Features of spherical torus plasmas

Peng, Yueng Kay Martin; Strickler, D. J.

doi:10.1088/0029-5515/26/6/005

Cited by 440 publications

(282 citation statements)

References 17 publications

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“…It should be noted that the high elongation achieved on TCV was a much more impressive technical effort than the higher elongation achieved on NSTX. Low aspect ratio has higher natural elongation, which has been pointed out as an advantage of the ST [1]. The maintenance of the current profile so that low values of l i are maintained is crucial for achieving high κ as shown in [10].…”

Section: Shape Control and Global Performancementioning

confidence: 99%

Progress towards steady state at low aspect ratio on the National Spherical Torus Experiment (NSTX)

Gates¹,

Ménard²,

Maingi³

et al. 2007

Nucl. Fusion

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Modifications to the plasma control capabilities and poloidal field coils of the National Spherical Torus Experiment (NSTX) have enabled a significant enhancement in shaping capability which has led to the transient achievement of a record shape factor (S ≡ q 95 (I p /aB t )) of ∼41 (MA m −1 T −1 ) simultaneous with a record plasma elongation of κ ≡ b/a ∼ 3. This result was obtained using isoflux control and real-time equilibrium reconstruction. Achieving high shape factor together with tolerable divertor loading is an important result for future ST burning plasma experiments as exemplified by studies for future ST reactor concepts, as well as neutron producing devices, which rely on achieving high shape factors in order to achieve steady state operation while maintaining MHD stability. Statistical evidence is presented which demonstrates the expected correlation between increased shaping and improved plasma performance. Plasmas with high shape factor have been sustained for pulse lengths which correspond to τ pulse = 1.6s ∼ 50τ E ∼ 5τ CR , where τ CR is the current relaxation time and τ E is the energy confinement time. Plasmas with higher β t ∼ 20% have been sustained for τ pulse = 1.2s ∼ 25τ E ∼ 3τ CR with noninductive current fractions f NI ∼ 50%, with ∼40% pressure driven current and ∼10% neutral beam driven current. An interesting feature of these discharges is the observation that the central value of the safety factor q(0) remains elevated and constant for several current diffusion times without sawteeth, similar to the 'hybrid mode'. Calculations of the profiles of inductive and non-inductive current are compared with measurements of the total current profile and are shown to be in quantitative agreement. Results are shown from experiments which investigate the applicability of high harmonic fast waves (HHFWs) and electron Bernstein waves (EBWs) as current drive and heating sources, and the possibility of LHCD for future ST devices is raised. A calculated scenario which provides 100% non-inductive current drive is described. NSTX operates with peak divertor heat fluxes which are in the same range as those expected for the ITER device, i.e. with P heat max ∼ 10 MW m −2 . High triangularity, high elongation plasmas on NSTX have been demonstrated to have reduced peak heat flux to the divertor plates to <3 MW m −2 .

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Section: Shape Control and Global Performancementioning

confidence: 99%

Progress towards steady state at low aspect ratio on the National Spherical Torus Experiment (NSTX)

Gates¹,

Ménard²,

Maingi³

et al. 2007

Nucl. Fusion

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show abstract

“…NSTX [41] is a medium size ST, designed to test the predicted advantageous confinement and stability properties of the ST [42]. It has a major radius of R = 0.85 m, and typically operates at aspect ratios R/a = 1.3-1.4 (here, R is the major radius of the plasma and a is the minor radius, defined as half the distance between the inboard and outboard plasma boundaries at the midplane).…”

Section: Experimental Backgroundmentioning

confidence: 99%

First observation of ELM pacing with vertical jogs in a spherical torus

et al. 2010

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Experiments in a number of conventional aspect ratio tokamaks have been successful in pacing edge localized modes (ELMs) by rapid vertical jogging of the plasma. This paper demonstrates the first pacing of ELMs in a spherical torus plasma. Applied 30 Hz vertical jogs synchronized the ELMs with the upward motion of the plasma. 45 Hz jogs also lead to an increase in the ELM frequency, though the synchronization of the ELMs and jogs was unclear. A reduction in the ELM energy was observed at the higher driven ELM frequencies.

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“…With the advent of significant levels of auxiliary heating and maturing diagnostic and operational capabilities over the last two years, the National Spherical Torus Experiment [1] (NSTX) has begun intensive research aimed at establishing the physics basis for high performance, long pulse, solenoid-free operations of the spherical torus [2] (ST) concept. This research is directed at developing an understanding of the physics of the ST operational space, developing tools to expand this space, and contributing broadly to toroidal science.…”

Section: Introductionmentioning

confidence: 99%

The National Spherical Torus Experiment (NSTX) Research Program and Progress Towards High Beta, Long Pulse Operating Scenarios

Synakowski¹,

Bell²,

Bell³

et al. 2002

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-A major research goal of the National Spherical Torus Experiment is establishing long-pulse, high beta, high confinement operation and its physics basis. This research has been enabled by facility capabilities developed over the last two years, including neutral beam (up to 7 MW) and high harmonic fast wave heating (up to 6 MW), toroidal fields up to 6 kG, plasma currents up to 1.5 MA, flexible shape control, and wall preparation techniques. These capabilities have enabled the generation of plasmas with <β T > up to 35%. Normalized beta values often exceed the no wall limit, and studies suggest that passive wall mode stabilization is enabling this for broad pressure profiles characteristic of H mode plasmas. The viability of long, high bootstrap current fraction operations has been established for ELMing H mode plasmas with toroidal beta values in excess of 15% and sustained for several current relaxation times. Improvements in wall conditioning and fueling are likely contributing to a reduction in H mode power thresholds. Electron thermal conduction is the dominant thermal loss channel in auxiliary heated plasmas examined thus far. HHFW effectively heats electrons, and its acceleration of fast beam ions has been observed. Evidence for HHFW current drive is by comparing of the loop voltage evolution in plasmas with matched density and temperature profiles but varying phases of launched HHFW waves. A peak heat flux of 10 MW/m 2 has been measured in the H mode, with large asymmetries in the power deposition being observed between the inner and outer strike points. Noninductive plasma startup studies have focused on coaxial helicity injection. With this technique, toroidal currents up to 400 kA have been driven, and studies to assess flux closure and coupling to other current drive techniques have begun.

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Features of spherical torus plasmas

Cited by 440 publications

References 17 publications

Progress towards steady state at low aspect ratio on the National Spherical Torus Experiment (NSTX)

Progress towards steady state at low aspect ratio on the National Spherical Torus Experiment (NSTX)

First observation of ELM pacing with vertical jogs in a spherical torus

The National Spherical Torus Experiment (NSTX) Research Program and Progress Towards High Beta, Long Pulse Operating Scenarios

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