A power-balance model for local helicity injection startup in a spherical tokamak

J. L. Barr; M. W. Bongard; M. G. Burke; R. J. Fonck; E. T. Hinson; J. M. Perry; J. A. Reusch

doi:10.1088/1741-4326/aac0a0

A power-balance model for local helicity injection startup in a spherical tokamak

J. L. Barr, M. W. Bongard, M. G. Burke, R. J. Fonck, E. T. Hinson, J. M. Perry, J. A. Reusch

Power Exhaust and Particle Control

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8 Scopus citations

Abstract

A 0D circuit model for predicting Ip(t) in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface ?ux of fnite-A plasmas developed by Hirshman and Neilson (1986 Phys. Fluids 29 790) are modifed and expanded to treat highly shaped, ultralow-A tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the A ∼ 1 Pegasus spherical tokamak, and are found to agree within 15% of experimental Ip(t). High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the frst half of the current ramp, or Ip ≲ 75 kA. The second half of the current ramp follows a limit imposed by power-balance as plasmas expand from high-A to ultralow-A. This shape evolution generates a signifcant drop in external plasma inductance, effectively using the plasma's initially high inductance to drive the current ramp and provide >70% of the current drive V-s. Projections using this model indicate the relative in?uences of higher helicity input rate and injector current on the attainable total plasma current.

Original language	English
Article number	076011
Journal	Nuclear Fusion
Volume	58
Issue number	7
DOIs	https://doi.org/10.1088/1741-4326/aac0a0
State	Published - May 15 2018

Funding

The authors thank: B.A. Kujak-Ford, B.T. Lewicki, and G.R. Winz for their technical assistance; D.J. Schlossberg, G.M. Bodner, T.M. Townsend, and D.M. Kreite for their support in experimental operations; and C.J. Everson and K.E. Thome for their work supporting the development and verification of the ShapeFIT boundary reconstruction code. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, under Award Numbers DE-FG02-96ER54375 and DE-SC0006928.

Keywords

Plasma startup
Taylor relaxation
current drive
helicity injection
spherical tokamaks
tokamaks

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10.1088/1741-4326/aac0a0

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title = "A power-balance model for local helicity injection startup in a spherical tokamak",

abstract = "A 0D circuit model for predicting Ip(t) in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface ?ux of fnite-A plasmas developed by Hirshman and Neilson (1986 Phys. Fluids 29 790) are modifed and expanded to treat highly shaped, ultralow-A tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the A ∼ 1 Pegasus spherical tokamak, and are found to agree within 15% of experimental Ip(t). High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the frst half of the current ramp, or Ip ≲ 75 kA. The second half of the current ramp follows a limit imposed by power-balance as plasmas expand from high-A to ultralow-A. This shape evolution generates a signifcant drop in external plasma inductance, effectively using the plasma's initially high inductance to drive the current ramp and provide >70% of the current drive V-s. Projections using this model indicate the relative in?uences of higher helicity input rate and injector current on the attainable total plasma current.",

keywords = "Plasma startup, Taylor relaxation, current drive, helicity injection, spherical tokamaks, tokamaks",

author = "Barr, {J. L.} and Bongard, {M. W.} and Burke, {M. G.} and Fonck, {R. J.} and Hinson, {E. T.} and Perry, {J. M.} and Reusch, {J. A.}",

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AU - Fonck, R. J.

AU - Hinson, E. T.

AU - Perry, J. M.

AU - Reusch, J. A.

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N2 - A 0D circuit model for predicting Ip(t) in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface ?ux of fnite-A plasmas developed by Hirshman and Neilson (1986 Phys. Fluids 29 790) are modifed and expanded to treat highly shaped, ultralow-A tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the A ∼ 1 Pegasus spherical tokamak, and are found to agree within 15% of experimental Ip(t). High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the frst half of the current ramp, or Ip ≲ 75 kA. The second half of the current ramp follows a limit imposed by power-balance as plasmas expand from high-A to ultralow-A. This shape evolution generates a signifcant drop in external plasma inductance, effectively using the plasma's initially high inductance to drive the current ramp and provide >70% of the current drive V-s. Projections using this model indicate the relative in?uences of higher helicity input rate and injector current on the attainable total plasma current.

AB - A 0D circuit model for predicting Ip(t) in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface ?ux of fnite-A plasmas developed by Hirshman and Neilson (1986 Phys. Fluids 29 790) are modifed and expanded to treat highly shaped, ultralow-A tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the A ∼ 1 Pegasus spherical tokamak, and are found to agree within 15% of experimental Ip(t). High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the frst half of the current ramp, or Ip ≲ 75 kA. The second half of the current ramp follows a limit imposed by power-balance as plasmas expand from high-A to ultralow-A. This shape evolution generates a signifcant drop in external plasma inductance, effectively using the plasma's initially high inductance to drive the current ramp and provide >70% of the current drive V-s. Projections using this model indicate the relative in?uences of higher helicity input rate and injector current on the attainable total plasma current.

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A power-balance model for local helicity injection startup in a spherical tokamak

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