Ignitor

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Ignition

Physics

Ignitor can sustain a burning plasma over a time longer than any of the physically relevant time scales.
The plasma current and density should be programmed to be risen during a relatively long  transient phase (3 to 4 s) while gradually increasing also the cross section of the plasma column. Ignition is expected to be reached near the end of the current rise, mainly thanks to the ohmic heating. The burning phase starts when the temperature reaches 5-6 keV. The peak temperature is expected to be T e0T i0 ≈ 11 keV for an energy confinement time
~ 0.6 s.
The achievement of ignition with ohmic heating only relies on the possibility of reaching a critical temperature (
~ 4 keV), at which the contribution of the α-particles heating compensates for a less effective ohmic heating in the central part of the plasma column as the temperature increases. This result can be attained more easily by means of small amounts of ICRF heating (~ 3 MW) that can be injected during the current rise and turned off before the beginning of the flat top, so that ignition is reached when only ohmic heating is present [3].
The α-particle are expected to be well confined in the central part of the plasma column, and given the relatively low temperature and high density, their slowing down time will be considerably shorter than the energy confinement time. This represents a peculiar feature of Ignitor with respect to other Tokamaks (including ITER-FEAT) [1]. In particular, the very short α-particle slowing down time provides a good margin of stability relative to fast particle induced modes.
Another peculiar result is that, according to the simulations performed, the plasma pressure profile at ignition would be nearly of a unique type, independently of the kind of expression of the electron thermal diffusivity adopted to describe the plasma evolution [
3 and references therein].
In the fully ignited scenario, the main goal essentially concerns the possibility of controlling the thermonuclear instability, which develops after reaching ignition, as self-heating of the plasma by fusion produced particles can lead to a significant rise of the plasma temperature and pressure. Then, internal plasma modes may be excited and saturate thermonuclear instability at acceptable levels without external intervention. In the case where an internal process may not be effective, a scenario is considered whereby Ignitor is led to operate in a slightly sub-critical regime, i.e. the plasma parameters are so chosen that the thermonuclear heating power is slightly less than the power lost, and a small fraction of  3He is added to the optimal Deuterium-Tritium mixture. The difference be
tween power lost and α-heating is compensated by additional ICRH heating that should be able to energize the minority species (minority heating) directly, which can transfer the power to the main plasma species by collisions. In order to study this problem for Ignitor the considered energy balance equation includes the ICRH heating term [2].


1) B. Coppi et al, Nucl. Fusion 41, 1253 (2001)
2) B. Coppi et al, Overview Paper OV/P-02, Proceedings of the 24 th IAEA Fusion Energy Conference, San Diego, US, 8-13/10/2012
3) F. Bombarda et al, Braz. J. Phys. 34, 1786 (2004)




 
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