K. E. Miller

Flux generation and sustainment of a field reversed configuration with rotating magnetic field current drive

A new experimental device has been constructed to study the flux build-up and sustainment of a field reversed configuration (FRC) with a rotating magnetic field (RMF). Even though complete penetration was expected from RMF theory, the RMF field was observed to penetrate only a few centimeters inside the FRC separatrix. Despite the limited penetration, significantly larger toroidal currents (40 kA) were driven than in previous experiments (∼2 kA) with the same RMF field. The high currents and lack of deep penetration allowed the axial field to be the dominant field throughout the FRC. The radially inward pondermotive force arising from axial screening currents at the FRC edge had a significant influence on energy and particle confinement, reducing convective losses to the limit of observability. With only ohmic heating, the measured low ion temperatures (2 eV) left the ions unmagnetized while the electrons (∼40 eV) were well magnetized. No destructive instability was observed for the RMF driven FRC despite...

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.

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...

Edge-driven rotating magnetic field current drive of field-reversed configurations

Field-reversed configurations (FRCs) are created and sustained using a rotating magnetic field (RMF) in the Translation Confinement and Sustainment experiment. Normally this experiment is operated in a manner where the RMF only partially penetrates the plasma column. This method of operation may have significant advantages in producing less disturbances to the bulk of the FRC, but requires driving an overall radially inward flow to maintain Eθ(r)=0 everywhere (through the VrBz term in the generalized Ohm’s law). However, some RMF penetration is still required at the field null R, where Bz=0. For some experimental conditions it appears that the RMF does not even penetrate as far as the null, raising the question as to how Eθ(r=R) can be maintained at zero despite a finite η⊥jθ(r=R). Numerical simulations with a resistivity profile that is sharply peaked near the plasma edge yield similar profiles, and provide insight into this physical process. An inner magnetic structure forms, which rotates at a much low...

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...

Children's use of sequence information in partially predictable reaction-time sequences☆

Abstract Second-, fourth-, and sixth-grade children and adults performed a four-choice reaction time task with partially predictable sequences and 250-, 500-, and 750-msec response-to-stimulus intervals. The relative advantage for in-sequence as opposed to out-of-sequence events was independent of the response-to-stimulus interval for all ages. Children, but not adults, were slower for nonrepeated than repeated out-of sequence events and this advantage for repeated signals decreased as age increased. A second experiment extended the range of intervals tested to zero. Second graders and adults responded to four-choice partially predictable sequences with 0-, 250-, and 500-msec response-to-stimulus intervals. As in the first experiment, the difference between in-sequence and out-of-sequence events did not vary with the response-to-stimulus interval. The results suggest that both children and adults are able to process advance sequence probability information about a subsequent event in parallel with an ongoing response.

An overview of the star thrust experiment

The Field Reversed Configuration, FRC, is a closed field fusion confinement geometry with great potential to be used as a space propulsive device and power source. Present formation techniques are cumbersome and severely constrain the resultant FRC. An experiment is presently under construction to study the formation and sustainment of the FRC using a rotating magnetic field. If successful, this technique would vastly simplify and enable future FRC endeavors. An overview of the STX experiment is presented.