Transport Processes in the Plasma

Higher electron temperature (Te>350eV) and reduced electron thermal diffusivity (χe<10m2∕s) is achieved in the Sustained Spheromak Physics Experiment (SSPX) by increasing the discharge current=Igun and gun bias flux=ψgun in a prescribed manner. The internal current and q=safety factor profile derived from equilibrium reconstruction as well as the measured magnetic fluctuation amplitude can be controlled by programming the ratio λgun=μ0Igun∕ψgun. Varying λgun above and below the minimum energy eigenvalue=λFC of the flux conserver (∇×B=λFCB) varies the q profile and produces the m∕n=poloidal/toroidal magnetic fluctuation mode spectrum expected from mode-rational surfaces with q=m∕n. The highest Te is measured when the gun is driven with λgun slightly less than λFC, producing low fluctuation amplitudes (<1%) and 1∕2<q<2∕3. Transport analysis shows a reduction in χe as Te increases, differing from Bohm or open field line transport models where χe increases with Te. Detailed resistive magnetohydrodynamic sim...

/pdf/transport-and-fluctuations-in-high-temperature-spheromak-5ey59p6ygh.pdf

Transport and fluctuations in high temperature spheromak plasmas

The Space Plasma Environment Research Facility (SPERF) is a new ground-based experimental device for fundamental research studies on space plasma currently under construction at Harbin Institute of Technology in China. Scientific objectives of the SPERF include studying the asymmetric reconnection dynamics relevant to the interaction between the interplanetary and magnetospheric plasmas, reproducing the inner magnetosphere to simulate the processes of trapping, acceleration, and transport of energetic charged particles restrained in a dipole magnetic field configuration, and revealing the physical mechanism of the dipolarization front in the magnetotail. The device comprises a vacuum chamber, 11 coils consisting of 18 groups of sub-coils that are independently programmablely energized, and the plasma source system to provide the magnetic field and the plasma required by the physical experiments. Thus, each of these 18 groups of sub-coils requires a separate pulsed power supply; furthermore, the 18 pulsed power supplies constitute the pulsed power supply system of the SPERF of which the total storage energy is up to 18.3 MJ, and the technical challenges have to be overcome. The power supply energizing a dipole field coil (labeled OJC coil) wired by the copper wire to provide a dipole magnetic field is the most energetic power supply (labeled OJC power supply) with a 2.42 MJ, 16.8 mF capacitor bank charged to 20 kV. The OJC power supply delivers a current with a peak of 18 kA for a rise time of ∼26.69 ms, and the duration of the current is not less than 95% of the peak over 10 ms to the OJC coil. Meanwhile, the most challenging power supply is the power supply labeled poloidal field power supply with a 5.04 mF capacitor bank charged to 20 kV, which provides the excitation current for the load coil set with the current not less than 360 kA at the typical time of 0.11 ms to produce the sufficient growth of the magnetic field that the experiments need. In this paper, the overall design of the pulsed power supply system, the design concept of the modularization, and the principle selection basis of the key components are presented. The technical details of each power supply will be demonstrated in the future.

An 18.3 MJ charging and discharging pulsed power supply system for the Space Plasma Environment Research Facility (SPERF). I. The overall design.

The magnetic compression experiment at General Fusion was a repetitive non-destructive test to study plasma physics applicable to magnetic target fusion compression. A compact torus (CT) is formed with a co-axial gun into a containment region with an hour-glass shaped inner flux conserver, and an insulating outer wall. External coil currents keep the CT off the outer wall (radial levitation) and then rapidly compress it inwards. The optimal external coil configuration greatly improved both the levitated CT lifetime and the recurrence rate of shots with good compressional flux conservation. As confirmed by spectrometer data, the improved levitation field profile reduced plasma impurity levels by suppressing the interaction between plasma and the insulating outer wall during the formation process. Significant increases in magnetic field, electron density, and ion temperature were routinely observed at magnetic compression in the final external coil configuration tested, despite the prevalence of an instability, thought be an external kink mode, at compression. Matching the decay rate of the levitation currents to that of the CT currents resulted in a reduced level of MHD activity associated with unintentional compression by the levitation field, and a higher probability of long-lived CTs. The DELiTE (Differential Equations on Linear Triangular Elements) framework was developed for spatial discretisation of partial differential equations on an unstructured triangular grid in axisymmetric geometry. The framework is based on discrete differential operators in matrix form, which are derived using linear finite elements and mimic some of the properties of their continuous counterparts. A single-fluid two-temperature MHD model is implemented in this framework. The inherent properties of the operators are used in the code to ensure global conservation of energy, particle count, toroidal flux, and angular momentum. The development of the discrete forms of the equations solved is presented. The code was applied to study the magnetic compression experiment. The numerical models developed to simulate CT formation, levitation, and magnetic compression are reported. A model for anisotropic thermal diffusion has been formulated and implemented to the code. A method for determining the q profile of the CT was established simulated CT q profiles indicate that the magnetic compression experiment could be improved by modifying the q profile to regimes with increased stability against kink modes. Comparisons between simulated and experimental diagnostics are presented. A model of plasma/neutral fluid interaction was developed and included in the framework. The source rates of species momentum and energy due to ionization and recombination were derived using a simple method that enables determination of the volumetric rate of thermal energy transfer from electrons to photons and neutral particles due to radiative recombination, which has been neglected in other studies. The implementation of the model has enabled clarification of the mechanisms behind the increases in CT electron density that are routinely observed on the SPECTOR plasma injector well after CT formation. This understanding helps account for the exceptionally significant increase in electron temperature, and markedly reduced electron density, observed during the electrode edge biasing experiment conducted on SPECTOR. It is thought that edge fueling impediment, a consequence of a biasing-induced transport barrier, is largely responsible for the observed modifications to temperature and density.

Magnetic Compression of Compact Tori, Experiment and Simulation

A design point is presented here for a prototype fusion neutron source for waste transmutation ( n/s), based on the adiabatic compression of a compact torus (spheromak). The design utilizes the CORSICA (2D equilibrium) and NIMROD (3D time-dependent MHD) codes as well as analytic modeling with target parameters Rinitial = 0.5 m, Rfinal = 0.167 m, Tinitial = 0.4 keV, Tfinal = 4 keV, ninitial = 2 × 1020 m–3 and nfinal = 50 × 1020 m–3, with radial convergence of C = 3. 3D time-dependent simulations of spheromak compression agree well with analytic models for adiabatic compression, if the run-in time . Knowing required, we design coils and passive structure (with CORSICA) to ensure stability; then design the capacitor bank needed to both form the target plasma and drive coils. We specify target parameters for the compression in terms of plasma beta, formation efficiency and energy confinement.

Adiabatic Compression of a Compact Torus

The Facility for Laboratory Reconnection Experiments (FLARE) is an intermediate laboratory experiment currently under construction at Princeton University by a consortium of five universities and two Department of Energy (DoE) national laboratories, located at the Princeton Plasma Physics Laboratory (PPPL). The goal of FLARE is to provide experimental accesses to new regimes of the magnetic reconnection process and related phenomena directly relevant to heliophysics, astrophysics, and fusion plasmas. The device comprises a vacuum chamber and 9 coils sets that are independently programmable to provide the poloidal and toroidal magnetic fields required to form plasma and study the effects of magnetic reconnection. Each of these 9 coil sets requires a separate pulsed power system, it is the design of the power systems that is reported here. The 9 separate pulsed power systems combine to produce over 5.5MJ of energy to the experiment and each presented their own unique challenges. The most energetic power system is a 3.4MJ, 19.2mF capacitor bank charged to 20kV that provides the guide field, with a rise time of approximately 12ms it delivers an average peak current of 40kA over 5.3ms to 12 coils wired in series. The poloidal field coils consist of two separate coilsets each requiring 540kA peak current which is produced by two 20kV, 2.64mF capacitor banks. The design of the two driver coilsets each charged to 60kV will also be presented.

Design of 5.5MJ charge dump power supply for the PPPL FLARE experiment

Physicsdesign point for a 1MW fusion neutron source

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Adiabatic Compression of a Compact Torus

Design of 5.5MJ charge dump power supply for the PPPL FLARE experiment

Physicsdesign point for a 1MW fusion neutron source