High-Θ (Θ≡Bϑ(a)/〈Bφ〉) toroidal reversed-field pinch discharges in the ZT-40M device are characterized by soft x-ray sawtooth oscillations. It is demonstrated that a one-dimensional transport description is sufficient to model the rise time of sawtooth oscillations, provided the resistivity profile has significant gradients. This result suggests that the toroidal magnetic flux generation (dynamo) observed in the reversed-field pinch (RFP) may be restricted to the m=1 tearing events associated with the sawtooth crash (decrease in the soft x-ray flux).
Energy confinement within the reversed field pinch (RFP) is governed by three plasma regions: a poorly confined plasma core characterized by parallel (radial) transport and flux surfaces destroyed by m = 1 tearing mode activity associated with dynamo relaxation; a medium confined edge limited by ideal pressure gradient driven modes; and a good confinement region located near the reversal layer. The good confinement region determines the global confinement characteristics of the RFP, and, if limited by resistive interchange modes, would be consistent with the Connor-Taylor scaling that has provided a good fit to international RFP results. After establishing a two parameter fit of confinement scaling to the RFP database, the scaling relation is used to project the physical characteristics of, and costs associated with, next step and ignition experiments for ohmically heated RFPs. The RFP projects to smaller and less expensive machines than the tokamak with comparable performance,
Plasma models are described and used to calculate numerically the transport confinement (nτE) requirements and steady state operation points for both the reversed field pinch (RFP) and the tokamak. The models are used to examine the CIT tokamak ignition conditions and the RFP experimental and ignition conditions. Physics differences between RFPs and tokamaks and their consequences for a D-T ignition machine are discussed. Compared with a tokamak, the ignition RFP has many physics advantages, including Ohmic heating to ignition (no need for auxiliary heating systems), higher beta, lower ignition current, less sensitivity of ignition requirements to impurity effects, no hard disruptions (associated with beta or density limits) and successful operation with high radiation fractions (fRAD ∼ 0.95). These physics advantages, coupled with important engineering advantages associated with lower external magnetic fields, larger aspect ratios and smaller plasma cross-sections, translate to significant cost reductions for both ignition and reactor applications. The primary drawback of the RFP is the uncertainty that the present scaling will extrapolate to reactor regimes. Devices that are under construction should go a long way towards resolving this scaling uncertainty. The 4 MA ZTH is expected to extend the nτE transport scaling data by three orders of magnitude above the results of ZT-40M, and, if the present scaling holds, ZTH is expected to achieve a D-T equivalent scientific energy breakeven, Q = 1. A base case RFP ignition point is identified with a plasma current of 8.1 MA and no auxiliary heating.
The results of a significantly more efficient model (molecular model) for computing particle and energy sources from recycling in edge plasma transport codes, such as b2, are compared with results from more detailed calculations using the b2 code coupled with the neutral transport code, DEGAS. The molecular model considers the reflux both of cold molecules and of energetic backscattered atomic neutrals from the divertor target plates and has been implemented to function intemally within the b2 code. The molecular model calculations are shown to be in reasonable agreement with more detailed calculations done with the coupled b2/DEGAS codes for seven cases typical of conditions in the scrape-off layer of the Doublet III (DIII-D) tokamak. Because the molecular model is a far less expensive computational tool, it represents a significant improvement over the existing (coupled) edge transport recycling model. Thus, it may find wider application in design studies for future devices, such as the International Thermonuclear Experimental Reactor (ITER).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.