Visible bremsstrahlung tomographic diagnostic for the pulsed high density field-reversed configuration experiment Rev. Sci. Instrum. 77, 10F319 (2006); 10.1063/1.2220018Modeling of field-reversed configuration experiment with large safety factora) Phys. Plasmas 13, 056119 (2006); 10.1063/1.2177635 Rotating magnetic quadrupole current drive for field-reversed configurationsDeformation of the internal structure of a field-reversed configuration ͑FRC͒ was studied using a tomographic reconstruction technique. A simple and configurable tomographic system was developed, with which the time evolution of the FRC internal structure was reconstructed. In the latter phase of equilibrium, a FRC has a well-known global rotational instability with toroidal mode number n = 2. It has been believed that elliptical deformation of the FRC allows interaction between the wall and the plasma, which terminates this configuration. However, these experiments revealed the FRC to deform into a dumbbell-like structure before the edge hits the chamber wall, leading to the disruption phase. In addition, an internal shift ͑toroidal mode number n =1͒ mode was observed in the equilibrium phase, followed by growth of n = 2 rotational instability.

A constructed diagnostic system consisting of a 60-channel set of optical detectors with flexible viewing configurations is realized to investigate three-dimensional magnetohydrodynamic (MHD) motions and the internal structure of a field-reversed configuration (FRC) plasma. The system can detect radiation from the plasma in the wavelength range of 420–820 nm. Optical filters are used to select the wavelength ranges required in the experiment. The sensitivities of all the optical detectors are calibrated using radiation from the FRC plasma at a quiescent phase. Radiation profiles measured by orthogonal viewing configuration of the detectors are shown at three toroidal cross sections. From these profiles, the time evolution of the three-dimensional MHD motion of the plasma is depicted. The radiation profile measured by a one-dimensional viewing configuration yields not only an electron density profile inside the separatrix but also the width of an edge-layer plasma. A bright halo around the edge-layer plasma is observed using a Balmer-α line filter. The orthogonal viewing configuration can also be used to analyze the internal structure of the FRC. The deviated position of the major axis is estimated from the comparison between the measured radiation profiles and the nonconcentric density profile based on the rigid rotor profile model.

The separatrix shape of a field-reversed configuration plasma is determined in comparison with measured fluxes surrounding the plasma and the solution of the Grad–Shafranov equation with an edge-layer plasma in the open field region. A reconnection point of the bias field, an outgoing flow of torn plasmas and a large amplitude ripple on the separatrix surface are clearly observed at the formation phase. A smoothed separatrix shape having definite ends is observed at the quiescent phase. It is also estimated that the beta value at the separatrix and the thickness of the edge-layer plasma are, respectively, βs=0.5–0.7 and 4–6 ρi (ρi: ion gyroradius). The magnetic structure inside the separatrix is investigated by solving the Grad–Shafranov equation with the measured separatrix shape and βs. It is found that magnetic islands are formed near the magnetic axis not only at the formation phase but also at the quiescent phase. The appearance and coalescence of the islands are repeated during the discharge.

The plasma density of a field reversed configuration (FRC) needs to be decreased below the present experimental regime in order to heat the plasma and sustain the configuration by a high energy neutral beam in an FRC reactor. However, as the plasma is produced in a linear vacuum vessel, there exists a severe breakdown limit at a low fill pressure as compared with a toroidal system. A method to form FRCs beyond the breakdown limit is proposed here. The preionized plasma is compressed by a strong bias field to enhance the plasma flow from the confinement region to the outside region and is then diluted before the start of the confinement field on the NUCTE device. The use of a diluted plasma enables the critical density of the FRC to be lowered from 1.1 × 1021 to 5.6 × 1020m-3 and the sum of the electron and ion temperatures to be increased from 0.35 to 0.81 keV.

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