D. Borba

High fusion performance from deuterium-tritium plasmas in JET

High fusion power experiments using DT mixtures in ELM-free H mode and optimized shear regimes in JET are reported. A fusion power of 16.1 MW has been produced in an ELM-free H mode at 4.2 MA/3.6 T. The transient value of the fusion amplification factor was 0.95±0.17, consistent with the high value of nDT(0)τEdiaTi(0) = 8.7 × 1020±20% m-3 s keV, and was maintained for about half an energy confinement time until excessive edge pressure gradients resulted in discharge termination by MHD instabilities. The ratio of DD to DT fusion powers (from separate but otherwise similar discharges) showed the expected factor of 210, validating DD projections of DT performance for similar pressure profiles and good plasma mixture control, which was achieved by loading the vessel walls with the appropriate DT mix. Magnetic fluctuation spectra showed no evidence of Alfvenic instabilities driven by alpha particles, in agreement with theoretical model calculations. Alpha particle heating has been unambiguously observed, its effect being separated successfully from possible isotope effects on energy confinement by varying the tritium concentration in otherwise similar discharges. The scan showed that there was no, or at most a very weak, isotope effect on the energy confinement time. The highest electron temperature was clearly correlated with the maximum alpha particle heating power and the optimum DT mixture; the maximum increase was 1.3±0.23 keV with 1.3 MW of alpha particle heating power, consistent with classical expectations for alpha particle confinement and heating. In the optimized shear regime, clear internal transport barriers were established for the first time in DT, with a power similar to that required in DD. The ion thermal conductivity in the plasma core approached neoclassical levels. Real time power control maintained the plasma core close to limits set by pressure gradient driven MHD instabilities, allowing 8.2 MW of DT fusion power with nDT(0)τEdiaTi(0) ≈ 1021 m-3 s keV, even though full optimization was not possible within the imposed neutron budget. In addition, quasi-steady-state discharges with simultaneous internal and edge transport barriers have been produced with high confinement and a fusion power of up to 7 MW; these double barrier discharges show a great potential for steady state operation.

The HAGIS self-consistent nonlinear wave-particle interaction model

Abstract The problem of modelling the self-consistent interaction of an energetic particle ensemble with a wave spectrum specific to magnetically confined plasmas in a torus is discussed. Particle motion in a magnetic field coordinate system, whose surfaces are perturbed by a spectrum of finite amplitude magnetohydrodynamical (MHD) waves, is described using a Hamiltonian formulation. Employing the δƒ method enables the simulation particles to only represent the change in the total particle distribution function and consequently possesses significant computational advantages over standard techniques. Changes to the particle distribution function subsequently affect the wave spectrum through wave-particle interactions. The model is validated using large aspect-ratio asymptotic limits as well as through a comparison with other numerical work. A consideration of the Kinetic Toroidal Alfven Eigenmode instability driven by fusion born α -particles in a D-T JET plasma illustrates a use of the code and demonstrates nonlinear saturation of the instability, together with the resultant redistribution of particles both in energy and across the plasma cross section.

Modeling the excitation of global Alfvén modes by an external antenna in the Joint European Torus (JET)

The active excitation of global Alfven modes using the saddle coils in the Joint European Torus (JET) [Plasma Physics and Controlled Nuclear Fusion Research 1984, Proceedings of the 10th International Conference, London (International Atomic Energy Agency, Vienna, 1985), Vol. 1, p. 11] as the external antenna, will provide information on the damping of global modes without the need to drive the modes unstable. For the modeling of the Alfven mode excitation, the toroidal resistive magnetohydrodynamics (MHD) code CASTOR (Complex Alfven Spectrum in TORoidal geometry) [18th EPS Conference On Controlled Fusion and Plasma Physics, Berlin, 1991, edited by P. Bachmann and D. C. Robinson (The European Physical Society, Petit‐Lancy, 1991), Vol. 15, Part IV, p. 89] has been extended to calculate the response to an external antenna. The excitation of a high‐performance, high beta JET discharge is studied numerically. In particular, the influence of a finite pressure is investigated. Weakly damped low‐n global modes d...

Fast particles-wave interaction in the Alfvén frequency range on the Joint European Torus tokamak

Wave-particle interaction phenomena in the Alfven Eigenmode (AE) frequency range are investigated at the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] using active and passive diagnostic methods. Fast particles are generated by neutral beam injection, ion cyclotron resonance heating, and fusion reactions. External antennas are used to excite stable AEs and measure fast particle drive and damping separately. Comparisons with numerical calculations lead to an identification of the different damping mechanisms. The use of the active AE diagnostic system to generate control signals based on the proximity to marginal stability limits for AE and low-frequency magnetohydrodynamic (MHD) modes is explored. Signatures of the different nonlinear regimes of fast particle driven AE instabilities predicted by theory are found in the measured spectra. The diagnostic use of AE measurements to get information both on the plasma bulk and the fast particle distribution is assessed.

Nonlinear interaction of fast particles with Alfvén waves in toroidal plasmas

A numerical algorithm to study the nonlinear, resonant interaction of fast particles with Alfven waves in tokamak geometry has been developed. When the instability is sufficiently weak, it is known that the wave-particle trapping nonlinearity will lead to mode saturation before wave–wave nonlinearities are appreciable. The spectrum of linear modes can thus be calculated using a magnetohydrodynamic normal-mode code, then nonlinearly evolved in time in an efficient way according to a two-timescale Lagrangian dynamical wave model. The fast particle kinetic equation, including the effect of orbit nonlinearity arising from the mode perturbation, is simultaneously solved for the deviation, δf=f−f0, from an initial analytic distribution f0. High statistical resolution allows linear growth rates, frequency shifts, resonance broadening effects, and nonlinear saturation to be calculated quickly and precisely. The results have been applied to an International Thermonuclear Experimental Reactor [ITER EDA Doc. Series ...

Modelling of Alfven waves in JET plasmas with the CASTOR-K code

A hybrid magnetohydrodynamic (MHD)-gyro-kinetic model CASTOR-K developed for the study of Alfven eigenmode (AE) stability in the presence of energetic ions has been applied to the interpretation of recent measurements of Alfven waves in JET. These include the detailed AE damping measurements performed using the AE antenna excitation system and also the observations of Alfven cascades in strongly reversed shear scenarios at JET. The mode conversion between the AEs and kinetic Alfven waves and the relation to the Alfven continuum is studied and the calculated damping is compared with the experimental data. The contribution of ion cyclotron resonant heating driven minority ions to the growth rate of the novel-type mode localized around the point of zero magnetic shear is calculated. This mode is shown to be clearly linked to the ideal MHD `Alfven continuum, computed with the CSCAS code and consistent with the observation of a quasi-periodic pattern of upward frequency sweeping Alfven cascades in JET.

Particle and energy transport during the first tritium experiments on JET

The particle and energy transport properties of the high fusion performance JET pulses that were obtained before and during the first tritium experiments are discussed. The particle diffusion coefficient of tritium is determined by monitoring the decay of a small quantity of injected tritium in a deuterium background plasma. A good simulation of the measured 14 MeV neutron emissivity profile is obtained throughout the decay phase if the mixing of the two species is described by a model in which the tritium diffusion coefficient is similar to that of deuterium. The energy confinement of these low density, hot ion, H mode discharges is found to have both improved central and edge confinement over the conventional medium to high density H mode discharges, regardless of the presence or absence of tritium in the discharge. As the tritium concentration of these D-T discharges is small (varying from <1% to 10%), no isotopic dependence was expected and indeed none is observed. Enhancement factors of at least twice the value predicted by H mode scaling expressions are observed but only transiently. A local transport analysis is completed to try and establish the reason for the improved confinement and its transient nature. Similarities between these pulses and DIII-D VH mode discharges have been noticed, and common characteristics are discussed. In particular, the expansion of the region with access to the second stability regime certainly appears to be a possibility for the enhanced confinement. The stabilization of the ηi mode by the peaked density profile seems unlikely to be the cause of the improved confinement. Finally, for the discharge with a high concentration of tritium, it has been suggested that alpha particle driven instabilities could affect the energy confinement. A comparison is made with tile stability threshold of toroidicity induced Alfven eigenmodes (TAE), which appear to have been stable. The alpha particle statistics are also presented

MHD Activity in JET Hot Ion H Mode Discharges

The MHD effects observed in the hot ion H modes in the pre-divertor configuration of JET, including those generated in the preliminary tritium experiment, are described. Some observations were found to be similar to those in high beta regimes while others are new and appear to be pertinent to high performance discharges only. The high performance phase is largely sawtooth free and dominated by fishbone activity, which increases in amplitude throughout this phase. During termination of the high performance phase, the growth of a large variety of MHD activity with low mode numbers is observed. Also, edge instabilities possibly associated with much larger mode numbers are seen in the Dalpha emission. In some cases, the unusual structure of two central m=n=1 islands was found, and resistive MHD modelling indicates that this observation is consistent with a nearly flat, non-monotonic q profile. In some discharges a sawtooth collapse immediately followed by an edge localized mode (ELM) is observed at, or shortly following, the termination of the high performance

Alfven eigenmode induced energetic particle transport in JET

A Hamiltonian guiding centre particle following code has been developed to study the fast particle motion in the presence of arbitrary time dependent electromagnetic perturbations. In conjunction with an MHD stability code, this code was used to analyse TAE/KTAE induced alpha orbit diffusion and alpha losses in JET plasmas. Resonant alpha orbits are studied below and above the stochasticity thresholds, in the presence of single or several TAEs and KTAEs. Monte Carlo randomized ensembles of alpha particles in the presence of finite amplitude TAE/KTAEs are followed and estimates for the stochastic diffusion coefficients are obtained. Generalization of the method towards the self-consistent wave-particle evolution is described