Alpha particle loss in TFTR deuterium-tritium plasmas with reversed magnetic shear

The confinement and loss of fusion alpha particles are examined for reversed magnetic shear plasmas in TFTR. Such plasmas, with high central q and non-monotonic q profiles can exhibit remarkably reduced energy and particle transport of the thermal ions. However, these same conditions are theoretically predicted to produce high levels of stochastic ripple loss of suprathermal particles, which may reduce the efficiency of plasma heating by the alpha particles and other heating schemes involving fast ions. This paper presents calculations of guiding-center code alpha particle orbit loss from deuterium-tritium (DT) simulations of TFTR deuterium-only experiments. They are compared to results of measurements made in DT reversed shear plasmas of both the confined alpha particle distribution and the alpha particles lost from the plasma. Large fast particle losses have also been found in reversed shear ITER simulations (up to 20%) and from measurements of triton burnup in reversed shear experiments on JT-60U (12%)

Multispecies Density and Temperature Gradient Dependence of Quasilinear Particle and Energy Fluxes

The variations of the normalized quasilinear particle and energy fluxes with artificial changes in the density and temperature gradients, as well as the variations of the linear growth rates and real frequencies, for ion temperature gradient and trapped-electron modes, are calculated. The quasilinear fluxes are normalized to the total energy flux, summed over all species. Here, realistic cases for tokamaks and spherical torii are considered which have two impurity species. For situations where there are substantial changes in the normalized fluxes, the ''diffusive approximation,'' in which the normalized fluxes are taken to be linear in the gradients, is seen to be inaccurate. Even in the case of small artificial changes in density or temperature gradients, changes in the fluxes of different species (''off-diagonal'') generally are significant, or even dominant, compared to those for the same species (''diagonal'')

Measurements of the production and transport of helium ash on the TFTR Tokamak

Helium ash production and transport have been measured in TFTR deuterium-tritium plasmas using charge-exchange recombination spectroscopy. The helium ash confinement time, including recycling effects, is 6--10 times the energy confinement time and is compatible with sustained ignition in a reactor. The ash confinement time is dominated by edge pumping rates rather than core transport. The measured evolution of the local thermal ash density agrees with modeling, indicating that alpha particle slowing-down calculations used in the modeling are reasonable

Coherent fluctuations in the initial TFTR D-T experiments

The initial operation of TFTR with approximately equal power in the tritium and deuterium neutral beam injectors has resulted in the production of fusion power in excess of 9MW and central {beta}{sub {alpha}} > 0.25%. This {beta}{sub {alpha}} is within a factor of 2--3 of the {beta}{sub {alpha}} in projections of ITER performance. Effects of this {alpha} population on TAE modes, sawteeth and fishbone activity are being searched for. The D-T plasmas are also being studied for evidence of changes in MHD activity which might be attributed to the fast {alpha} population. This paper reports on the activity in the Alfven range of frequencies in the D-T plasmas and on detailed measurements of the MHD activity preceding major disruptions in D-D and D-T

Non-linear evolution of double tearing modes in tokamaks

The delta prime formalism with neoclassical modifications has proven to be a useful tool in the study of tearing modes in high beta, collisionless plasmas. In this paper the formalism developed for the inclusion of neoclassical effects on tearing modes in monotonic q-profile plasmas is extended to plasmas with hollow current profiles and double rational surfaces. First, the classical formalism of tearing modes in the Rutherford regime in low beta plasmas is extended to q profiles with two rational surfaces. Then it is shown that this formalism is readily extended to include neoclassical effects

Tomography of (2, 1) and (3, 2) magnetic island structures on Tokamak Fusion Test Reactor

High-resolution electron cyclotron emission (ECE) image reconstruction has been used to observe (m,n)=(2,1) and (3, 2) island structures on Tokamak Fusion Test Reactor [Plasma Phys. Controlled. Fusion 33, 1509 (1991)], where m and n are the poloidal and the toroidal mode number, respectively. The observed island structure is compared with other diagnostics, such as soft x-ray tomography and magnetic measurements. A cold elliptic island is observed after lithium pellet injection. Evidence for the enhancement of the heat transfer due to the island is observed. A relaxation phenomenon due to the m=2 mode is newly observed in Ohmic plasmas

Gyrokinetic Stability Studies of the Microtearing Mode in the National Spherical Torus Experiment H-mode

Insight into plasma microturbulence and transport is being sought using linear simulations of drift waves on the National Spherical Torus Experiment (NSTX), following a study of drift wave modes on the Alcator C-Mod Tokamak. Microturbulence is likely generated by instabilities of drift waves, which cause transport of heat and particles. Understanding this transport is important because the containment of heat and particles is required for the achievement of practical nuclear fusion. Microtearing modes may cause high heat transport through high electron thermal conductivity. It is hoped that microtearing will be stable along with good electron transport in the proposed low collisionality International Thermonuclear Experimental Reactor (ITER). Stability of the microtearing mode is investigated for conditions at mid-radius in a high density NSTX high performance (H-mode) plasma, which is compared to the proposed ITER plasmas. The microtearing mode is driven by the electron temperature gradient, and believed to be mediated by ion collisions and magnetic shear. Calculations are based on input files produced by TRXPL following TRANSP (a time-dependent transport analysis code) analysis. The variability of unstable mode growth rates is examined as a function of ion and electron collisionalities using the parallel gyrokinetic computational code GS2. Results show the microtearing mode stability dependence for a range of plasma collisionalities. Computation verifies analytic predictions that higher collisionalities than in the NSTX experiment increase microtearing instability growth rates, but that the modes are stabilized at the highest values. There is a transition of the dominant mode in the collisionality scan to ion temperature gradient character at both high and low collisionalities. The calculations suggest that plasma electron thermal confinement may be greatly improved in the low-collisionality ITER

Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part 1. Measurements of Magnetic Islands and Delta Prime.

Tearing-type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from the electron cyclotron emission (ECE) diagnostic, and an experiment where the plasma major radius is rapidly shifted ("Jog" experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing-mode eigenfunction mapped from the jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory

Marginal Stability Studies of Microturbulence Near ITB Onset on Alcator C-Mod

Insight into microturbulence and transport in tokamak plasmas is being sought using linear simulations of drift waves near the onset time of an internal transport barrier (ITB) on Alcator C-Mod. Microturbulence is likely generated by instabilities of drift waves and causes transport of heat and particles. This transport is studied because the containment of heat and particles is important for the achievement of practical nuclear fusion. We investigate nearness to marginal stability of ion temperature gradient (ITG) modes for conditions in the ITB region at the trigger time for ITB formation. Data from C-Mod, analyzed by TRANSP (a time dependent transport analysis code), is read by the code TRXPL and made into input files for the parallel gyrokinetic model code GS2. Temperature and density gradients in these input files are modified to produce new input files. Results from these simulations show a weak ITG instability in the barrier region at the time of onset, above marginal stability; the normalized critical temperature gradient is 80% of the experimental temperature gradient. The growth rate increases linearly above the critical value, with the spectrum of ITG modes remaining parabolic up to a multiplicative factor of 2. The effect of varying density gradients is found to be much weaker and causes the fastest growing drift mode to change from ITG to trapped electron mode character. Simulations were carried out on the NERSC IBM 6000 SP using 4 nodes, 16 processors per node. Predictive simulations were examined for converged instability after 10,000-50,000 timesteps in each case. Each simulation took approximately 30 minutes to complete on the IBM SP