Abstract
Stationary 3D equilibria can form in fusion plasmas via saturation of magnetohydrodynamic (MHD) instabilities or stimulated by external 3D fields. In these cases the current profile is anomalously broad due to magnetic flux pumping produced by the MHD modes. Flux pumping plays an important role in hybrid tokamak plasmas, maintaining the minimum safety factor above unity and thus removing sawteeth. It also enables steady-state hybrid operation, by redistributing non-inductive current driven near the center by electron cyclotron waves. A validated flux pumping model is not yet available, but it would be necessary to extrapolate hybrid operation to future devices. In this work flux pumping physics is investigated for helical core equilibria stimulated by external 3D fields in DIII-D hybrid plasmas. We show that flux pumping can be produced in a continuous way by an MHD dynamo emf. The same effect maintains helical equilibria in reversed-field pinch (RFP) plasmas. The effective MHD dynamo loop voltage is calculated for experimental 3D equilibrium reconstructions, by balancing Ohm's law over helical flux surfaces, and is consistent with the expected current redistribution. Similar results are also obtained with more sophisticated nonlinear MHD simulations. The same modelling approach is applied to helical RFP states forming spontaneously in RFX-mod as the plasma current is raised above 0.8-1 MA. This comparison allows to identify the underlying physics common to tokamak and RFP: a helical core displacement modulates parallel current density along flux tubes, which requires a helical electrostatic potential to build up, giving rise to a helical MHD dynamo flow.
| Original language | English |
|---|---|
| Article number | 076014 |
| Journal | Nuclear Fusion |
| Volume | 57 |
| Issue number | 7 |
| DOIs | |
| State | Published - May 18 2017 |
Funding
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This material is based upon work supported in part by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under awards DE-FC02-04ER54698, DE-AC05-00OR22725, DE-AC05-06OR23100, DE-FG02-04ER54761, and DE-AC52-07NA27344. DIII-D data shown in this paper can be obtained in digital format by following the links at https://fusion.gat.com/global/D3D-DMP
| Funders | Funder number |
|---|---|
| DOE Office of Science user facility | DE-AC05-00OR22725, DE-FG02-04ER54761, DE-FC02-04ER54698, DE-AC52-07NA27344, DE-AC05-06OR23100 |
| U.S. Department of Energy | |
| Office of Science | |
| Fusion Energy Sciences | |
| Horizon 2020 Framework Programme | 633053 |
| H2020 Euratom |
Keywords
- 3D equilibrium reconstruction
- Helical RFP states
- Hybrid tokamak scenario
- MHD dynamo
