David W. Swain

Exploration of Spherical Torus Physics in the NSTX Device

The National Spherical Torus Experiment (NSTX) is being built at the Princeton Plasma Physics Laboratory to test the fusion physics principles for the Spherical Torus (ST) concept at the MA level. The NSTX nominal plasma parameters are R {sub 0} = 85 cm, a = 67 cm, R/a greater than or equal to 1.26, B {sub T} = 3 kG, I {sub p} = 1 MA, q {sub 95} = 14, elongation {kappa} less than or equal to 2.2, triangularity {delta} less than or equal to 0.5, and plasma pulse length of up to 5 sec. The plasma heating/current drive (CD) tools are High Harmonic Fast Wave (HHFW) (6 MW, 5 sec), Neutral Beam Injection (NBI) (5 MW, 80 keV, 5 sec), and Coaxial Helicity Injection (CHI). Theoretical calculations predict that NSTX should provide exciting possibilities for exploring a number of important new physics regimes including very high plasma beta, naturally high plasma elongation, high bootstrap current fraction, absolute magnetic well, and high pressure driven sheared flow. In addition, the NSTX program plans to explore fully noninductive plasma start-up, as well as a dispersive scrape-off layer for heat and particle flux handling.

An efficient technique for magnetic analysis of non-circular, high-beta tokamak equilibria

A method for determining plasma shape and global properties of high-beta non-circular tokamak equilibria (βp, β, q and li) from magnetic sensor data is described. The technique uses a least-squares fitting procedure to find the plasma boundary and a global force balance analysis (as opposed to a complete solution of the MHD equilibrium equation) to determine plasma pressure. Estimates of the uncertainties in the computed quantities are also obtained, and the method is fast enough that it can be used to provide a complete time history (up to 100 time points) of ISX-B high-beta discharges within two to three minutes of each shot. Values obtained using this method are in excellent agreement with more detailed measurements and with free-boundary MHD equilibrium computations.

H-mode research in NSTX

H-modes are routinely obtained in the National Spherical Torus Experiment (NSTX) and have become a standard operational scenario. L–H transitions triggered by NBI heating have been obtained over a wide parameter range in Ip, Bt, and e in either lower-single-null (LSN) or double-null (DN) diverted discharges. Edge localized modes are observed in both configurations but the characteristics differ between DN and LSN, which also have different triangularities (δ). An H-mode duration of 500 ms was obtained in LSN, with a total pulse length of ~1 s. Preliminary power threshold studies indicate that the L–H threshold is between 600 kW and 1.2 MW, depending on the target parameters. Gas injector fuelling from the centre stack (i.e. the high toroidal field side) has enabled routine H-mode access, and comparisons with low-field side (LFS) fuelled H-mode discharges show that the LFS fuelling delays the L–H transition and alters the pre-transition plasma profiles. Gas puff imaging and reflectometry show that the H-mode edge is usually more quiescent than the L-mode edge. Divertor infrared camera measurements indicate up to 70% of available power flows to the divertor targets in quiescent H-mode discharges.

Simulation of high-power electromagnetic wave heating in the ITER burning plasma

The next step toward fusion as a practical energy source is the design and construction of ITER [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)], a device capable of producing and controlling the high-performance plasma required for self-sustaining fusion reactions, i.e., “burning plasma.” ITER relies in part on ion-cyclotron radio frequency power to heat the deuterium and tritium fuel to fusion temperatures. In order to heat effectively, the radio frequency wave fields must couple efficiently to the dense core plasma. Calculations in this paper support the argument that this will be the case. Three-dimensional full-wave simulations show that fast magnetosonic waves in ITER propagate radially inward with strong central focusing and little toroidal spreading. Energy deposition, current drive, and plasma flow are all highly localized near the plasma center. Very high resolution, two-dimensional calculations reveal the presence of mode conversion layers, where fast waves can be converted to slow ion cyclotron...

Effect of plasma shaping on performance in the National Spherical Torus Experiment

The National Spherical Torus Experiment (NSTX) has explored the effects of shaping on plasma performance as determined by many diverse topics including the stability of global magnetohydrodynamic (MHD) modes (e.g., ideal external kinks and resistive wall modes), edge localized modes (ELMs), bootstrap current drive, divertor flux expansion, and heat transport. Improved shaping capability has been crucial to achieving βt∼40%. Precise plasma shape control has been achieved on NSTX using real-time equilibrium reconstruction. NSTX has simultaneously achieved elongation κ∼2.8 and triangularity δ∼0.8. Ideal MHD theory predicts increased stability at high values of shaping factor S≡q95Ip∕(aBt), which has been observed at large values of the S∼37[MA∕(m∙T)] on NSTX. The behavior of ELMs is observed to depend on plasma shape. A description of the ELM regimes attained as shape is varied will be presented. Increased shaping is predicted to increase the bootstrap fraction at fixed Ip. The achievement of strong shaping ...

Reflectometer sensing of rf waves in front of the high harmonic fast wave antenna on NSTX

The ability to measure rf driven waves in the edge of the plasma can help to elucidate the role that surface waves and parametric decay instabilities (PDIs) play in rf power losses on NSTX. A microwave reflectometer has recently been modified to monitor rf plasma waves in the scrape-off layer in front of the 30MHz high harmonic fast wave antenna array on NSTX. In rf heated plasmas, the plasma-reflected microwave signal exhibits 30MHz sidebands, due primarily to the modulation of the cutoff layer by the electrostatic component of the heating wave. Similarly, electrostatic parametric decay waves (when present) are detected at frequencies below the heating frequency, near 28, 26,…MHz, separated from the heating frequency by harmonics of the local ion cyclotron frequency of about 2MHz. In addition, a corresponding frequency matched set of decay waves is also detected near the ion cyclotron harmonics at 2, 4,…MHz. The rf plasma-wave sensing capability is useful for determination of the PDI power threshold as a...

The KSTAR tokamak

The KSTAR (Korea Superconducting Tokamak Advanced Research) project is the major effort of the Korean National Fusion Program to design, construct, and operate a steady-state-capable superconducting tokamak. The project is led by Korea Basic Science Institute and shared by national laboratories, universities, and industry along with international collaboration. It is in the conceptual design phase and aims for the first plasma by mid 2002. The key design features of KSTAR are: major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 T, plasma current 2 MA, and flexible plasma shaping (elongation 2.0; triangularity 0.8; double-null poloidal divertor). Both the toroidal and the poloidal field magnets are superconducting coils. The device is configured to be initially capable of 20 s pulse operation and then to be upgraded for operation up to 300 s with non-inductive current drive. The auxiliary heating and current drive system consists of neutral beam, ICRF, lower hybrid, and ECRF. Deuterium operation is planned with a full radiation shielding.

Fast ion absorption of the high harmonic fast wave in the National Spherical Torus Experiment

Ion absorption of the high harmonic fast wave in a spherical torus [Y.-K. M. Peng et al., Nucl. Fusion 26, 769 (1986)] is of critical importance to assessing the viability of the wave as a means of heating and driving current. Analysis of recent National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)] shots has revealed that under some conditions when neutral beam and rf power are injected into the plasma simultaneously, a fast ion population with energy above the beam injection energy is sustained by the wave. In agreement with modeling, these experiments find the rf-induced fast ion tail strength and neutron rate at lower B-fields to be less enhanced, likely due to a larger β profile, which promotes greater off-axis absorption where the fast ion population is small. Ion loss codes find the increased loss fraction with decreased B insufficient to account for the changes in tail strength, providing further evidence that this is a rf interaction effect. Though greater ion absorption...

Radial transport of high-energy runaway electrons in ORMAK

The transport of high-energy runaway electrons near the outside of a low-density ORMAK discharge is investigated by measuring the flux of runaways to the outer limiter during and after an inward shift of the plasma column. The experimental results are interpreted through a runaway confinement model which includes both the classical outward displacement of the runaway orbit with increasing energy and an additional runaway spatial diffusion coefficient which simulates an unspecified source of anomalous transport. Diffusion coefficients in the range D 102–104 cm s−1 are found under various discharge conditions indicatinga significant non-collisional runaway transport near the outside of the discharge, particularly in the presence of MHD instability.

Exploration of high harmonic fast wave heating on the National Spherical Torus Experiment

High harmonic fast wave (HHFW) heating has been proposed as a particularly attractive means for plasma heating and current drive in the high beta plasmas that are achievable in spherical torus (ST) devices. The National Spherical Torus Experiment (NSTX) [M. Ono, S. M. Kaye, S. Neumeyer et al., in Proceedings of the 18th IEEE/NPSS Symposium on Fusion Engineering, Albuquerque, 1999 (IEEE, Piscataway, NJ, 1999), p. 53] is such a device. An rf heating system has been installed on the NSTX to explore the physics of HHFW heating, current drive via rf waves and for use as a tool to demonstrate the attractiveness of the ST concept as a fusion device. To date, experiments have demonstrated many of the theoretical predictions for HHFW. In particular, strong wave absorption on electrons over a wide range of plasma parameters and wave parallel phase velocities, wave acceleration of energetic ions, and indications of current drive for directed wave spectra have been observed. In addition HHFW heating has been used to ...