the CGM model was found to be responsible for 75% of measured AE induced neutron deficit [1]. A fully kinetic HINST is used to compute mode stability for the non-perturbative version of CGM (or nCGM). We have found that AEs show strong local instability drive up to / γ ω ∼ 20% violating assumptions of perturbative approaches used in NOVA-K code. We demonstrate that both models agree with each other and both underestimate the neutron deficit measured in DIII-D shot by approximately a factor of 2.On the other hand in NSTX the application of CGM shows good agreement for the measured flux deficit predictions. We attempt to understand these results with the help of the so-called kick model which is based on the guiding center code ORBIT. The kick model comparison gives important insight into the underlying velocity space dependence of the AE induced EP transport as well as it allows the estimate of the neutron deficit in the presence of the low frequency Alfvénic modes. Within the limitations of used models we infer that there are missing modes in the analysis which could improve the agreement with the experiments.

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Fast-ion energy loss during TAE avalanches in the National Spherical Torus Experiment

Cited by 41 publications

References 63 publications

Overview of physics results from the conclusive operation of the National Spherical Torus Experiment

Overview of physics results from the conclusive operation of the National Spherical Torus Experiment

Mitigation of Alfvénic activity by 3D magnetic perturbations on NSTX

Validating predictive models for fast ion profile relaxation in burning plasmas

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