Abstract
We present evidence that the fast-ion diffusion coefficient changes across the TFTR plasma column. Two MHD quiescent discharges are analysed: a high power D - T plasma heated with 21 MW of deuterium and tritium beams, and an ohmic plasma into which a 10 MW, 20 ms deuterium beam pulse was injected. The localized charge-exchange measurements in the ohmic plasma, and the neutron flux measurements in the D - T plasma are compared with predictions from the transport simulation code TRANSP. We have modified the code to allow modelling with arbitrary fast-ion diffusion profiles . Significant improvement in the agreement between measurement and simulation is obtained with a profile that has low values in the inner half of the plasma column and then rises rapidly towards the plasma periphery. This suggests a common underlying mechanism of enhanced beam ion transport in the two discharges, such as stochastic ripple diffusion.
Highlights
Good confinement of fast ions is essential for fusion energy production in tokamak reactors
The 3.5 MeV alphas from the d(t, n)4He nuclear reaction sustain the burning of the thermal deuterium and tritium
The evidence is based on analysis of spatially localized measurements in two quite different discharges: one is produced by injecting short beam pulses into an ohmic plasma (#49113), the other is a high power D–T plasma heated with 21 MW of deuterium and tritium beams (#73457)
Summary
Good confinement of fast ions is essential for fusion energy production in tokamak reactors. The slowing down takes place primarily on electrons (for full-energy ions), and strongly depends on the fast-ion energy and the electron temperature: the 90 keV beam ions have a steep profile, while the 30 keV beam ions have almost the same slowing time over the entire plasma column, figure 8 These features are responsible for the characteristics of the measured charge-exchange signals at 50 and 30 keV, along the chords CX–D (R = 2.44 m, ξ ∼= 0.1) and CX–F (R = 2.97 m, ξ ∼= 0.6), figures 9 and 10. The predictions for chord CX–F differ very little both in peak values and in decay rates These results confirm that the charge-exchange signals depend primarily on the beam ion density and that the beam ion transport is responsible for their features.
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