Deuterium-Tritium Operation in Magnetic Confinement Experiments: Results and Underlying Physics

A review of experimental results obtained in JET D-T plasmas is presented. In discussing the underlying physics, results previously obtained on TFTR are also taken into account. In JET, the maximum fusion power output (Pfus) of 16.1 MW has been obtained in an ELM-free hot-ion H-mode featuring an edge confinement barrier in a single-null divertor plasma with a Q(≡Pfus/Pin) ≈ 0.62 where Pin is the total input power to torus. A steady-state H-mode discharge, with plasma shape and safety factor q similar to that of ITER, produced 4MW for 5s (22MJ). The steady-state results extrapolate well to ignition with ITER parameters using the normalized plasma pressure (βN) achieved on JET. Also, the Advanced Tokamak regime using optimized magnetic shear configuration featuring an internal transport barrier produced 8.2MW of fusion power. With regards to reactor physics issues, a clear identification of electron heating by fusion born alpha particles has been made both in JET and TFTR. The JET experiments show that the H-mode threshold power has approximately an inverse isotopic mass dependence and that it does not depend on the method of auxiliary heating. The global energy confinement time in the TFTR D-T supershot regime scales as ~A0.85 but in the JET H-modes, it is found to be practically independent of isotopic mass (~A0.03±0.2) where A is the atomic mass of the hydrogenic species. In JET, the plasma core and the edge appear to have different underlying confinement physics, the former follows the gyro-Bohm transport (~A-0.2) model whereas the edge pedestal energy scales as ~A0.5±0.2. The maximum edge pressure in H-modes is analyzed in relation to the ion poloidal Larmor radius at the edge. The fast ions driven by NBI or ICRH could play an important role in setting the width of the edge pedestal. The thermal ELMy H-mode confinement both in D or T gas fuelled plasmas decreases significantly when the plasma density exceeds 0.75 of the Greenwald (nGW) limit and the maximum density achieved is 0.85nGW. The ICRH scenarios for a reactor have been evaluated. For example, He3-minority in 50:50 D:T and tritium dominated plasmas showed strong bulk ion heating leading to ion temperatures up to 13keV with ICRH alone. Deuterium minority ion cyclotron heating in tritium plasmas at a power level of 6MW produced a steady-state record values of Q ≈ 0.22 for more than 2.5s. Finally, the on-site closed-cycle tritium reprocessing plant and remote handling tools at JET have been used routinely and provided an integrated demonstration of safe and reliable operations of a tokamak device in reactor-relevant conditions.
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