Observation of Beam Driven Modes during Neutral Beam Heating on the National Spherical Torus Experiment

Fredrickson, E.; Gorelenkov, N. N.; Cheng, C. Z.; Bell, R. E.; Darrow, D. S.; Johnson, Donald F.; Kaye, S. M.; LeBlanc, B.; Ménard, J.; Kubota, S.; Peebles, W. A.

doi:10.2172/788447

Cited by 8 publications

(10 citation statements)

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“…One recognizes that for rigid body rotation with a small rotation frequency Ω, only the two first lines remain, and the solutions are the same as to (8), except that the frequency is shifted by an amount −nΩ. The eigenmode equation (21) has been split into its three vector components in Appendix A, and the resulting equations for the three components of the perturbed magnetic field have been implemented in the CAE3B code.…”

Section: The Eigenmode Equationmentioning

confidence: 99%

“…In conventional tokamaks, the fast ions can excite compressional waves close to harmonics of the ion cyclotron frequency [1,2], and the phenomenon is termed ion cyclotron emission (ICE) [3,4,5,6]. In spherical tokamaks [7,8,9,10], and also in some conventional tokamak experiments [11], the magnetic field is lower and the fast ion drive causes compressional eigenmodes to appear at frequencies comparable to but below the on-axis ion cyclotron frequency.…”

Section: Introductionmentioning

confidence: 99%

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Compressional Alfvén eigenmodes in rotating spherical tokamak plasmas

Smith

Fredrickson

2017

Plasma Phys. Control. Fusion

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Abstract. Spherical tokamaks often have a considerable toroidal plasma rotation of several tens of kHz. Compressional Alfvén eigenmodes (CAEs) in such devices therefore experience a frequency shift, which if the plasma were rotating as a rigid body, would be a simple Doppler shift. However, since the rotation frequency depends on minor radius, the eigenmodes are affected in a more complicated way. The eigenmode solver CAE3B [Smith et al. Plasma Phys. Control. Fusion 51, 075001 (2009)] has been extended to account for toroidal plasma rotation. The results show that the eigenfrequency shift due to rotation can be approximated by a rigid body rotation with a frequency computed from a spatial average of the real rotation profile weighted with the eigenmode amplitude. To investigate the effect of extending the computational domain to the vessel wall, a simplified eigenmode equation, yet retaining plasma rotation, is solved by a modified version of the CAE code used in [Fredrickson et al. Phys. Plasmas 20, 042112 (2013)]. In summary, both solving the full eigenmode equation, as in the CAE3B code, and placing the boundary at the vessel wall, as in the CAE code, significantly influences the calculated eigenfrequencies.

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Section: The Eigenmode Equationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Compressional Alfvén eigenmodes in rotating spherical tokamak plasmas

Smith

Fredrickson

2017

Plasma Phys. Control. Fusion

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“…The CAEs and GAEs appear in a broad spectrum of nearly equally spaced peaks in the frequency range from $0.2 X i to $1.2 X i (or 500 kHz to a few MHz) in NSTX. 314 The CAEs are compressional Alfv en waves which can exist above and below the ion cyclotron frequency, and GAEs represent the shear Alfv en wave roots that exist below the ion cyclotron frequency (f ci $ 3 MHz for this case). The CAE dispersion relation is x CAE ¼ k V A , where k is mainly defined by the radial wave number.…”

Section: B Energetic-particle-driven Modesmentioning

confidence: 99%

Recent progress on spherical torus research

Ono

Kaita

2015

Physics of Plasmas

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The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R0/a) reduced to A ∼ 1.5, well below the normal tokamak operating range of A ≥ 2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β ∼ 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era.

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“…1͑a͔͒, the compressional and the global Alfvén eigenmodes ͑CAE and GAE, respectively, both in orange͒. [16][17][18][19][20][21][22][23][24][25] These are also weakly damped natural modes of the plasma and exhibit behavior similar to that of the TAE. The GAE are shear waves ͑␦B perpendicular to the equilibrium field͒, but can couple to compressional waves.…”

Section: Physics Of Plasmas 13 056109 ͑2006͒mentioning

confidence: 99%

Collective fast ion instability-induced losses in National Spherical Tokamak Experiment

et al. 2006

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A wide variety of fast ion driven instabilities are excited during neutral beam injection ͑NBI͒ in the National Spherical Torus Experiment ͑NSTX͒ ͓Nucl. Fusion 40, 557 ͑2000͔͒ due to the large ratio of fast ion velocity to Alfvén velocity, V fast / V Alfvén , and high fast ion beta. The ratio V fast / V Alfvén in ITER ͓Nucl. Fusion 39, 2137 ͑1999͔͒ and NSTX is comparable. The modes can be divided into three categories: chirping energetic particle modes ͑EPM͒ in the frequency range 0 to 120 kHz, the toroidal Alfvén eigenmodes ͑TAE͒ with a frequency range of 50 kHz to 200 kHz, and the compressional and global Alfvén eigenmodes ͑CAE and GAE, respectively͒ between 300 kHz and the ion cyclotron frequency. Fast ion driven modes are of particular interest because of their potential to cause substantial fast ion losses. In all regimes of NBI heated operation we see transient neutron rate drops, correlated with bursts of TAE or fishbone-like EPMs. The fast ion loss events are predominantly correlated with the EPMs, although losses are also seen with bursts of multiple, large amplitude TAE. The latter is of particular significance for ITER; the transport of fast ions from the expected resonance overlap in phase space of a "sea" of large amplitude TAE is the kind of physics expected in ITER. The internal structure and amplitude of the TAE and EPMs has been measured with quadrature reflectometry and soft x-ray cameras. The TAE bursts have internal amplitudes of ñ / n = 1% and toroidal mode numbers 2 Ͻ n Ͻ 7. The EPMs are core localized, kink-like modes similar to the fishbones in conventional aspect ratio tokamaks. Unlike the fishbones, the EPMs can be present with q͑0͒ Ͼ 1 and can have a toroidal mode number n Ͼ 1. The range of the frequency chirp can be quite large and the resonance can be through a fishbone-like precessional drift resonance, or through a bounce resonance.

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Observation of Beam Driven Modes during Neutral Beam Heating on the National Spherical Torus Experiment

Cited by 8 publications

References 1 publication

Compressional Alfvén eigenmodes in rotating spherical tokamak plasmas

Compressional Alfvén eigenmodes in rotating spherical tokamak plasmas

Recent progress on spherical torus research

Collective fast ion instability-induced losses in National Spherical Tokamak Experiment

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