H. Y. Guo

Long pulse FRC sustainment with enhanced edge driven rotating magnetic field current drive

Field reversed configurations (FRCs) have been formed and sustained for up to 50 normal flux decay times by rotating magnetic fields (RMFs) in the translation, confinement, and sustainment experiment. For these longer pulse times a new phenomenon has been observed: switching to a higher performance mode delineated by shallower RMF penetration, higher ratios of generated poloidal to RMF drive field, and lower overall plasma resistivity. This mode switching is always accompanied by, and perhaps triggered by, the spontaneous development of a toroidal field with a magnitude up to 20% of the peak poloidal field. The global data cannot be explained by previous RMF theory based on uniform electron rotational velocities or by numerical calculations based on uniform plasma resistivity, but agrees in many respects with new calculations made using strongly varying resistivity profiles. In order to more realistically model RMF driven FRCs with such non-uniform resistivity profiles, a double rigid rotor model has been developed with separate inner and outer electron rotational velocities and resistivities. The results of this modelling suggest that the RMF drive results in very high resistivity in a narrow edge layer, and that the higher performance mode is characterized by a sharp reduction in resistivity over the bulk of the FRC.

Rotating magnetic field current drive of high-temperature field reversed configurations with high ζ scaling

Greatly reduced recycling and impurity ingestion in the Translation, Confinement, and Sustainment—Upgrade (TCSU) device has allowed much higher plasma temperatures to be achieved in the field reversed configurations (FRC) under rotating magnetic field (RMF) formation and sustainment. The hotter plasmas have higher magnetic fields and much higher diamagnetic electron rotation rates so that the important ratio of average electron rotation frequency to RMF frequency, called ζ, approaches unity, for the first time, in TCSU. A large fraction of the RMF power is absorbed by an as yet unexplained (anomalous) mechanism directly proportional to the square of the RMF magnitude. It becomes of relatively lesser significance as the FRC current increases, and simple resistive heating begins to dominate, but the anomalous absorption is useful for initial plasma heating. Measurements of total absorbed power, and comparisons of applied RMF torque to torque on the electrons due to electron-ion friction under high-ζ operati...

Principal physics of rotating magnetic-field current drive of field reversed configurations

After extensive experimentation on the Translation, Confinement, and Sustainment rotating magnetic-field (RMF)-driven field reversed configuration (FRC) device [A. L. Hoffman et al., Fusion Sci. Technol. 41, 92 (2002)], the principal physics of RMF formation and sustainment of standard prolate FRCs inside a flux conserver is reasonably well understood. If the RMF magnitude Bω at a given frequency ω is high enough compared to other experimental parameters, it will drive the outer electrons of a plasma column into near synchronous rotation, allowing the RMF to penetrate into the plasma. If the resultant azimuthal current is strong enough to reverse an initial axial bias field Bo a FRC will be formed. A balance between the RMF applied torque and electron-ion friction will determine the peak plasma density nm∝Bω∕η1∕2ω1∕2rs, where rs is the FRC separatrix radius and η is an effective weighted plasma resistivity. The plasma total temperature Tt is free to be any value allowed by power balance as long as the rat...

Observations of improved confinement in field reversed configurations sustained by antisymmetric rotating magnetic fields

Rotating magnetic fields (RMF) have been employed to both form and sustain currents in field reversed configurations (FRC). A major concern about this method has been the fear of opening up magnetic field lines with even small ratios of vacuum RMF Bω to external confinement field Be. A recently proposed innovation was to use an antisymmetric arrangement of RMF, but vacuum calculations with full RMF penetration showed that very low values of Bω∕Be would still be required to provide field-line closure. Recent comparisons of symmetric and antisymmetric RMF drive on the translation, confinement, and sustainment (TCS) facility [A. L. Hoffman, H. Y. Guo, J. T. Slough et al., Fusion Sci. Technol. 41, 92 (2002)] have shown strong improvements in the basic confinement properties of the FRCs when using antisymmetric drive, even with ratios of Bω∕Be as high as 0.3. This is due to normal standard operation with only partial penetration of the RMF beyond the FRC separatrix. The uniform transverse RMF in vacuum is shie...

Resistivity scaling of rotating magnetic field current drive in FRCs

Rotating magnetic fields (RMFs) have been used to both form and sustain low density, prolate FRCs in the translation confinement and sustainment (TCS) facility. The two most important factors governing performance are the plasma resistivity, which sets the maximum density for which toroidal current can be maintained, and the energy loss rate, which sets the plasma temperature. The plasma resistivity has been determined by carefully measuring the amount of RMF power absorbed by the FRC. When the ratio of RMF magnitude, Bω, to external poloidal confinement field, Be, is high, this resistivity is very adversely affected by the RMF drive process. However, when Bω/Be falls below about 0.3, the resistivity returns to values typical of non-driven FRCs. The observed scaling leads to a density dependence of ne ∼ Bω/rsω 1/2 where rs is the FRC separatrix radius and ω is the RMF frequency. Since the FRC contains little or no toroidal field, Be is proportional to (neTt) 1/2 where Tt = Te + Ti is the sum of the electron and ion temperatures. In the present experiments, except for the initial start-up phase where Tt can exceed 100 eV, the plasma temperature is limited to about 40 eV by high oxygen impurity levels. Thus, low Bω/Be, low resistivity operation was only realized by operating at low values of Bω. The RMF drive sustains particles as well as flux, and resistive input powers can be in the MW range at higher values of Bω, so that high temperature, steady-state operation should be possible once impurity levels are reduced. Changes are being made to the present ‘O-ring’ sealed, quartz chambered TCS to provide bakable metal walls and wall conditioning as in other quasi-steady fusion facilities.

Improved confinement and current drive of high temperature field reversed configurations in the new translation, confinement, and sustainment upgrade devicea)

Previous work in the translation, confinement, and sustainment (TCS) device [Hoffman, Guo, Slough et al., Fusion Sci. Technol. 41, 92 (2002)] demonstrated formation and steady-state sustainment of field reversed configurations (FRC) by rotating magnetic fields (RMF). However, in TCS the plasma temperature was limited to several 10s of eV due to high impurity content. These impurities are greatly reduced in the new TCS upgrade device (TCSU), which was built with a bakable, ultrahigh vacuum chamber, and advanced wall conditioning capabilities. This led to dramatic improvements in TCSU with temperatures well over 200eV, using simple even-parity RMF drive. The higher temperatures, coupled with reduced recycling, allowed plasma to enter into a collisionless, high-ζ (ratio of average electron rotation frequency to RMF frequency) regime. These new FRC states exhibit the following key features: (1) Dramatic improvement in current drive efficiency with ζ approaching 100%, for the first time in TCSU; (2) up to thre...

Nearby-fluids equilibria. II. Zonal flows in a high-β, self-organized plasma experiment

The field and flow structure observed in a high-β field reversed configuration (FRC) produced in the translation, confinement, and sustainment (TCS) experiment are modeled using the newly developed nearby-fluids equilibrium model. These results are the first evidence that experimental FRCs have complex flows, that is nonrigid rotational flow and poloidal flow, both with maximum speeds nearly half the Alfven speed. The interpretive approach is an innovative “backwards” method using the nearby-fluids platform for two-fluid equilibria. The most remarkable outcome is the prediction of a poloidal flow structure with significant zonal features. The poloidal flow has never been directly measured in FRCs; thus this discovery results from applying the flowing equilibrium model as an interpretive tool. The poloidal flows explain the unusual toroidal field structure observed in TCS. Zonal features in the rotational flow are also inferred from the unfolding of chord-integrated measurements. The results also indicated...

Rotating magnetic quadrupole current drive for field-reversed configurations

In the translation, confinement, and sustainment experiment [A. L. Hoffman, H. Y. Guo, J. T. Slough, S. J. Tobin, L. S. Schrank, W. A. Reass, and G. A. Wurden, Fusion Sci. Technol. 41, 92 (2002)], field-reversed configurations (FRCs) are created and sustained using a rotating magnetic field (RMF). The RMF is usually in the form of a rotating dipole, which in vacuum penetrates uniformly to the axis of symmetry. However, plasma conditions in the FRC normally adjust so that the RMF only partially penetrates the plasma column. We have investigated the possibility of using a rotating quadrupole rather than a rotating dipole magnetic field. The vacuum field from a quadrupole is proportional to radius and cannot penetrate to the axis of symmetry; however, this is not a disadvantage if the current drive is confined to the outer region of the FRC. It was found that the quadrupole drive efficiency is comparable to that of a dipole, but the rotating dipole is more effective at stabilizing the n=2 rotational instabil...