Abstract
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 during 2001 and 2002, including neutral beam (up to 7 MW) and high harmonic fast wave (HHFW) 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 of up to 35%. Normalized beta values often exceed the no-wall limit, and studies suggest that passive wall mode stabilization enables this for H mode plasmas with broad pressure profiles. 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 fuelling 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 obtained by comparision of the loop voltage evolution in plasmas with matched density and temperature profiles but varying phases of launched HHFW waves. Studies of emissions from electron Bernstein waves indicate a density scale length dependence of their transmission across the upper hybrid resonance near the plasma edge that is consistent with theoretical predictions. A peak heat flux to the divertor targets of 10 MW m−2 has been measured in the H mode, with large asymmetries being observed in the power deposition between the inner and outer strike points. Non-inductive 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.
Highlights
With the advent of significant levels of auxiliary heating and maturing diagnostic and operational capabilities in 2001 and 2002, the national spherical torus experiment (NSTX) [1] has begun intensive research aimed at establishing the physics basis for high performance, long pulse, solenoidfree operations of the spherical torus (ST) [2] concept
Research has focused on high beta MHD stability, confinement, high harmonic fast wave (HHFW) heating and current drive, boundary physics, solenoid-free startup, and exploration of scenarios that integrate favourable confinement, stability, and non-inductive current drive properties
Particular attention is given to those elements relevant to establishing the physics and operational basis for long pulse, high beta, high confinement regimes with high fractions of non-inductive current drive
Summary
With the advent of significant levels of auxiliary heating and maturing diagnostic and operational capabilities in 2001 and 2002, the national spherical torus experiment (NSTX) [1] has begun intensive research aimed at establishing the physics basis for high performance, long pulse, solenoidfree operations of the spherical torus (ST) [2] 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 the science of toroidal confinement To these ends, research has focused on high beta MHD stability, confinement, high harmonic fast wave (HHFW) heating and current drive, boundary physics, solenoid-free startup, and exploration of scenarios that integrate favourable confinement, stability, and non-inductive current drive properties. Research has focused on high beta MHD stability, confinement, high harmonic fast wave (HHFW) heating and current drive, boundary physics, solenoid-free startup, and exploration of scenarios that integrate favourable confinement, stability, and non-inductive current drive properties Some results of these efforts include the following:. Particular attention is given to those elements relevant to establishing the physics and operational basis for long pulse, high beta, high confinement regimes with high fractions of non-inductive current drive
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