RESUMO
Many fields of basic and applied science require efficiently exploring complex systems with high dimensionality. An example of such a challenge is optimising the performance of plasma fusion experiments. The highly-nonlinear and temporally-varying interaction between the plasma, its environment and external controls presents a considerable complexity in these experiments. A further difficulty arises from the fact that there is no single objective metric that fully captures both plasma quality and equipment constraints. To efficiently optimise the system, we develop the Optometrist Algorithm, a stochastic perturbation method combined with human choice. Analogous to getting an eyeglass prescription, the Optometrist Algorithm confronts a human operator with two alternative experimental settings and associated outcomes. A human operator then chooses which experiment produces subjectively better results. This innovative technique led to the discovery of an unexpected record confinement regime with positive net heating power in a field-reversed configuration plasma, characterised by a >50% reduction in the energy loss rate and concomitant increase in ion temperature and total plasma energy.
RESUMO
Developing a stable plasma state with high-beta (ratio of plasma to magnetic pressures) is of critical importance for an economic magnetic fusion reactor. At the forefront of this endeavour is the field-reversed configuration. Here we demonstrate the kinetic stabilizing effect of fast ions on a disruptive magneto-hydrodynamic instability, known as a tilt mode, which poses a central obstacle to further field-reversed configuration development, by energetic beam injection. This technique, combined with the synergistic effect of active plasma boundary control, enables a fully stable ultra-high-beta (approaching 100%) plasma with a long lifetime.
RESUMO
Field reversed configurations (FRCs) with high confinement are obtained in the C-2 device by combining plasma gun edge biasing and neutral beam injection. The plasma gun creates an inward radial electric field that counters the usual FRC spin-up. The n = 2 rotational instability is stabilized without applying quadrupole magnetic fields. The FRCs are nearly axisymmetric, which enables fast ion confinement. The plasma gun also produces E × B shear in the FRC edge layer, which may explain the observed improved particle transport. The FRC confinement times are improved by factors 2 to 4, and the plasma lifetimes are extended from 1 to up to 4 ms.
RESUMO
A compact high-sensitivity second-harmonic interferometer for line-integrated electron density measurements on a large plasma machine is presented. The device is based on a fiber coupled near-infrared continuous-wave Nd:YAG laser and is remotely controlled. The performances of the instrument are tested on the Irvine field-reversed configuration machine, and a sensitivity of few 10(14) cm(-2) in measuring line integrated electron density is demonstrated with a time resolution of a few microseconds. The interferometer is self calibrated, has an impressive stability, and it does not require any further alignment after proper installation. These features make this device a real turn-key system suitable for electron density measurement in large plasma machines.
RESUMO
A time-of-flight diagnostic has been implemented on the Irvine field reversed configuration (IFRC) to obtain an energy distribution function from charge-exchanged neutral hydrogen. The diagnostic includes a 13 cm radius slotted disk rotating at 165 Hz in vacuum which chops the emitted neutrals at a rate of 26 kHz. In situ timing verification was performed with a dc xenon discharge lamp with an uncertainty less than 100 ns for a 38 micros chopping period. Energy calibration was accomplished with a singly ionized lithium source in the range of 300-1500 eV, achieving an average energy uncertainty, DeltaE/E, of 0.11. The diagnostic has measured neutrals in the range of 20-80 eV from the IFRC and the corresponding energy distribution function has been obtained.
