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Paul Bonoli is the Principal Investigator of the SciDac ”Center for Simulation of Wave Plasma Interactions” Project. fields that are fully three-dimensional rather than axisymmetric. Results have been obtained for optimized stellarators that are quasi-symmetric or quasi-isodynamic. Quasi-symmetric stellarators share many of the properties of tokamaks, but without the need of a transformer to drive an Ohmic current. General expressions have been obtained for the flows, heat transport, and currents in these devices and in quasi-isodynamic configurations. In a quasi-isodynamic stellarator the surfaces of constant magnetic field, B, close poloidally such that the minimum and maximum of B are the same for Columbia University scientist Darren Garnier works on the Levitated Dipole Experiment (LDX). every field line on the flux surface. The PSFC theory group also has a strong participation in the multi-institutional SciDAC “Center for Simulation of Wave Plasma Interactions,” where Dr. Paul Bonoli serves as the Principal Investigator of this entire SciDAC Project. We are also involved in several other SciDACs and prototype simulation projects. Their main purpose is to implement advanced fluid, kinetic and hybrid descriptions in large-scale nonlinear simulations. In 2007 a new high-performance parallel computing cluster (Loki) was developed and implemented at the PSFC by the theory group, becoming a major tool in aiding scientific discovery within the PSFC theory program. It is employed for theoretical fusion research initiatives in which simulation plays a significant or dominant role. It also supports the Alcator C-Mod, LDX, and VTF experiments through simulation of experimental conditions and interpretation of diagnostic measurements. Loki is now heavily used by about 50 graduate students and researchers, including some of our external domestic and international collaborators. The cluster has enabled many Alcator C-Mod students to carry out sophisticated, quantitative simulations of microturbulence and radiofrequency heating for direct comparisons with their measurements. The cluster was initially comprised of 230 processors in 65 compute nodes. In 2009 the number of processors in the cluster was increased from 260 to 600, the total cluster memory was more than tripled from 260 GB to 940 GB, and 10 new nodes were added to improve operational flexibility and cluster scheduling. In 2010 an additional upgrade was started and is nearing completion. The storage capacity on the head node is being increased from 2 to 25 TB, and the operating system has been upgraded. PSFC Progress Report 09–11 17
Levitated Dipole Experiment (LDX) Physicists at the PSFC have been exploring novel magnetic confinement configurations that are fundamentally different from the tokamak. These experiments have broadened the understanding of the physics of plasma confinement and may lead to alternate reactor concepts. In addition they provide insights into space and astrophysical plasma phenomenon. Dipole confinement of relatively highpressure plasma has been observed in nature, e.g., in the magnetospheres surrounding Earth or Jupiter. In the Levitated Dipole Experiment (LDX) a superconducting floating current ring arrangement generates a dipole magnetic field in which we create a torus of hot plasma. LDX permitted the study of stability and confinement properties of a dipole in a laboratory setting. Unfortunately, the US DOE recently terminated LDX. The experiment was a joint collaborative project with Columbia University at MIT, led by Principal Investigators Senior Research Scientist Dr. Jay Kesner (MIT) and Professor Michael Mauel (Columbia University). During the initial LDX experimental campaign, which began in 2004, the dipole coil was mechanically supported within the vacuum chamber. These experiments provided a database for supported operation, later to be compared with levitated experiments. During supported experiments plasma was primarily lost to the supports, dominating cross-field transport processes that are of scientific interest. Experiments with the floating coil fully levitated began in 2007. A launching structure lifted the floating coil into the center of the vacuum chamber, the levitation system was turned on and the launcher was then moved outside of the plasma. The plasma was heated by up to 25 kW of electron cyclotron heating at multiple frequencies: 2.45, 6.4, 10.5 and 28.0 GHz, respectively. As compared to previous studies in which the internal coil was supported, levitation resulted in improved Senior Research Engineer Paul Woskov works on a dual receiver millimeter-wave system used for hot electron cyclotron emission measurements in conjunction with the Levitated Dipole Experiment. particle confinement, which allowed high-density, high-beta discharges to be maintained at significantly reduced gas fueling. Elimination of parallel losses coupled with reduced gas fueling was observed to lead to improved energy confinement. During levitation of the half-ton superconducting current ring a strong particle pinch was observed, which was the basis of an article in Nature-Physics [Boxer et al, Nature Physics, 3 (2010) 207] and an accompanying press release. Theoretical studies supported the view that the pinch was driven by plasma turbulence; these observations serve as an example of turbulence driving density inwards. A weak turbulent pinch is thought to play an important role in fueling other plasma devices, but the demonstration in LDX was clear and dramatic. The pinch was observed to produce stationary density profiles with a peak-to-edge density ratio of up to 30. Additionally, the improved particle confinement that accompanied levitation was seen to improve the stability of the hot electron component, and was seen to produce significantly improved dipole discharges. A video of the first levitated plasma operation may be viewed on the LDX website (www. psfc.mit.edu/ldx/). In these experiments we utilized only electron heating. Recently the PSFC obtained a 1 MW transmitter, which has been installed near the experimental bay. As fusion energy requires hot ions, we were preparing to initiate direct ion heating 18 PSFC Progress Report 09–11
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