A discovery made in the RFX-mod Reversed Field Pinch (RFP) fusion device demonstrated that if one keeps on increasing the current, above 1.5 million amperes, the helix becomes very smooth, the trapping of the plasma in the magnetic fields improves and the plasma gets hotter. Moreover, the helical state appears to be the preferred one at high current and is spontaneously chosen, or self-organized, by the plasma. RFX is a large RFP device (ten cubic meter plasma) located in Padova, Italy.
An attractive feature of the RFP fusion device is that most of its magnetic field is produced by the current flowing in the plasma. This eliminates many of the costs and technical difficulties associated with producing strong magnetic fields using high-tech superconducting coils, such as those used in tokamak and stellarator fusion containment designs. The helical shape of the plasma comes with an additional bonus: the current lines are also helical. This greatly increases the length of the electrical circuit with respect to the tokamak. This could make reaching thermonuclear temperatures possible with only the electric power dissipated in the plasma. In principle, no additional heating is necessary—an additional positive economic and technical feature.
The large range of plasma currents (Ip = 0.2–1.6 MA) and feedback-controlled magnetic boundary conditions of the RFX-mod experiment make it well suited to performing scaling studies. The assessment of such scaling, in particular those on temperature and energy confinement, is crucial both for improving the operating reversed-field pinch (RFP) devices and for validating the RFP configuration as a candidate for the future fusion reactors. For such a purpose scaling laws for magnetic fluctuations, temperature and energy confinement have been evaluated in stationary operation. RFX-mod scaling laws have been compared with those obtained from other RFP devices and numerical simulations. The role of the magnetic boundary has been analysed, comparing discharges performed with different active control schemes of the edge radial magnetic field.
In RFX-mod the role of the graphite wall on density control during experiments is under investigation. It has been found that when the graphite is sufficiently loaded with H2 particles n/nG is determined by the wall and settles at a value of ~0.5 when graphite saturation is reached. At low wall loading a wider range of densities is achievable, the value depending on the strength and uniformity of plasma wall interaction. Wall preloading by means of H2 GDC is proposed as a way to obtain a plasma with desired density even if for a limited number of shots.
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