Experiments in the Tokamak Fusion Test Reactor ͑TFTR͒ ͓Phys. Plasmas 2, 2176 ͑1995͔͒ have explored several novel regimes of improved tokamak confinement in deuterium-tritium ͑D-T͒ plasmas, including plasmas with reduced or reversed magnetic shear in the core and high-current plasmas with increased shear in the outer region ͑high l i ͒. New techniques have also been developed to enhance the confinement in these regimes by modifying the plasma-limiter interaction through in situ deposition of lithium. In reversed-shear plasmas, transitions to enhanced confinement have been observed at plasma currents up to 2.2 MA (q a Ϸ4.3), accompanied by the formation of internal transport barriers, where large radial gradients develop in the temperature and density profiles. Experiments have been performed to elucidate the mechanism of the barrier formation and its relationship with the magnetic configuration and with the heating characteristics. The increased stability of high-current, high-l i plasmas produced by rapid expansion of the minor cross section, coupled with improvement in the confinement by lithium deposition has enabled the achievement of high fusion power, up to 8.7 MW, with D-T neutral beam heating. The physics of fusion alpha-particle confinement has been investigated in these regimes, including the interactions of the alphas with endogenous plasma instabilities and externally applied waves in the ion cyclotron range of frequencies. In D-T plasmas with q 0 Ͼ1 and weak magnetic shear in the central region, a toroidal Alfvén eigenmode instability driven purely by the alpha particles has been observed for the first time. The interactions of energetic ions with ion Bernstein waves produced by mode conversion from fast waves in mixed-species plasmas have been studied as a possible mechanism for transferring the energy of the alphas to fuel ions.

We report a significant enhancement in both the energy and the flux of relativistic electrons accelerated by ultra-intense laser pulse irradiation (>1×10 21 W cm −2 ) of near solid density aligned CD 2 nanowire arrays in comparison to those from solid CD 2 foils irradiated with the same laser pulses. Ultrahigh contrast femtosecond laser pulses penetrate deep into the nanowire array creating a large interaction volume. Detailed three dimensional relativistic particle-in-cell simulations show that electrons originating anywhere along the nanowire length are first driven towards the laser to reach a lower density plasma region near the tip of the nanowires, where they are accelerated to the highest energies. Electrons that reach the lower density plasma experience direct laser acceleration up to the dephasing length, where they outrun the laser pulse. This yields an electron beam characterized by a 3× higher electron temperature and an integrated flux 22.4× larger respect to foil targets. Additionally, the generation of>1 MeV photons were observed to increase up to 4.5×.

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International audienceSeeding soft-X-ray plasma amplifiers with high harmonics has been demonstrated to generate high-brightness soft-X-ray laser pulses with full spatial and temporal coherence. The interaction between the injected coherent field and the swept-gain medium has been modelled. However, no experiment has been conducted to probe the gain dynamics when perturbed by a strong external seed field. Here, we report the first X-ray pump–X-ray probe measurement of the nonlinear response of a plasma amplifier perturbed by a strong soft-X-ray ultra-short pulse. We injected a sequence of two time-delayed high-harmonic pulses (λ = 18.9 nm) into a collisionally excited nickel-like molybdenum plasma to measure with femtosecond resolution the gain depletion induced by the saturated amplification of the high-harmonic pump and its subsequent recovery. The measured fast gain recovery in 1.5–1.75 ps confirms the possibility to generate ultra-intense, fully phase-coherent soft-X-ray lasers by chirped pulse amplification in plasma amplifiers