Electric fields and transport in optimized stellarators

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 119-124).Recent stellarator experiments have been designed with one of two types of neoclassical optimization: quasisymmetry or quasi-isodynamism. Both types of stellarator have perfectly confined collisionless particle orbits as well as one additional feature. Quasisymmetric plasmas have minimal flow damping, which may lead to reduced turbulent transport. Quasi-isodynamic plasmas can have vanishing bootstrap current, implying less variation in the magnetic configuration as the pressure changes and also implying greater stability. Analytical expressions for neoclassical transport in a general stellarator are complicated, so it is desirable to find reduced expressions for ideal limiting cases to provide insight. Here, new neoclassical expressions are derived for a quasi-isodynamic plasma. The Pfirsch-Schliiter flow and current can be written concisely as an integral of B. The remaining components of the flow and bootstrap current are identical to those in a quasi-poloidally symmetric device. A compact expression is derived for the radial electric field Er which is largely independent of the details of the magnetic field. Another issue in the neoclassical theory of stellarators which has not been fully resolved is the validity of the so-called monoenergetic approximation, in which ad-hoc changes are made to Er terms in the kinetic equation to expedite numerical computations. Here we show that at least in a quasisymmetric plasma, this approximate treatment of Er leads to a significant and systematic underestimation of the trapped particle fraction. This distortion of the collisionless orbits is independent of any approximations made to the collision operator. For ideal quasisymmetric and quasi-isodynamic plasmas, new neoclassical expressions are derived in which this problematic monoenergetic approximation is avoided. In the quasisymmetric case, results are presented in both the banana regime and plateau regime for the ion flow, ion radial heat flux, and bootstrap current. The bootstrap current is found to be enhanced. For the quasi-isodynamic case, new Er-driven contributions to the distribution function are obtained. The flow and bootstrap current turn out to be modified by the same numerical coefficient as in the quasisymmetric case.by Matthew Joseph Landreman.Ph.D

Three-Dimensional Structure Of Magnetic Reconnection In A Laboratory Plasma

The local three-dimensional structure of magnetic reconnection has been measured for the first time in a magnetohydrodynamic (MHD) laboratory plasma at the Swarthmore Spheromak Experiment. An array of 600 magnetic probes which resolve ion inertial length and MHD time scale dynamics on a single shot basis measured the magnetic structure of partial spheromak merging events. Counter-helicity spheromaks merge rapidly, and reconnection activity clearly self-generates a local component of B which breaks the standard 2D symmetry at the ion inertial scale. Consistent with prior results, no reconnection is observed for co-helicity merging

Generalized Ohm\u27s Law In A 3-D Reconnection Experiment

We report the measurement of non-ideal terms of the generalized Ohm\u27s law at a reconnection site of a weakly collisional laboratory magnetohydrodynamic plasma. Results show that the Hall term dominates the measured terms; resistive and electron inertia terms are small. We suggest that electron pressure (not measured) supports the observed quasistatic reconnection rate, and that anomalous resistivity, while not ruled out, is not required to account for the results

Experimental Observation Of Energetic Ions Accelerated By Three-Dimensional Magnetic Reconnection In A Laboratory Plasma

Magnetic reconnection is widely believed responsible for heating the solar corona as well as for generating X-rays and energetic particles in solar flares. On astrophysical scales, reconnection in the intergalactic plasma is a prime candidate for a local source (Mpc) of cosmic rays exceeding the Greisen-Zatsepin-Kuzmin cutoff (∼10(19) eV). In a laboratory astrophysics experiment, we have made the first observation of particles accelerated by magnetic reconnection events to energies significantly above both the thermal and the characteristic magnetohydrodynamic energies. These particles are correlated temporally and spatially with the formation of three-dimensional magnetic structures in the reconnection region

Fluid And Kinetic Structure Of Magnetic Merging In The Swarthmore Spheromak Experiment

Measurement of the in-plane Lorentz force and the out-of-plane magnetic field associated with the Hall electric field near the reconnection zone in the Swarthmore Spheromak Experiment (SSX) confirms expectations, based on simulation, theory and spacecraft data, that the quadrupolar out-of-plane magnetic field is a signature of collisionless effects in magnetic reconnection with a weak guide field

Rapid Multiplexed Data Acquisition: Application To Three-Dimensional Magnetic Field Measurements In A Turbulent Laboratory Plasma

Multiplexing electronics have been constructed to reduce the cost of high-speed data acquisition at the Swarthmore Spheromak Experiment (SSX) and Redmond Plasma Physics Laboratory. An application of the system is described for a three-dimensional magnetic probe array designed to resolve magnetohydrodynamic time scale and ion inertial spatial scale structure of magnetic reconnection in a laboratory plasma at SSX. Multiplexing at 10 MHz compresses 600 pick-up coil signals in the magnetic probe array into 75 digitizer channels. An external master timing system maintains synchronization of the multiplexers and digitizers. The complete system, calibrated and tested with Helmholtz, line current, and magnetofluid fields, reads out the entire 5 x 5 x 8 probe array every 800 ns with an absolute accuracy of approximately 20 G, limited mainly by bit error. (C) 2003 American Institute of Physics

Poloidal asymmetries in edge transport barriers

Measurements of impurities in Alcator C-Mod indicate that in the pedestal region, significant poloidal asymmetries can exist in the impurity density, ion temperature, and main ion density. In light of the observation that ion temperature and electrostatic potential are not constant on a flux surface [Theiler et al., Nucl. Fusion 54, 083017 (2014)], a technique based on total pressure conservation to align profiles measured at separate poloidal locations is presented and applied. Gyrokinetic neoclassical simulations with XGCa support the observed large poloidal variations in ion temperature and density, and that the total pressure is approximately constant on a flux surface. With the updated alignment technique, the observed in-out asymmetry in impurity density is reduced from previous publishing [Churchill et al., Nucl. Fusion 53, 122002 (2013)], but remains substantial (nz,H/nz,L∼6). Candidate asymmetry drivers are explored, showing that neither non-uniform impurity sources nor localized fluctuation-driven transport are able to explain satisfactorily the impurity density asymmetry. Since impurity density asymmetries are only present in plasmas with strong electron density gradients, and radial transport timescales become comparable to parallel transport timescales in the pedestal region, it is suggested that global transport effects relating to the strong electron density gradients in the pedestal are the main driver for the pedestal in-out impurity density asymmetry.United States. Department of Energy (DE-FC02-99ER54512)United States. Department of Energy (DE-FG02-06ER54845)United States. Department of Energy (DE-FG02-86ER53223)United States. Department of Energy (DE-AC02-09CH11466

Ion Runaway in Lightning Discharges

Runaway ions can be produced in plasmas with large electric fields, where the accelerating electric force is augmented by the low mean ionic charge due to the imbalance between the number of electrons and ions. Here we derive an expression for the high-energy tail of the ion distribution function in lightning discharges and investigate the energy range that the ions can reach. We also estimate the corresponding energetic proton and neutron production due to fusion reactions.Oak Ridge Institute for Science and Education (Fusion Energy Postdoctoral Research Program

Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000

Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy

Major results from the first plasma campaign of the Wendelstein 7-X stellarator

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 1019 m-3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.Peer reviewe