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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 10 s 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 200 eV, 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 threefold increase in global energy confinement time; and (3) significant reduction in transport rates, accompanied by a striking transition from a Bohm-type transport to a lower hybrid driftlike transport that scales better than gyro-Bohm and is very favorable for the next step FRC development. [ABSTRACT FROM AUTHOR]

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 ratio of FRC diamagnetic current, I′dia≈2Be/μo, is less than the maximum possible synchronous current, I′sync〈ne〉eωrs2/2. The RMF will self-consistently penetrate a distance δ* governed by the ratio ζ=I′dia/I′sync. Since the FRC is a diamagnetic entity, its peak pressure pm=nmkTt determines its external magnetic field Be≈(2μopm)1/2. Higher FRC currents, magnetic fields, and poloidal fluxes can thus be obtained, with the same RMF parameters, simply by raising the plasma temperature. Higher temperatures have also been noted to reduce the effective plasma resistivity, so that these higher currents can be supported with surprisingly little increase in absorbed RMF power. [ABSTRACT FROM AUTHOR]

In previous experiments on high temperature (>50 eV), high density (>10[sup 20] m[sup -3]) plasmas such as the field-reversed configuration (FRC), it has not been possible to obtain direct information of the internal field structure in a nondestructive way. The probe surface would vaporize due to high electron thermal transport as well as ablate due to high energy ion bombardment. To minimize these processes, the smallest possible probes made from materials with the longest thermal time to melting were constructed and tested. In order to measure fast magnetic field changes (∼several MHz), as well as not influence the FRC internal electric fields, the probe wall material was constructed from a nonconducting material. Of several insulating materials tested, beryllia was the only material that was found to be suitable. The probe wall consisted of a 0.3-m-long 2-mm-diam beryllia tube bored out to 1.5 mm. Inside the small bore, a “chain” probe of 24 loops was constructed out of 50-μm-diam magnet wire. The two axis probe measured axial and azimuthal FRC magnetic fields as small as a few gauss with centimeter resolution and a frequency response of 1 MHz or better. With the probe inserted, no changes in FRC confinement or behavior were observed over the entire 1 ms lifetime of the discharge. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the flux generation and sustainment of a field reversed configuration with rotating magnetic field current drive. Impact of high currents and lack of deep penetration on axial field; Influence of radially inward pondermotive force on energy and particle confinement; Focus on the tokamak concept.

A radio frequency (rf) system using solid state switching has been designed and constructed that is capable of driving a pair of loop antenna with a circulating power of 60 MW for millisecond pulses. The primary application of this system is to produce an oscillating magnetic field in the frequency range of 0.2-0.7 MHz, to generate and sustain a large plasma current. The maximum power transfer from the rf source was measured to be greater than 10 MW. The driver consisted of 12 insulated gate bipolar transistor modules driven in parallel, and was capable of switching 20 kA at 1700 V. A sinusoidal current waveform with large circulating currents was obtained on the antenna by incorporating the antenna in a high Q (60), parallel LCR resonant circuit. The low impedance switch driver was matched to the antenna load through a 20:1 step-up air core transformer with a coupling efficiency of 99%. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]