Comparison of the flows and radial electric field in the HSX stellarator to neoclassical calculations

Briesemeister, A.; Zhai, K.; Anderson, David T.; Anderson, F. S. B.; Talmadge, J. N.

doi:10.1088/0741-3335/55/1/014002

Cited by 22 publications

(33 citation statements)

References 19 publications

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“…This mechanism could explain the decrease in I tor ðt ¼ 1Þ with increasing ECRH power. However, data from Charge Exchange Recombination Spectroscopy (ChERS) 27 shows no indication of such a large radial electric field, although this measurement is unable to adequately resolve the magnitude of the radial electric field very close to the magnetic axis because of the finite width of the neutral beam that is employed.…”

Section: Summary and Discussionmentioning

confidence: 99%

Modeling, measurement, and 3-D equilibrium reconstruction of the bootstrap current in the Helically Symmetric Experiment

et al. 2014

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The bootstrap current for three electron cyclotron resonance heated plasma scenarios in a quasihelically symmetric stellarator (the Helically Symmetric Experiment) are analyzed and compared to a neoclassical transport code PENTA. The three conditions correspond to 50 kW input power with a resonance that is off-axis, 50 kW on-axis heating and 100 kW on-axis heating. When the heating location was moved from off-axis to on-axis with 50 kW heating power, the stored energy and the extrapolated steady-state current were both observed to increase. When the on-axis heating power was increased from 50 kW to 100 kW, the stored energy continued to increase while the bootstrap current slightly decreased. This trend is qualitatively in agreement with the calculations which indicate that a large positive electric field for the 100 kW case was driving the current negative in a small region close to the magnetic axis and accounting for the decrease in the total integrated current. This trend in the calculations is only observed to occur when momentum conservation between particle species is included. Without momentum conservation, the calculated bootstrap current increases monotonically. We show that the magnitude of the bootstrap current as calculated by PENTA agrees better with the experiment when momentum conservation between plasma species is included in the calculation. The total current was observed in all cases to flow in a direction to unwind the transform, unlike in a tokamak in which the bootstrap current adds to the transform. The 3-D inductive response of the plasma is simulated to predict the evolution of the current profile during the discharge. The 3-D equilibrium reconstruction code V3FIT is used to reconstruct profiles of the plasma pressure and current constrained by measurements with a set of magnetic diagnostics. The reconstructed profiles are consistent with the measured plasma pressure profile and the simulated current profile when the reconstruction is constrained by the measured data from a diagnostic array that is internal to the vacuum chamber.

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Section: Summary and Discussionmentioning

confidence: 99%

Modeling, measurement, and 3-D equilibrium reconstruction of the bootstrap current in the Helically Symmetric Experiment

et al. 2014

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“…Thus, a nonzero dΦ 0 /dψ gives a singular perturbation to the dΦ 0 /dψ = 0 limit in this partial trajectory model: the dΦ 0 /dψ = 0 solution for f a1 need not satisfy (23) can also be seen from the fact that when the dΦ 0 /dψ terms inẋ a andξ a are retained in the more accurate trajectories (17), these terms precisely cancel (23).…”

Section: Particle and Energy Moment Equations Conservation Propmentioning

confidence: 98%

Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas

et al. 2014

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In this work, we examine the validity of several common simplifying assumptions used in numerical neoclassical calculations for nonaxisymmetric plasmas, both by using a new continuum drift-kinetic code and by considering analytic properties of the kinetic equation. First, neoclassical phenomena are computed for the LHD and W7-X stellarators using several versions of the drift-kinetic equation, including the commonly used incompressible-E × B-drift approximation and two other variants, corresponding to different effective particle trajectories. It is found that for electric fields below roughly one third of the resonant value, the different formulations give nearly identical results, demonstrating the incompressible E ×B-drift approximation is quite accurate in this regime. However, near the electric field resonance, the models yield substantially different results. We also compare results for various collision operators, including the full linearized Fokker-Planck operator. At low collisionality, the radial transport driven by radial gradients is nearly identical for the different operators, while in other cases it is found to be important that collisions conserve momentum.

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“…With regard to the electrons, it was found that there were no major differences in core electron transport between the Standard configuration and Increased Iota configurations at W7-X due to the mitigating presence of a positive radial electric field 55,56 . At HSX the role of the radial electric field 57,58 in mitigating transport is still an open question. Finally, EMC3-EIRENE simulations contain no information about potential core neoclassical transport changes that may have occurred as a result of the change in magnetic configuration (i.e.…”

Section: Final Considerations and Conclusionmentioning

confidence: 99%

Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators

et al. 2018

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The edge magnetic structure in the Helically Symmetric eXperiment (HSX) and Wendelstein 7X (W7-X) stellarators has been shown to have a significant impact on the particle fueling and exhaust of the plasma main species (hydrogen) as well as impurity helium. For HSX, the plasma sourcing to exhaust ratio, quantified by the effective and global particle confinement times τ * p and τ p,H , respectively, increases when a magnetic island chain is located in the plasma edge. The fueling efficiency is reduced by 25% when the plasma boundary is deformed by the magnetic islands. The X-point geometry also yields higher plasma temperatures in front of the main recycling region. When the island is moved radially inward, both τ * p and τ p decrease by 10 − 25% depending on plasma density. The τ p,H results rely heavily on EMC3-EIRENE modeling which confirms reduced fueling efficiency due to more rapid ionization in the outward shifted island position. These findings suggest that for a helically optimized system like HSX, the plasma fueling from the recycling source, as well as from active gas injection, can be controlled by the magnetic island chain in the plasma edge-which is a basic requirement for a divertor system. This process is also effective for the control of effective helium exhaust times, as τ * p,He measured by perturbative gas puff experiments is reduced by up to 40% when the islands are shifted inwards. For Wendelstein 7-X, a similar reduction of τ * p,He was inferred when magnetic islands were moved from the far plasma edge into the confined plasma region. However, the effective confinement features of H as the main plasma species were not affected due to the non-optimal position of the magnetic islands with respect to the highly localized ionization domain during the limiter startup campaign.

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Comparison of the flows and radial electric field in the HSX stellarator to neoclassical calculations

Cited by 22 publications

References 19 publications

Modeling, measurement, and 3-D equilibrium reconstruction of the bootstrap current in the Helically Symmetric Experiment

Modeling, measurement, and 3-D equilibrium reconstruction of the bootstrap current in the Helically Symmetric Experiment

Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas

Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators

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