A multi-species fluid model is described for the steady state parallel and radial force balance equations in axisymmetric tokamak plasmas. The bootstrap current, electrical resistivity, and particle and heat fluxes are evaluated in terms of the rotation velocities and friction and viscosity coefficients. A recent formulation of the neoclassical plasma viscosity for arbitrary shape and aspect ratio (including the unity aspect ratio limit), arbitrary collisionality, and orbit squeezing from strong radial electric fields is used to illustrate features of the model. The bootstrap current for the very low aspect ratio National Spherical Torus Experiment [J. Spitzer et al., Fusion Technol. 30, 1337 (1996)] is compared with other models; the largest differences occur near the plasma edge from treatment of the collisional contributions. The effects of orbit squeezing on bootstrap current, thermal and particle transport, and poloidal rotation are illustrated for an enhanced reverse shear plasma in the Tokamak Fusion Test Reactor [D. Meade and the TFTR Group, Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, p. 9]. Multiple charge states of impurities are incorporated using the reduced ion charge state formalism for computational efficiency. Because the force balance equations allow for inclusion of external momentum and heat sources and sinks they can be used for general plasma rotation studies while retaining the multi-species neoclassical effects.

Bootstrap current and neoclassical transport in tokamaks of arbitrary collisionality and aspect ratio

A nominally axisymmetric plasma configuration, such as a tokamak or a spherical torus, is highly sensitive to nonaxisymmetric magnetic perturbations due to currents outside of the plasma. The high sensitivity means that the primary interest is in the response of the plasma to very small perturbations, i.e., ∣b∕B∣≈10−2 to 10−4, which can be calculated using the theory of perturbed equilibria. The ideal perturbed equilibrium code (IPEC) is described and applied to the study of the plasma response in a spherical torus to such external perturbations.

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Computation of three-dimensional tokamak and spherical torus equilibria

The sensitivity of tokamak plasmas to very small deviations from the axisymmetry of the magnetic field |δ→(over)Β/→(over)Β|≈ 10–4 is well known. What was not understood until very recently is the importance of the perturbation to the plasma equilibrium in assessing the effects of externally produced asymmetries in the magnetic field, even far from a stability limit. DIII-D and NSTX experiments find that when the deleterious effects of asymmetries are mitigated, the external asymmetric field was often made stronger and with an increased interaction with the magnetic field of the unperturbed equilibrium fields. This paper explains these counter intuitive results. The explanation using ideal perturbed equilibria has important implications for the control of field errors in all toroidal plasmas.

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Control of Asymmetric Magnetic Perturbations in Tokamaks

Dependence of divertor heat flux widths on heating power, flux expansion, and plasma current in the NSTX

A scintillator based energetic ion loss detector has been built and installed on the National Spherical Torus Experiment (NSTX) to measure the loss of neutral beam ions. The detector is able to resolve the pitch angle and gyroradius of the lost energetic ions. It has a wide acceptance range in pitch angle and energy, and is able to resolve the full, one-half, and one-third energy components of the 80 keV D neutral beams up to the maximum toroidal magnetic field of NSTX. Multiple Faraday cups have been embedded behind the scintillator to allow easy absolute calibration of the diagnostic and to measure the energetic ion loss to several ranges of pitch angle with good time resolution. Several small, vacuum compatible lamps allow simple calibration of the scintillator position within the field of view of the diagnostic's video camera.

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Scintillator based energetic ion loss diagnostic for the National Spherical Torus Experiment

The National Spherical Tokamak Experiment (NSTX) is an ultra low aspect ratio device with a plasma current of 1 MA. The tokamak features auxiliary heating and current drive with a close-fitting conducting shell to maximize the plasma pressure. NSTX is designed for an experimental pulse length that will demonstrate quasi-steady state non-inductively driven advanced tokamak operation. The design also takes maximum advantage of existing facilities and components from previous Princeton devices to reduce the overall program costs.

Engineering Design of the National Spherical Tokamak Experiment

Engineering design of the National Spherical Torus Experiment

The NSTX project will provide a national facility for the study of plasma confinement, heating, and current drive in a low aspect ratio, spherical torus (ST) configuration, the ST configuration is an alternate confinement concept which is characterized by high /spl beta/, high elongation, high bootstrap fraction, and low B/sub T/ compared to conventional tokamaks, NSTX is the next step ST experiment following smaller experiments such as the PPPL CDX-U (Current Drive Experiment, Upgrade), the START (Small Tight Aspect Ratio Tokamak) at Culham, and the HIT (Helicity Injected Tokamak) at University of Washington, and is similar in scale to the MAST (Meg-Amp Spherical Tokamak) machine now under construction at Culham. This paper provides a description of the mission and gives an overview of the engineering features of the design of the machine and facility and discusses some of the key design solutions.

R. Wilson

Papers

Engineering Design of the National Spherical Tokamak Experiment

Engineering design of the National Spherical Torus Experiment

Engineering overview of the National Spherical Torus Experiment (NSTX)